American Institute of Physics: Physics of Fluids: Table of Contents
Table of Contents for Physics of Fluids. List of articles from both the latest and ahead of print issues.
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American Institute of Physics: Physics of Fluids: Table of Contents
American Institute of Physics
enUS
Physics of Fluids
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https://aip.scitation.org/loi/phf?af=R&feed=mostrecent

Kitchen flows: Making science more accessible, affordable, and curiosity driven
https://aip.scitation.org/doi/10.1063/5.0131565?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>
Kitchen flows: Making science more accessible, affordable, and curiosity driven
10.1063/5.0131565
Physics of Fluids
20221110T12:17:36Z
© 2022 Author(s).
Gerald G. Fuller
Maciej Lisicki
Arnold J. T. M. Mathijssen
Endre J. L. Mossige
Rossana Pasquino
Vivek N. Prakash
Laurence Ramos

A review on deep reinforcement learning for fluid mechanics: An update
https://aip.scitation.org/doi/10.1063/5.0128446?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In the past couple of years, the interest of the fluid mechanics community for deep reinforcement learning techniques has increased at fast pace, leading to a growing bibliography on the topic. Due to its ability to solve complex decisionmaking problems, deep reinforcement learning has especially emerged as a valuable tool to perform flow control, but recent publications also advertise the great potential for other applications, such as shape optimization or microfluidics. The present work proposes an exhaustive review of the existing literature and is a followup to our previous review on the topic. The contributions are regrouped by the domain of application and are compared together regarding algorithmic and technical choices, such as state selection, reward design, time granularity, and more. Based on these comparisons, general conclusions are drawn regarding the current stateoftheart, and perspectives for future improvements are sketched.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In the past couple of years, the interest of the fluid mechanics community for deep reinforcement learning techniques has increased at fast pace, leading to a growing bibliography on the topic. Due to its ability to solve complex decisionmaking problems, deep reinforcement learning has especially emerged as a valuable tool to perform flow control, but recent publications also advertise the great potential for other applications, such as shape optimization or microfluidics. The present work proposes an exhaustive review of the existing literature and is a followup to our previous review on the topic. The contributions are regrouped by the domain of application and are compared together regarding algorithmic and technical choices, such as state selection, reward design, time granularity, and more. Based on these comparisons, general conclusions are drawn regarding the current stateoftheart, and perspectives for future improvements are sketched.
A review on deep reinforcement learning for fluid mechanics: An update
10.1063/5.0128446
Physics of Fluids
20221117T12:25:10Z
© 2022 Author(s).
J. Viquerat
P. Meliga
A. Larcher
E. Hachem

Analytical model for predicting the length scale of shock/boundary layer interaction with curvature
https://aip.scitation.org/doi/10.1063/5.0125439?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In practical aerodynamic problems, curved shock/boundary layer interaction (CSBLI) is more frequently encountered than the canonical shock/boundary layer interaction (SBLI). Owing to the topological complexity of the flow field brought about by shock curvature, accurate prediction of the interaction length scale of CSBLI is a challenging task. In this work, streamwise and spanwise curvatures are introduced, in turn, with the aim of establishing an analytical model for the interaction length scale of CSBLI based on conservation of mass. The validity and universality of the model are verified, which reveal the impact of the shock curvatures on the interaction, acting as the form of nonuniformity. As an example, the CSBLI with different curvatures is compared, demonstrating that a streamwise curvature of 0.071 will bring a reduction of about 16.5% of the interaction length. The proposed method can be regarded as providing a foundation for further research on CSBLI, opening new perspectives for the investigation of SBLI flow structures.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In practical aerodynamic problems, curved shock/boundary layer interaction (CSBLI) is more frequently encountered than the canonical shock/boundary layer interaction (SBLI). Owing to the topological complexity of the flow field brought about by shock curvature, accurate prediction of the interaction length scale of CSBLI is a challenging task. In this work, streamwise and spanwise curvatures are introduced, in turn, with the aim of establishing an analytical model for the interaction length scale of CSBLI based on conservation of mass. The validity and universality of the model are verified, which reveal the impact of the shock curvatures on the interaction, acting as the form of nonuniformity. As an example, the CSBLI with different curvatures is compared, demonstrating that a streamwise curvature of 0.071 will bring a reduction of about 16.5% of the interaction length. The proposed method can be regarded as providing a foundation for further research on CSBLI, opening new perspectives for the investigation of SBLI flow structures.
Analytical model for predicting the length scale of shock/boundary layer interaction with curvature
10.1063/5.0125439
Physics of Fluids
20221102T02:43:50Z
© 2022 Author(s).

Correspondence between the number of noslip critical points and nature of rear stagnation point of a symmetric object
https://aip.scitation.org/doi/10.1063/5.0122016?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>For flow around an isolated object, the points of zero vorticity/shear stress located at fluid–solid interface, i.e., the separation, reattachment points inclusive of forward and rear stagnation points are refered to as noslip critical points. The total number, [math], of such points is an even number. For flow past a diamondsection object, it is shown here that a change of the value of n by 2 alters the nature of its rear stagnation point. The rear stagnation point acts as a separation point for n = 2, 6, 10, etc. and as an attachment point for n = 4, 8, 12, etc. A pair of hypothetical mean wakes is proposed and their viability discussed with reference to results available in literature. Concerning flow past two inline diamond cylinders, the formation of an “antiwake” at the leading edge of the downstream cylinder renders its forward stagnation point to act as a separation point, which, otherwise for an isolated object, invariably serves as an attachment point. The forebody and afterbody of a symmetric object act as independent entities in influencing the nature of noslip stagnation points.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>For flow around an isolated object, the points of zero vorticity/shear stress located at fluid–solid interface, i.e., the separation, reattachment points inclusive of forward and rear stagnation points are refered to as noslip critical points. The total number, [math], of such points is an even number. For flow past a diamondsection object, it is shown here that a change of the value of n by 2 alters the nature of its rear stagnation point. The rear stagnation point acts as a separation point for n = 2, 6, 10, etc. and as an attachment point for n = 4, 8, 12, etc. A pair of hypothetical mean wakes is proposed and their viability discussed with reference to results available in literature. Concerning flow past two inline diamond cylinders, the formation of an “antiwake” at the leading edge of the downstream cylinder renders its forward stagnation point to act as a separation point, which, otherwise for an isolated object, invariably serves as an attachment point. The forebody and afterbody of a symmetric object act as independent entities in influencing the nature of noslip stagnation points.
Correspondence between the number of noslip critical points and nature of rear stagnation point of a symmetric object
10.1063/5.0122016
Physics of Fluids
20221102T02:44:15Z
© 2022 Author(s).
Shravan Kumar Mishra
Pavan Kumar Yadav
Himalaya Sarkar
Subhankar Sen

Laminar–turbulent intermittency in pipe flow for an Herschel–Bulkley fluid: Radial receptivity to finiteamplitude perturbations
https://aip.scitation.org/doi/10.1063/5.0128748?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We investigate the laminartoturbulent transition for nonNewtonian Herschel–Bulkley fluids that exhibit either a shearthinning or shearthickening behavior. The reducedorder model developed in this study also includes the effect of yieldstress for the fluid. Within our model framework, we investigate how the Newtonian dynamics change when significant nonNewtonian effects are considered either via the flow index n or the yieldstress τ0 or both. We find that an increase in τ0 as well as a decrease in n lead to a delayed transition if a perturbation of the given turbulent intensity is injected at various radial locations. As the radial position of the injection for the perturbation is varied in this study, our reducedorder model allows for the investigation of the flow receptivity to the finiteamplitude perturbations and to their radial position of inception. We observe that, for a given mean flow profile, the same perturbation becomes more prone to induce turbulence the closer it approaches the wall because of its initial amplitude being relatively higher with respect to the local mean flow. An opposite trend is found when the perturbation amplitude is rescaled on the local mean flow.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We investigate the laminartoturbulent transition for nonNewtonian Herschel–Bulkley fluids that exhibit either a shearthinning or shearthickening behavior. The reducedorder model developed in this study also includes the effect of yieldstress for the fluid. Within our model framework, we investigate how the Newtonian dynamics change when significant nonNewtonian effects are considered either via the flow index n or the yieldstress τ0 or both. We find that an increase in τ0 as well as a decrease in n lead to a delayed transition if a perturbation of the given turbulent intensity is injected at various radial locations. As the radial position of the injection for the perturbation is varied in this study, our reducedorder model allows for the investigation of the flow receptivity to the finiteamplitude perturbations and to their radial position of inception. We observe that, for a given mean flow profile, the same perturbation becomes more prone to induce turbulence the closer it approaches the wall because of its initial amplitude being relatively higher with respect to the local mean flow. An opposite trend is found when the perturbation amplitude is rescaled on the local mean flow.
Laminar–turbulent intermittency in pipe flow for an Herschel–Bulkley fluid: Radial receptivity to finiteamplitude perturbations
10.1063/5.0128748
Physics of Fluids
20221111T12:47:39Z
© 2022 Author(s).
Antoine Charles
Francesco Romanò
Thierry Ribeiro
Sam Azimi
Vincent Rocher
JeanChristophe Baudez
S. Amir Bahrani

Mitigating jitter in droplet stream by uniform charging
https://aip.scitation.org/doi/10.1063/5.0129057?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Monodisperse droplets induced by Plateau–Rayleigh instability of liquid jet are widely applied. Due to spatial jitter, the spacing between droplets becomes uneven as the working distance increases. We found that the jitter can be ameliorated by uniformly charging the droplets. Under the electrostatic forces, the droplets align at uniform spacing over a long distance. Nevertheless, radial jitter emerges when the charging voltage is too high. The effect of charging on the jitter was modeled and validated by experiments. A recommended charging parameters configuration is given considering a tradeoff between axial and radial jitter to obtain evenly distributed droplets.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Monodisperse droplets induced by Plateau–Rayleigh instability of liquid jet are widely applied. Due to spatial jitter, the spacing between droplets becomes uneven as the working distance increases. We found that the jitter can be ameliorated by uniformly charging the droplets. Under the electrostatic forces, the droplets align at uniform spacing over a long distance. Nevertheless, radial jitter emerges when the charging voltage is too high. The effect of charging on the jitter was modeled and validated by experiments. A recommended charging parameters configuration is given considering a tradeoff between axial and radial jitter to obtain evenly distributed droplets.
Mitigating jitter in droplet stream by uniform charging
10.1063/5.0129057
Physics of Fluids
20221111T12:47:40Z
© 2022 Author(s).

The erythrocyte destruction mechanism in nonphysiological shear mechanical hemolysis
https://aip.scitation.org/doi/10.1063/5.0112967?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Increasingly heart failure patients need to use Ventricular Assist Devices (VADs) to keep themselves alive. During treatment, hemolysis is an inevitable complication of interventional devices. The most common method for evaluating mechanical hemolysis is to calculate Hemolysis Index (HI) by the powerlaw formula. However, the HI formula still has obvious flaws. With an intention of further understanding the phenomenon of mechanical hemolysis in nonphysiological flow, our study developed a coarsegrained erythrocyte destruction model at the cellular scale and explored the mechanism of the single erythrocyte shear destruction utilizing the Dissipative Particle Dynamics, including the erythrocyte stretching destruction process and the erythrocyte nonphysiological shearing destruction process. In the process of stretching and shearing, the highstrain distribution areas of erythrocytes are entirely different. The highstrain areas during stretching are concentrated on the central axis. After the stretch failure, the erythrocyte changes from fusiform to shriveled biconcave. In the shear breaking process, the high strain areas are focused on the erythrocyte edge, causing the red blood cells to evolve from an ellipsoid shape to a plate shape. In addition to the flow shear stress, the shear rate acceleration is also an important factor in the erythrocyte shear damage. The erythrocyte placed in low shear stress flow is still unstably destroyed under high shear rate acceleration. Consequently, the inclusion of flowbuffering structures in the design of VADs may improve nonphysiological hemolysis.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Increasingly heart failure patients need to use Ventricular Assist Devices (VADs) to keep themselves alive. During treatment, hemolysis is an inevitable complication of interventional devices. The most common method for evaluating mechanical hemolysis is to calculate Hemolysis Index (HI) by the powerlaw formula. However, the HI formula still has obvious flaws. With an intention of further understanding the phenomenon of mechanical hemolysis in nonphysiological flow, our study developed a coarsegrained erythrocyte destruction model at the cellular scale and explored the mechanism of the single erythrocyte shear destruction utilizing the Dissipative Particle Dynamics, including the erythrocyte stretching destruction process and the erythrocyte nonphysiological shearing destruction process. In the process of stretching and shearing, the highstrain distribution areas of erythrocytes are entirely different. The highstrain areas during stretching are concentrated on the central axis. After the stretch failure, the erythrocyte changes from fusiform to shriveled biconcave. In the shear breaking process, the high strain areas are focused on the erythrocyte edge, causing the red blood cells to evolve from an ellipsoid shape to a plate shape. In addition to the flow shear stress, the shear rate acceleration is also an important factor in the erythrocyte shear damage. The erythrocyte placed in low shear stress flow is still unstably destroyed under high shear rate acceleration. Consequently, the inclusion of flowbuffering structures in the design of VADs may improve nonphysiological hemolysis.
The erythrocyte destruction mechanism in nonphysiological shear mechanical hemolysis
10.1063/5.0112967
Physics of Fluids
20221101T02:45:01Z
© 2022 Author(s).

Effects of phase difference on hydrodynamic interactions and wake patterns in highdensity fish schools
https://aip.scitation.org/doi/10.1063/5.0113826?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this study, we numerically investigate the effects of the tailbeat phase differences between the trailing fish and its neighboring fish on the hydrodynamic performance and wake dynamics in a twodimensional highdensity school. Foils undulating with a wavylike motion are employed to mimic swimming fish. The phase difference varies from 0[math] to 360[math]. A sharpinterface immersed boundary method is used to simulate flows over the fishlike bodies and provide quantitative analysis of the hydrodynamic performance and wakes of the school. It is found that the highest net thrust and swimming efficiency can be reached at the same time in the fish school with a phase difference of 180[math]. In particular, when the phase difference is 90[math], the trailing fish achieves the highest efficiency, 58% enhancement compared with a single fish, while it has the highest thrust production, increased by 108% over a single fish, at a phase difference of 0[math]. The performance and flow visualization results suggest that the phase of the trailing fish in the dense school can be controlled to improve thrust and propulsive efficiency, and these improvements occur through the hydrodynamic interactions with the vortices shed by the neighboring fish and the channel formed by the side fish. In addition, the investigation of the phase difference effects on the wake dynamics of schools performed in this work represents the first study in which the wake patterns for systems consisting of multiple undulating bodies are categorized. In particular, a reversed Bénard–von Kármán vortex wake is generated by the trailing fish in the school with a phase difference of 90[math], while a Bénard–von Kármán vortex wake is produced when the phase difference is 0[math]. Results have revealed that the wake patterns are critical to predicting the hydrodynamic performance of a fish school and are highly dependent on the phase difference.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this study, we numerically investigate the effects of the tailbeat phase differences between the trailing fish and its neighboring fish on the hydrodynamic performance and wake dynamics in a twodimensional highdensity school. Foils undulating with a wavylike motion are employed to mimic swimming fish. The phase difference varies from 0[math] to 360[math]. A sharpinterface immersed boundary method is used to simulate flows over the fishlike bodies and provide quantitative analysis of the hydrodynamic performance and wakes of the school. It is found that the highest net thrust and swimming efficiency can be reached at the same time in the fish school with a phase difference of 180[math]. In particular, when the phase difference is 90[math], the trailing fish achieves the highest efficiency, 58% enhancement compared with a single fish, while it has the highest thrust production, increased by 108% over a single fish, at a phase difference of 0[math]. The performance and flow visualization results suggest that the phase of the trailing fish in the dense school can be controlled to improve thrust and propulsive efficiency, and these improvements occur through the hydrodynamic interactions with the vortices shed by the neighboring fish and the channel formed by the side fish. In addition, the investigation of the phase difference effects on the wake dynamics of schools performed in this work represents the first study in which the wake patterns for systems consisting of multiple undulating bodies are categorized. In particular, a reversed Bénard–von Kármán vortex wake is generated by the trailing fish in the school with a phase difference of 90[math], while a Bénard–von Kármán vortex wake is produced when the phase difference is 0[math]. Results have revealed that the wake patterns are critical to predicting the hydrodynamic performance of a fish school and are highly dependent on the phase difference.
Effects of phase difference on hydrodynamic interactions and wake patterns in highdensity fish schools
10.1063/5.0113826
Physics of Fluids
20221102T02:44:35Z
© 2022 Author(s).

Study of nonNewtonian synovial fluid flow by a recursive approach
https://aip.scitation.org/doi/10.1063/5.0121918?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This study analyzes the nonNewtonian synovial fluid flow between the joints in a synovitis, which is a diseased condition due to inflammation of synovial membrane. It is assumed in this study that the secretion of synovial fluid through the inflamed synovial membrane is a linear function of the membrane length. The mathematical modeling of synovial fluid through a synovial membrane is made by the nonNewtonian Linear PhanThien–Tanner (LPTT) fluid model through a thin conduit having permeable walls. The nonlinear flow of LPTT fluid gives the nonhomogeneous complex boundary value problem, and the recursive approach is used to solve the problem. The flow of synovial fluid along and across the membrane is calculated under the inflamed membrane, and results are displayed through graphs. The axial pressure required for the nonNewtonian fluid flow and deformation of synovial fluid that produces the shearing forces near the synovial membrane are also calculated. The purpose of this research is to observe the shear stress on the synovial fluid and inflammation rate on the flow along the membrane at different position and pressure required for the flow of synovial fluid in diseased condition. The mathematical and graphical results for pressure, flow, volume flux, and streamline are calculated and plotted using the software MATHEMATICA. This study is very helpful for the biomedical engineers to measure the compression force and shear stress on the synovial fluid in a diseased condition and can be controlled by the viscosity of the synovial fluid.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This study analyzes the nonNewtonian synovial fluid flow between the joints in a synovitis, which is a diseased condition due to inflammation of synovial membrane. It is assumed in this study that the secretion of synovial fluid through the inflamed synovial membrane is a linear function of the membrane length. The mathematical modeling of synovial fluid through a synovial membrane is made by the nonNewtonian Linear PhanThien–Tanner (LPTT) fluid model through a thin conduit having permeable walls. The nonlinear flow of LPTT fluid gives the nonhomogeneous complex boundary value problem, and the recursive approach is used to solve the problem. The flow of synovial fluid along and across the membrane is calculated under the inflamed membrane, and results are displayed through graphs. The axial pressure required for the nonNewtonian fluid flow and deformation of synovial fluid that produces the shearing forces near the synovial membrane are also calculated. The purpose of this research is to observe the shear stress on the synovial fluid and inflammation rate on the flow along the membrane at different position and pressure required for the flow of synovial fluid in diseased condition. The mathematical and graphical results for pressure, flow, volume flux, and streamline are calculated and plotted using the software MATHEMATICA. This study is very helpful for the biomedical engineers to measure the compression force and shear stress on the synovial fluid in a diseased condition and can be controlled by the viscosity of the synovial fluid.
Study of nonNewtonian synovial fluid flow by a recursive approach
10.1063/5.0121918
Physics of Fluids
20221102T02:44:27Z
© 2022 Author(s).
K. Maqbool
A. M. Siddiqui
H. Mehboob
Q. Jamil

Propulsive performance of a twodimensional elliptic foil undergoing interlinked pitching and heaving
https://aip.scitation.org/doi/10.1063/5.0113647?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We computationally study the propulsive performance of a twodimensional elliptic foil undergoing interlinked pitchingheaving motion. This motion is realized by pitching the foil about an axis on its centerline outside the foil and by varying the distance between the pitching point and the leading edge. A distance of 0 and [math] corresponds to leading edge pitching and pure heaving. An inhouse fluidstructure interaction solver based on the sharp interface immersed boundary method is employed to resolve the flow field around the foil. We conducted simulations for different cases of the location of the pitching axis and pitching frequency at a Reynolds number of 100. The thrust generation is explained by the dynamics of leadingedge and trailingedge vortices. The wake corresponding to thrust is either reverse von Kármán or a deflected reverse von Kármán vortex street. Analysis revealed the existence of an optimal pitching point for maximum thrust or propulsive efficiency at a given reduced pitching frequency. The optimal regions of the thrust and propulsive efficiency are quantified as a function of reduced pitching frequency and the location of the pitching axis. The pitching point for the maximum thrust and efficiency is found to be different. We discuss the fluidmechanical reasons for the variation of propulsive performance with the location of the pitching point and the pitching frequency and corroborate our reasoning with the wake signatures.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We computationally study the propulsive performance of a twodimensional elliptic foil undergoing interlinked pitchingheaving motion. This motion is realized by pitching the foil about an axis on its centerline outside the foil and by varying the distance between the pitching point and the leading edge. A distance of 0 and [math] corresponds to leading edge pitching and pure heaving. An inhouse fluidstructure interaction solver based on the sharp interface immersed boundary method is employed to resolve the flow field around the foil. We conducted simulations for different cases of the location of the pitching axis and pitching frequency at a Reynolds number of 100. The thrust generation is explained by the dynamics of leadingedge and trailingedge vortices. The wake corresponding to thrust is either reverse von Kármán or a deflected reverse von Kármán vortex street. Analysis revealed the existence of an optimal pitching point for maximum thrust or propulsive efficiency at a given reduced pitching frequency. The optimal regions of the thrust and propulsive efficiency are quantified as a function of reduced pitching frequency and the location of the pitching axis. The pitching point for the maximum thrust and efficiency is found to be different. We discuss the fluidmechanical reasons for the variation of propulsive performance with the location of the pitching point and the pitching frequency and corroborate our reasoning with the wake signatures.
Propulsive performance of a twodimensional elliptic foil undergoing interlinked pitching and heaving
10.1063/5.0113647
Physics of Fluids
20221102T02:44:37Z
© 2022 Author(s).
Aayush Patel
Rajneesh Bhardwaj

Estimating forces from crosssectional data in the wake of flows past a plate using theoretical and datadriven models
https://aip.scitation.org/doi/10.1063/5.0125374?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We report a comparative study of theoretical and datadriven models for estimating forces from velocity data in the wake of threedimensional flows past a plate. The datasets with a range of angles of attack are calculated using the immersed boundary method. First, we develop a theoretical model to estimate forces on a flat plate from crosssectional velocity data in the far wake. This algebraic model incorporates the local momentum deficit and pressure variation. Second, we develop several datadriven models based on the convolutional neural network (CNN) for force estimation by regarding the velocity field on a series of cross sections as images. In particular, we design three CNN architectures for integrating physical information or attention mechanism, and use different training datasets for interpolation and extrapolation tasks. The model performances indicate that the optimized CNN can identify important flow regions and learn empirical physical laws. The theoretical and CNN models are assessed by multiple criteria. In general, both models are accurate (with errors less than 10%), robust, and applicable to complex wake flows. The theoretical model is superior to the CNN model in terms of the completeness, cost, and interpretability, and the CNN model with the appropriate training data and optimized CNN architecture has better description and accuracy.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We report a comparative study of theoretical and datadriven models for estimating forces from velocity data in the wake of threedimensional flows past a plate. The datasets with a range of angles of attack are calculated using the immersed boundary method. First, we develop a theoretical model to estimate forces on a flat plate from crosssectional velocity data in the far wake. This algebraic model incorporates the local momentum deficit and pressure variation. Second, we develop several datadriven models based on the convolutional neural network (CNN) for force estimation by regarding the velocity field on a series of cross sections as images. In particular, we design three CNN architectures for integrating physical information or attention mechanism, and use different training datasets for interpolation and extrapolation tasks. The model performances indicate that the optimized CNN can identify important flow regions and learn empirical physical laws. The theoretical and CNN models are assessed by multiple criteria. In general, both models are accurate (with errors less than 10%), robust, and applicable to complex wake flows. The theoretical model is superior to the CNN model in terms of the completeness, cost, and interpretability, and the CNN model with the appropriate training data and optimized CNN architecture has better description and accuracy.
Estimating forces from crosssectional data in the wake of flows past a plate using theoretical and datadriven models
10.1063/5.0125374
Physics of Fluids
20221103T12:35:13Z
© 2022 Author(s).

Amplitude reflections and interaction solutions of linear and nonlinear acoustic waves with hard and soft boundaries
https://aip.scitation.org/doi/10.1063/5.0126558?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this study, the propagation of a fundamental plane mode in a bifurcated waveguide structure with soft–hard boundaries is analyzed by using the Helmholtz equation. The explicit solution is given to this bifurcated spaced waveguide problem by means of matching the potential across the boundary of continuity. Amplitudes of the reflected field in all those regions have been evaluated, and the energy balance has been derived. We have observed the reflection of the acoustic wave against the wavenumber and shown its variation with the duct width. Convergence of the problem has been shown graphically. In our analysis, we notice that the reflected amplitude decreases as the duct spacing increases; as a result, the acoustic energy will increase as the duct spacing increases. It is expected that our analysis could be helpful to give better understanding of wave reflection in an exhaust duct system. We then reduce the linear acoustic wave equation to the Kadomtsev–Petviashvili (KP) equation. Multipleperiodic wave interaction solutions of the KP nonlinear wave equation are investigated, and the energy transfer mechanism between the primary and higher harmonics is explained, which, to the best of our knowledge, is overlooked.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this study, the propagation of a fundamental plane mode in a bifurcated waveguide structure with soft–hard boundaries is analyzed by using the Helmholtz equation. The explicit solution is given to this bifurcated spaced waveguide problem by means of matching the potential across the boundary of continuity. Amplitudes of the reflected field in all those regions have been evaluated, and the energy balance has been derived. We have observed the reflection of the acoustic wave against the wavenumber and shown its variation with the duct width. Convergence of the problem has been shown graphically. In our analysis, we notice that the reflected amplitude decreases as the duct spacing increases; as a result, the acoustic energy will increase as the duct spacing increases. It is expected that our analysis could be helpful to give better understanding of wave reflection in an exhaust duct system. We then reduce the linear acoustic wave equation to the Kadomtsev–Petviashvili (KP) equation. Multipleperiodic wave interaction solutions of the KP nonlinear wave equation are investigated, and the energy transfer mechanism between the primary and higher harmonics is explained, which, to the best of our knowledge, is overlooked.
Amplitude reflections and interaction solutions of linear and nonlinear acoustic waves with hard and soft boundaries
10.1063/5.0126558
Physics of Fluids
20221104T12:50:42Z
© 2022 Author(s).
Muhammad Ishaq
ZhiMin Chen

Unsteady aerodynamic performance of a tandem flapping–fixed airfoil configuration at low Reynolds number
https://aip.scitation.org/doi/10.1063/5.0119554?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In nature, insects with their forewings and hindwings undergoing smallgap flapping motion experience strong aerodynamic interaction. Conventional studies mainly focus on the propulsion performance of tandem flapping wings, while the interaction between a flapping wing and a fixed wing in the tandem configuration at low Reynolds numbers (Re) is unclear. In this paper, we numerically studied the aerodynamic performance and vortex structure of this tandem flapping–fixed airfoil configuration. The effects of horizontal distance (LX), vertical distance (LY), and geometric angle of attack (α) of the fixed wing on the thrust and lift performance are investigated. The results show that LX dominates the propulsion performance, while LY and α control the lift performance. The thrust enhancement of the flapping airfoil is effective only within a small range of LX, and the thrust is mainly determined by the changing rate of the impulse of the vortices directly connected to the airfoils. The lift reaches its peak when LY approaches the plunging amplitude. Compared with a fixed airfoil, the flapping–fixed configuration shows a larger lifttodrag ratio, indicating a lift enhancement led by the interaction with the upstream flapping airfoil. Moreover, increasing LY and α simultaneously can lead to additional advantages in lift generation. Further analysis shows that changes of LY and α both manifest in a variation of the effective angle of attack of the fixed airfoil, thereby manipulating its lift generation. This paper provides an aerodynamic database and guidance for the design of micro air vehicles using tandem flapping–fixed wings.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In nature, insects with their forewings and hindwings undergoing smallgap flapping motion experience strong aerodynamic interaction. Conventional studies mainly focus on the propulsion performance of tandem flapping wings, while the interaction between a flapping wing and a fixed wing in the tandem configuration at low Reynolds numbers (Re) is unclear. In this paper, we numerically studied the aerodynamic performance and vortex structure of this tandem flapping–fixed airfoil configuration. The effects of horizontal distance (LX), vertical distance (LY), and geometric angle of attack (α) of the fixed wing on the thrust and lift performance are investigated. The results show that LX dominates the propulsion performance, while LY and α control the lift performance. The thrust enhancement of the flapping airfoil is effective only within a small range of LX, and the thrust is mainly determined by the changing rate of the impulse of the vortices directly connected to the airfoils. The lift reaches its peak when LY approaches the plunging amplitude. Compared with a fixed airfoil, the flapping–fixed configuration shows a larger lifttodrag ratio, indicating a lift enhancement led by the interaction with the upstream flapping airfoil. Moreover, increasing LY and α simultaneously can lead to additional advantages in lift generation. Further analysis shows that changes of LY and α both manifest in a variation of the effective angle of attack of the fixed airfoil, thereby manipulating its lift generation. This paper provides an aerodynamic database and guidance for the design of micro air vehicles using tandem flapping–fixed wings.
Unsteady aerodynamic performance of a tandem flapping–fixed airfoil configuration at low Reynolds number
10.1063/5.0119554
Physics of Fluids
20221108T01:21:31Z
© 2022 Author(s).

Research on the hydrodynamic performance of double manta ray gliding in groups with variable attack angles
https://aip.scitation.org/doi/10.1063/5.0123371?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>To investigate the effect of arrangement and attack angle on the hydrodynamic performance of double manta rays in group gliding, a manta ray model was first established based on the actual shape of manta rays, and then, numerical simulations were carried out with Fluent software to simulate the group gliding of double manta rays in three arrangements of tandem, parallel, and vertical with variable attack angle and variable spacing. Then, the average lift/drag of the group system and the lift/drag of each individual in the group were analyzed by combining with the flow field pressure cloud. From the drag performance, a systematic drag reduction was observed for the double manta rays in tandem and parallel group gliding; in vertical cluster gliding, with the change of attack angle, the individual in the group alternately gained drag reduction, but no systematic drag reduction was found. In terms of lift performance, the average system lift is basically the same as that received by the single body when gliding, regardless of the arrangement, and the difference in lift received by each individual in the group decreases with the increase in the arrangement spacing. This study provides useful results for the formation arrangement of group gliding with twin underwater vehicles.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>To investigate the effect of arrangement and attack angle on the hydrodynamic performance of double manta rays in group gliding, a manta ray model was first established based on the actual shape of manta rays, and then, numerical simulations were carried out with Fluent software to simulate the group gliding of double manta rays in three arrangements of tandem, parallel, and vertical with variable attack angle and variable spacing. Then, the average lift/drag of the group system and the lift/drag of each individual in the group were analyzed by combining with the flow field pressure cloud. From the drag performance, a systematic drag reduction was observed for the double manta rays in tandem and parallel group gliding; in vertical cluster gliding, with the change of attack angle, the individual in the group alternately gained drag reduction, but no systematic drag reduction was found. In terms of lift performance, the average system lift is basically the same as that received by the single body when gliding, regardless of the arrangement, and the difference in lift received by each individual in the group decreases with the increase in the arrangement spacing. This study provides useful results for the formation arrangement of group gliding with twin underwater vehicles.
Research on the hydrodynamic performance of double manta ray gliding in groups with variable attack angles
10.1063/5.0123371
Physics of Fluids
20221109T12:03:10Z
© 2022 Author(s).

Effect of stomach motility on food hydrolysis and gastric emptying: Insight from computational models
https://aip.scitation.org/doi/10.1063/5.0120933?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The peristaltic motion of stomach walls combines with the secretion of digestive enzymes to initiate the process that breaks down food. In this study, the mixing, breakdown, and emptying of a liquid meal containing protein is simulated in a model of a human stomach. In this model, pepsin, the gastric enzyme responsible for protein hydrolysis, is secreted from the proximal region of the stomach walls and allowed to react with the contents of the stomach. The velocities of the retropulsive jet induced by the peristaltic motion, the emptying rate, and the extent of hydrolysis are quantified for a control case as well as for three other cases with reduced motility of the stomach, which may result from conditions such as diabetes mellitus. This study quantifies the effect of stomach motility on the rate of food breakdown and its emptying into the duodenum and we correlate these observations with the mixing in the stomach induced by the wall motion.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The peristaltic motion of stomach walls combines with the secretion of digestive enzymes to initiate the process that breaks down food. In this study, the mixing, breakdown, and emptying of a liquid meal containing protein is simulated in a model of a human stomach. In this model, pepsin, the gastric enzyme responsible for protein hydrolysis, is secreted from the proximal region of the stomach walls and allowed to react with the contents of the stomach. The velocities of the retropulsive jet induced by the peristaltic motion, the emptying rate, and the extent of hydrolysis are quantified for a control case as well as for three other cases with reduced motility of the stomach, which may result from conditions such as diabetes mellitus. This study quantifies the effect of stomach motility on the rate of food breakdown and its emptying into the duodenum and we correlate these observations with the mixing in the stomach induced by the wall motion.
Effect of stomach motility on food hydrolysis and gastric emptying: Insight from computational models
10.1063/5.0120933
Physics of Fluids
20221109T12:03:22Z
© 2022 Author(s).
Sharun Kuhar
Pankaj J Pasricha
Rajat Mittal

Flexible polymeric tail for micro robot drag reduction bioinspired by the nature microorganisms
https://aip.scitation.org/doi/10.1063/5.0107085?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In nature, most microorganisms have flexible micro/nanostructure tails, which help them create propulsion, reduce drag, or search for food. Previous studies investigated these flexible structures mostly from the propulsion creation perspective. However, the drag reduction and the underlying physical mechanisms of such tails are less known. This scientific gap is more significant when multipolymeric/hierarchical structures are used. To fill the gap, we use the dissipative particle dynamics (DPD) method as a powerful fluid–polymer interaction technique to study the flexible tails' influences on drag reduction. Note that the flow regime for these microorganisms is in the range of laminar low Reynolds number; hence, the effects of both pressure and viscous drag forces are crucial. On the other hand, in the DPD method, only the total drag force is obtained. Therefore, this paper first proposes a way to determine the contribution of viscous and pressure drags and then investigates their effects on the body of the microrobot separately. As a bioinspiredtemplated microrobot simulation, the flow over a circular cylinder with an attached flexible tail is investigated. The problem is carried out for the Reynolds numbers from 10 to 25 for different polymer lengths (single/multi) and hierarchical structure tails. Our results show that long polymer tails strongly affect pressure drag, such that the longer polymeric tails (single/multi), the more drag reduction, particularly the pressure drag. Moreover, the hierarchical structures (containing short and long tails) caused the total drag reduction mainly by decreasing the viscous drag rather than the pressure one.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In nature, most microorganisms have flexible micro/nanostructure tails, which help them create propulsion, reduce drag, or search for food. Previous studies investigated these flexible structures mostly from the propulsion creation perspective. However, the drag reduction and the underlying physical mechanisms of such tails are less known. This scientific gap is more significant when multipolymeric/hierarchical structures are used. To fill the gap, we use the dissipative particle dynamics (DPD) method as a powerful fluid–polymer interaction technique to study the flexible tails' influences on drag reduction. Note that the flow regime for these microorganisms is in the range of laminar low Reynolds number; hence, the effects of both pressure and viscous drag forces are crucial. On the other hand, in the DPD method, only the total drag force is obtained. Therefore, this paper first proposes a way to determine the contribution of viscous and pressure drags and then investigates their effects on the body of the microrobot separately. As a bioinspiredtemplated microrobot simulation, the flow over a circular cylinder with an attached flexible tail is investigated. The problem is carried out for the Reynolds numbers from 10 to 25 for different polymer lengths (single/multi) and hierarchical structure tails. Our results show that long polymer tails strongly affect pressure drag, such that the longer polymeric tails (single/multi), the more drag reduction, particularly the pressure drag. Moreover, the hierarchical structures (containing short and long tails) caused the total drag reduction mainly by decreasing the viscous drag rather than the pressure one.
Flexible polymeric tail for micro robot drag reduction bioinspired by the nature microorganisms
10.1063/5.0107085
Physics of Fluids
20221110T12:17:32Z
© 2022 Author(s).
Salar Heyat Davoudian
Khodayar Javadi

Peristaltic transport of a powerlaw fluid induced by a single wave: A numerical analysis using the cumulant lattice Boltzmann method
https://aip.scitation.org/doi/10.1063/5.0122182?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Peristaltic pumping is the primary mechanism of food transport in the human intestine. Intestinal contents are often modeled as powerlaw fluids with lowbehavior indices (n < 1). Peristaltic flows were studied for periodic contraction waves ([math]) with infinitely long wavelengths ([math]) in the Stokes flow regime ([math]). However, the peristaltic flow generated by an isolated contraction wave with a short wavelength at nonzero Reynolds numbers is more relevant to physiological conditions. In this study, we investigated the peristaltic transport of a powerlaw fluid with a low behavior index of n = 0.21 at nonzero Reynolds numbers up to Re = 10, generated by a single short contraction wave. First, we investigated the analytical solution for the peristaltic transport of the powerlaw fluid for [math] and [math]. The analytical solution shows that the discharge flow rate of a powerlaw fluid generated by a single contraction wave is much smaller than that of a Newtonian fluid (n = 1). Next, we investigated the peristaltic transport for [math] 10 using the cumulant lattice Boltzmann method. The numerical results demonstrate that the discharge flow rate for the powerlaw fluid sharply increased owing to the inertia effect. The powerlaw fluid induces an asymmetric flow field with respect to the contraction wave at smaller Reynolds numbers than Newtonian fluids. The inertia effect was increased by the sharpness of the contraction wave. These results suggest that intestinal contents can be transported more quickly by an isolated contraction wave with a shorter wavelength when the contents have low consistency indices or when the contraction wave has a large propagation velocity.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Peristaltic pumping is the primary mechanism of food transport in the human intestine. Intestinal contents are often modeled as powerlaw fluids with lowbehavior indices (n < 1). Peristaltic flows were studied for periodic contraction waves ([math]) with infinitely long wavelengths ([math]) in the Stokes flow regime ([math]). However, the peristaltic flow generated by an isolated contraction wave with a short wavelength at nonzero Reynolds numbers is more relevant to physiological conditions. In this study, we investigated the peristaltic transport of a powerlaw fluid with a low behavior index of n = 0.21 at nonzero Reynolds numbers up to Re = 10, generated by a single short contraction wave. First, we investigated the analytical solution for the peristaltic transport of the powerlaw fluid for [math] and [math]. The analytical solution shows that the discharge flow rate of a powerlaw fluid generated by a single contraction wave is much smaller than that of a Newtonian fluid (n = 1). Next, we investigated the peristaltic transport for [math] 10 using the cumulant lattice Boltzmann method. The numerical results demonstrate that the discharge flow rate for the powerlaw fluid sharply increased owing to the inertia effect. The powerlaw fluid induces an asymmetric flow field with respect to the contraction wave at smaller Reynolds numbers than Newtonian fluids. The inertia effect was increased by the sharpness of the contraction wave. These results suggest that intestinal contents can be transported more quickly by an isolated contraction wave with a shorter wavelength when the contents have low consistency indices or when the contraction wave has a large propagation velocity.
Peristaltic transport of a powerlaw fluid induced by a single wave: A numerical analysis using the cumulant lattice Boltzmann method
10.1063/5.0122182
Physics of Fluids
20221122T11:09:59Z
© 2022 Author(s).

Electrolytic flow in partially saturated charged microchannels: Electrocapillarity vs electroosmosis
https://aip.scitation.org/doi/10.1063/5.0100261?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Wettability is the main factor controlling the fluid flow in an electrically neutral partially saturated microchannel. If the microchannel body carries electric charges and is fully saturated by a conductive fluid, electroosmosis is considered the driving force for fluid movement. The flow of electrolytes in an electrically charged partially saturated microchannel, however, needs further attention where the electrocapillary and electroosmosis can simultaneously exist. We, thus, investigated the movement of KCl electrolytes with different concentrations (0.1, 0.5, 1, and 3 M) in a partially saturated (airfilled) and electrically charged microchannel fabricated in a conductive substrate (aluminum) using microfluidics. We additionally studied the contact anglebased wettability alteration of an electrolyte/air/aluminum substrate system under an electric field. This allowed us to link the change in capillary forces due to the electricityinduced wettability alteration to microfluidic flow observations, i.e., a link between electroosmosis and capillary forces. Our theoretical analysis revealed that at low concentration, the role of electroosmosis and electrocapillarity on fluid flow in partially saturated charged microchannel is relatively comparable. At 0.1 M KCl concentration, the change in wettability due to the applied electric field contributed to over 42% of the induced flow of the solution in the microchannel. As the ionic concentration increases, the role of capillary pressure fades and electroosmosis becomes the dominant process controlling the flow. At 3.0 M KCl concentration, electrocapillarity contributed only 23% to the induced flow under the applied electric field in the microchannel. The results reveal the importance of electroosmosis along with electrocapillary flow in partially saturated electrically charged microchannels.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Wettability is the main factor controlling the fluid flow in an electrically neutral partially saturated microchannel. If the microchannel body carries electric charges and is fully saturated by a conductive fluid, electroosmosis is considered the driving force for fluid movement. The flow of electrolytes in an electrically charged partially saturated microchannel, however, needs further attention where the electrocapillary and electroosmosis can simultaneously exist. We, thus, investigated the movement of KCl electrolytes with different concentrations (0.1, 0.5, 1, and 3 M) in a partially saturated (airfilled) and electrically charged microchannel fabricated in a conductive substrate (aluminum) using microfluidics. We additionally studied the contact anglebased wettability alteration of an electrolyte/air/aluminum substrate system under an electric field. This allowed us to link the change in capillary forces due to the electricityinduced wettability alteration to microfluidic flow observations, i.e., a link between electroosmosis and capillary forces. Our theoretical analysis revealed that at low concentration, the role of electroosmosis and electrocapillarity on fluid flow in partially saturated charged microchannel is relatively comparable. At 0.1 M KCl concentration, the change in wettability due to the applied electric field contributed to over 42% of the induced flow of the solution in the microchannel. As the ionic concentration increases, the role of capillary pressure fades and electroosmosis becomes the dominant process controlling the flow. At 3.0 M KCl concentration, electrocapillarity contributed only 23% to the induced flow under the applied electric field in the microchannel. The results reveal the importance of electroosmosis along with electrocapillary flow in partially saturated electrically charged microchannels.
Electrolytic flow in partially saturated charged microchannels: Electrocapillarity vs electroosmosis
10.1063/5.0100261
Physics of Fluids
20221101T11:30:12Z
© 2022 Author(s).
Mohammed Abdul Qadeer Siddiqui
Emad Sadeghinezhad
Klaus RegenauerLieb
Hamid Roshan

Alternatingcurrent nonlinear electrokinetics in microfluidic insulatordecorated bipolar electrochemistry
https://aip.scitation.org/doi/10.1063/5.0119608?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We proposed herein a unique method of insulatordecorated bipolar electrochemistry (IDBE), for realizing largescale separation of bioparticles in microchannels driven by AC dielectrophoresis (DEP). In IDBE, a pair of planar driving electrodes is placed at the bottom of channel sidewalls, between which an array of the rectangular floating electrode (FE) strips without external Ohmic contact are evenly spaced along transversal direction, and a series of insulating dielectric blocks are periodically deposited above all the interelectrode gaps and in full contact with the channel bottom surface. By creating local field maximum and minimum at multiple sites, IDBE extends well the actuating range of DEP force field from the immediate vicinity of electrode tips in traditional bipolar electrochemistry to current fluid bulk. Considering DEP force plays the dominant role around 1 MHz, we utilize Lagrange particle tracing algorithm to calculate motion trajectories of incoming samples for testing the feasibility of microchip in continuous separation of live and dead yeast cells. By applying suitable voltage parameters, highly efficient DEP sorting is theoretically achievable under a moderate inlet flow rate, where most of the viable yeasts are trapped by positiveDEP to sharp dielectric edges, while all the incoming nonviable yeasts are repelled by negativeDEP to the top surface of both FE and insulating block to form multiple thin beams coflowing into the channel outlet. The microfluidic device exploiting insulators on bipolar FE effectively expands the actuating range of nonlinear electrodynamics and provides invaluable guidelines for developing flexible electrokinetic frameworks in modern microfluidic systems.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We proposed herein a unique method of insulatordecorated bipolar electrochemistry (IDBE), for realizing largescale separation of bioparticles in microchannels driven by AC dielectrophoresis (DEP). In IDBE, a pair of planar driving electrodes is placed at the bottom of channel sidewalls, between which an array of the rectangular floating electrode (FE) strips without external Ohmic contact are evenly spaced along transversal direction, and a series of insulating dielectric blocks are periodically deposited above all the interelectrode gaps and in full contact with the channel bottom surface. By creating local field maximum and minimum at multiple sites, IDBE extends well the actuating range of DEP force field from the immediate vicinity of electrode tips in traditional bipolar electrochemistry to current fluid bulk. Considering DEP force plays the dominant role around 1 MHz, we utilize Lagrange particle tracing algorithm to calculate motion trajectories of incoming samples for testing the feasibility of microchip in continuous separation of live and dead yeast cells. By applying suitable voltage parameters, highly efficient DEP sorting is theoretically achievable under a moderate inlet flow rate, where most of the viable yeasts are trapped by positiveDEP to sharp dielectric edges, while all the incoming nonviable yeasts are repelled by negativeDEP to the top surface of both FE and insulating block to form multiple thin beams coflowing into the channel outlet. The microfluidic device exploiting insulators on bipolar FE effectively expands the actuating range of nonlinear electrodynamics and provides invaluable guidelines for developing flexible electrokinetic frameworks in modern microfluidic systems.
Alternatingcurrent nonlinear electrokinetics in microfluidic insulatordecorated bipolar electrochemistry
10.1063/5.0119608
Physics of Fluids
20221101T11:30:09Z
© 2022 Author(s).

Imbibition dynamics and steady flows in graphene nanochannels with sparse geometric and chemical defects
https://aip.scitation.org/doi/10.1063/5.0114940?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Geometric and chemical defects are frequently found or created on smooth graphene for applications of nanofluidics. In this work, imbibition dynamics and steady flows of water in graphene nanochannels with sparse defects are explored by molecular dynamics. The water contact angle is raised slightly by geometric defects (hole and protrusion) but lowered significantly by chemical defects (hydroxyl and epoxide groups). In steady flows, the mean velocity and slip length are always reduced by sparse defects and the effect of chemical defects is more significant than that of geometric defects. Moreover, it is interesting to find that the velocity profile is pluglike for geometric defects but becomes parabolic for chemical defects, regardless of the slip length. Sparse defects on graphene nanoslits also affect the imbibition dynamics remarkably, which generally follows Washburn's equation with the slip length. For chemical defects, surface friction (slip length) dominates over the driving force associated with surface wettability (contact angle). Nonetheless, for protrusion defects, the stickslip behavior caused by contact line pinning and thermal fluctuations can be observed. Our new and novel findings indicate that the defect nature is crucial in nanoscale flows and imbibition processes, which the conventional hydrodynamic theory fails to depict.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Geometric and chemical defects are frequently found or created on smooth graphene for applications of nanofluidics. In this work, imbibition dynamics and steady flows of water in graphene nanochannels with sparse defects are explored by molecular dynamics. The water contact angle is raised slightly by geometric defects (hole and protrusion) but lowered significantly by chemical defects (hydroxyl and epoxide groups). In steady flows, the mean velocity and slip length are always reduced by sparse defects and the effect of chemical defects is more significant than that of geometric defects. Moreover, it is interesting to find that the velocity profile is pluglike for geometric defects but becomes parabolic for chemical defects, regardless of the slip length. Sparse defects on graphene nanoslits also affect the imbibition dynamics remarkably, which generally follows Washburn's equation with the slip length. For chemical defects, surface friction (slip length) dominates over the driving force associated with surface wettability (contact angle). Nonetheless, for protrusion defects, the stickslip behavior caused by contact line pinning and thermal fluctuations can be observed. Our new and novel findings indicate that the defect nature is crucial in nanoscale flows and imbibition processes, which the conventional hydrodynamic theory fails to depict.
Imbibition dynamics and steady flows in graphene nanochannels with sparse geometric and chemical defects
10.1063/5.0114940
Physics of Fluids
20221101T11:29:27Z
© 2022 Author(s).

Study of flow of Buongiorno nanofluid in a conical gap between a cone and a disk
https://aip.scitation.org/doi/10.1063/5.0121642?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The cone–disk apparatus consists of a cone that touches the disk at its apex and is used in medical evices, viscosimeters, conical diffusers, etc. Theoretically, a threedimensional flow of a nanofluid in a conical gap of a cone–disk apparatus is studied for four different physical configurations. Buongiorno nanofluid model, consisting of thermophoresis and Brownian diffusion mechanisms, is used to describe the convective heat transport of the nanofluid. The continuity equation, the Navier–Stokes momentum equation, the heat equation, and the conservation of nanoparticle volume fraction equation constitute the governing system for the flow of nanofluids. The Lie group approach is used to obtain selfsimilar equations. Solutions are computed for an appropriate rotational Reynolds number and four different gap angles to examine flow, mass, and heat transport features. The skin friction coefficients and torque are computed and analyzed. Multivariate nonlinear regression analysis is also performed. A corotating disk and cone configuration has been shown to produce less torque due to the increased centrifugal force. Of the four cone–disk apparatus configurations, the maximum heat/mass transport occurs for a rotating disk with a static cone for all selected gap angles, and the least drag in the radial direction is attained for a rotating cone with a static disk. In addition, there is a minimal drag along the tangential direction for the counterrotating disk and cone configuration. Brownian diffusion and thermophoresis of the nanoparticles lead to a higher fluid temperature and, thus, lower Nusselt numbers are obtained.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The cone–disk apparatus consists of a cone that touches the disk at its apex and is used in medical evices, viscosimeters, conical diffusers, etc. Theoretically, a threedimensional flow of a nanofluid in a conical gap of a cone–disk apparatus is studied for four different physical configurations. Buongiorno nanofluid model, consisting of thermophoresis and Brownian diffusion mechanisms, is used to describe the convective heat transport of the nanofluid. The continuity equation, the Navier–Stokes momentum equation, the heat equation, and the conservation of nanoparticle volume fraction equation constitute the governing system for the flow of nanofluids. The Lie group approach is used to obtain selfsimilar equations. Solutions are computed for an appropriate rotational Reynolds number and four different gap angles to examine flow, mass, and heat transport features. The skin friction coefficients and torque are computed and analyzed. Multivariate nonlinear regression analysis is also performed. A corotating disk and cone configuration has been shown to produce less torque due to the increased centrifugal force. Of the four cone–disk apparatus configurations, the maximum heat/mass transport occurs for a rotating disk with a static cone for all selected gap angles, and the least drag in the radial direction is attained for a rotating cone with a static disk. In addition, there is a minimal drag along the tangential direction for the counterrotating disk and cone configuration. Brownian diffusion and thermophoresis of the nanoparticles lead to a higher fluid temperature and, thus, lower Nusselt numbers are obtained.
Study of flow of Buongiorno nanofluid in a conical gap between a cone and a disk
10.1063/5.0121642
Physics of Fluids
20221101T11:29:37Z
© 2022 Author(s).
Mahanthesh Basavarajappa
Dambaru Bhatta

A macroscopic and mesoscopic model of Newtonian and nonNewtonian nanofluids with a twoenergy equation method
https://aip.scitation.org/doi/10.1063/5.0124292?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We present an updated comprehensive macroscopic model of nanofluids, considering a revisited local thermal nonequilibrium (LTNE) condition to study the temperature difference between carrier fluid and nanoparticles. A new relation for thermal conductivity of solid and liquid phases in the LTNE condition is introduced which considers the possible particle aggregation. This model is thermodynamically consistent and covers the nonNewtonian models of nanofluids, including powerlaw and viscoplastic ones. A mesoscopic scheme based on the lattice Boltzmann method (LBM) which satisfies the presented macroscopic equations is introduced and derived. This investigation is a further development of our recent studies[G. H. R. Kefayati and A. Bassom, “A lattice Boltzmann method for single and two phase models of nanofluids: Newtonian and nonNewtonian nanofluids,” Phys. Fluids 33, 102008 (2021); G. H. R. Kefayati, “A two and threedimensional mesoscopic method for an updated nonhomogeneous model of Newtonian and nonNewtonian nanofluids,” Phys. Fluids 34, 032003 (2022).] for simulating and analyzing nanofluids by a twophase model. To assess the present numerical method, it is studied for a benchmark problem of natural convection in a cavity. The dimensional and nondimensional macroscopic equations for the mentioned benchmark are defined and the implemented nondimensional relations of LBM are shown. The present approach is verified with the obtained results of the mixture approach and a previous twophase model, which demonstrated the accuracy of the presented method. The results including the temperature distributions of the solid and fluid phases, the nanoparticles distributions, and fluid flow behavior as well as the yielded/unyielded sections for the viscoplastic nanofluids are shown and discussed for the defined nondimensional parameters. It was also demonstrated that the previous proposed thermal conductivity model of nanofluids in the LTNE approach generates a significantly different value compared to experimental results, and the current suggested model produces reliable results to the experimental ones.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We present an updated comprehensive macroscopic model of nanofluids, considering a revisited local thermal nonequilibrium (LTNE) condition to study the temperature difference between carrier fluid and nanoparticles. A new relation for thermal conductivity of solid and liquid phases in the LTNE condition is introduced which considers the possible particle aggregation. This model is thermodynamically consistent and covers the nonNewtonian models of nanofluids, including powerlaw and viscoplastic ones. A mesoscopic scheme based on the lattice Boltzmann method (LBM) which satisfies the presented macroscopic equations is introduced and derived. This investigation is a further development of our recent studies[G. H. R. Kefayati and A. Bassom, “A lattice Boltzmann method for single and two phase models of nanofluids: Newtonian and nonNewtonian nanofluids,” Phys. Fluids 33, 102008 (2021); G. H. R. Kefayati, “A two and threedimensional mesoscopic method for an updated nonhomogeneous model of Newtonian and nonNewtonian nanofluids,” Phys. Fluids 34, 032003 (2022).] for simulating and analyzing nanofluids by a twophase model. To assess the present numerical method, it is studied for a benchmark problem of natural convection in a cavity. The dimensional and nondimensional macroscopic equations for the mentioned benchmark are defined and the implemented nondimensional relations of LBM are shown. The present approach is verified with the obtained results of the mixture approach and a previous twophase model, which demonstrated the accuracy of the presented method. The results including the temperature distributions of the solid and fluid phases, the nanoparticles distributions, and fluid flow behavior as well as the yielded/unyielded sections for the viscoplastic nanofluids are shown and discussed for the defined nondimensional parameters. It was also demonstrated that the previous proposed thermal conductivity model of nanofluids in the LTNE approach generates a significantly different value compared to experimental results, and the current suggested model produces reliable results to the experimental ones.
A macroscopic and mesoscopic model of Newtonian and nonNewtonian nanofluids with a twoenergy equation method
10.1063/5.0124292
Physics of Fluids
20221101T12:38:28Z
© 2022 Author(s).
Gholamreza Kefayati

Particle clusters within inertial vortical flows in microcrossshaped channels
https://aip.scitation.org/doi/10.1063/5.0119418?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Inertial vortical flows can be used as a tool to capture and manipulate microparticles, vesicles, or cells. Current work follows our previous report [Zhang et al., Int. J. Multiphase Flow 150, 104030 (2022)] to study the flow of diluted particle suspension in microcrossshaped channels at 20 < Re < 500 by microlaserinduced fluorescence and highspeed photography. Effects of inlet aspect ratio (α) and Reynolds numbers (Re) on flow regimes and particle capture were studied. Numerical simulation was adopted to reveal vortex breakdown dynamics associated with particle capture. For each α, as Re increases, segregated flows, steady engulfment flows, vortex shedding flows, and unsteady engulfment flows appear in turns. Experimental results demonstrate a flowinduced, Re and αdependent particle cluster within steady engulfment and vortex shedding flows, and an increase in α decreases the onset Re of the cluster. With increasing Re, an interesting oscillation of the cluster is established, which triggers the escape of captured particles. Simulation results show that the oscillation frequencies of the cluster and fluid are comparable. Although isolated particles display brief recirculating paths under unsteady engulfment flows, the particle cluster disappears.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Inertial vortical flows can be used as a tool to capture and manipulate microparticles, vesicles, or cells. Current work follows our previous report [Zhang et al., Int. J. Multiphase Flow 150, 104030 (2022)] to study the flow of diluted particle suspension in microcrossshaped channels at 20 < Re < 500 by microlaserinduced fluorescence and highspeed photography. Effects of inlet aspect ratio (α) and Reynolds numbers (Re) on flow regimes and particle capture were studied. Numerical simulation was adopted to reveal vortex breakdown dynamics associated with particle capture. For each α, as Re increases, segregated flows, steady engulfment flows, vortex shedding flows, and unsteady engulfment flows appear in turns. Experimental results demonstrate a flowinduced, Re and αdependent particle cluster within steady engulfment and vortex shedding flows, and an increase in α decreases the onset Re of the cluster. With increasing Re, an interesting oscillation of the cluster is established, which triggers the escape of captured particles. Simulation results show that the oscillation frequencies of the cluster and fluid are comparable. Although isolated particles display brief recirculating paths under unsteady engulfment flows, the particle cluster disappears.
Particle clusters within inertial vortical flows in microcrossshaped channels
10.1063/5.0119418
Physics of Fluids
20221102T02:44:07Z
© 2022 Author(s).

Propagativerhythmic membrane contraction modulated efficient micropumping of nonNewtonian fluids
https://aip.scitation.org/doi/10.1063/5.0121704?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We here discuss a novel bioinspired pumping mechanism of nonNewtonian fluids in a microfluidic configuration, consistent with the propagative rhythmic contraction–expansion of a membrane attached to the wall of the fluidic channel. We consider the Rabinowitsch model to represent the rheology of nonNewtonian fluids. By employing lubrication theory and approximating the underlying flow to be in the creeping regime, the transport equations governing the pumping process are framed pertaining to the chosen setup. The transport equations are then evaluated by employing a wellestablished perturbation technique. By depicting the flow velocity components, streamline patterns, and velocity contours graphically, we aptly discuss the flow structure developed in the flow pathway and demonstrate the eventual consequence of these flow parameters to the net throughput during both compression and expansion phases of the pumping process. Finally, by demonstrating a phasespace diagram, we also discuss the impact of fluid rheology and membrane kinematics on the pumping capacity. The results obtained from the proposed model establish that the net flow owing to propagative rhythmic membrane contraction strongly relies on exponent parameter M and rheological parameter β. These consequences are expected to be of substantial practical relevance in designing micropumps intended to yield unidirectional flow of the complex fluids with improved efficiency, commonly used in biochemical/biomicrofluidic applications.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We here discuss a novel bioinspired pumping mechanism of nonNewtonian fluids in a microfluidic configuration, consistent with the propagative rhythmic contraction–expansion of a membrane attached to the wall of the fluidic channel. We consider the Rabinowitsch model to represent the rheology of nonNewtonian fluids. By employing lubrication theory and approximating the underlying flow to be in the creeping regime, the transport equations governing the pumping process are framed pertaining to the chosen setup. The transport equations are then evaluated by employing a wellestablished perturbation technique. By depicting the flow velocity components, streamline patterns, and velocity contours graphically, we aptly discuss the flow structure developed in the flow pathway and demonstrate the eventual consequence of these flow parameters to the net throughput during both compression and expansion phases of the pumping process. Finally, by demonstrating a phasespace diagram, we also discuss the impact of fluid rheology and membrane kinematics on the pumping capacity. The results obtained from the proposed model establish that the net flow owing to propagative rhythmic membrane contraction strongly relies on exponent parameter M and rheological parameter β. These consequences are expected to be of substantial practical relevance in designing micropumps intended to yield unidirectional flow of the complex fluids with improved efficiency, commonly used in biochemical/biomicrofluidic applications.
Propagativerhythmic membrane contraction modulated efficient micropumping of nonNewtonian fluids
10.1063/5.0121704
Physics of Fluids
20221103T12:35:41Z
© 2022 Author(s).
Jaikishan Mansukhani
Arijeet Tripathy
Mahesh Kumar
Pranab Kumar Mondal

Novel converging–diverging microchannel heat sink with porous fins for combined thermohydraulic performance
https://aip.scitation.org/doi/10.1063/5.0118700?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The optimum design of the microchannel heat sinks needs to consider both the heat transfer and pressure drop limitations of the microchannel. In this paper, a novel configuration of the microchannel heat sink is proposed to obtain improved thermohydraulic performance. The proposed microchannel includes porous fins that form adjacent converging–diverging channels. Threedimensional steady laminar simulations were conducted to access the performance of this novel microchannel and compare it with the conventional parallel ones with porous and solid fins. The results showed that by using this novel design, a 9.75% decrease in pressure drop is observed when compared to conventional solid fin parallel microchannel. Also, the mean Nusselt number of the microchannel heat sink with converging–diverging porous fins showed a maximum improvement of 16.5% compared to the parallel microchannel with solid fins. The overall thermohydraulic performance evaluation factor of the converging–diverging microchannel showed also a significant 20% improvement compared to conventional designs. The analysis of the flow fields showed that the converging–diverging design with porous fins leads to a local pressure difference between two adjacent neighboring channels inducing a crosswise velocity component within the porous fins leading to enhanced thermal performance. Moreover, it was shown that only for converging–diverging angles above 0.5 [math], performance enhancement was observed compared with a microchannel with solid fins showing the existence of an optimum range for converging–diverging angles. The response surface method was used to find the optimum range of fin porosity and converging–diverging angle where the performance of the microchannel heat sink is maximum.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The optimum design of the microchannel heat sinks needs to consider both the heat transfer and pressure drop limitations of the microchannel. In this paper, a novel configuration of the microchannel heat sink is proposed to obtain improved thermohydraulic performance. The proposed microchannel includes porous fins that form adjacent converging–diverging channels. Threedimensional steady laminar simulations were conducted to access the performance of this novel microchannel and compare it with the conventional parallel ones with porous and solid fins. The results showed that by using this novel design, a 9.75% decrease in pressure drop is observed when compared to conventional solid fin parallel microchannel. Also, the mean Nusselt number of the microchannel heat sink with converging–diverging porous fins showed a maximum improvement of 16.5% compared to the parallel microchannel with solid fins. The overall thermohydraulic performance evaluation factor of the converging–diverging microchannel showed also a significant 20% improvement compared to conventional designs. The analysis of the flow fields showed that the converging–diverging design with porous fins leads to a local pressure difference between two adjacent neighboring channels inducing a crosswise velocity component within the porous fins leading to enhanced thermal performance. Moreover, it was shown that only for converging–diverging angles above 0.5 [math], performance enhancement was observed compared with a microchannel with solid fins showing the existence of an optimum range for converging–diverging angles. The response surface method was used to find the optimum range of fin porosity and converging–diverging angle where the performance of the microchannel heat sink is maximum.
Novel converging–diverging microchannel heat sink with porous fins for combined thermohydraulic performance
10.1063/5.0118700
Physics of Fluids
20221103T12:35:26Z
© 2022 Author(s).
Fatemeh Bagherighajari
Mohammadmahdi Abdollahzadehsangroudi
Mehdi Esmaeilpour
Farid Dolati
José Páscoa

Effect of aspect ratio on entrance length in rectangular minichannels with plenum
https://aip.scitation.org/doi/10.1063/5.0119897?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In the case of a long, straight rectangular channel, hydrodynamic development of flows is influenced by the growth of the boundary layer along the walls of the channel. Though such a geometry is wellstudied in the literature, in reality, the flow often happens in channels with plenums on each end and is not studied extensively. This work addresses this gap. There is a sudden contraction from the plenum to the channel which causes the flow to separate at the entrance of the channel. Hence, the flow development is influenced not only by the boundary layer growth but also by recirculation and the presence of a continuous wall along one direction in the case of planar geometries. This causes the centerline velocity in the entrance region to overshoot the value at the fully developed region, which makes the conventional usage of 99% of the fully developed value difficult. Hence, an alternate method of defining entrance length, based on the slowest development across the channel cross section, is proposed. Based on this approach, the entrance length value shows a nonmonotonic variation with the aspect ratio (AR)—its value reduces between 0.6 and 1.66; beyond 1.66, it increases up to 20 before becoming flat. The entrance length also shows a weak dependence on the Reynolds number for AR between 2 and 20. A new set of correlations of entrance and recirculation lengths are proposed.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In the case of a long, straight rectangular channel, hydrodynamic development of flows is influenced by the growth of the boundary layer along the walls of the channel. Though such a geometry is wellstudied in the literature, in reality, the flow often happens in channels with plenums on each end and is not studied extensively. This work addresses this gap. There is a sudden contraction from the plenum to the channel which causes the flow to separate at the entrance of the channel. Hence, the flow development is influenced not only by the boundary layer growth but also by recirculation and the presence of a continuous wall along one direction in the case of planar geometries. This causes the centerline velocity in the entrance region to overshoot the value at the fully developed region, which makes the conventional usage of 99% of the fully developed value difficult. Hence, an alternate method of defining entrance length, based on the slowest development across the channel cross section, is proposed. Based on this approach, the entrance length value shows a nonmonotonic variation with the aspect ratio (AR)—its value reduces between 0.6 and 1.66; beyond 1.66, it increases up to 20 before becoming flat. The entrance length also shows a weak dependence on the Reynolds number for AR between 2 and 20. A new set of correlations of entrance and recirculation lengths are proposed.
Effect of aspect ratio on entrance length in rectangular minichannels with plenum
10.1063/5.0119897
Physics of Fluids
20221103T12:35:18Z
© 2022 Author(s).
Oswald Jason Lobo
Dhiman Chatterjee

Influences of electroosmotic flow on ionic current through nanopores: A comprehensive understanding
https://aip.scitation.org/doi/10.1063/5.0123396?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Continuum simulations become an important tool to uncover the mysteries in nanofluidic experiments. However, fluid flow in simulation models is usually unconsidered. Here, systematical simulations are conducted to provide a quantitative understanding of influences from electroosmotic flow (EOF) on ionic transport through nanopores by both types of models with and without consideration of EOF. In nanopores of less than ∼10 nm in diameter, counterions dominate ionic current, which is always promoted obviously by the convective effect of EOF. In the diameter range from ∼10 to ∼30 nm, strong EOF induces ion concentration polarization or ion depletion inside nanopores, which causes significant decreases in ionic current. For nanopores larger than ∼30 nm, due to convective promotion and inhibition of EOF on the transport of counterions and anions, considerable nanopore selectivity to counterions maintains in cases with EOF. Though the difference in total current between both cases decreases with further pore size increasing, the difference in cation/anion current is still considerable. From our results under various pore parameters and applied conditions, the fluid flow should be considered in the simulation cases when EOF is strong. Our work may provide useful guidance for simulation conductance.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Continuum simulations become an important tool to uncover the mysteries in nanofluidic experiments. However, fluid flow in simulation models is usually unconsidered. Here, systematical simulations are conducted to provide a quantitative understanding of influences from electroosmotic flow (EOF) on ionic transport through nanopores by both types of models with and without consideration of EOF. In nanopores of less than ∼10 nm in diameter, counterions dominate ionic current, which is always promoted obviously by the convective effect of EOF. In the diameter range from ∼10 to ∼30 nm, strong EOF induces ion concentration polarization or ion depletion inside nanopores, which causes significant decreases in ionic current. For nanopores larger than ∼30 nm, due to convective promotion and inhibition of EOF on the transport of counterions and anions, considerable nanopore selectivity to counterions maintains in cases with EOF. Though the difference in total current between both cases decreases with further pore size increasing, the difference in cation/anion current is still considerable. From our results under various pore parameters and applied conditions, the fluid flow should be considered in the simulation cases when EOF is strong. Our work may provide useful guidance for simulation conductance.
Influences of electroosmotic flow on ionic current through nanopores: A comprehensive understanding
10.1063/5.0123396
Physics of Fluids
20221104T12:21:56Z
© 2022 Author(s).

Effect of electromigration dispersion and nonNewtonian rheology of a charged solute in a microcapillary
https://aip.scitation.org/doi/10.1063/5.0110118?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The present work is concerned with the electromigration interaction of nonNewtonian fluid in a rectangular microcapillary under the influence of an external electric field to predict the spatiotemporal dynamics of the solute concentration due to an effective dispersion and migration velocity. The solute concentration is optimized by dispersion and a driving force exploiting the interplay between the sequential ionic distribution and the local electrical conductivity coupled with the characteristics of the fluid. The incompressible Navier–Stokes equation combined with the Poisson equation for the electric field is considered for the flow transport incorporated with the Nernst–Planck equation for the ion transport. The numerical computations are performed for the coupled electroosmosis/electrophoresis migrated nonlinear equations by a control volume approach for effective dispersion. The analytical observation of electrical conductivity in the case of a planar uniformly charged substrate is found to be varied locally near the sample peak and majorly concentration dependent. The asymptotic analysis for the velocity is made by using the lubrication approximation. The solutal species calculation is made from an area averaged nonlinear advection diffusion equation incorporating the coupled momentum equation. It is observed that the Taylor–Aries dispersion effect is dependent on the flow behavior index of the power law fluid, the flow strength, and the local sample concentration. The study of the time regime and the flow strength dependent instantaneous dispersion has also been conducted.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The present work is concerned with the electromigration interaction of nonNewtonian fluid in a rectangular microcapillary under the influence of an external electric field to predict the spatiotemporal dynamics of the solute concentration due to an effective dispersion and migration velocity. The solute concentration is optimized by dispersion and a driving force exploiting the interplay between the sequential ionic distribution and the local electrical conductivity coupled with the characteristics of the fluid. The incompressible Navier–Stokes equation combined with the Poisson equation for the electric field is considered for the flow transport incorporated with the Nernst–Planck equation for the ion transport. The numerical computations are performed for the coupled electroosmosis/electrophoresis migrated nonlinear equations by a control volume approach for effective dispersion. The analytical observation of electrical conductivity in the case of a planar uniformly charged substrate is found to be varied locally near the sample peak and majorly concentration dependent. The asymptotic analysis for the velocity is made by using the lubrication approximation. The solutal species calculation is made from an area averaged nonlinear advection diffusion equation incorporating the coupled momentum equation. It is observed that the Taylor–Aries dispersion effect is dependent on the flow behavior index of the power law fluid, the flow strength, and the local sample concentration. The study of the time regime and the flow strength dependent instantaneous dispersion has also been conducted.
Effect of electromigration dispersion and nonNewtonian rheology of a charged solute in a microcapillary
10.1063/5.0110118
Physics of Fluids
20221104T12:22:24Z
© 2022 Author(s).
A. Chatterjee
A. K. Nayak
B. Weigand

Formation of highviscosity microdroplets in Tchannels with neck structure induced by surface acoustic waves
https://aip.scitation.org/doi/10.1063/5.0118015?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In the fields of organ printing and drug preparation, highprecision and stable dispersion of highviscosity biomaterials enable precise control of organ morphology and drug release rate. This paper proposes the use of an acoustic surface wave to overcome the problem of unstable interface breakup and weak size controllability when the traditional passive droplet microfluidics is applied to highviscosity (higher than 0.4 Pa·s) dispersed phases. This paper studies the internal flow behavior of highviscosity fluid under the influence of an acoustic field and realizes the accurate prediction of formation regime and droplet size. Experimental results show that with the increase in acoustic power, three unique droplet generation regimes (e.g., long jetting, transition, and dripping) exist. The transition regime is most suitable for highthroughput preparation of highviscosity droplets, and its corresponding flow and acoustic conditions can be predicted by equation μd/μc = 4.8 × 10−8 (μc × vc/A[math] × w)−3.32. Affected by the regime transition, the droplet size increases with the increase in acoustic power. The droplet size prediction can be realized based on the capillary number Caf, which represents the intensity of the acoustic field.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In the fields of organ printing and drug preparation, highprecision and stable dispersion of highviscosity biomaterials enable precise control of organ morphology and drug release rate. This paper proposes the use of an acoustic surface wave to overcome the problem of unstable interface breakup and weak size controllability when the traditional passive droplet microfluidics is applied to highviscosity (higher than 0.4 Pa·s) dispersed phases. This paper studies the internal flow behavior of highviscosity fluid under the influence of an acoustic field and realizes the accurate prediction of formation regime and droplet size. Experimental results show that with the increase in acoustic power, three unique droplet generation regimes (e.g., long jetting, transition, and dripping) exist. The transition regime is most suitable for highthroughput preparation of highviscosity droplets, and its corresponding flow and acoustic conditions can be predicted by equation μd/μc = 4.8 × 10−8 (μc × vc/A[math] × w)−3.32. Affected by the regime transition, the droplet size increases with the increase in acoustic power. The droplet size prediction can be realized based on the capillary number Caf, which represents the intensity of the acoustic field.
Formation of highviscosity microdroplets in Tchannels with neck structure induced by surface acoustic waves
10.1063/5.0118015
Physics of Fluids
20221107T12:49:11Z
© 2022 Author(s).

Bullseye focusing of cylindrical waves at a liquid–solid interface
https://aip.scitation.org/doi/10.1063/5.0127709?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Two pairs of converging and superimposing shock and Rayleigh waves are generated on a glass substrate by focusing laser pulses on two concentric rings in a bullseye configuration (67 and 96 μm radii). We experimentally study the threshold for the substrate damage as a function of the number of repetitions and the delay (0–20 ns). The bullseye focusing experiments are compared to a single focusing ring. Additionally, fluid–structure interaction simulations using a volumeoffluid framework are utilized to estimate the stresses. The lowest number of repetitions to attain surface damage is found for constructive superposition of the Rayleigh waves, i.e., here for a delay of 10 ns. The observed damage is consistent with the simulations where the largest positive stresses ([math] GPa) are achieved for bullseye focusing with [math] ns followed by [math] ns, which corresponds to a simultaneous shock wave focusing. In all these cases, the positive stresses are followed (a few nanoseconds later) by the negative stresses that can reach [math] GPa.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Two pairs of converging and superimposing shock and Rayleigh waves are generated on a glass substrate by focusing laser pulses on two concentric rings in a bullseye configuration (67 and 96 μm radii). We experimentally study the threshold for the substrate damage as a function of the number of repetitions and the delay (0–20 ns). The bullseye focusing experiments are compared to a single focusing ring. Additionally, fluid–structure interaction simulations using a volumeoffluid framework are utilized to estimate the stresses. The lowest number of repetitions to attain surface damage is found for constructive superposition of the Rayleigh waves, i.e., here for a delay of 10 ns. The observed damage is consistent with the simulations where the largest positive stresses ([math] GPa) are achieved for bullseye focusing with [math] ns followed by [math] ns, which corresponds to a simultaneous shock wave focusing. In all these cases, the positive stresses are followed (a few nanoseconds later) by the negative stresses that can reach [math] GPa.
Bullseye focusing of cylindrical waves at a liquid–solid interface
10.1063/5.0127709
Physics of Fluids
20221108T05:44:23Z
© 2022 Author(s).
Ulisses J. GutiérrezHernández
Hendrik Reese
ClausDieter Ohl
Pedro A. QuintoSu

Unsteady flow and pressure pulsation characteristics in centrifugal pump based on dynamic mode decomposition method
https://aip.scitation.org/doi/10.1063/5.0097223?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Considering the computing accuracy and resources of numerical calculation, a hybrid Reynoldsaveraged Navier–Stokes/largeeddy simulations method based on the von Kármán scale and the corrected eddy viscosity has been used to study the unsteady flow structure and pressure pulsation characteristics in a centrifugal pump. The unsteady flow characteristics of the vertical twostage marine centrifugal pumps with complex structure have been studied. The dynamic mode decomposition method is used to study the internal unsteady flow structure and analyze the mechanism of pressure pulsation in the centrifugal pump. The results show that the unstable flow in impeller is mainly affected by the inflow state, system rotation, and the structure of the impeller. Different inflow states lead to obvious differences of the internal flow states and unsteady flow structures between the firststage and secondstage impellers. There are complex pressure pulsation characteristics dominated by different frequencies in different parts of a twostage centrifugal pump. The impeller blade main pass frequency has different causes at different locations in the downstream flow passages. The mutual matching of different numbers of impeller blades and guide vane blades will result in a kind of impeller guide vane blade interaction frequency in guide vane and volute, which will excite higher harmonics of the impeller blade frequency. Other important characteristic frequencies in centrifugal pumps had been analyzed. The pressure pulsation mechanism analysis of the centrifugal pump will help researchers to optimize the design of the centrifugal pump and improve the operation stability of the centrifugal pump. Some possible improvement measures for typical frequency pressure pulsation are recommended.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Considering the computing accuracy and resources of numerical calculation, a hybrid Reynoldsaveraged Navier–Stokes/largeeddy simulations method based on the von Kármán scale and the corrected eddy viscosity has been used to study the unsteady flow structure and pressure pulsation characteristics in a centrifugal pump. The unsteady flow characteristics of the vertical twostage marine centrifugal pumps with complex structure have been studied. The dynamic mode decomposition method is used to study the internal unsteady flow structure and analyze the mechanism of pressure pulsation in the centrifugal pump. The results show that the unstable flow in impeller is mainly affected by the inflow state, system rotation, and the structure of the impeller. Different inflow states lead to obvious differences of the internal flow states and unsteady flow structures between the firststage and secondstage impellers. There are complex pressure pulsation characteristics dominated by different frequencies in different parts of a twostage centrifugal pump. The impeller blade main pass frequency has different causes at different locations in the downstream flow passages. The mutual matching of different numbers of impeller blades and guide vane blades will result in a kind of impeller guide vane blade interaction frequency in guide vane and volute, which will excite higher harmonics of the impeller blade frequency. Other important characteristic frequencies in centrifugal pumps had been analyzed. The pressure pulsation mechanism analysis of the centrifugal pump will help researchers to optimize the design of the centrifugal pump and improve the operation stability of the centrifugal pump. Some possible improvement measures for typical frequency pressure pulsation are recommended.
Unsteady flow and pressure pulsation characteristics in centrifugal pump based on dynamic mode decomposition method
10.1063/5.0097223
Physics of Fluids
20221110T12:18:04Z
© 2022 Author(s).

A simple and accessible approach for processing photopolymer master molds for the fabrication of microfluidic polydimethylsiloxane devices
https://aip.scitation.org/doi/10.1063/5.0122055?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The use of threedimensional (3D) printing for fabrication of master molds for microfluidic devices is very attractive due to its availability and simplicity and replaces the standard methods of soft lithography. However, the commercially available photopolymer resins inhibit the curing of polydimethylsiloxane (PDMS), preventing reliable replication of 3D printed master mold structures. Here, we present a simple and safe method to postprocess 3D printed photopolymer master molds for PDMS microfluidic devices. This approach expands the possibilities of prototyping microfluidic PDMS devices for a wider research community without complex postprocessing tools currently required for fabrication of 3D photopolymer master molds.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The use of threedimensional (3D) printing for fabrication of master molds for microfluidic devices is very attractive due to its availability and simplicity and replaces the standard methods of soft lithography. However, the commercially available photopolymer resins inhibit the curing of polydimethylsiloxane (PDMS), preventing reliable replication of 3D printed master mold structures. Here, we present a simple and safe method to postprocess 3D printed photopolymer master molds for PDMS microfluidic devices. This approach expands the possibilities of prototyping microfluidic PDMS devices for a wider research community without complex postprocessing tools currently required for fabrication of 3D photopolymer master molds.
A simple and accessible approach for processing photopolymer master molds for the fabrication of microfluidic polydimethylsiloxane devices
10.1063/5.0122055
Physics of Fluids
20221114T11:44:50Z
© 2022 Author(s).
A. Otroshchenko
M. V. Zyuzin

Analysis of electroviscous effect and heat transfer for flow of nonNewtonian fluids in a microchannel with surface chargedependent slip at high zeta potentials
https://aip.scitation.org/doi/10.1063/5.0123964?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In hydrophobic surfaces, pressuredriven flows induce electrokinetic flow retardation, where the slip length decreases due to the surface charge. In the current work, we investigate the thermal transport and fluid flow behavior of a pressuredriven flow of shearthinning fluid with an electroviscous effect, accounting for the influence of surface charge on the slip. The electrical potential field induced in the electrical double layer (EDL), velocity, streaming potential, and temperature is obtained after solving the Poisson–Boltzmann equation, mass, momentum, and energy conservation equations without invoking the Debye–Hückel linearization. Results are presented for a broad range of dimensionless parameters, such as surface chargeindependent slip length, Debye–Hückel parameter, zeta potential, heat flux, and flow consistency index (n). The flow velocity decreases after considering the effect of surface charge on slip, and such decrement is more for lower value of n, higher magnitude of zeta potential, and thicker EDL. Moreover, for lower value of n (1/3), the alteration of the Nusselt number with the surface charge is nonmonotonic, whereas it increases with the surface charge magnitude for higher value of n (1/2). Further, for lower value of n, the Nusselt number enhances by the surface charge effect on the slip, whereas, for higher value of n, the trend is the opposite. Also, there is a strong interplay of the rheology of the fluid and EDL thickness in dictating the variation of the Nusselt number.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In hydrophobic surfaces, pressuredriven flows induce electrokinetic flow retardation, where the slip length decreases due to the surface charge. In the current work, we investigate the thermal transport and fluid flow behavior of a pressuredriven flow of shearthinning fluid with an electroviscous effect, accounting for the influence of surface charge on the slip. The electrical potential field induced in the electrical double layer (EDL), velocity, streaming potential, and temperature is obtained after solving the Poisson–Boltzmann equation, mass, momentum, and energy conservation equations without invoking the Debye–Hückel linearization. Results are presented for a broad range of dimensionless parameters, such as surface chargeindependent slip length, Debye–Hückel parameter, zeta potential, heat flux, and flow consistency index (n). The flow velocity decreases after considering the effect of surface charge on slip, and such decrement is more for lower value of n, higher magnitude of zeta potential, and thicker EDL. Moreover, for lower value of n (1/3), the alteration of the Nusselt number with the surface charge is nonmonotonic, whereas it increases with the surface charge magnitude for higher value of n (1/2). Further, for lower value of n, the Nusselt number enhances by the surface charge effect on the slip, whereas, for higher value of n, the trend is the opposite. Also, there is a strong interplay of the rheology of the fluid and EDL thickness in dictating the variation of the Nusselt number.
Analysis of electroviscous effect and heat transfer for flow of nonNewtonian fluids in a microchannel with surface chargedependent slip at high zeta potentials
10.1063/5.0123964
Physics of Fluids
20221114T11:44:57Z
© 2022 Author(s).
Debanjan Banerjee
Sukumar Pati
Pankaj Biswas

Simulation of electrospray emission processes for low to moderate conductivity liquids
https://aip.scitation.org/doi/10.1063/5.0120737?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The leakydielectric model is incorporated in the Finite Volume Method (FVM) code, OpenFOAM, to investigate the electrospray emission behavior of low to moderate conductivity liquids. This work extends FVM modeling to moderate conductivities by employing a new interface interpolation scheme that is devised in the volume of fluid method to ensure charge conservation for accurate reproduction of charge accumulation and resulting meniscus shape in the conetojet region and jet breakup. The model results agree well with experiments and scaling laws for droplet diameter and total current for low and moderate conductivity fluids, i.e., heptane and tributyl phosphate, respectively. The droplet diameter is shown to increase as the dimensionless flow rate increases or the electric Reynolds number decreases. The results are also consistent with a parametric investigation of the meniscus shape and the maximum charge density for key operating conditions (flow rate and extraction potential) and liquid properties (conductivity, surface tension, viscosity, and relative permittivity). These results show that the new interface interpolation scheme provides accurate results for a wide range of conductivities, fluid properties, and operating conditions. The results also provide valuable physical insight for varying liquid conductivity in the electrospray emission process. In particular, low dimensionless flow rate or high electric Reynolds number leads to the emergence of convexoutward menisci associated with a high charge density in the conetojet region, resulting in high jetting velocity and high specific charge droplets.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The leakydielectric model is incorporated in the Finite Volume Method (FVM) code, OpenFOAM, to investigate the electrospray emission behavior of low to moderate conductivity liquids. This work extends FVM modeling to moderate conductivities by employing a new interface interpolation scheme that is devised in the volume of fluid method to ensure charge conservation for accurate reproduction of charge accumulation and resulting meniscus shape in the conetojet region and jet breakup. The model results agree well with experiments and scaling laws for droplet diameter and total current for low and moderate conductivity fluids, i.e., heptane and tributyl phosphate, respectively. The droplet diameter is shown to increase as the dimensionless flow rate increases or the electric Reynolds number decreases. The results are also consistent with a parametric investigation of the meniscus shape and the maximum charge density for key operating conditions (flow rate and extraction potential) and liquid properties (conductivity, surface tension, viscosity, and relative permittivity). These results show that the new interface interpolation scheme provides accurate results for a wide range of conductivities, fluid properties, and operating conditions. The results also provide valuable physical insight for varying liquid conductivity in the electrospray emission process. In particular, low dimensionless flow rate or high electric Reynolds number leads to the emergence of convexoutward menisci associated with a high charge density in the conetojet region, resulting in high jetting velocity and high specific charge droplets.
Simulation of electrospray emission processes for low to moderate conductivity liquids
10.1063/5.0120737
Physics of Fluids
20221115T12:22:38Z
© 2022 Author(s).
Henry Huh
Richard E. Wirz

Surfacechargemobilitymodulated electrokinetic energy conversion in graphene nanochannels
https://aip.scitation.org/doi/10.1063/5.0124153?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In recent years, electrokinetic energy conversion for pressuredriven flow through hydrophobic nanochannels has attracted increasing attention from numerous researchers. However, the reported electrokinetic energy conversion efficiencies may be overestimated owing to neglect of the surface charge mobility effect of hydrophobic nanochannels. In fact, both the effective slip length and the induced streaming potential are influenced by the surface charge mobility. In this paper, a theoretical model for electrokinetic energy conversion through graphene nanochannels is developed with consideration of the influence of surface charge mobility. The surface charge density σs varies from very low to considerably high. A numerical solution to the electric potential is obtained by using the finite difference method. We also derive analytical solutions for two limiting cases, namely, the case with a low zeta potential and the case without considerable electric double layer overlap. Our results reveal that consideration of the surface charge mobility leads to a 44% reduction in the maximum conversion efficiency. The predicted maximum efficiency is approximately 5.9% at σs = −0.0162 C/m2. Our results may prove useful for predicting and optimizing the electrokinetic conversion efficiency in hydrophobic nanochannels.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In recent years, electrokinetic energy conversion for pressuredriven flow through hydrophobic nanochannels has attracted increasing attention from numerous researchers. However, the reported electrokinetic energy conversion efficiencies may be overestimated owing to neglect of the surface charge mobility effect of hydrophobic nanochannels. In fact, both the effective slip length and the induced streaming potential are influenced by the surface charge mobility. In this paper, a theoretical model for electrokinetic energy conversion through graphene nanochannels is developed with consideration of the influence of surface charge mobility. The surface charge density σs varies from very low to considerably high. A numerical solution to the electric potential is obtained by using the finite difference method. We also derive analytical solutions for two limiting cases, namely, the case with a low zeta potential and the case without considerable electric double layer overlap. Our results reveal that consideration of the surface charge mobility leads to a 44% reduction in the maximum conversion efficiency. The predicted maximum efficiency is approximately 5.9% at σs = −0.0162 C/m2. Our results may prove useful for predicting and optimizing the electrokinetic conversion efficiency in hydrophobic nanochannels.
Surfacechargemobilitymodulated electrokinetic energy conversion in graphene nanochannels
10.1063/5.0124153
Physics of Fluids
20221116T10:41:48Z
© 2022 Author(s).

Electrophoresis of a soft particle with a hydrophobic rigid core decorated with a softstep and partially ionpenetrable polymer layer
https://aip.scitation.org/doi/10.1063/5.0124145?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>On the basis of flatplate formalism, we present an analytical theory for the electrophoresis of soft particles consisting of a hydrophobic inner core decorated with a layer of inhomogeneously distributed polymer segments. Biocolloids or biocompatible drug delivery vehicles often carry the nonwettable or hydrophobic inner core. In addition, due to electrostatic swelling/shrinking processes, a spatially varying heterogeneity can be seen in the monomer distribution as well as charge properties of the peripheral polyelectrolyte layer (PEL). We adopt the softstep function to model the chemical and structural anisotropy of the peripheral PEL. In addition, the PEL for the aforementioned biosystems immersed in aquatic microenvironment often induces dielectric gradientmediated ion partitioning effect, which in turn leads to the PEL to be partially ion penetrable. Within the Debye–Hückel electrostatic framework, we derive a general expression for electrophoretic mobility of a soft particle considering the combined impacts of hydrophobicity of the inner core, inhomogeneously distributed segment distribution accompanied by chemical heterogeneity and ion partitioning effect. We further derived asymptotic limits of the more generic results detailed here under several electrostatic and hydrodynamic conditions.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>On the basis of flatplate formalism, we present an analytical theory for the electrophoresis of soft particles consisting of a hydrophobic inner core decorated with a layer of inhomogeneously distributed polymer segments. Biocolloids or biocompatible drug delivery vehicles often carry the nonwettable or hydrophobic inner core. In addition, due to electrostatic swelling/shrinking processes, a spatially varying heterogeneity can be seen in the monomer distribution as well as charge properties of the peripheral polyelectrolyte layer (PEL). We adopt the softstep function to model the chemical and structural anisotropy of the peripheral PEL. In addition, the PEL for the aforementioned biosystems immersed in aquatic microenvironment often induces dielectric gradientmediated ion partitioning effect, which in turn leads to the PEL to be partially ion penetrable. Within the Debye–Hückel electrostatic framework, we derive a general expression for electrophoretic mobility of a soft particle considering the combined impacts of hydrophobicity of the inner core, inhomogeneously distributed segment distribution accompanied by chemical heterogeneity and ion partitioning effect. We further derived asymptotic limits of the more generic results detailed here under several electrostatic and hydrodynamic conditions.
Electrophoresis of a soft particle with a hydrophobic rigid core decorated with a softstep and partially ionpenetrable polymer layer
10.1063/5.0124145
Physics of Fluids
20221117T12:25:17Z
© 2022 Author(s).
Sourav Chowdhury
Paramita Mahapatra
H. Ohshima
Partha P. Gopmandal

Droplet impact on sparse hydrophobic pillar surface: Impact phenomena,
spreading mode, and droplet breakup
https://aip.scitation.org/doi/10.1063/5.0111786?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Functional surfaces with controllable droplet spreading and breakup dynamics have
received widespread attention in selfcleaning, spraying cooling, 3D printing, etc. The
arrangement of a microstructure is of great value for the design of functional surfaces.
Here, we numerically investigated the droplet impact dynamics on the sparse hydrophobic
pillar surface with OpenFOAM. We investigated the effect of Weber number, impact
locations, and pillar spacing. Outcomes are most strongly influenced by impact locations,
pillar pitch, Weber number, and eight spreading patterns were registered, including
circle, square, crossshaped, Chinese knot, octopus, ellipse, dumbbell, and hexagram.
Furthermore, a set of theoretical models were developed for the spreading pattern
transition to predict the critical Weber number for different droplet spreading patterns.
The breakup dynamics of droplets strongly depend on the spreading patterns and the impact
location, which can emit secondary droplets in specific directions. The cross pattern
significantly reduces the threshold for secondary droplet generation. The results obtained
some essential characteristics for droplet impinging sparse hydrophobic pillar surface,
which could provide valuable insights into functional surface design, fluidicbased
systems and applications.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Functional surfaces with controllable droplet spreading and breakup dynamics have
received widespread attention in selfcleaning, spraying cooling, 3D printing, etc. The
arrangement of a microstructure is of great value for the design of functional surfaces.
Here, we numerically investigated the droplet impact dynamics on the sparse hydrophobic
pillar surface with OpenFOAM. We investigated the effect of Weber number, impact
locations, and pillar spacing. Outcomes are most strongly influenced by impact locations,
pillar pitch, Weber number, and eight spreading patterns were registered, including
circle, square, crossshaped, Chinese knot, octopus, ellipse, dumbbell, and hexagram.
Furthermore, a set of theoretical models were developed for the spreading pattern
transition to predict the critical Weber number for different droplet spreading patterns.
The breakup dynamics of droplets strongly depend on the spreading patterns and the impact
location, which can emit secondary droplets in specific directions. The cross pattern
significantly reduces the threshold for secondary droplet generation. The results obtained
some essential characteristics for droplet impinging sparse hydrophobic pillar surface,
which could provide valuable insights into functional surface design, fluidicbased
systems and applications.
Droplet impact on sparse hydrophobic pillar surface: Impact phenomena,
spreading mode, and droplet breakup
10.1063/5.0111786
Physics of Fluids
20221101T12:38:24Z
© 2022 Author(s).

Forces and charge analysis of a water droplet dragged by an electric field
https://aip.scitation.org/doi/10.1063/5.0111817?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Droplet removal from solid surfaces is particularly important for heat and mass transfer, corrosion protection, and certain technological requirements in production. In this study, we investigate droplet removal from a solid surface using an electric field. First, a visual platform was established to capture a video of the droplet deforming and jumping motion in an electric field, and a deformed ellipse equation was applied to fit the liquid droplet profiles. Second, the electric charge distribution was obtained, and the electric forces on the droplet surface before and after jumping were calculated. The result indicates that the charge only accumulates on the upper surface of the droplet, mostly at the top point, and the maximum charge of the 7 μl droplet is about 2 × 10−4 μC in this experiment. The forces on the droplet are almost constant and maintain a constant acceleration (greater than 10 m/s2) after leaving the surface. Third, the effects of droplet volume, electric field intensity, and electrode plate distance on droplet jumping were quantitatively studied. The experiments show that the electric field intensity required for droplet jumping is independent of the droplet volume but positive with the distance between the plates, when the distance between plates increases from 10 to 18 mm, the critical jumping electric field intensity increases by 0.1 kV/mm. The droplet acceleration decreases by about 20% with the increase in volume (5–10 μl) but increases with the increase in electric field intensity. The charge increases with the increase in electric field intensity, but the charge–mass ratio decreases by about 30% with the increase in volume (5–10 μl). Finally, the results show that a small volume and plate distance are more favorable to stimulating the droplets jumping under the electric field.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Droplet removal from solid surfaces is particularly important for heat and mass transfer, corrosion protection, and certain technological requirements in production. In this study, we investigate droplet removal from a solid surface using an electric field. First, a visual platform was established to capture a video of the droplet deforming and jumping motion in an electric field, and a deformed ellipse equation was applied to fit the liquid droplet profiles. Second, the electric charge distribution was obtained, and the electric forces on the droplet surface before and after jumping were calculated. The result indicates that the charge only accumulates on the upper surface of the droplet, mostly at the top point, and the maximum charge of the 7 μl droplet is about 2 × 10−4 μC in this experiment. The forces on the droplet are almost constant and maintain a constant acceleration (greater than 10 m/s2) after leaving the surface. Third, the effects of droplet volume, electric field intensity, and electrode plate distance on droplet jumping were quantitatively studied. The experiments show that the electric field intensity required for droplet jumping is independent of the droplet volume but positive with the distance between the plates, when the distance between plates increases from 10 to 18 mm, the critical jumping electric field intensity increases by 0.1 kV/mm. The droplet acceleration decreases by about 20% with the increase in volume (5–10 μl) but increases with the increase in electric field intensity. The charge increases with the increase in electric field intensity, but the charge–mass ratio decreases by about 30% with the increase in volume (5–10 μl). Finally, the results show that a small volume and plate distance are more favorable to stimulating the droplets jumping under the electric field.
Forces and charge analysis of a water droplet dragged by an electric field
10.1063/5.0111817
Physics of Fluids
20221101T11:29:42Z
© 2022 Author(s).

Equilibrium shapes of two and threedimensional twophase rotating fluid drops with surface tension: Effects of inner drop displacement
https://aip.scitation.org/doi/10.1063/5.0121208?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The shapes of rotating fluid drops held together by surface tension are an important field of study in fluid mechanics. Recently, experiments with micrometerscale droplets of liquid helium have been undertaken and it has proven useful to compare the shapes of the resultant superfluid droplets with classical analogs. If the helium is a mixture of He3 and He4, two phases are present. In a recent paper, the shapes of rotating twophase fluid droplets were calculated where the inner drop was constrained to stay at the drop center. The outer shapes and dimensionless rotation rate–angular momentum relationships were shown to be similar to singlephase drops, providing that the density and surface tension scales were chosen appropriately. In the current paper, I investigate models in which the inner drop can displace from the center. In order to simplify the analyses, twodimensional drops are first investigated. I show that the inner drop is unstable in the center position if its density is greater than the outer density and that the inner drop will move toward the outer boundary of the drop in these cases. When the inner drop has a higher density than the outer drop, the moment of inertia of displaced inner drops is increased relative to centered drops, and hence, the kinetic energy is decreased. Shapes of two and threedimensional drops, rotation rate–angular momentum, and kinetic and surface energy relationships are investigated for offaxis inner drops with parameters relevant to recent liquid He experiments.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The shapes of rotating fluid drops held together by surface tension are an important field of study in fluid mechanics. Recently, experiments with micrometerscale droplets of liquid helium have been undertaken and it has proven useful to compare the shapes of the resultant superfluid droplets with classical analogs. If the helium is a mixture of He3 and He4, two phases are present. In a recent paper, the shapes of rotating twophase fluid droplets were calculated where the inner drop was constrained to stay at the drop center. The outer shapes and dimensionless rotation rate–angular momentum relationships were shown to be similar to singlephase drops, providing that the density and surface tension scales were chosen appropriately. In the current paper, I investigate models in which the inner drop can displace from the center. In order to simplify the analyses, twodimensional drops are first investigated. I show that the inner drop is unstable in the center position if its density is greater than the outer density and that the inner drop will move toward the outer boundary of the drop in these cases. When the inner drop has a higher density than the outer drop, the moment of inertia of displaced inner drops is increased relative to centered drops, and hence, the kinetic energy is decreased. Shapes of two and threedimensional drops, rotation rate–angular momentum, and kinetic and surface energy relationships are investigated for offaxis inner drops with parameters relevant to recent liquid He experiments.
Equilibrium shapes of two and threedimensional twophase rotating fluid drops with surface tension: Effects of inner drop displacement
10.1063/5.0121208
Physics of Fluids
20221101T11:29:46Z
© 2022 Author(s).
S. L. Butler

Thirdorder less oscillatory and less diffusive compact stencilbased upwind schemes, and their applications to incompressible flows and free surface flows
https://aip.scitation.org/doi/10.1063/5.0112953?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We propose novel thirdorder less oscillatory and less diffusive compact stencilbased upwind schemes for the approximation of the continuity equation. The proposed schemes are based on the constrained interpolation profileconservative semiLagrangian schemes. An important feature of the proposed schemes is that the interpolation functions are constructed using only variables within one upwind cell (a cell average and two boundary values). Furthermore, the proposed schemes have thirdorder accuracy and are also less oscillatory, less diffusive, and fully conservative. The proposed schemes are validated through various benchmark problems and comparisons with experiments of two droplets collision/separation and droplet splashing. The numerical results have shown that the proposed schemes have thirdorder accuracy for smooth solution, and capture discontinuities and smooth solutions simultaneously without numerical oscillations. The proposed schemes can capture the secondary vorticity of liddriven cavity flow of Re = 7500 with a Cartesian grid of 64 × 64. The numerical results of two droplets collision/separation of We = 40 show that the proposed schemes can reproduce droplets collision/separation with quite coarse grids. These numerical results of droplet splashing have demonstrated that proposed schemes can reduce numerical diffusions well against existing schemes and robust.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We propose novel thirdorder less oscillatory and less diffusive compact stencilbased upwind schemes for the approximation of the continuity equation. The proposed schemes are based on the constrained interpolation profileconservative semiLagrangian schemes. An important feature of the proposed schemes is that the interpolation functions are constructed using only variables within one upwind cell (a cell average and two boundary values). Furthermore, the proposed schemes have thirdorder accuracy and are also less oscillatory, less diffusive, and fully conservative. The proposed schemes are validated through various benchmark problems and comparisons with experiments of two droplets collision/separation and droplet splashing. The numerical results have shown that the proposed schemes have thirdorder accuracy for smooth solution, and capture discontinuities and smooth solutions simultaneously without numerical oscillations. The proposed schemes can capture the secondary vorticity of liddriven cavity flow of Re = 7500 with a Cartesian grid of 64 × 64. The numerical results of two droplets collision/separation of We = 40 show that the proposed schemes can reproduce droplets collision/separation with quite coarse grids. These numerical results of droplet splashing have demonstrated that proposed schemes can reduce numerical diffusions well against existing schemes and robust.
Thirdorder less oscillatory and less diffusive compact stencilbased upwind schemes, and their applications to incompressible flows and free surface flows
10.1063/5.0112953
Physics of Fluids
20221101T11:30:28Z
© 2022 Author(s).
Kensuke Yokoi

Enhanced spreading of surfactantcontaining, selfrewetting fluids in pulmonary drug delivery
https://aip.scitation.org/doi/10.1063/5.0116016?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We study the enhanced spreading and internal diffusion of a cold, selfrewetting droplet laden with both surfactant and medicine that is placed over a hot liquid film. Spreading is induced by solutocapillary and thermocapillary effects simultaneously. A numerical simulation based on Stokes flow is performed, and the internal velocity map is obtained. The horizontal velocity flux and total medicine absorption are calculated to examine the internal diffusion and transport behaviors for a lowviscosity case and a highviscosity mucus case. The results show that solutocapillary and thermocapillary effects contribute to droplet spreading positively and negatively, respectively. Selfrewetting fluids enhance spreading by increasing the surface tension gradient and prolonging the time required for spreading to reach a steady regime. For the selfrewetting fluid case at the final calculation time, the thermoMarangoni number ΣT = 0.03, and the solutoMarangoni number ΣS = 0.9, the internal diffusion and medicine absorption are enhanced by 9.1% and 8.3% relative to the ordinary fluid, respectively. When a droplet spreads on a highviscosity mucus at the same Marangoni numbers, both spreading and diffusion are hindered. The spreading enhancement provided by selfrewetting fluids is much smaller than in lowviscosity cases. However, medicine absorption still increases by 11%.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We study the enhanced spreading and internal diffusion of a cold, selfrewetting droplet laden with both surfactant and medicine that is placed over a hot liquid film. Spreading is induced by solutocapillary and thermocapillary effects simultaneously. A numerical simulation based on Stokes flow is performed, and the internal velocity map is obtained. The horizontal velocity flux and total medicine absorption are calculated to examine the internal diffusion and transport behaviors for a lowviscosity case and a highviscosity mucus case. The results show that solutocapillary and thermocapillary effects contribute to droplet spreading positively and negatively, respectively. Selfrewetting fluids enhance spreading by increasing the surface tension gradient and prolonging the time required for spreading to reach a steady regime. For the selfrewetting fluid case at the final calculation time, the thermoMarangoni number ΣT = 0.03, and the solutoMarangoni number ΣS = 0.9, the internal diffusion and medicine absorption are enhanced by 9.1% and 8.3% relative to the ordinary fluid, respectively. When a droplet spreads on a highviscosity mucus at the same Marangoni numbers, both spreading and diffusion are hindered. The spreading enhancement provided by selfrewetting fluids is much smaller than in lowviscosity cases. However, medicine absorption still increases by 11%.
Enhanced spreading of surfactantcontaining, selfrewetting fluids in pulmonary drug delivery
10.1063/5.0116016
Physics of Fluids
20221101T11:30:18Z
© 2022 Author(s).

Deposition of micro/macroscale water droplets on grooved hydrophobic surfaces
https://aip.scitation.org/doi/10.1063/5.0119558?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Hydrophobic surfaces containing groove structures are frequently found in nature. Understanding the contact line dynamics of water droplets on such surfaces is important for analyzing the droplet motion and utilizing it for directional droplet transport. Although the visualization technique has been significantly improved, less attention has been paid to the contact line dynamics of droplets on grooved hydrophobic surfaces. Here, we fabricated hydrophobic surfaces containing grooves using a facile laser technique and visualized the advancing and receding contact line dynamics on the surfaces through highspeed imaging. In addition, the geometry of the groove structure, the droplet volume, and the inclination angle of the surface were systematically varied, and their effects on the sizes and shapes of the residual droplets deposited on the groove structures because of the sliding droplet were studied. Minute and uniform water droplets were deposited on the grooved surfaces when the structures were perpendicular to the droplet's moving path. As the droplet volume, surface inclination angle, and solid fraction of the grooved surfaces increased, the sizes of the residual droplets deposited on the grooves increased. A simple dimensionless analysis indicated that the size of the residual droplet could be predicted using the parameters tested in this study.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Hydrophobic surfaces containing groove structures are frequently found in nature. Understanding the contact line dynamics of water droplets on such surfaces is important for analyzing the droplet motion and utilizing it for directional droplet transport. Although the visualization technique has been significantly improved, less attention has been paid to the contact line dynamics of droplets on grooved hydrophobic surfaces. Here, we fabricated hydrophobic surfaces containing grooves using a facile laser technique and visualized the advancing and receding contact line dynamics on the surfaces through highspeed imaging. In addition, the geometry of the groove structure, the droplet volume, and the inclination angle of the surface were systematically varied, and their effects on the sizes and shapes of the residual droplets deposited on the groove structures because of the sliding droplet were studied. Minute and uniform water droplets were deposited on the grooved surfaces when the structures were perpendicular to the droplet's moving path. As the droplet volume, surface inclination angle, and solid fraction of the grooved surfaces increased, the sizes of the residual droplets deposited on the grooves increased. A simple dimensionless analysis indicated that the size of the residual droplet could be predicted using the parameters tested in this study.
Deposition of micro/macroscale water droplets on grooved hydrophobic surfaces
10.1063/5.0119558
Physics of Fluids
20221102T02:44:00Z
© 2022 Author(s).

The liquid film behaviors created by an inclined jet impinging on a vertical wall
https://aip.scitation.org/doi/10.1063/5.0122541?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Liquid films created by inclined jetwall impingement are commonly seen in industrial applications. We investigated the liquid film behaviors created by an inclined jet impinging on a vertical glass wall using a brightnessbased laserinduced fluorescence method. It was found that the typical liquid film by an inclined jetwall impingement consists of the thin layer zone, the raised zone, the liquid node, and the trailing edge. The liquid film expands with higher impingement velocity but keeps the same elliptical shape. A normalized linear correlation is proposed to estimate the liquid film thickness. Based on the continuity equation and the empirical convection model, the Reynolds number distribution is deduced from the film thickness distribution. The Reynolds number in the thin layer zone is less than the critical Reynolds number. The surface waves in the thin layer zone are divided into the ripple waves and the disturbance waves. The disturbance waves have a larger wavelength and amplitude than the ripple waves. The quantitative measurement of the disturbance waves shows that the wavelength and amplitude increase linearly along the radial distance. The smaller impingement velocity does not change the growth rate of the wavelength but accelerates the development of the amplitude.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Liquid films created by inclined jetwall impingement are commonly seen in industrial applications. We investigated the liquid film behaviors created by an inclined jet impinging on a vertical glass wall using a brightnessbased laserinduced fluorescence method. It was found that the typical liquid film by an inclined jetwall impingement consists of the thin layer zone, the raised zone, the liquid node, and the trailing edge. The liquid film expands with higher impingement velocity but keeps the same elliptical shape. A normalized linear correlation is proposed to estimate the liquid film thickness. Based on the continuity equation and the empirical convection model, the Reynolds number distribution is deduced from the film thickness distribution. The Reynolds number in the thin layer zone is less than the critical Reynolds number. The surface waves in the thin layer zone are divided into the ripple waves and the disturbance waves. The disturbance waves have a larger wavelength and amplitude than the ripple waves. The quantitative measurement of the disturbance waves shows that the wavelength and amplitude increase linearly along the radial distance. The smaller impingement velocity does not change the growth rate of the wavelength but accelerates the development of the amplitude.
The liquid film behaviors created by an inclined jet impinging on a vertical wall
10.1063/5.0122541
Physics of Fluids
20221102T02:43:12Z
© 2022 Author(s).

Spreading and retraction of the concentric impact of a drop with a sessile drop of the same liquid: Effect of surface wettability
https://aip.scitation.org/doi/10.1063/5.0117964?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The concentric impact on a sessile drop is relevant in many applications, including spray coating and icing phenomena. Herein, the spreading and retraction phases yielded during the impact of a coaxial drop with a sessile drop on a solid substrate were empirically and analytically examined. We analyzed the effects of surface wettability on the impact outcomes utilizing five distinctive surfaces (i.e., smooth glass, aluminum, copper, Teflon, and coated glass). The results showed that the merged drop takes longer to attain its maximum spreading diameter at a relatively higher contact angle of the sessile drop with the solid surface. Furthermore, based on energy balance, a model for predicting the maximum spreading diameter of the drop with varying surface wettability was presented. This model considers the assumption of viscous energy loss during the merging of falling and sessile drops and at the maximum spreading diameter. Additionally, the maximum retraction height during the impact on the coated glass surface was investigated. Our model results matched well with the experimental data.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The concentric impact on a sessile drop is relevant in many applications, including spray coating and icing phenomena. Herein, the spreading and retraction phases yielded during the impact of a coaxial drop with a sessile drop on a solid substrate were empirically and analytically examined. We analyzed the effects of surface wettability on the impact outcomes utilizing five distinctive surfaces (i.e., smooth glass, aluminum, copper, Teflon, and coated glass). The results showed that the merged drop takes longer to attain its maximum spreading diameter at a relatively higher contact angle of the sessile drop with the solid surface. Furthermore, based on energy balance, a model for predicting the maximum spreading diameter of the drop with varying surface wettability was presented. This model considers the assumption of viscous energy loss during the merging of falling and sessile drops and at the maximum spreading diameter. Additionally, the maximum retraction height during the impact on the coated glass surface was investigated. Our model results matched well with the experimental data.
Spreading and retraction of the concentric impact of a drop with a sessile drop of the same liquid: Effect of surface wettability
10.1063/5.0117964
Physics of Fluids
20221102T02:44:30Z
© 2022 Author(s).
Mostafa Abouelsoud
Vinod A. Thale
Ahmed N. Shmroukh

The interfacial swirling motion of twolayer liquids in a tank under orbital excitations
https://aip.scitation.org/doi/10.1063/5.0121771?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The swirling motion of the interface of twolayer liquids in a tank with a square base is investigated experimentally in this study. The tank was fixed on a platform and the horizontal circularorbital excitations were applied. Both resonant and offresonant responses were studied and the profiles of the interface between two liquids along two adjacent vertical walls of the tank were recorded and processed by using the image processing method. When the frequency of the excitation was equal to the lowest natural frequency of the interface between two liquids, the resonant response of the interface was triggered as swirling waves with strong nonlinearity. Instead of being a circular shape, the parametric curve was more of a triangular shape. The modal analysis revealed that this is caused by the additional contributions from the nonlinear modes of sloshing waves with multiple times of the natural frequency. For offresonant cases, the nonlinear contribution is weaker and the parametric curve is, thus, close to a circular shape.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The swirling motion of the interface of twolayer liquids in a tank with a square base is investigated experimentally in this study. The tank was fixed on a platform and the horizontal circularorbital excitations were applied. Both resonant and offresonant responses were studied and the profiles of the interface between two liquids along two adjacent vertical walls of the tank were recorded and processed by using the image processing method. When the frequency of the excitation was equal to the lowest natural frequency of the interface between two liquids, the resonant response of the interface was triggered as swirling waves with strong nonlinearity. Instead of being a circular shape, the parametric curve was more of a triangular shape. The modal analysis revealed that this is caused by the additional contributions from the nonlinear modes of sloshing waves with multiple times of the natural frequency. For offresonant cases, the nonlinear contribution is weaker and the parametric curve is, thus, close to a circular shape.
The interfacial swirling motion of twolayer liquids in a tank under orbital excitations
10.1063/5.0121771
Physics of Fluids
20221103T12:34:55Z
© 2022 Author(s).

Experimental study on the performance of a miniscale Ytype mixer with two liquid metalenabled pumps
https://aip.scitation.org/doi/10.1063/5.0106409?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The mixing process of two liquids inside an open minichannel was experimentally studied in the presence of liquid metal and an electric field. The Ytype mixers under study were made of Plexiglas, and two liquid metalenabled pumping systems (based on electrically induced surface tension gradients) were placed at the inlets of the mixer instead of conventional syringe pumps. The effects of the mixing angle, the voltage applied to the liquid metals, and the Reynolds number on the mixing process were investigated. To accurately determine the mixing index, the image processing toolbox of MATLAB software was employed. The results showed that the mixing intensity increased as the applied voltage signal increased, thereby creating a chaotic advection in the minichannel. Furthermore, although the Reynolds number of induced flow and the applied voltages were directly proportional, the input angle plays an important role in the mixing. Among the considered models, in the constant voltage, the 30° and 90° had the best and the worst mixing, respectively. The maximum mixing intensity of 94% was obtained at an input angle of 30° and voltage of 14 V, where, in the absence of an electric field, the maximum mixing intensity was 55%.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The mixing process of two liquids inside an open minichannel was experimentally studied in the presence of liquid metal and an electric field. The Ytype mixers under study were made of Plexiglas, and two liquid metalenabled pumping systems (based on electrically induced surface tension gradients) were placed at the inlets of the mixer instead of conventional syringe pumps. The effects of the mixing angle, the voltage applied to the liquid metals, and the Reynolds number on the mixing process were investigated. To accurately determine the mixing index, the image processing toolbox of MATLAB software was employed. The results showed that the mixing intensity increased as the applied voltage signal increased, thereby creating a chaotic advection in the minichannel. Furthermore, although the Reynolds number of induced flow and the applied voltages were directly proportional, the input angle plays an important role in the mixing. Among the considered models, in the constant voltage, the 30° and 90° had the best and the worst mixing, respectively. The maximum mixing intensity of 94% was obtained at an input angle of 30° and voltage of 14 V, where, in the absence of an electric field, the maximum mixing intensity was 55%.
Experimental study on the performance of a miniscale Ytype mixer with two liquid metalenabled pumps
10.1063/5.0106409
Physics of Fluids
20221103T12:35:29Z
© 2022 Author(s).
A. Mohammad Jafarpour
A. Rostamzadeh Khosroshahi
M. Hanifi
F. Sadegh Moghanlou

Experimental study on the collapse of tail cavity induced by underwater ventilation
https://aip.scitation.org/doi/10.1063/5.0117711?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Tail cavities are common in gaspropelled underwater cold emission and jetpropelled underwater vehicles. They can also provide a stable working environment for solid rocket motors. In this study, a 2mmdiameter ventilation hole was provided at the vehicle's rear for a ventilationinduced cavity. Then, the effect of different initial ventilation flows (Q) and Froude numbers (Fr) on cavity collapse after the air supply was stopped was studied. Three different tail cavity closure types are observed: the intact cavity (IC), partially broken cavity (PBC), and pulsating foam cavity (PFC). The IC changes from a twin vortex tube closure to a reentrant jet closure, eventually collapsing entirely. The cavity collapse time decreases with increasing Fr and increases with increasing Q. The dimensionless cavity length (L/D) has an exponential relationship with time when Fr is small and becomes linear with time when Fr is large. The cavity collapse velocity increases with increasing Fr, while Q has little effect. For PBC collapses, the cavity first transforms into an IC and then collapses as an IC. L/D first increases to a local maximum and then decreases. The effect of the reflux gas on the cavity length is critical. During PFC collapses, the cavity first transforms into a PBC, then into an IC, and finally collapses as an IC. L/D first increases to a local maximum and then decreases exponentially.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Tail cavities are common in gaspropelled underwater cold emission and jetpropelled underwater vehicles. They can also provide a stable working environment for solid rocket motors. In this study, a 2mmdiameter ventilation hole was provided at the vehicle's rear for a ventilationinduced cavity. Then, the effect of different initial ventilation flows (Q) and Froude numbers (Fr) on cavity collapse after the air supply was stopped was studied. Three different tail cavity closure types are observed: the intact cavity (IC), partially broken cavity (PBC), and pulsating foam cavity (PFC). The IC changes from a twin vortex tube closure to a reentrant jet closure, eventually collapsing entirely. The cavity collapse time decreases with increasing Fr and increases with increasing Q. The dimensionless cavity length (L/D) has an exponential relationship with time when Fr is small and becomes linear with time when Fr is large. The cavity collapse velocity increases with increasing Fr, while Q has little effect. For PBC collapses, the cavity first transforms into an IC and then collapses as an IC. L/D first increases to a local maximum and then decreases. The effect of the reflux gas on the cavity length is critical. During PFC collapses, the cavity first transforms into a PBC, then into an IC, and finally collapses as an IC. L/D first increases to a local maximum and then decreases exponentially.
Experimental study on the collapse of tail cavity induced by underwater ventilation
10.1063/5.0117711
Physics of Fluids
20221103T12:35:22Z
© 2022 Author(s).

Normal and tangential contact models for mixed lubrication of mechanical interface
https://aip.scitation.org/doi/10.1063/5.0125283?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Lubricating oil is usually injected in the gap between mechanical interfaces to reduce the friction and wear and improve the normal bearing capacity of the interface. The state of the mixed lubrication is induced from the inadequate lubrication. However, in the investigation of this state, the irregular oil pits of rough surfaces are usually simplified to be the cylindrical or conical in traditional contact models, which is disagreed with the actual contact situations and makes errors in the contact analysis when it is used to reveal the contact performance of the mixed lubrication. To investigate the normal and tangential contact performances for the real mixed lubrication, the normal and tangential contact models reproducing the effects of both the surface roughness and the lubrication viscosity on the normal contact force as well as the tangential fraction force are proposed in this work. Based on the statistical theory, the total area of oil pits is presented to describe irregular oil pits of rough surfaces. Then, the contact performance between the solid and liquid parts involved in the mixed lubrication is analyzed referring to the elastic–plastic theory and the hydrodynamics theory. Finally, several key influencing factors (including the surface roughness, the film thickness, and the lubrication oil viscosity) on the normal and tangential contact performances of mechanical interface are revealed. The main contribution of this work is providing some guidance on the improvement of the normal and tangential contact performances of the mechanical interface by adjusting the normal pressure, the surface roughness, the relative movement speed, and the viscosity of the lubricating medium.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Lubricating oil is usually injected in the gap between mechanical interfaces to reduce the friction and wear and improve the normal bearing capacity of the interface. The state of the mixed lubrication is induced from the inadequate lubrication. However, in the investigation of this state, the irregular oil pits of rough surfaces are usually simplified to be the cylindrical or conical in traditional contact models, which is disagreed with the actual contact situations and makes errors in the contact analysis when it is used to reveal the contact performance of the mixed lubrication. To investigate the normal and tangential contact performances for the real mixed lubrication, the normal and tangential contact models reproducing the effects of both the surface roughness and the lubrication viscosity on the normal contact force as well as the tangential fraction force are proposed in this work. Based on the statistical theory, the total area of oil pits is presented to describe irregular oil pits of rough surfaces. Then, the contact performance between the solid and liquid parts involved in the mixed lubrication is analyzed referring to the elastic–plastic theory and the hydrodynamics theory. Finally, several key influencing factors (including the surface roughness, the film thickness, and the lubrication oil viscosity) on the normal and tangential contact performances of mechanical interface are revealed. The main contribution of this work is providing some guidance on the improvement of the normal and tangential contact performances of the mechanical interface by adjusting the normal pressure, the surface roughness, the relative movement speed, and the viscosity of the lubricating medium.
Normal and tangential contact models for mixed lubrication of mechanical interface
10.1063/5.0125283
Physics of Fluids
20221104T01:20:26Z
© 2022 Author(s).

Threedimensional printed liquid diodes with tunable velocity: Design guidelines and applications for liquid collection and transport
https://aip.scitation.org/doi/10.1063/5.0122281?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Directional and selfpropelled flow in open channels has a variety of applications, including microfluidic and medical devices, industrial filtration processes, fogharvesting, and condensing apparatuses. Here, we present versatile threedimensionalprinted liquid diodes that enable spontaneous unidirectional flow over long distances for a wide range of liquid contact angles (CAs). Typically, we can achieve average flow velocities of several millimeters per second over a distance of tens to hundreds millimeters. The diodes have two key design principles. First, a sudden widening in the channels' width, in combination with a small bump, the pitch, ensure pinning of the liquid in the backward direction. Second, an adjustable reservoir with differing expansion angles, the bulga, is introduced to manipulate the liquid velocity. Using a combination of experiments and lattice Boltzmann simulations, we provide a comprehensive analysis of the flow behavior and speed within the channels depending on CAs, pitch heights, and bulga angles. This provides guidelines for the fabrication of bespoke liquid diodes with optimal design for their potential applications. As a feasibility investigation, we test our design for condensation of water from fog and subsequent transport uphill.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Directional and selfpropelled flow in open channels has a variety of applications, including microfluidic and medical devices, industrial filtration processes, fogharvesting, and condensing apparatuses. Here, we present versatile threedimensionalprinted liquid diodes that enable spontaneous unidirectional flow over long distances for a wide range of liquid contact angles (CAs). Typically, we can achieve average flow velocities of several millimeters per second over a distance of tens to hundreds millimeters. The diodes have two key design principles. First, a sudden widening in the channels' width, in combination with a small bump, the pitch, ensure pinning of the liquid in the backward direction. Second, an adjustable reservoir with differing expansion angles, the bulga, is introduced to manipulate the liquid velocity. Using a combination of experiments and lattice Boltzmann simulations, we provide a comprehensive analysis of the flow behavior and speed within the channels depending on CAs, pitch heights, and bulga angles. This provides guidelines for the fabrication of bespoke liquid diodes with optimal design for their potential applications. As a feasibility investigation, we test our design for condensation of water from fog and subsequent transport uphill.
Threedimensional printed liquid diodes with tunable velocity: Design guidelines and applications for liquid collection and transport
10.1063/5.0122281
Physics of Fluids
20221107T12:48:53Z
© 2022 Author(s).
Camilla Sammartino
Michael Rennick
Halim Kusumaatmaja
BatEl Pinchasik

Influence of the interaction of capillary waves on the dynamics of two drops falling sidebyside on a liquid pool
https://aip.scitation.org/doi/10.1063/5.0121615?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We experimentally examine the coalescence dynamics of two ethanol drops of equal and unequal size, impacting a deep ethanol pool at low impact velocity using a highspeed shadowgraph. By altering the separation distance between the drops and their size ratios, different coalescence outcomes, such as total coalescence, interacting partial coalescence, and noninteracting partial coalescence, have been observed. Two distinct dynamics have been identified, namely, (i) when the primary drops coalesce first before the resulting conglomerate coalesces into the liquid pool and (ii) when the drops coalesce in the liquid pool separately, resulting in capillary waves interaction and affecting the coalescence outcomes. We also observe another fascinating phenomenon for certain parameters as the satellite drops coalesce as they ascend from the liquid pool. It is found that the coalescence time delay between the drops influences the size of the secondary drops by changing the dynamics from the interacting to noninteracting partial coalescence behavior at the coalescence time delay of 1.31. Our results also indicate that when the normalized separation distance between the dispensing needles is greater than 3.2, the capillary waves produced from both the drops do not interact, and the drops exhibit a usual partial coalescence like two single individual drops.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We experimentally examine the coalescence dynamics of two ethanol drops of equal and unequal size, impacting a deep ethanol pool at low impact velocity using a highspeed shadowgraph. By altering the separation distance between the drops and their size ratios, different coalescence outcomes, such as total coalescence, interacting partial coalescence, and noninteracting partial coalescence, have been observed. Two distinct dynamics have been identified, namely, (i) when the primary drops coalesce first before the resulting conglomerate coalesces into the liquid pool and (ii) when the drops coalesce in the liquid pool separately, resulting in capillary waves interaction and affecting the coalescence outcomes. We also observe another fascinating phenomenon for certain parameters as the satellite drops coalesce as they ascend from the liquid pool. It is found that the coalescence time delay between the drops influences the size of the secondary drops by changing the dynamics from the interacting to noninteracting partial coalescence behavior at the coalescence time delay of 1.31. Our results also indicate that when the normalized separation distance between the dispensing needles is greater than 3.2, the capillary waves produced from both the drops do not interact, and the drops exhibit a usual partial coalescence like two single individual drops.
Influence of the interaction of capillary waves on the dynamics of two drops falling sidebyside on a liquid pool
10.1063/5.0121615
Physics of Fluids
20221107T12:48:42Z
© 2022 Author(s).
Pavan Kumar Kirar
Sumedha D. Pokale
Kirti Chandra Sahu
Bahni Ray
Gautam Biswas

Isothermal and nonisothermal spreading of a viscous droplet on a solid surface in total wetting condition
https://aip.scitation.org/doi/10.1063/5.0122220?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We study the isothermal and nonisothermal spreading of viscous silicone oil droplets on a glass surface in total wetting condition. In particular, the effects of viscosity, impact velocity, and substrate temperature on the spreading dynamics are reported. We employ highspeed photography to record timevarying droplet shapes from the side. An infrared camera maps the temperature distribution on the liquid–gas interface. In the isothermal inertialcapillary or early regime, the initial spreading is driven by inertial forces, and kinetic energy converts into surface energy and gets dissipated by bulk viscosity. The later stage is governed by the balance of surface energy and viscosity dissipation, i.e., capillary–viscous or late regime. The characteristics timescales of the two regimes are obtained using scaling arguments. The measured crossover time from early to late spreading regimes for different cases of impact velocity and viscosity corroborates with a scaling analysis developed in the present work. Measurements confirm the value of exponents of established powerlaw spreading with time in early and late regimes [math]. At a larger substrate temperature, the spreading magnitude is larger for droplets with larger viscosity and is explained by the reduction of viscous dissipation by heating the droplet. However, in the case of nonisothermal spreading of a low viscosity droplet, recoiling after the early spreading reduces the spreading magnitude compared to the isothermal case. We explain the recoiling and spreading rates obtained in different cases. We analyze unsteady heat transfer between the droplet and substrate by combining measurements and a numerical model.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We study the isothermal and nonisothermal spreading of viscous silicone oil droplets on a glass surface in total wetting condition. In particular, the effects of viscosity, impact velocity, and substrate temperature on the spreading dynamics are reported. We employ highspeed photography to record timevarying droplet shapes from the side. An infrared camera maps the temperature distribution on the liquid–gas interface. In the isothermal inertialcapillary or early regime, the initial spreading is driven by inertial forces, and kinetic energy converts into surface energy and gets dissipated by bulk viscosity. The later stage is governed by the balance of surface energy and viscosity dissipation, i.e., capillary–viscous or late regime. The characteristics timescales of the two regimes are obtained using scaling arguments. The measured crossover time from early to late spreading regimes for different cases of impact velocity and viscosity corroborates with a scaling analysis developed in the present work. Measurements confirm the value of exponents of established powerlaw spreading with time in early and late regimes [math]. At a larger substrate temperature, the spreading magnitude is larger for droplets with larger viscosity and is explained by the reduction of viscous dissipation by heating the droplet. However, in the case of nonisothermal spreading of a low viscosity droplet, recoiling after the early spreading reduces the spreading magnitude compared to the isothermal case. We explain the recoiling and spreading rates obtained in different cases. We analyze unsteady heat transfer between the droplet and substrate by combining measurements and a numerical model.
Isothermal and nonisothermal spreading of a viscous droplet on a solid surface in total wetting condition
10.1063/5.0122220
Physics of Fluids
20221108T03:56:32Z
© 2022 Author(s).
Prathamesh G. Bange
Gaurav Upadhyay
Nagesh D. Patil
Rajneesh Bhardwaj

Deformation and necking of liquid droplets in a magnetic field
https://aip.scitation.org/doi/10.1063/5.0119614?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Pendant droplets of water and paramagnetic solutions are studied in the presence of uniform and nonuniform magnetic fields produced by small permanent magnet arrays, both in static conditions and during dynamic pinchoff. Static measurements of the droplet shape are analyzed in terms of an apparent surface tension γapp or an effective density ρeff. The change of surface tension of deionized water in a uniform field of 450 mT is insignificant, 0.19 ± 0.21 mNm−1. Measurements on droplets of compensated zerosusceptibility solutions of Cu2+, Mn2+, and Dy3+, where the shape is unaffected by any magnetic body force, show changes of surface tension of about −1% in 500 mT. Magnetic field gradients of up to 100 T2 m−1 deform the droplets and lead to changes of ρeff that are negative for diamagnetic solutions (buoyancy effect) and positive for paramagnetic solutions. The droplet profile of strongly paramagnetic 0.1 molar DyCl3 solution is analyzed, treating the nonuniform vertical field gradient as a spatial variation of gravity. The influence of Maxwell stress on the droplet shape is discussed. In dynamic measurements, the droplet shape at pinchoff is recorded by highspeed photography and analyzed in terms of a relative change of dynamic surface tension in the presence of a magnetic field. The surfacetensiondependent prefactor of the scaling law that governs the pinchoff dynamics shows no difference for pure water or 0.11 M DyCl3 solutions in the field. The nonuniform field has no influence in the pinchoff region because the filament diameter is much less than the capillary length.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Pendant droplets of water and paramagnetic solutions are studied in the presence of uniform and nonuniform magnetic fields produced by small permanent magnet arrays, both in static conditions and during dynamic pinchoff. Static measurements of the droplet shape are analyzed in terms of an apparent surface tension γapp or an effective density ρeff. The change of surface tension of deionized water in a uniform field of 450 mT is insignificant, 0.19 ± 0.21 mNm−1. Measurements on droplets of compensated zerosusceptibility solutions of Cu2+, Mn2+, and Dy3+, where the shape is unaffected by any magnetic body force, show changes of surface tension of about −1% in 500 mT. Magnetic field gradients of up to 100 T2 m−1 deform the droplets and lead to changes of ρeff that are negative for diamagnetic solutions (buoyancy effect) and positive for paramagnetic solutions. The droplet profile of strongly paramagnetic 0.1 molar DyCl3 solution is analyzed, treating the nonuniform vertical field gradient as a spatial variation of gravity. The influence of Maxwell stress on the droplet shape is discussed. In dynamic measurements, the droplet shape at pinchoff is recorded by highspeed photography and analyzed in terms of a relative change of dynamic surface tension in the presence of a magnetic field. The surfacetensiondependent prefactor of the scaling law that governs the pinchoff dynamics shows no difference for pure water or 0.11 M DyCl3 solutions in the field. The nonuniform field has no influence in the pinchoff region because the filament diameter is much less than the capillary length.
Deformation and necking of liquid droplets in a magnetic field
10.1063/5.0119614
Physics of Fluids
20221108T01:21:37Z
© 2022 Author(s).
Sruthy Poulose
Jennifer A. Quirke
Plamen Stamenov
Matthias E. Möbius
J. M. D. Coey

Contact angle ageing and anomalous capillary flow of a molten metal
https://aip.scitation.org/doi/10.1063/5.0123707?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Recent capillary flow experiments, conducted on a combined wetting/nonwetting assemble, consistently feature an anomalous flow over the nonwetting substrate: (i) apparent abrupt or gradual recession stages in the motion of the contact line, (ii) nonmonotonic abrupt changes in the receding contact angle, and (iii) contact angle overshoot above the nominal equilibrium contact angle. We find that such behavior of a liquid metal alloy cannot be explained by the standard capillary flow models. However, a model that includes the ageing of the equilibrium contact angle predicts all the observed features qualitatively. We use the phase field formulation for capillary flows with a diffusive motion of the triple line to accommodate the novel diffusive boundary condition with the timeevolving quasiequilibrium contact angle. We discover that the observed anomalies in capillary flow are qualitatively explained by two factors: (1) time evolution (ageing) of the quasiequilibrium contact angle and (2) high viscosity of the partially molten braze. We also discover that for the given flow geometry, the transition from the initial to the final configuration may follow two distinct topological paths: one is characterized by a coalescence of liquid–solid contact domains, the other by a contact separation. The selection of the two paths in the configurational space is dependent on both contact ageing parameters and viscosity.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Recent capillary flow experiments, conducted on a combined wetting/nonwetting assemble, consistently feature an anomalous flow over the nonwetting substrate: (i) apparent abrupt or gradual recession stages in the motion of the contact line, (ii) nonmonotonic abrupt changes in the receding contact angle, and (iii) contact angle overshoot above the nominal equilibrium contact angle. We find that such behavior of a liquid metal alloy cannot be explained by the standard capillary flow models. However, a model that includes the ageing of the equilibrium contact angle predicts all the observed features qualitatively. We use the phase field formulation for capillary flows with a diffusive motion of the triple line to accommodate the novel diffusive boundary condition with the timeevolving quasiequilibrium contact angle. We discover that the observed anomalies in capillary flow are qualitatively explained by two factors: (1) time evolution (ageing) of the quasiequilibrium contact angle and (2) high viscosity of the partially molten braze. We also discover that for the given flow geometry, the transition from the initial to the final configuration may follow two distinct topological paths: one is characterized by a coalescence of liquid–solid contact domains, the other by a contact separation. The selection of the two paths in the configurational space is dependent on both contact ageing parameters and viscosity.
Contact angle ageing and anomalous capillary flow of a molten metal
10.1063/5.0123707
Physics of Fluids
20221109T12:03:25Z
© 2022 Author(s).
Konstantinos Lazaridis
Yangyang Wu
Santhosh K. Muniyal Krishna
ChengNien Yu
Mikhail D. Krivilyov
Dusan P. Sekulic
Sinisa Dj. Mesarovic

Formation and breakup of twisting ligaments in a viscous swirling liquid jet
https://aip.scitation.org/doi/10.1063/5.0122754?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We analyze the successive steps of the breakup morphology of a swirling liquid jet. Threedimensional numerical simulations are carried out using the Volume of Fluid method with adaptive mesh refinement for axial Reynolds numbers of 50 and swirl numbers of [math]. We present fundamental flow features of the swirling jet in terms of timeaveraged axial and azimuthal velocity profiles for the considered range of swirl numbers. The provision of a swirl induces helical disturbance at the interface of the jet, which exhibits an azimuthal mode number of m = 4. We identified that viscous forces are the most dominant force in the flow, which causes the suppression of Kelvin–Helmholtz instability at the interface. In contrast, we found the existence of centrifugal instability, which destabilizes the helical rim developing at the interface. As a result, centrifugally induced corrugations in the form of tiny protrusions develop along each of the helical rims, which triggers Rayleigh–Taylor instability. Subsequently, these tiny protrusions get stretched in the radially outward direction and transform into twisting ligaments that break into droplets. We have elucidated the mechanism for the twisting of ligaments and its further disintegration into firstgeneration droplets, which has not been reported in previous studies.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We analyze the successive steps of the breakup morphology of a swirling liquid jet. Threedimensional numerical simulations are carried out using the Volume of Fluid method with adaptive mesh refinement for axial Reynolds numbers of 50 and swirl numbers of [math]. We present fundamental flow features of the swirling jet in terms of timeaveraged axial and azimuthal velocity profiles for the considered range of swirl numbers. The provision of a swirl induces helical disturbance at the interface of the jet, which exhibits an azimuthal mode number of m = 4. We identified that viscous forces are the most dominant force in the flow, which causes the suppression of Kelvin–Helmholtz instability at the interface. In contrast, we found the existence of centrifugal instability, which destabilizes the helical rim developing at the interface. As a result, centrifugally induced corrugations in the form of tiny protrusions develop along each of the helical rims, which triggers Rayleigh–Taylor instability. Subsequently, these tiny protrusions get stretched in the radially outward direction and transform into twisting ligaments that break into droplets. We have elucidated the mechanism for the twisting of ligaments and its further disintegration into firstgeneration droplets, which has not been reported in previous studies.
Formation and breakup of twisting ligaments in a viscous swirling liquid jet
10.1063/5.0122754
Physics of Fluids
20221109T12:03:33Z
© 2022 Author(s).
Toshan Lal Sahu
Ujjwal Chetan
Jagannath Mahato
Prabir Kumar Kar
Prasanta Kumar Das
Rajaram Lakkaraju

Migration dynamics of an initially spherical deformable bubble in the vicinity of a corner
https://aip.scitation.org/doi/10.1063/5.0115162?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Threedimensional numerical simulations are performed to study the migration dynamics of a deformable bubble that is spherical at an initial time near a corner formed by two vertical walls. Nearwall dynamics of this rising bubble are studied by investigating its path, wake, shape and velocity. A finite volume approach coupled with VOF (volumeoffluid) method is adopted to solve the incompressible Navier–Stokes equation and track the gas–liquid interface. From the study, it is found that bubble inertia is dictated by initial bubble–walls interaction as bubble progressively migrates away in a diagonal direction from both the vertical walls in threedimensional space. This influenced bubble inertia, in turn, affects the conformity of the bubble to a specific bubble regime. Five regimes are identified based on the bubble's migrating trajectory among which three of them are fully developedsteady, zigzag, and spiral and two of them are transitionalsteady to zigzag and zigzag to spiral. The point of complete transformation of bubble from steady to zigzag transition to fully developed path instability is evaluated by varying a certain dimensionless parameter, Galilei number Ga. It is found that the path instability occurs at a lower Ga than what it is for the unbounded situations, and the onset of planar zigzag motion is not the result of vortex shedding rather the critical amount of wake accumulation on bubble surface and bubble inertia modulated by walls. Furthermore, the overall dynamics found in the current study show distinguishable characteristics when compared to single wall and unbounded situations.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Threedimensional numerical simulations are performed to study the migration dynamics of a deformable bubble that is spherical at an initial time near a corner formed by two vertical walls. Nearwall dynamics of this rising bubble are studied by investigating its path, wake, shape and velocity. A finite volume approach coupled with VOF (volumeoffluid) method is adopted to solve the incompressible Navier–Stokes equation and track the gas–liquid interface. From the study, it is found that bubble inertia is dictated by initial bubble–walls interaction as bubble progressively migrates away in a diagonal direction from both the vertical walls in threedimensional space. This influenced bubble inertia, in turn, affects the conformity of the bubble to a specific bubble regime. Five regimes are identified based on the bubble's migrating trajectory among which three of them are fully developedsteady, zigzag, and spiral and two of them are transitionalsteady to zigzag and zigzag to spiral. The point of complete transformation of bubble from steady to zigzag transition to fully developed path instability is evaluated by varying a certain dimensionless parameter, Galilei number Ga. It is found that the path instability occurs at a lower Ga than what it is for the unbounded situations, and the onset of planar zigzag motion is not the result of vortex shedding rather the critical amount of wake accumulation on bubble surface and bubble inertia modulated by walls. Furthermore, the overall dynamics found in the current study show distinguishable characteristics when compared to single wall and unbounded situations.
Migration dynamics of an initially spherical deformable bubble in the vicinity of a corner
10.1063/5.0115162
Physics of Fluids
20221110T12:18:43Z
© 2022 Author(s).
S. M. Mahfuzul Hasan
A. B. M. Toufique Hasan

Numerical simulation of hydrogen bubble growth and mass transfer on horizontal microelectrode surface under electrodenormal magnetic field
https://aip.scitation.org/doi/10.1063/5.0127299?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Based on our previous visual experiments and the volume of fluid (VOF) multiphase model, the growth and detachment characteristics of a single hydrogen bubble on the horizontal microelectrode surface under the electrodenormal magnetic field have been numerically investigated. The mass transfer contributions of supersaturated dissolved hydrogen to the bubble growth from the liquid microlayer under the direct injection model and from the bulk bubble interface under the gas–liquid diffusioncontrolled model are adopted. The bubble shapes and diameters predicted from the numerical investigation agree well with experimental results under the same conditions. The simulated results indicate that the supersaturated dissolved hydrogen concentration and the mass transfer source at the wedgeshaped areas adjacent to the bubble foot are obviously higher than those in the wider bulk bubble interface regions. The mass transfer contribution to the bubble growth from the liquid microlayer beneath the bubble base directly plays a dominant role. The higher current density and corresponding Lorentz force mainly appears in the wedgeshaped areas, while the higher rotational electrolyte flow velocity appear at oblique positions of the bubble equator. The bubble detachment behavior makes the rotational electrolyte flows is significantly more complex.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Based on our previous visual experiments and the volume of fluid (VOF) multiphase model, the growth and detachment characteristics of a single hydrogen bubble on the horizontal microelectrode surface under the electrodenormal magnetic field have been numerically investigated. The mass transfer contributions of supersaturated dissolved hydrogen to the bubble growth from the liquid microlayer under the direct injection model and from the bulk bubble interface under the gas–liquid diffusioncontrolled model are adopted. The bubble shapes and diameters predicted from the numerical investigation agree well with experimental results under the same conditions. The simulated results indicate that the supersaturated dissolved hydrogen concentration and the mass transfer source at the wedgeshaped areas adjacent to the bubble foot are obviously higher than those in the wider bulk bubble interface regions. The mass transfer contribution to the bubble growth from the liquid microlayer beneath the bubble base directly plays a dominant role. The higher current density and corresponding Lorentz force mainly appears in the wedgeshaped areas, while the higher rotational electrolyte flow velocity appear at oblique positions of the bubble equator. The bubble detachment behavior makes the rotational electrolyte flows is significantly more complex.
Numerical simulation of hydrogen bubble growth and mass transfer on horizontal microelectrode surface under electrodenormal magnetic field
10.1063/5.0127299
Physics of Fluids
20221110T12:18:11Z
© 2022 Author(s).

A novel pattern in nonlinear interfacial stability for a magnetic fluid column subject to an axial rotation
https://aip.scitation.org/doi/10.1063/5.0121989?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Nonlinear differential equations that control the propagation of a surface wave through the surface disconnection between two fluids are described by the Helmholtz–Duffing oscillator having imaginary damping forces. This oscillator is solved without using any perturbation techniques. This study is relevant in many fields such as nanotechnology. Along with the nonlinear analysis, the periodic solution and the stability criteria are established. Numerical calculations for stability conditions showed vital changes in the stability behavior due to the presence of the rotation ratio.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Nonlinear differential equations that control the propagation of a surface wave through the surface disconnection between two fluids are described by the Helmholtz–Duffing oscillator having imaginary damping forces. This oscillator is solved without using any perturbation techniques. This study is relevant in many fields such as nanotechnology. Along with the nonlinear analysis, the periodic solution and the stability criteria are established. Numerical calculations for stability conditions showed vital changes in the stability behavior due to the presence of the rotation ratio.
A novel pattern in nonlinear interfacial stability for a magnetic fluid column subject to an axial rotation
10.1063/5.0121989
Physics of Fluids
20221114T11:44:52Z
© 2022 Author(s).
Yusry O. ElDib
L. S. ElSherif

Axisymmetric Riemann–smoothed particle hydrodynamics modeling of highpressure bubble dynamics with a simple shifting scheme
https://aip.scitation.org/doi/10.1063/5.0123106?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Highpressure bubble dynamics often involves many complex issues, including large deformations and inhomogeneities, strong compression, moving interfaces, and large discontinuities, that bring challenges to numerical simulations. In this work, an axisymmetric Riemann–smoothed particle hydrodynamics (SPH) method is used to simulate highpressure bubbles near different boundaries. This Riemann–SPH can adopt the real sound speed instead of the artificial one for the air phase in the bubble. Therefore, the real compressibility of the air phase can be considered, and the corresponding time step is significantly increased. To avoid unphysical interface penetration and maintain relatively homogeneous particle distribution, a new and simple particle shifting scheme for multiphase flows is proposed. Additionally, to minimize the influence of the unphysical boundary on the bubble, a large fluid domain with an optimized initial particle distribution is adopted to reduce the particle number. Several highpressure bubbles under different boundary conditions are considered, including in a free field, near a free surface, near a solid boundary, and near a rigid sphere. Numerical results show that these bubble dynamic behaviors can be reproduced with satisfactory accuracy.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Highpressure bubble dynamics often involves many complex issues, including large deformations and inhomogeneities, strong compression, moving interfaces, and large discontinuities, that bring challenges to numerical simulations. In this work, an axisymmetric Riemann–smoothed particle hydrodynamics (SPH) method is used to simulate highpressure bubbles near different boundaries. This Riemann–SPH can adopt the real sound speed instead of the artificial one for the air phase in the bubble. Therefore, the real compressibility of the air phase can be considered, and the corresponding time step is significantly increased. To avoid unphysical interface penetration and maintain relatively homogeneous particle distribution, a new and simple particle shifting scheme for multiphase flows is proposed. Additionally, to minimize the influence of the unphysical boundary on the bubble, a large fluid domain with an optimized initial particle distribution is adopted to reduce the particle number. Several highpressure bubbles under different boundary conditions are considered, including in a free field, near a free surface, near a solid boundary, and near a rigid sphere. Numerical results show that these bubble dynamic behaviors can be reproduced with satisfactory accuracy.
Axisymmetric Riemann–smoothed particle hydrodynamics modeling of highpressure bubble dynamics with a simple shifting scheme
10.1063/5.0123106
Physics of Fluids
20221114T11:44:55Z
© 2022 Author(s).
Abbas Khayyer

Study of flow and heat transfer characteristics of saturated flow film boiling over two inline cylinders
https://aip.scitation.org/doi/10.1063/5.0125192?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this article, a numerical investigation carried out on saturated flow film boiling over two cylinders placed in an inline arrangement has been discussed. Water at near critical condition, [math] = 0.99, is considered for all simulations, where pc is the critical pressure of water. In this study, buoyancy assisted flow is considered. The effects of different liquid Reynolds number ReD and wall superheat on heat transfer rates are studied for different spacings between the two cylinders. Interface structures and their modes of evolution greatly vary for different cases investigated. For some cases, especially for those with lower spacing between the cylinders, a steady vapor column connecting both cylinders in the gap region between them is formed. At higher flow rates, the vapor column in the gap region is unstable. At some cases where recirculation zones are formed, the vapor column breaks off. The heat transfer rate from the rear cylinder is significantly affected by the modes of the phaseinterface evolution and the types of flow structures formed in the gap region between the cylinders. It is found that the flow structures and the phaseinterface evolution are strongly dependent on the distance between the two cylinders for a given Reynolds number and a nondimensional wall superheat.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this article, a numerical investigation carried out on saturated flow film boiling over two cylinders placed in an inline arrangement has been discussed. Water at near critical condition, [math] = 0.99, is considered for all simulations, where pc is the critical pressure of water. In this study, buoyancy assisted flow is considered. The effects of different liquid Reynolds number ReD and wall superheat on heat transfer rates are studied for different spacings between the two cylinders. Interface structures and their modes of evolution greatly vary for different cases investigated. For some cases, especially for those with lower spacing between the cylinders, a steady vapor column connecting both cylinders in the gap region between them is formed. At higher flow rates, the vapor column in the gap region is unstable. At some cases where recirculation zones are formed, the vapor column breaks off. The heat transfer rate from the rear cylinder is significantly affected by the modes of the phaseinterface evolution and the types of flow structures formed in the gap region between the cylinders. It is found that the flow structures and the phaseinterface evolution are strongly dependent on the distance between the two cylinders for a given Reynolds number and a nondimensional wall superheat.
Study of flow and heat transfer characteristics of saturated flow film boiling over two inline cylinders
10.1063/5.0125192
Physics of Fluids
20221116T10:41:29Z
© 2022 Author(s).
S. M. Thamil Kumaran
B. Premachandran

Evaluation of different interfacecapturing methods for cryogenic twophase flows under microgravity
https://aip.scitation.org/doi/10.1063/5.0127146?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The distribution of the gas–liquid interface is crucial to the accurate calculation of the flow and heat transfer of inorbit cryogenic propellants, for which the surface tension force overtakes the gravitational force. As an essential oxidant, liquid oxygen has a lower surface tension coefficient and viscosity than most roomtemperature fluids, causing a greater possibility of interface instability and breakage. Conventional numerical methods have seldom been assessed in terms of cryogenic twophase flows under microgravity, and commercial software cannot provide a consistent platform for the assessment. In this study, a unified code based on OpenFOAM has been developed for evaluating four interfacecapturing methods for twophase flows, namely, the algebraic volume of fluid (VoF), geometric VoF, coupled level set and VoF (CLSVoF), and densityscaled CLSVoF with a balanced force (CLSVoFDSB) methods. The results indicate that the CLSVoFDSB method is most accurate in predicting the interface motion, because it uses the level set function to represent the gas and liquid phases. The gas–liquid interface predicted by the CLSVoFDSB method is the most stable because it adopts the scaling Heaviside function to weaken the effects of spurious currents and increases the stability. The numerical algorithm of the algebraic VoF method is the most simple, so it has the highest efficiency. The geometric VoF uses the isoface to locate the gas–liquid interface in a grid cell, so it can obtain the thinnest interface. In applications of liquid oxygen, the CLSVoFDSB method should be used if the overall accuracy is required.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The distribution of the gas–liquid interface is crucial to the accurate calculation of the flow and heat transfer of inorbit cryogenic propellants, for which the surface tension force overtakes the gravitational force. As an essential oxidant, liquid oxygen has a lower surface tension coefficient and viscosity than most roomtemperature fluids, causing a greater possibility of interface instability and breakage. Conventional numerical methods have seldom been assessed in terms of cryogenic twophase flows under microgravity, and commercial software cannot provide a consistent platform for the assessment. In this study, a unified code based on OpenFOAM has been developed for evaluating four interfacecapturing methods for twophase flows, namely, the algebraic volume of fluid (VoF), geometric VoF, coupled level set and VoF (CLSVoF), and densityscaled CLSVoF with a balanced force (CLSVoFDSB) methods. The results indicate that the CLSVoFDSB method is most accurate in predicting the interface motion, because it uses the level set function to represent the gas and liquid phases. The gas–liquid interface predicted by the CLSVoFDSB method is the most stable because it adopts the scaling Heaviside function to weaken the effects of spurious currents and increases the stability. The numerical algorithm of the algebraic VoF method is the most simple, so it has the highest efficiency. The geometric VoF uses the isoface to locate the gas–liquid interface in a grid cell, so it can obtain the thinnest interface. In applications of liquid oxygen, the CLSVoFDSB method should be used if the overall accuracy is required.
Evaluation of different interfacecapturing methods for cryogenic twophase flows under microgravity
10.1063/5.0127146
Physics of Fluids
20221117T12:25:50Z
© 2022 Author(s).

Effects of spanwise length and sidewall boundary condition on plunging breaking waves
https://aip.scitation.org/doi/10.1063/5.0124895?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A systematic study of the effect of the spanwise length and the sidewall boundary condition of a numerical wave flume (NWF) on direct numerical simulation of a plunging breaking wave is performed. To deal with the topological changes of free surfaces, a highfidelity numerical model is employed to solve the Navier–Stokes equations together with the volume of fluid function. After verification by twodimensional (2D) simulations of a plunging breaker on a sloping beach, ten NWFs with different spanwise extents and sidewall boundary conditions are studied. Special attention is devoted to the threedimensionality of the plunging breaker. Compared with threedimensional (3D) models, the 2D model accurately reproduces the dynamics of a breaking solitary wave in the early stage, but it is inadequate for the study of the postbreaking process. For a 3D NWF with nonslip sidewall boundary condition, the wave domain can be divided into two regions with different physical properties. In the nearwall region, the nonslip boundary condition on the sidewall plays a crucial role in the wave hydrodynamics, while in the central region, the properties of the breaking wave are similar to those for the periodic boundary condition, which provide a closer representation of the real sea environment. The spanwise length of the NWF plays only a minor role in simulations under the periodic boundary condition. Furthermore, lateral boundaries and spanwise length show more influences on a plunging breaker with larger incident wave steepness. This study provides valuable support for the design of numerical simulations of wave breaking.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A systematic study of the effect of the spanwise length and the sidewall boundary condition of a numerical wave flume (NWF) on direct numerical simulation of a plunging breaking wave is performed. To deal with the topological changes of free surfaces, a highfidelity numerical model is employed to solve the Navier–Stokes equations together with the volume of fluid function. After verification by twodimensional (2D) simulations of a plunging breaker on a sloping beach, ten NWFs with different spanwise extents and sidewall boundary conditions are studied. Special attention is devoted to the threedimensionality of the plunging breaker. Compared with threedimensional (3D) models, the 2D model accurately reproduces the dynamics of a breaking solitary wave in the early stage, but it is inadequate for the study of the postbreaking process. For a 3D NWF with nonslip sidewall boundary condition, the wave domain can be divided into two regions with different physical properties. In the nearwall region, the nonslip boundary condition on the sidewall plays a crucial role in the wave hydrodynamics, while in the central region, the properties of the breaking wave are similar to those for the periodic boundary condition, which provide a closer representation of the real sea environment. The spanwise length of the NWF plays only a minor role in simulations under the periodic boundary condition. Furthermore, lateral boundaries and spanwise length show more influences on a plunging breaker with larger incident wave steepness. This study provides valuable support for the design of numerical simulations of wave breaking.
Effects of spanwise length and sidewall boundary condition on plunging breaking waves
10.1063/5.0124895
Physics of Fluids
20221117T12:25:35Z
© 2022 Author(s).

Pair of particle chain selforganization in a square channel flow of Giesekus viscoelastic fluid
https://aip.scitation.org/doi/10.1063/5.0125738?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Pair of particle chain selforganization in a square channel flow of Giesekus viscoelastic fluid is studied by the direct forcing/fictitious domain method. The effects of particle diameter, initial particle distance, shearthinning (n), Weissenberg number (Wi), and Reynolds number (Re) are explored to analyze the mechanism of particle chain selforganization in Giesekus viscoelastic fluid. The results show that the small particle at the equilibrium position moves faster than the larger one and then catches up with it to form a particle chain, in which the large and small particles are located at the front and the end of the chain, respectively. The particle pair with the same diameter cannot form the chain in Giesekus viscoelastic fluid. In addition, the larger the diameter ratio and the initial particle distance, the larger the absolute value of the particle velocity difference, the earlier the particle chain is formed. The particle chain will be formed early with increasing n, Re, and Wi.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Pair of particle chain selforganization in a square channel flow of Giesekus viscoelastic fluid is studied by the direct forcing/fictitious domain method. The effects of particle diameter, initial particle distance, shearthinning (n), Weissenberg number (Wi), and Reynolds number (Re) are explored to analyze the mechanism of particle chain selforganization in Giesekus viscoelastic fluid. The results show that the small particle at the equilibrium position moves faster than the larger one and then catches up with it to form a particle chain, in which the large and small particles are located at the front and the end of the chain, respectively. The particle pair with the same diameter cannot form the chain in Giesekus viscoelastic fluid. In addition, the larger the diameter ratio and the initial particle distance, the larger the absolute value of the particle velocity difference, the earlier the particle chain is formed. The particle chain will be formed early with increasing n, Re, and Wi.
Pair of particle chain selforganization in a square channel flow of Giesekus viscoelastic fluid
10.1063/5.0125738
Physics of Fluids
20221107T12:48:48Z
© 2022 Author(s).

Influence of wall slip in the radial displacement of a yield strength material in a Hele–Shaw cell
https://aip.scitation.org/doi/10.1063/5.0128287?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The growth of viscous fingers in the radial displacement of a yield strength material confined between the plates of a Hele–Shaw cell is investigated. The apparatus is filled with an aqueous solution of Carbopol® before air is injected to start the displacement process. In addition to striking fingering patterns, we identify unyielded residuals of the Carbopol solution arrested on the plates' surfaces with the assistance of digital mobile microscopes placed above the top plate. These unyielded residuals are subjected to slip conditions on the surface walls and appear in different forms. The experimental observations are correlated with the wall slip behavior detected in rheometric measurements, i.e., observed in the flow curve for shear rates below a critical value. This correlation provides an estimate of a critical propagating radius beyond which shear rates drop to values lower than the critical one, and the influence of wall slip becomes significant. We observe that these residuals are uniformly distributed and appear as thin films where the radii are smaller than the critical value and the wall slip is minimum. However, in locations where the radii are larger than the critical one, the residuals turn into isolated blobs of different sizes, which may propagate in a stickslip motion radially downstream inside the air fingers. In addition, we observe that the morphology of residuals depends on the gap width between the plates, the injection rate of the invading air, the yield strength of the Carpobol solution, and the wettability conditions of the surface walls.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The growth of viscous fingers in the radial displacement of a yield strength material confined between the plates of a Hele–Shaw cell is investigated. The apparatus is filled with an aqueous solution of Carbopol® before air is injected to start the displacement process. In addition to striking fingering patterns, we identify unyielded residuals of the Carbopol solution arrested on the plates' surfaces with the assistance of digital mobile microscopes placed above the top plate. These unyielded residuals are subjected to slip conditions on the surface walls and appear in different forms. The experimental observations are correlated with the wall slip behavior detected in rheometric measurements, i.e., observed in the flow curve for shear rates below a critical value. This correlation provides an estimate of a critical propagating radius beyond which shear rates drop to values lower than the critical one, and the influence of wall slip becomes significant. We observe that these residuals are uniformly distributed and appear as thin films where the radii are smaller than the critical value and the wall slip is minimum. However, in locations where the radii are larger than the critical one, the residuals turn into isolated blobs of different sizes, which may propagate in a stickslip motion radially downstream inside the air fingers. In addition, we observe that the morphology of residuals depends on the gap width between the plates, the injection rate of the invading air, the yield strength of the Carpobol solution, and the wettability conditions of the surface walls.
Influence of wall slip in the radial displacement of a yield strength material in a Hele–Shaw cell
10.1063/5.0128287
Physics of Fluids
20221111T12:49:39Z
© 2022 Author(s).
Behbood Abedi
Lara Schimith Berghe
Bruno S. Fonseca
Elias C. Rodrigues
Rafael M. Oliveira
Paulo R. de Souza Mendes

On the analytical approximations to the nonplanar damped Kawahara equation: Cnoidal and solitary waves and their energy
https://aip.scitation.org/doi/10.1063/5.0119630?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this work, the nonintegrable nonplanar (cylindrical and spherical) damped Kawahara equation (ndKE) is solved and analyzed analytically. The ansatz method is implemented for analyzing the ndKE in order to derive some highaccurate and more stable analytical approximations. Based on this method, twodifferent and general formulas for the analytical approximations are derived. The obtained solutions are applied for studying the distinctive features for both cylindrical and spherical dissipative dressed solitons and cnoidal waves in a complex plasma having superthermal ions. Moreover, the accuracy of the obtained approximations is numerically examined by estimating the global maximum residual error. Also, a general formula for the nonplanar dissipative dressed solitons energy is derived in detail. This formula can recover the energy of the nonplanar dissipative dressed solitons, the planar dressed solitons, the planar damped dressed solitons, and the nonplanar dressed solitons. Both the suggested method and obtained approximations can help a large sector of authors interested in studying the nonlinear and complicated phenomena in various fields of science such as the propagating of nonlinear phenomena in physics of plasmas, nonlinear optics, communications, oceans, and seas.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this work, the nonintegrable nonplanar (cylindrical and spherical) damped Kawahara equation (ndKE) is solved and analyzed analytically. The ansatz method is implemented for analyzing the ndKE in order to derive some highaccurate and more stable analytical approximations. Based on this method, twodifferent and general formulas for the analytical approximations are derived. The obtained solutions are applied for studying the distinctive features for both cylindrical and spherical dissipative dressed solitons and cnoidal waves in a complex plasma having superthermal ions. Moreover, the accuracy of the obtained approximations is numerically examined by estimating the global maximum residual error. Also, a general formula for the nonplanar dissipative dressed solitons energy is derived in detail. This formula can recover the energy of the nonplanar dissipative dressed solitons, the planar dressed solitons, the planar damped dressed solitons, and the nonplanar dressed solitons. Both the suggested method and obtained approximations can help a large sector of authors interested in studying the nonlinear and complicated phenomena in various fields of science such as the propagating of nonlinear phenomena in physics of plasmas, nonlinear optics, communications, oceans, and seas.
On the analytical approximations to the nonplanar damped Kawahara equation: Cnoidal and solitary waves and their energy
10.1063/5.0119630
Physics of Fluids
20221114T11:46:46Z
© 2022 Author(s).
S. A. ElTantawy
L. S. ElSherif
A. M. Bakry
Weaam Alhejaili
AbdulMajid Wazwaz

Theoretical and experimental investigation on local turbulence effect on mixedlubrication journal bearing during speeding up
https://aip.scitation.org/doi/10.1063/5.0122039?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Due to the relatively low loadcarrying capacity, lowviscosity lubricated bearings can easily operate in a mixedlubrication regime. Furthermore, low lubricant viscosity may cause local turbulence in the area with relatively thick film. To analyze the influence of local turbulence on the transient characteristics of mixedlubrication bearings, a simulation approach for unsteady mixedlubrication bearings considering local turbulence effects is proposed. Transient journal center locations are solved via journal dynamic equations and numerical integration. The ratio of the film thickness to the roughness determines whether each node of the bearing is in the mixedlubrication area or the pure hydrodynamic lubrication area. The liquid film in the mixedlubrication area is analyzed by a transient average Reynolds equation to account for the surface roughness effect. A liquid film in the hydrodynamic lubrication area is analyzed by a transient generalized Reynolds equation to consider local turbulence. The transient average Reynolds equation and transient generalized Reynolds equation are derived in a form convenient for coupling with the governing equations of the journal center. The proposed model is experimentally validated. The influence of local turbulence on unsteady mixedlubrication bearings is analyzed. The results show that local turbulence and asperity contact can coexist in lowviscosity lubricated bearings, even if the journal speed is relatively low. In the mixedlubrication regime, local turbulence increases the minimum film thickness and decreases the friction coefficient as well as the journal speed at which the mixedlubrication transitions to hydrodynamic lubrication.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Due to the relatively low loadcarrying capacity, lowviscosity lubricated bearings can easily operate in a mixedlubrication regime. Furthermore, low lubricant viscosity may cause local turbulence in the area with relatively thick film. To analyze the influence of local turbulence on the transient characteristics of mixedlubrication bearings, a simulation approach for unsteady mixedlubrication bearings considering local turbulence effects is proposed. Transient journal center locations are solved via journal dynamic equations and numerical integration. The ratio of the film thickness to the roughness determines whether each node of the bearing is in the mixedlubrication area or the pure hydrodynamic lubrication area. The liquid film in the mixedlubrication area is analyzed by a transient average Reynolds equation to account for the surface roughness effect. A liquid film in the hydrodynamic lubrication area is analyzed by a transient generalized Reynolds equation to consider local turbulence. The transient average Reynolds equation and transient generalized Reynolds equation are derived in a form convenient for coupling with the governing equations of the journal center. The proposed model is experimentally validated. The influence of local turbulence on unsteady mixedlubrication bearings is analyzed. The results show that local turbulence and asperity contact can coexist in lowviscosity lubricated bearings, even if the journal speed is relatively low. In the mixedlubrication regime, local turbulence increases the minimum film thickness and decreases the friction coefficient as well as the journal speed at which the mixedlubrication transitions to hydrodynamic lubrication.
Theoretical and experimental investigation on local turbulence effect on mixedlubrication journal bearing during speeding up
10.1063/5.0122039
Physics of Fluids
20221114T11:46:49Z
© 2022 Author(s).

Air entrainment dynamics of aqueous polymeric droplets from dilute to semidilute unentangled regimes
https://aip.scitation.org/doi/10.1063/5.0130251?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Recent studies have revealed the aircushioning effect of droplet impact upon various surfaces and although pure water droplets have extensively been studied, the air entrainment dynamics for aqueous polymeric droplets was the focus of this study. Herein, droplets of low to moderate Weber numbers, [math], displayed air film thickness gradients which was strongly influenced by the viscoelastic properties of the aqueous polymeric droplets in the dilute to the semidilute unentangled regimes. Aqueous polyethylene oxide droplets impacting a smooth thin oil film surface formed a submicrometer air layer, moments prior to impact, which was tracked by a highspeed total internal reflection microscopy technique. The radial changes in the air film thickness were related to the polymer concentration, thus providing an alternative tool for comparing the rheometerderived overlap concentrations with a contactless optical technique.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Recent studies have revealed the aircushioning effect of droplet impact upon various surfaces and although pure water droplets have extensively been studied, the air entrainment dynamics for aqueous polymeric droplets was the focus of this study. Herein, droplets of low to moderate Weber numbers, [math], displayed air film thickness gradients which was strongly influenced by the viscoelastic properties of the aqueous polymeric droplets in the dilute to the semidilute unentangled regimes. Aqueous polyethylene oxide droplets impacting a smooth thin oil film surface formed a submicrometer air layer, moments prior to impact, which was tracked by a highspeed total internal reflection microscopy technique. The radial changes in the air film thickness were related to the polymer concentration, thus providing an alternative tool for comparing the rheometerderived overlap concentrations with a contactless optical technique.
Air entrainment dynamics of aqueous polymeric droplets from dilute to semidilute unentangled regimes
10.1063/5.0130251
Physics of Fluids
20221115T10:30:43Z
© 2022 Author(s).
Ziwen He
Huy Tran
Min Y. Pack

Numerical verification of sharp corner behavior for Giesekus and PhanThien–Tanner fluids
https://aip.scitation.org/doi/10.1063/5.0125940?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We verify numerically the theoretical stress singularities for two viscoelastic models that occur at sharp corners. The models considered are the Giesekus and PhanThien–Tanner (PTT), both of which are shear thinning and are able to capture realistic polymer behaviors. The theoretical asymptotic behavior of these two models at sharp corners has previously been found to involve an integrable solvent and polymer elastic stress singularity, along with narrow elastic stress boundary layers at the walls of the corner. We demonstrate here the validity of these theoretical results through numerical simulation of the classical contraction flow and analyzing the [math] corner. Numerical results are presented, verifying both the solvent and polymer stress singularities, as well as the dominant terms in the constitutive equations supporting the elastic boundary layer structures. For comparison at Weissenberg order one, we consider both the Cartesian stress formulation and the alternative natural stress formulation of the viscoelastic constitutive equations. Numerically, it is shown that the natural stress formulation gives increased accuracy and convergence behavior at the stress singularity and, moreover, encounters no upper Weissenberg number limitation in the global flow simulation for sufficiently large solvent viscosity fraction. The numerical simulations with the Cartesian stress formulation cannot reach such high Weissenberg numbers and run into convergence failure associated with the socalled high Weissenberg number problem.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We verify numerically the theoretical stress singularities for two viscoelastic models that occur at sharp corners. The models considered are the Giesekus and PhanThien–Tanner (PTT), both of which are shear thinning and are able to capture realistic polymer behaviors. The theoretical asymptotic behavior of these two models at sharp corners has previously been found to involve an integrable solvent and polymer elastic stress singularity, along with narrow elastic stress boundary layers at the walls of the corner. We demonstrate here the validity of these theoretical results through numerical simulation of the classical contraction flow and analyzing the [math] corner. Numerical results are presented, verifying both the solvent and polymer stress singularities, as well as the dominant terms in the constitutive equations supporting the elastic boundary layer structures. For comparison at Weissenberg order one, we consider both the Cartesian stress formulation and the alternative natural stress formulation of the viscoelastic constitutive equations. Numerically, it is shown that the natural stress formulation gives increased accuracy and convergence behavior at the stress singularity and, moreover, encounters no upper Weissenberg number limitation in the global flow simulation for sufficiently large solvent viscosity fraction. The numerical simulations with the Cartesian stress formulation cannot reach such high Weissenberg numbers and run into convergence failure associated with the socalled high Weissenberg number problem.
Numerical verification of sharp corner behavior for Giesekus and PhanThien–Tanner fluids
10.1063/5.0125940
Physics of Fluids
20221122T11:10:00Z
© 2022 Author(s).
J. D. Evans
I. L. Palhares Junior
C. M. Oishi
F. Ruano Neto

Modeling the rheological behavior of crude oil–water emulsions
https://aip.scitation.org/doi/10.1063/5.0123274?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>During crude oil extraction, crude oil is often mixed with water, leading to the formation of waterinoil emulsions. Since these emulsions pose severe flow resistance, such as higher pressure drops, due to their complex fluid rheology, it is important to have in our arsenal a rheological constitutive model that accurately predicts their rheological response. In the present work, we propose such a model wherein the emulsions are modeled as deformable volumepreserving droplets via the use of a determinantpreserving contravariant secondrank tensor. We use the generalized bracket formalism of nonequilibrium thermodynamics to make sure that the derived model is by construction thermodynamically admissible. An additional scalar structural variable is considered to allow the prediction of a yielding point, following previous work. The predictions of the new model are shown to be in very good agreement with available experimental measurements.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>During crude oil extraction, crude oil is often mixed with water, leading to the formation of waterinoil emulsions. Since these emulsions pose severe flow resistance, such as higher pressure drops, due to their complex fluid rheology, it is important to have in our arsenal a rheological constitutive model that accurately predicts their rheological response. In the present work, we propose such a model wherein the emulsions are modeled as deformable volumepreserving droplets via the use of a determinantpreserving contravariant secondrank tensor. We use the generalized bracket formalism of nonequilibrium thermodynamics to make sure that the derived model is by construction thermodynamically admissible. An additional scalar structural variable is considered to allow the prediction of a yielding point, following previous work. The predictions of the new model are shown to be in very good agreement with available experimental measurements.
Modeling the rheological behavior of crude oil–water emulsions
10.1063/5.0123274
Physics of Fluids
20221122T11:09:55Z
© 2022 Author(s).
Maria K. Papadimitriou
Pavlos S. Stephanou

Effect of particle arrangement and density on aerodynamic interference
between twin particles interacting with a plane shock wave
https://aip.scitation.org/doi/10.1063/5.0101365?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Unsteady drag, unsteady lift, and movement of one or two moving particles caused by the
passage of a planar shock wave are investigated using particleresolved simulations of
viscous flows. The particle motion analysis is carried out based on particleresolved
simulations for one or two particles under a shock Mach number of 1.22 and a particle
Reynolds number of 49, and the particle migration and fluid forces are investigated. The
unsteady drag, unsteady lift, and particle behavior are investigated for different
densities and particle configurations. The time evolution of the unsteady drag and lift is
changed by interference by the planar shock wave, Mach stem convergence, and the shock
wave reflected from the other particle. These two particles become closer after the shock
wave passes than in the initial state under most conditions. Two particles placed in an
inline arrangement approach each other very closely due to the passage of a shock wave.
On the other hand, two particles placed in a sidebyside arrangement are only slightly
closer to each other after the shock wave passes between them. The pressure waves
resulting from Mach stem convergence of the upstream particle and the reflected shock
waves from the downstream particle are the main factors responsible for the force in the
direction that pushes the particles apart. The wide distance between the two particles
attenuates these pressure waves, and the particles reduce their motion away from each
other.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Unsteady drag, unsteady lift, and movement of one or two moving particles caused by the
passage of a planar shock wave are investigated using particleresolved simulations of
viscous flows. The particle motion analysis is carried out based on particleresolved
simulations for one or two particles under a shock Mach number of 1.22 and a particle
Reynolds number of 49, and the particle migration and fluid forces are investigated. The
unsteady drag, unsteady lift, and particle behavior are investigated for different
densities and particle configurations. The time evolution of the unsteady drag and lift is
changed by interference by the planar shock wave, Mach stem convergence, and the shock
wave reflected from the other particle. These two particles become closer after the shock
wave passes than in the initial state under most conditions. Two particles placed in an
inline arrangement approach each other very closely due to the passage of a shock wave.
On the other hand, two particles placed in a sidebyside arrangement are only slightly
closer to each other after the shock wave passes between them. The pressure waves
resulting from Mach stem convergence of the upstream particle and the reflected shock
waves from the downstream particle are the main factors responsible for the force in the
direction that pushes the particles apart. The wide distance between the two particles
attenuates these pressure waves, and the particles reduce their motion away from each
other.
Effect of particle arrangement and density on aerodynamic interference
between twin particles interacting with a plane shock wave
10.1063/5.0101365
Physics of Fluids
20221101T11:29:21Z
© 2022 Author(s).

Coalescenceinduced jumping of droplets from superhydrophobic surfaces—The effect of contactangle hysteresis
https://aip.scitation.org/doi/10.1063/5.0118645?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Droplets coalesce and jump from superhydrophobic surfaces, a result that stems from the dominance of capillary and inertial forces and the presence of high contact angles. This phenomenon has been a subject of intensive numerical research mostly for cases when the degree of hydrophobicity is described by a single contactangle value (a static contact angle). The introduction of various degrees of contactangle hysteresis complicates the numerical modeling of the jumping process due to the sensitivity of the results to the effective value of the contact angle. We have developed and validated a comprehensive volumeoffluid–immersed boundary numerical framework that accounts for the effect of hysteresis by focusing on the representation of actual (i.e., effective) values of contact angles. By comparing the behavior of jumping droplets on superhydrophobic surfaces with several degrees of hysteresis (up to 15°), we quantified the influence of hysteresis on the jumping process and identified various stages of the merged droplet's detachment and reattachment to the surface. The latter phenomena were observed in all our simulations with droplets of different initial radii. In all the cases with hysteresis, the merged droplet eventually jumps, but we point out the decrease in the jumping velocity as compared to cases with only a static contact angle imposed. Finally, by using the Kistler dynamic contactangle model, we demonstrate and quantify the importance of accurately capturing the dynamic receding contact angle when droplets jump from superhydrophobic surfaces with various degrees of hysteresis.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Droplets coalesce and jump from superhydrophobic surfaces, a result that stems from the dominance of capillary and inertial forces and the presence of high contact angles. This phenomenon has been a subject of intensive numerical research mostly for cases when the degree of hydrophobicity is described by a single contactangle value (a static contact angle). The introduction of various degrees of contactangle hysteresis complicates the numerical modeling of the jumping process due to the sensitivity of the results to the effective value of the contact angle. We have developed and validated a comprehensive volumeoffluid–immersed boundary numerical framework that accounts for the effect of hysteresis by focusing on the representation of actual (i.e., effective) values of contact angles. By comparing the behavior of jumping droplets on superhydrophobic surfaces with several degrees of hysteresis (up to 15°), we quantified the influence of hysteresis on the jumping process and identified various stages of the merged droplet's detachment and reattachment to the surface. The latter phenomena were observed in all our simulations with droplets of different initial radii. In all the cases with hysteresis, the merged droplet eventually jumps, but we point out the decrease in the jumping velocity as compared to cases with only a static contact angle imposed. Finally, by using the Kistler dynamic contactangle model, we demonstrate and quantify the importance of accurately capturing the dynamic receding contact angle when droplets jump from superhydrophobic surfaces with various degrees of hysteresis.
Coalescenceinduced jumping of droplets from superhydrophobic surfaces—The effect of contactangle hysteresis
10.1063/5.0118645
Physics of Fluids
20221101T11:29:32Z
© 2022 Author(s).
K. Konstantinidis
J. Göhl
A. Mark
S. Sasic

Modeling the mass transfer at acoustically generated bubble interface using Rayleigh–Plesset equation secondorder derivatives
https://aip.scitation.org/doi/10.1063/5.0124416?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>One of the many ways of cavitation utilized for process intensification is through acoustically inducing it. As acoustic cavitation gained traction in recent industrial works, numerical modeling became an important study tool to scrutinize and optimize acoustic cavitation applications. However, available hydrodynamic cavitation models are found incapable of accurately predicting acoustic cavitation structures and flow features. This could source from the oversimplification of the Rayleigh–Plesset equation or from obscure effects of empirical model constants. To address this issue, new mass transfer source terms for Zwart–Gerber–Belamri model were derived based on the consideration of Rayleigh–Plesset's secondorder derivatives. In addition, a design of experiments statistical approach, coupled with Monte Carlo simulations, was implemented to assess the influence of empirical model constants on the model's performance by examining variations in amplitude and frequency responses. Moreover, a set of optimized model constants was obtained: evaporation constant = 17.359 88, condensation constant = 0.1, Bubble Radius = 25 × 10−6 m, and Nucleation Site Volume Fraction = 5 × 10−4, to obtain a maximum pressure and frequency of 3.62 bar and 4128.73 Hz, respectively. The new model, with the new constants, was configured into ANSYS Fluent 22.1 and validated against experimental values. The new model resulted with maximum pressure and frequency of 3.48 bar and 4894.56 Hz, respectively, validating the statistical model and showing drastic improvement in qualitatively and quantitatively capturing acoustic cavitation.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>One of the many ways of cavitation utilized for process intensification is through acoustically inducing it. As acoustic cavitation gained traction in recent industrial works, numerical modeling became an important study tool to scrutinize and optimize acoustic cavitation applications. However, available hydrodynamic cavitation models are found incapable of accurately predicting acoustic cavitation structures and flow features. This could source from the oversimplification of the Rayleigh–Plesset equation or from obscure effects of empirical model constants. To address this issue, new mass transfer source terms for Zwart–Gerber–Belamri model were derived based on the consideration of Rayleigh–Plesset's secondorder derivatives. In addition, a design of experiments statistical approach, coupled with Monte Carlo simulations, was implemented to assess the influence of empirical model constants on the model's performance by examining variations in amplitude and frequency responses. Moreover, a set of optimized model constants was obtained: evaporation constant = 17.359 88, condensation constant = 0.1, Bubble Radius = 25 × 10−6 m, and Nucleation Site Volume Fraction = 5 × 10−4, to obtain a maximum pressure and frequency of 3.62 bar and 4128.73 Hz, respectively. The new model, with the new constants, was configured into ANSYS Fluent 22.1 and validated against experimental values. The new model resulted with maximum pressure and frequency of 3.48 bar and 4894.56 Hz, respectively, validating the statistical model and showing drastic improvement in qualitatively and quantitatively capturing acoustic cavitation.
Modeling the mass transfer at acoustically generated bubble interface using Rayleigh–Plesset equation secondorder derivatives
10.1063/5.0124416
Physics of Fluids
20221101T11:29:59Z
© 2022 Author(s).
Basel Al Bishtawi
Khameel Bayo Mustapha
Gianfranco Scribano

Central rebound jet of a droplet normal impact on a confined thin liquid film
https://aip.scitation.org/doi/10.1063/5.0113371?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The phenomenon of the impact of a droplet on a confined thin liquid film is encountered in a variety of industrial applications. Despite exhaustive research, the central rebound jet (CRJ) and its pinchoff are still far from being understood owing to their strong randomness and the uncertainty in secondary pinchoff droplet numbers. This study experimentally investigated the CRJ and its pinchoff formed by the normal impact of a single droplet on a confined thin liquid film. The dynamic evolution of CRJ formation along with its pinchoff is discussed for three typical Weber numbers (We). Its morphology (base diameter and height) was analyzed by focusing on the effects of We and film thickness on the formation mechanism for droplets, and a qualitative comparison of CRJ height with the previous results was made. The critical thresholds of the CRJ pinchoffs are characterized, and a novel concise prediction method was developed. The results show that the increase in the dome diameter is caused not only by the CRJ growth but also by its fallback. Its maximum value is positively correlated with the increase in We and film thickness. The pinchoff height of the CRJ column is characterized by the critical threshold of We (or K), decreasing with the increase in the film thickness. The maximum height of the CRJ increases with the increase in the Froude number (Fr) and shows a power function. An active region of the liquid film thickness taking a Gaussian normal distribution was found for CRJ formation and its pinchoff. The film thickness has a significant influence on the CRJ height in the active region, but little outside this region. A novel concise equation for predicting CRJ pinchoff and its droplet numbers was further obtained by a multiple inverse powerlaw function of We with Ohnesorge number (Oh), Re/Fr, and viscosity effects.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The phenomenon of the impact of a droplet on a confined thin liquid film is encountered in a variety of industrial applications. Despite exhaustive research, the central rebound jet (CRJ) and its pinchoff are still far from being understood owing to their strong randomness and the uncertainty in secondary pinchoff droplet numbers. This study experimentally investigated the CRJ and its pinchoff formed by the normal impact of a single droplet on a confined thin liquid film. The dynamic evolution of CRJ formation along with its pinchoff is discussed for three typical Weber numbers (We). Its morphology (base diameter and height) was analyzed by focusing on the effects of We and film thickness on the formation mechanism for droplets, and a qualitative comparison of CRJ height with the previous results was made. The critical thresholds of the CRJ pinchoffs are characterized, and a novel concise prediction method was developed. The results show that the increase in the dome diameter is caused not only by the CRJ growth but also by its fallback. Its maximum value is positively correlated with the increase in We and film thickness. The pinchoff height of the CRJ column is characterized by the critical threshold of We (or K), decreasing with the increase in the film thickness. The maximum height of the CRJ increases with the increase in the Froude number (Fr) and shows a power function. An active region of the liquid film thickness taking a Gaussian normal distribution was found for CRJ formation and its pinchoff. The film thickness has a significant influence on the CRJ height in the active region, but little outside this region. A novel concise equation for predicting CRJ pinchoff and its droplet numbers was further obtained by a multiple inverse powerlaw function of We with Ohnesorge number (Oh), Re/Fr, and viscosity effects.
Central rebound jet of a droplet normal impact on a confined thin liquid film
10.1063/5.0113371
Physics of Fluids
20221101T11:29:18Z
© 2022 Author(s).
Franz Durst

Phase separation during sedimentation of dilute bacterial suspensions
https://aip.scitation.org/doi/10.1063/5.0121649?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Numerous natural systems depend on the sedimentation of passive particles in the presence of swimming microorganisms. Here, we investigate the dynamics of the sedimentation of spherical colloids at various E. coli concentrations within the dilute regime. Results show the appearance of two sedimentation fronts: a spherical particle front and the bacteria front. We find that the bacteria front behave diffusive at short times, whereas at long times it decays linearly. The sedimentation speed of passive particles decays at a constant speed and decreases as bacteria concentration ([math]) is increased. As [math] is increased further, the sedimentation speed becomes independent of [math]. The timescales of the bacteria front are associated with the particle settling speeds. Remarkably, all experiments collapse onto a single master line by using the bacteria front timescale. A phenomenological model is proposed that captures the sedimentation of passive particles in active fluids.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Numerous natural systems depend on the sedimentation of passive particles in the presence of swimming microorganisms. Here, we investigate the dynamics of the sedimentation of spherical colloids at various E. coli concentrations within the dilute regime. Results show the appearance of two sedimentation fronts: a spherical particle front and the bacteria front. We find that the bacteria front behave diffusive at short times, whereas at long times it decays linearly. The sedimentation speed of passive particles decays at a constant speed and decreases as bacteria concentration ([math]) is increased. As [math] is increased further, the sedimentation speed becomes independent of [math]. The timescales of the bacteria front are associated with the particle settling speeds. Remarkably, all experiments collapse onto a single master line by using the bacteria front timescale. A phenomenological model is proposed that captures the sedimentation of passive particles in active fluids.
Phase separation during sedimentation of dilute bacterial suspensions
10.1063/5.0121649
Physics of Fluids
20221101T11:29:48Z
© 2022 Author(s).
Bryan O. Torres Maldonado
Ranjiangshang Ran
K. Lawrence Galloway
Quentin Brosseau
Shravan Pradeep
Paulo E. Arratia

Effect of the Stokes boundary layer on the dynamics of particle pairs in an oscillatory flow
https://aip.scitation.org/doi/10.1063/5.0115487?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The alignment of a pair of spherical particles perpendicular to a horizontally oscillating flow is attributed to a nonzero residual flow, known as steady streaming. This phenomenon is the basis of complex patterns in denser systems, such as particle chains and the initial stages of rollinggrain ripples. Previous studies on such selforganization processes used two distinct systems: an oscillating box filled with viscous fluid and an oscillating channel flow, where the fluid oscillates relative to the bottom boundary. In this paper, we show that particle pair dynamics in these two systems are fundamentally different, due to the presence of a Stokes boundary layer above the bottom in the oscillating channel flow. The results are obtained from direct numerical simulations in which the dynamics of a pair of particles are simulated using an immersed boundary method. The oscillating box and the oscillating channel flow are only equivalent in a limited region of the parameter space, where both the normalized Stokes boundary layer thickness and the normalized relative particle excursion length are small. Overall, the particle dynamics in the oscillating channel flow, compared to the oscillating box, are governed by an additional dimensionless parameter, that is, the particle–fluid density ratio.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The alignment of a pair of spherical particles perpendicular to a horizontally oscillating flow is attributed to a nonzero residual flow, known as steady streaming. This phenomenon is the basis of complex patterns in denser systems, such as particle chains and the initial stages of rollinggrain ripples. Previous studies on such selforganization processes used two distinct systems: an oscillating box filled with viscous fluid and an oscillating channel flow, where the fluid oscillates relative to the bottom boundary. In this paper, we show that particle pair dynamics in these two systems are fundamentally different, due to the presence of a Stokes boundary layer above the bottom in the oscillating channel flow. The results are obtained from direct numerical simulations in which the dynamics of a pair of particles are simulated using an immersed boundary method. The oscillating box and the oscillating channel flow are only equivalent in a limited region of the parameter space, where both the normalized Stokes boundary layer thickness and the normalized relative particle excursion length are small. Overall, the particle dynamics in the oscillating channel flow, compared to the oscillating box, are governed by an additional dimensionless parameter, that is, the particle–fluid density ratio.
Effect of the Stokes boundary layer on the dynamics of particle pairs in an oscillatory flow
10.1063/5.0115487
Physics of Fluids
20221101T11:30:14Z
© 2022 Author(s).
T. J. J. M. van Overveld
W.P. Breugem
H. J. H. Clercx
M. DuranMatute

A wellposed multilayer model for granular avalanches: Comparisons with laboratory experiments
https://aip.scitation.org/doi/10.1063/5.0106908?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Granular avalanches are dangerous phenomena characterized by the rapid gravitydriven motion of granular solids. The complex dynamics of these flows can be effectively modeled by a multilayer approach, which, however, requires particular attention to the derivation of the model equations in order to allow stable solutions. In this work, we use a wellposed multilayer model, in which the μ(I)rheology is employed and a dilatancy law, depending on the inertial number I, is also taken into account, and systematically compare it with various laboratory experiments. The model, whose wellposedness is guaranteed by a physically based viscous regularization, describes the evolution of a preset number of superimposed granular layers. As the sidewall friction is relevant under most experimental conditions, the model is fitted here with suitable resistance terms. Moreover, nontrivial closures for the mass exchanges are introduced to avoid any unrealistic partitioning of the flow domain during the avalanche evolution, and, hence, guarantee a regular spatial discretization along the normal to flow direction. The velocity fields are compared with different experiments in unsteady state, and comparisons of both velocity and volume fraction profiles are provided with steady uniform flow experiments. The results confirm the good capabilities of the multilayer model and the underlying μ(I)rheology in capturing the granular flow dynamics. The experimental volume fraction profiles are qualitatively well reproduced by the proposed dilatancy law, while an overestimation is observed only in the upper, more dilute flow region with a thickness of a few grain diameters.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Granular avalanches are dangerous phenomena characterized by the rapid gravitydriven motion of granular solids. The complex dynamics of these flows can be effectively modeled by a multilayer approach, which, however, requires particular attention to the derivation of the model equations in order to allow stable solutions. In this work, we use a wellposed multilayer model, in which the μ(I)rheology is employed and a dilatancy law, depending on the inertial number I, is also taken into account, and systematically compare it with various laboratory experiments. The model, whose wellposedness is guaranteed by a physically based viscous regularization, describes the evolution of a preset number of superimposed granular layers. As the sidewall friction is relevant under most experimental conditions, the model is fitted here with suitable resistance terms. Moreover, nontrivial closures for the mass exchanges are introduced to avoid any unrealistic partitioning of the flow domain during the avalanche evolution, and, hence, guarantee a regular spatial discretization along the normal to flow direction. The velocity fields are compared with different experiments in unsteady state, and comparisons of both velocity and volume fraction profiles are provided with steady uniform flow experiments. The results confirm the good capabilities of the multilayer model and the underlying μ(I)rheology in capturing the granular flow dynamics. The experimental volume fraction profiles are qualitatively well reproduced by the proposed dilatancy law, while an overestimation is observed only in the upper, more dilute flow region with a thickness of a few grain diameters.
A wellposed multilayer model for granular avalanches: Comparisons with laboratory experiments
10.1063/5.0106908
Physics of Fluids
20221102T02:44:16Z
© 2022 Author(s).
L. Sarno
Y. Wang
M. N. Papa
P. Villani
M. Oberlack

Patterns of convection and distribution of binary particles under vibration and airflow
https://aip.scitation.org/doi/10.1063/5.0107462?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Granular matter exists widely in nature and engineering practice and exhibits abundant and complex phenomena of convection and separation. Here, we focus on the pattern of convection and distribution of binary particles under vibration and airflow. Various patterns of convection and distribution were observed. It is found that the convection of binary particles shows four patterns that are similar, but not identical to those in the monocomponent granular system. The same pattern of particle convection is often obtained with different particle distribution patterns in different cases of vibration and airflow, and one of distribution patterns tends to appear with the same convection pattern. The pattern of particle convection has an important influence on the pattern of particle distribution. These findings are expected to have implications for a basic understanding of the convection and separation phenomena of granular material.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Granular matter exists widely in nature and engineering practice and exhibits abundant and complex phenomena of convection and separation. Here, we focus on the pattern of convection and distribution of binary particles under vibration and airflow. Various patterns of convection and distribution were observed. It is found that the convection of binary particles shows four patterns that are similar, but not identical to those in the monocomponent granular system. The same pattern of particle convection is often obtained with different particle distribution patterns in different cases of vibration and airflow, and one of distribution patterns tends to appear with the same convection pattern. The pattern of particle convection has an important influence on the pattern of particle distribution. These findings are expected to have implications for a basic understanding of the convection and separation phenomena of granular material.
Patterns of convection and distribution of binary particles under vibration and airflow
10.1063/5.0107462
Physics of Fluids
20221102T02:43:53Z
© 2022 Author(s).

A novel theoretical model of gas–solid twophase flow mixed dielectric discharge
https://aip.scitation.org/doi/10.1063/5.0124376?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A theoretical physical model of gas–solid twophase flow mixed dielectric discharge in a uniform field based on Townsend's discharge theory is presented. This model extends the classical Townsend's theory to be applicable to the quantitative analysis of dielectric discharge questions related to gas–solid twophase flow environments, reveals the influence mechanism of flowing gases and solidphase particles on discharge, and provides a theoretical basis for expanding the application of discharge plasma technology in various fields. In the model, based on the basic physical process of gas discharge and our previous studies, the effects of the attraction and obstructive factors of solidphase particles on the number density of electrons or ions and the local space electric field in the inception and development of gas discharge were taken into account. On this basis, the analytical expression of the breakdown voltage in a gas–solid twophase flow mixed dielectric is obtained, Paschen's law of gas breakdown is modified, and Townsend's breakdown criterion for gas–solid twophase flow situation is proposed. It is shown that the breakdown voltage of the gas–solid twophase flow mixed dielectric decreases with increasing gas flow velocity. The gas flow velocity is the main factor affecting the variation trend of the breakdown voltage. The concentration and size of solidphase particles determine the values of breakdown voltage. The breakdown voltage of the smaller size and higher concentration of solidphase particles is greater, which has a stronger suppression effect on the discharge.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A theoretical physical model of gas–solid twophase flow mixed dielectric discharge in a uniform field based on Townsend's discharge theory is presented. This model extends the classical Townsend's theory to be applicable to the quantitative analysis of dielectric discharge questions related to gas–solid twophase flow environments, reveals the influence mechanism of flowing gases and solidphase particles on discharge, and provides a theoretical basis for expanding the application of discharge plasma technology in various fields. In the model, based on the basic physical process of gas discharge and our previous studies, the effects of the attraction and obstructive factors of solidphase particles on the number density of electrons or ions and the local space electric field in the inception and development of gas discharge were taken into account. On this basis, the analytical expression of the breakdown voltage in a gas–solid twophase flow mixed dielectric is obtained, Paschen's law of gas breakdown is modified, and Townsend's breakdown criterion for gas–solid twophase flow situation is proposed. It is shown that the breakdown voltage of the gas–solid twophase flow mixed dielectric decreases with increasing gas flow velocity. The gas flow velocity is the main factor affecting the variation trend of the breakdown voltage. The concentration and size of solidphase particles determine the values of breakdown voltage. The breakdown voltage of the smaller size and higher concentration of solidphase particles is greater, which has a stronger suppression effect on the discharge.
A novel theoretical model of gas–solid twophase flow mixed dielectric discharge
10.1063/5.0124376
Physics of Fluids
20221103T12:35:37Z
© 2022 Author(s).
Zhipeng Shi
Yongqiang Kang
Jialin Zhang
Shuaibing Li
Hongwei Li

Spatial evolution of multiscale droplet clusters in an evaporating spray
https://aip.scitation.org/doi/10.1063/5.0120790?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Evaporative sprays are encountered in a wide range of engineering applications. Since clustering of droplets in sprays leads to strong inhomogeneity in the spatial distribution of droplet concentration that impacts mass, momentum, and energy exchange between the spray and the surrounding flow, a detailed investigation of droplet clustering in evaporating sprays is important. In the current research work, we experimentally investigate the spatial evolution of droplet cluster characteristics in an evaporating acetone spray injected from an airassist atomizer. The droplet size and velocity are measured using Interferometric Laser Imaging for Droplet Sizing technique. In detail, characterization of the droplet clusters is achieved by the application of Voronoi analysis to particle image velocimetry images of the spray droplets. This approach not only identifies the droplet clusters but also provides area, length scale, and local droplet number density within the clusters. The identified droplet clusters are multiscale and could be classified into either large or smallscale clusters, which scale with spray halfwidth and Kolmogorov length scale, respectively. Experiments are also conducted in water spray under the same operating conditions. Despite the similarity in the droplet clustering process between the two sprays at small scales of air turbulence, some distinct trends are observed for the largescale clusters in the acetone spray. This is attributed to the higher evaporation rate of acetone droplets, which promotes preferential accumulation of droplets.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Evaporative sprays are encountered in a wide range of engineering applications. Since clustering of droplets in sprays leads to strong inhomogeneity in the spatial distribution of droplet concentration that impacts mass, momentum, and energy exchange between the spray and the surrounding flow, a detailed investigation of droplet clustering in evaporating sprays is important. In the current research work, we experimentally investigate the spatial evolution of droplet cluster characteristics in an evaporating acetone spray injected from an airassist atomizer. The droplet size and velocity are measured using Interferometric Laser Imaging for Droplet Sizing technique. In detail, characterization of the droplet clusters is achieved by the application of Voronoi analysis to particle image velocimetry images of the spray droplets. This approach not only identifies the droplet clusters but also provides area, length scale, and local droplet number density within the clusters. The identified droplet clusters are multiscale and could be classified into either large or smallscale clusters, which scale with spray halfwidth and Kolmogorov length scale, respectively. Experiments are also conducted in water spray under the same operating conditions. Despite the similarity in the droplet clustering process between the two sprays at small scales of air turbulence, some distinct trends are observed for the largescale clusters in the acetone spray. This is attributed to the higher evaporation rate of acetone droplets, which promotes preferential accumulation of droplets.
Spatial evolution of multiscale droplet clusters in an evaporating spray
10.1063/5.0120790
Physics of Fluids
20221103T12:35:33Z
© 2022 Author(s).
Nandhakumar Pandurangan
Srikrishna Sahu

Drag of a single particle within a multiparticle system in supercritical water
https://aip.scitation.org/doi/10.1063/5.0120561?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Particle drag is a very important factor in reactor simulation. The complex physical properties of supercritical water (SCW) prevent some modeling methods of reactor simulation from being able to accurately simulate an SCW reactor. Therefore, in this work, the effects of particle interaction on single particle drag within a multiparticle system in SCW are investigated. The results show that the variation in the drag coefficient in SCW is special. This work indicates a control mechanism for the Reynolds number, volume fraction, temperature, and pressure on drag specificity of a single particle. This mechanism essentially exhibits an interaction of viscosity and velocity gradient. Furthermore, through a comparison of SCW and constant property flow, a drag specificity model can be initially developed. The results for SCW can be obtained by calculating the constant property flow, coupled with a drag specificity model. This model can be applied to modeling methods of reactor simulation after further improvement.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Particle drag is a very important factor in reactor simulation. The complex physical properties of supercritical water (SCW) prevent some modeling methods of reactor simulation from being able to accurately simulate an SCW reactor. Therefore, in this work, the effects of particle interaction on single particle drag within a multiparticle system in SCW are investigated. The results show that the variation in the drag coefficient in SCW is special. This work indicates a control mechanism for the Reynolds number, volume fraction, temperature, and pressure on drag specificity of a single particle. This mechanism essentially exhibits an interaction of viscosity and velocity gradient. Furthermore, through a comparison of SCW and constant property flow, a drag specificity model can be initially developed. The results for SCW can be obtained by calculating the constant property flow, coupled with a drag specificity model. This model can be applied to modeling methods of reactor simulation after further improvement.
Drag of a single particle within a multiparticle system in supercritical water
10.1063/5.0120561
Physics of Fluids
20221103T12:34:56Z
© 2022 Author(s).

Inertial focusing of a dilute suspension in pipe flow
https://aip.scitation.org/doi/10.1063/5.0111680?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The dynamics of rigid particle suspensions in a wallbounded laminar flow present several nontrivial and intriguing features, including particle ordering, lateral transport, and the appearance of stable, preferential locations like the Segré–Silberberg annulus. The formation of more than one annulus is a particularly puzzling phenomenon that is still not fully explained. Here, we present numerical simulation results of a dilute suspension of particles in (periodic) pipe flow based on the lattice Boltzmann and the discrete element methods. Our simulations provide access to the full radial position history of the particles while traveling downstream. This allows to accurately quantify the transient and steady states. We observe the formation of the secondary, inner annulus and show that its position invariably shifts toward the Segré–Silberberg one if the channel is sufficiently long, proving that it is, in fact, a transient feature for Reynolds numbers (Re) up to 600. We quantify the variation of the channel focusing length ([math]) with Re. Interestingly and unlike the theoretical prediction for a pointlike particle, we observe that [math] increases with Re for both the single particle and the suspension.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The dynamics of rigid particle suspensions in a wallbounded laminar flow present several nontrivial and intriguing features, including particle ordering, lateral transport, and the appearance of stable, preferential locations like the Segré–Silberberg annulus. The formation of more than one annulus is a particularly puzzling phenomenon that is still not fully explained. Here, we present numerical simulation results of a dilute suspension of particles in (periodic) pipe flow based on the lattice Boltzmann and the discrete element methods. Our simulations provide access to the full radial position history of the particles while traveling downstream. This allows to accurately quantify the transient and steady states. We observe the formation of the secondary, inner annulus and show that its position invariably shifts toward the Segré–Silberberg one if the channel is sufficiently long, proving that it is, in fact, a transient feature for Reynolds numbers (Re) up to 600. We quantify the variation of the channel focusing length ([math]) with Re. Interestingly and unlike the theoretical prediction for a pointlike particle, we observe that [math] increases with Re for both the single particle and the suspension.
Inertial focusing of a dilute suspension in pipe flow
10.1063/5.0111680
Physics of Fluids
20221103T12:35:46Z
© 2022 Author(s).
Othmane Aouane
Marcello Sega
Bastian Bäuerlein
Kerstin Avila
Jens Harting

Airentrained vortex in open intake: Time–frequency analysis and the interaction with subsurface vortices
https://aip.scitation.org/doi/10.1063/5.0112245?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, the simple coupled levelset and volume of fluid and bifurcation models are used for the accurate prediction of the flow in an open pump intake with a vertical pipe. The continuous wavelet transform, which is suitable for the vortex detection, is applied to the pressure signals near both airentrained and subsurface vortices. Lowfrequency with long duration for airentrained vortex due to the vortex wandering and broadband with short duration for floor vortices (dominant in subsurface vortices) due to the generation of extreme strong vortex are observed. The vortex motion mechanism is revealed by the analysis of the transport equation of the vertical vorticity's enstrophy. Different from the previous results in which the tilting effect controls the vortex motion, the stretching effect is found to be dominating when it is large enough. When going through the bell mouth, the airentrained vortex's vorticity changes the sign and strengthens the vortex with the same sign. On the plane near the bell mouth, three vortex patterns, including corotating pair, merging, and counterrotating pair, are observed. The onset criterion of the vortex merging at a/b = 0.29–0.32 is found to be applicable to the present case. The counterrotating pair is found to be more stable. Airentrained vortex serves as an amplification of the strong vortices generated from the subsurface.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, the simple coupled levelset and volume of fluid and bifurcation models are used for the accurate prediction of the flow in an open pump intake with a vertical pipe. The continuous wavelet transform, which is suitable for the vortex detection, is applied to the pressure signals near both airentrained and subsurface vortices. Lowfrequency with long duration for airentrained vortex due to the vortex wandering and broadband with short duration for floor vortices (dominant in subsurface vortices) due to the generation of extreme strong vortex are observed. The vortex motion mechanism is revealed by the analysis of the transport equation of the vertical vorticity's enstrophy. Different from the previous results in which the tilting effect controls the vortex motion, the stretching effect is found to be dominating when it is large enough. When going through the bell mouth, the airentrained vortex's vorticity changes the sign and strengthens the vortex with the same sign. On the plane near the bell mouth, three vortex patterns, including corotating pair, merging, and counterrotating pair, are observed. The onset criterion of the vortex merging at a/b = 0.29–0.32 is found to be applicable to the present case. The counterrotating pair is found to be more stable. Airentrained vortex serves as an amplification of the strong vortices generated from the subsurface.
Airentrained vortex in open intake: Time–frequency analysis and the interaction with subsurface vortices
10.1063/5.0112245
Physics of Fluids
20221103T12:35:10Z
© 2022 Author(s).

Numerical investigation of subcooled flow boiling in an inclined rectangular minichannel at a low flow rate
https://aip.scitation.org/doi/10.1063/5.0115599?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Under a low flow rate, gravity may become prominent for bubble behavior and heat transfer of flowing boiling because of the weakness of drag force from liquid, and its effect changes with the inclination angle of the minichannel but without consensus. In this paper, based on a reasonable nucleus site density model and considering conjugate heat transfer, the coupled volumeoffluid and level set method is adopted to study the subcooled flow boiling in an inclined threedimensional rectangular minichannel (0°–180°) with a characteristic size of 1.0 mm at a low flow rate of 88.8 kg m−2 s−1. The inclination angle is found to have a slight effect on the flow boiling, which is different from the conclusion drawn based on the traditionalmacro channel. A bubbly flow appears when a heat flux of 300 kW/m2 is added. An unconventional impact force is proposed, which presses large bubbles to slip along the heating wall, with slight differences in the flow pattern under different inclination angles. When the inclination angle is close to 0°, the upstream small/medium bubbles leave the heating wall under gravitational effects, which is conducive to heat transfer. As the inclination angle approaches 90°, gravity pushes the large bubbles downstream to leave the channel, favoring the rewetting of the dry patches below. These two positive effects fail as the inclination angle approaches 180°, leading to slightly worse overall heat exchange efficiency. However, the maximum differences in the average and local wall superheating of the minichannel are only 8.4% and 22.5%, respectively, across the range of inclination angles because the flow pattern remains similar under the effect of the impact force. In addition, the effect of inclination angle on flow boiling becomes weaker with the increase in heat flux because of happening of slug flow.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Under a low flow rate, gravity may become prominent for bubble behavior and heat transfer of flowing boiling because of the weakness of drag force from liquid, and its effect changes with the inclination angle of the minichannel but without consensus. In this paper, based on a reasonable nucleus site density model and considering conjugate heat transfer, the coupled volumeoffluid and level set method is adopted to study the subcooled flow boiling in an inclined threedimensional rectangular minichannel (0°–180°) with a characteristic size of 1.0 mm at a low flow rate of 88.8 kg m−2 s−1. The inclination angle is found to have a slight effect on the flow boiling, which is different from the conclusion drawn based on the traditionalmacro channel. A bubbly flow appears when a heat flux of 300 kW/m2 is added. An unconventional impact force is proposed, which presses large bubbles to slip along the heating wall, with slight differences in the flow pattern under different inclination angles. When the inclination angle is close to 0°, the upstream small/medium bubbles leave the heating wall under gravitational effects, which is conducive to heat transfer. As the inclination angle approaches 90°, gravity pushes the large bubbles downstream to leave the channel, favoring the rewetting of the dry patches below. These two positive effects fail as the inclination angle approaches 180°, leading to slightly worse overall heat exchange efficiency. However, the maximum differences in the average and local wall superheating of the minichannel are only 8.4% and 22.5%, respectively, across the range of inclination angles because the flow pattern remains similar under the effect of the impact force. In addition, the effect of inclination angle on flow boiling becomes weaker with the increase in heat flux because of happening of slug flow.
Numerical investigation of subcooled flow boiling in an inclined rectangular minichannel at a low flow rate
10.1063/5.0115599
Physics of Fluids
20221104T12:22:09Z
© 2022 Author(s).

Neural stochastic differential equations for particle dispersion in largeeddy simulations of homogeneous isotropic turbulence
https://aip.scitation.org/doi/10.1063/5.0121344?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In dilute turbulent particleladen flows, such as atmospheric dispersion of pollutants or virus particles, the dynamics of tracerlike to low inertial particles are significantly altered by the fluctuating motion of the carrier fluid phase. Neglecting the effects of fluid velocity fluctuations on particle dynamics causes poor prediction of particle transport and dispersion. To account for the effects of fluid phase fluctuating velocity on the particle transport, stochastic differential equations coupled with largeeddy simulation are proposed to model the fluid velocity seen by the particle. The drift and diffusion terms in the stochastic differential equation are modeled using neural networks (“neural stochastic differential equations”). The neural networks are trained with direct numerical simulations (DNS) of decaying homogeneous isotropic turbulence at low and moderate Reynolds numbers. The predictability of the proposed models is assessed against DNS results through a priori analyses and a posteriori simulations of decaying homogeneous isotropic turbulence at lowtohigh Reynolds numbers. Total particle fluctuating kinetic energy is underpredicted by 40% with no model, compared to the DNS data. In contrast, the proposed model predictions match total particle fluctuating kinetic energy to within 5% of the DNS data for low to highinertia particles. For inertial particles, the model matches the variance of uncorrelated particle velocity to within 10% of DNS results, compared to 60%–70% underprediction with no model. It is concluded that the proposed model is applicable for flow configurations involving tracer and inertial particles, such as transport and dispersion of pollutants or virus particles.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In dilute turbulent particleladen flows, such as atmospheric dispersion of pollutants or virus particles, the dynamics of tracerlike to low inertial particles are significantly altered by the fluctuating motion of the carrier fluid phase. Neglecting the effects of fluid velocity fluctuations on particle dynamics causes poor prediction of particle transport and dispersion. To account for the effects of fluid phase fluctuating velocity on the particle transport, stochastic differential equations coupled with largeeddy simulation are proposed to model the fluid velocity seen by the particle. The drift and diffusion terms in the stochastic differential equation are modeled using neural networks (“neural stochastic differential equations”). The neural networks are trained with direct numerical simulations (DNS) of decaying homogeneous isotropic turbulence at low and moderate Reynolds numbers. The predictability of the proposed models is assessed against DNS results through a priori analyses and a posteriori simulations of decaying homogeneous isotropic turbulence at lowtohigh Reynolds numbers. Total particle fluctuating kinetic energy is underpredicted by 40% with no model, compared to the DNS data. In contrast, the proposed model predictions match total particle fluctuating kinetic energy to within 5% of the DNS data for low to highinertia particles. For inertial particles, the model matches the variance of uncorrelated particle velocity to within 10% of DNS results, compared to 60%–70% underprediction with no model. It is concluded that the proposed model is applicable for flow configurations involving tracer and inertial particles, such as transport and dispersion of pollutants or virus particles.
Neural stochastic differential equations for particle dispersion in largeeddy simulations of homogeneous isotropic turbulence
10.1063/5.0121344
Physics of Fluids
20221104T12:50:45Z
© 2022 Author(s).
J. Williams
U. Wolfram
A. Ozel

Characterization of the infocus droplets in shadowgraphy systems via deep learningbased image processing method
https://aip.scitation.org/doi/10.1063/5.0121174?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>It is important to accurately identify and measure infocus droplets from shadowgraph droplet images that typically contain a large number of defocused droplets for the research of multiphase flow. However, conventional infocus droplet identification methods are timeconsuming and laborious due to the noise and background illumination in experimental data. In this paper, a deep learningbased method called focusdroplet generative adversarial network (FocGAN) is developed to automatically detect and characterize the focused droplets in shadow images. A generative adversarial network framework is adopted by our model to output binarized images containing only infocus droplets, and inception blocks are used in the generator to enhance the extraction of multiscale features. To emulate the real shadow images, an algorithm based on the Gauss blur method is developed to generate paired datasets to train the networks. The detailed architecture and performance of the model were investigated and evaluated by both the synthetic data and spray experimental data. The results show that the present learningbased method is far superior to the traditional adaptive threshold method in terms of effective extraction rate and accuracy. The comprehensive performance of FocGAN, including detection accuracy and robustness to noise, is higher than that of the model based on a convolutional neural network. Moreover, the identification results of spray images with different droplet number densities clearly exhibit the feasibility of FocGAN in real experiments. This work indicates that the proposed learningbased approach is promising to be widely applied as an efficient and universal tool for processing particle shadowgraph images.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>It is important to accurately identify and measure infocus droplets from shadowgraph droplet images that typically contain a large number of defocused droplets for the research of multiphase flow. However, conventional infocus droplet identification methods are timeconsuming and laborious due to the noise and background illumination in experimental data. In this paper, a deep learningbased method called focusdroplet generative adversarial network (FocGAN) is developed to automatically detect and characterize the focused droplets in shadow images. A generative adversarial network framework is adopted by our model to output binarized images containing only infocus droplets, and inception blocks are used in the generator to enhance the extraction of multiscale features. To emulate the real shadow images, an algorithm based on the Gauss blur method is developed to generate paired datasets to train the networks. The detailed architecture and performance of the model were investigated and evaluated by both the synthetic data and spray experimental data. The results show that the present learningbased method is far superior to the traditional adaptive threshold method in terms of effective extraction rate and accuracy. The comprehensive performance of FocGAN, including detection accuracy and robustness to noise, is higher than that of the model based on a convolutional neural network. Moreover, the identification results of spray images with different droplet number densities clearly exhibit the feasibility of FocGAN in real experiments. This work indicates that the proposed learningbased approach is promising to be widely applied as an efficient and universal tool for processing particle shadowgraph images.
Characterization of the infocus droplets in shadowgraphy systems via deep learningbased image processing method
10.1063/5.0121174
Physics of Fluids
20221104T12:22:01Z
© 2022 Author(s).

Discontinuous shear thickening (DST) transition with spherical iron particles coated by adsorbed brush polymer
https://aip.scitation.org/doi/10.1063/5.0120502?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We explore the rheology of very concentrated (0.55 < Φ < 0.67) suspensions of carbonyl iron particles coated by a small polymer. A strong discontinuous shear thickening (DST) is observed in a large range of volume fraction presenting some specific behaviors in comparison with other systems. In particular, the DST transition can appear suddenly without being preceded by shear thickening. The presence of a frictional network of particles is confirmed by a simultaneous measurement of the electric resistance of the suspension and of the rheological curve. Using the Wyart–Cates (W–C) model, we show that with increasing the volume fraction, the fraction of frictional contacts increases more quickly with the stress, contrary to the prediction of numerical simulations. The same behavior is observed in the presence of a magnetic field with a strong increase in the viscosity before the DST transition. We interpret this behavior by the interpenetration of the polymer layer under the effect of the shear stress—and of the magnetic stress—followed by the expulsion of the polymer out of the surfaces between two particles in contact. We point out that above the DST transition, we do not observe a jamming in the range of volume fraction, whereas it is predicted by the W–C model. The frictional contacts are created by a shear stress and not by a static stress, so in the absence of shear flow, the polymer can adsorb again on the surface and lubricate the frictional contacts. We thus predict an asymptotic nonzero shear rate reproducing the experimental behavior.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We explore the rheology of very concentrated (0.55 < Φ < 0.67) suspensions of carbonyl iron particles coated by a small polymer. A strong discontinuous shear thickening (DST) is observed in a large range of volume fraction presenting some specific behaviors in comparison with other systems. In particular, the DST transition can appear suddenly without being preceded by shear thickening. The presence of a frictional network of particles is confirmed by a simultaneous measurement of the electric resistance of the suspension and of the rheological curve. Using the Wyart–Cates (W–C) model, we show that with increasing the volume fraction, the fraction of frictional contacts increases more quickly with the stress, contrary to the prediction of numerical simulations. The same behavior is observed in the presence of a magnetic field with a strong increase in the viscosity before the DST transition. We interpret this behavior by the interpenetration of the polymer layer under the effect of the shear stress—and of the magnetic stress—followed by the expulsion of the polymer out of the surfaces between two particles in contact. We point out that above the DST transition, we do not observe a jamming in the range of volume fraction, whereas it is predicted by the W–C model. The frictional contacts are created by a shear stress and not by a static stress, so in the absence of shear flow, the polymer can adsorb again on the surface and lubricate the frictional contacts. We thus predict an asymptotic nonzero shear rate reproducing the experimental behavior.
Discontinuous shear thickening (DST) transition with spherical iron particles coated by adsorbed brush polymer
10.1063/5.0120502
Physics of Fluids
20221104T01:20:22Z
© 2022 Author(s).
Georges Bossis
Yan Grasselli
Olga Volkova

Experimental and computational investigation of solid suspension and gas dispersion in a stirred vessel
https://aip.scitation.org/doi/10.1063/5.0122635?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Hydrodynamics and residence time distribution (RTD) of fluid elements are key parameters to characterize the performance of stirred vessel. They are governed by geometric and operating parameters of the stirred vessel (SV). In the present work, the performance of the stirred vessel is studied using computational fluid dynamics (CFD) with realizable k−ε turbulence model. The multiple reference frame and sliding mesh approach are used for impeller motion. The solid–liquid flow and associated solid suspension characteristics are predicted using the twofluid model (Euler–Granular). The performance of the stirred vessel is characterized by analyzing predicted velocity magnitude, solid concentration (suspension quality), and solid sedimentation. This is compared with the stirred vessel with draft tube baffle configuration (three inner baffles and six outer baffles). The recirculatory flow in draft tube SV helps to achieve uniform suspension and less sedimentation. Further, CFD simulations are carried out in Lagrangian way to analyze chaotic mixing among fluid elements. This is qualitatively analyzed using Poincaré map and quantitatively evaluated using Shannon entropy. The extent of chaotic mixing in draft tube SV is found to be high. The performance of the stirred vessel is further investigated through stimulus–response tracer techniques (RTD) to detect design flaws such as bypass and dead zones. This is analyzed for a wide range of operating parameters and identified optimum conditions (flow rate, impeller speed) for the operation of SV. The four different outlet pipe locations are chosen in SV. The bypass and dead volume are analyzed accordingly, and an optimum outlet pipe location is found. To reduce the extent of nonideal parameters, three different gas source locations are considered and gases are dispersed in the form of bubbles. The gas dispersion at optimum gas injection point is found to reduce nonideal parameters and improve the design of stirred vessel.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Hydrodynamics and residence time distribution (RTD) of fluid elements are key parameters to characterize the performance of stirred vessel. They are governed by geometric and operating parameters of the stirred vessel (SV). In the present work, the performance of the stirred vessel is studied using computational fluid dynamics (CFD) with realizable k−ε turbulence model. The multiple reference frame and sliding mesh approach are used for impeller motion. The solid–liquid flow and associated solid suspension characteristics are predicted using the twofluid model (Euler–Granular). The performance of the stirred vessel is characterized by analyzing predicted velocity magnitude, solid concentration (suspension quality), and solid sedimentation. This is compared with the stirred vessel with draft tube baffle configuration (three inner baffles and six outer baffles). The recirculatory flow in draft tube SV helps to achieve uniform suspension and less sedimentation. Further, CFD simulations are carried out in Lagrangian way to analyze chaotic mixing among fluid elements. This is qualitatively analyzed using Poincaré map and quantitatively evaluated using Shannon entropy. The extent of chaotic mixing in draft tube SV is found to be high. The performance of the stirred vessel is further investigated through stimulus–response tracer techniques (RTD) to detect design flaws such as bypass and dead zones. This is analyzed for a wide range of operating parameters and identified optimum conditions (flow rate, impeller speed) for the operation of SV. The four different outlet pipe locations are chosen in SV. The bypass and dead volume are analyzed accordingly, and an optimum outlet pipe location is found. To reduce the extent of nonideal parameters, three different gas source locations are considered and gases are dispersed in the form of bubbles. The gas dispersion at optimum gas injection point is found to reduce nonideal parameters and improve the design of stirred vessel.
Experimental and computational investigation of solid suspension and gas dispersion in a stirred vessel
10.1063/5.0122635
Physics of Fluids
20221107T12:49:32Z
© 2022 Author(s).
Basheer Ashraf Ali
Kumar B
Venkata Sai Teja Madana

Numerical study of drop spread and rebound on heated surfaces with consideration of high pressure
https://aip.scitation.org/doi/10.1063/5.0124794?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The impact of a drop on a solid surface has been studied for many years. However, most of the previous numerical simulations were focused on the drop impact on a surface at room temperature and standard atmospheric pressure. This paper presents a numerical study of nheptane and ndecane drops impacting solid surfaces with the consideration of high temperature and high pressure using smoothed particle hydrodynamics (SPH). The SPH method is validated against experiments from our work and literature. This work is focused on two typical dropimpact regimes, namely, spread and rebound. Different drop impact sequences were simulated at the wall temperature in the range of 27–400 °C and the ambient pressure between 1–20 bars. The difference between the inception of film boiling and liquid saturation temperature was found to decrease with elevating ambient pressure. The spread factor and apex height are investigated for the regime of spread. The results indicate that the lower viscosity fluid has a smaller spread factor as compared to the fluid with higher viscosity. The variation of Leidenfrost temperature with ambient pressure for both nheptane and ndecane droplets is established numerically and compared with the trend observed in the experiment. The simulation outcomes of drop rebound for high boiling point liquid (ndecane) in the film boiling regime at atmospheric pressure show that with the increasing wall temperature, the drop rebound height and vapor layer height increase. Finally, the effect of ambient pressure on drop rebound height and velocity is investigated. The numerical results indicate that the increase in ambient pressure reduces the droplet rebound velocity and rebound height.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The impact of a drop on a solid surface has been studied for many years. However, most of the previous numerical simulations were focused on the drop impact on a surface at room temperature and standard atmospheric pressure. This paper presents a numerical study of nheptane and ndecane drops impacting solid surfaces with the consideration of high temperature and high pressure using smoothed particle hydrodynamics (SPH). The SPH method is validated against experiments from our work and literature. This work is focused on two typical dropimpact regimes, namely, spread and rebound. Different drop impact sequences were simulated at the wall temperature in the range of 27–400 °C and the ambient pressure between 1–20 bars. The difference between the inception of film boiling and liquid saturation temperature was found to decrease with elevating ambient pressure. The spread factor and apex height are investigated for the regime of spread. The results indicate that the lower viscosity fluid has a smaller spread factor as compared to the fluid with higher viscosity. The variation of Leidenfrost temperature with ambient pressure for both nheptane and ndecane droplets is established numerically and compared with the trend observed in the experiment. The simulation outcomes of drop rebound for high boiling point liquid (ndecane) in the film boiling regime at atmospheric pressure show that with the increasing wall temperature, the drop rebound height and vapor layer height increase. Finally, the effect of ambient pressure on drop rebound height and velocity is investigated. The numerical results indicate that the increase in ambient pressure reduces the droplet rebound velocity and rebound height.
Numerical study of drop spread and rebound on heated surfaces with consideration of high pressure
10.1063/5.0124794
Physics of Fluids
20221107T12:49:14Z
© 2022 Author(s).
Md. M. A. Sohag
Abhijeet Chausalkar

How SARSCoV2 Omicron droplets transport and deposit in realistic extrathoracic airways
https://aip.scitation.org/doi/10.1063/5.0123213?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The SARSCoV2 Omicron variant is more highly transmissible and causes a higher mortality rate compared to the other eleven variants despite the high vaccination rate. The Omicron variant also establishes a local infection at the extrathoracic airway level. For better health risk assessment of the infected patients, it is essential to understand the transport behavior and the toxicity of the Omicron variant droplet deposition in the extrathoracic airways, which is missing in the literature. Therefore, this study aims to develop a numerical model for the Omicron droplet transport to the extrathoracic airways and to analyze that transport behavior. The finite volume method and ANSYS Fluent 2020 R2 solver were used for the numerical simulation. The Lagrangian approach, the discrete phase model, and the species transport model were employed to simulate the Omicron droplet transport and deposition. Different breathing rates, the mouth and nose inhalation methods were employed to analyze the viral toxicity at the airway wall. The results from this study indicated that there was a 33% of pressure drop for a flow rate at 30 l/min, while there was only a 3.5% of pressure drop for a 7.5 l/min. The nose inhalation of SARSCoV2 Omicron droplets is significantly more harmful than through the mouth due to a high deposition rate at the extrathoracic airways and high toxicity in the nasal cavities. The findings of this study would potentially improve knowledge of the health risk assessment of Omicroninfected patients.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The SARSCoV2 Omicron variant is more highly transmissible and causes a higher mortality rate compared to the other eleven variants despite the high vaccination rate. The Omicron variant also establishes a local infection at the extrathoracic airway level. For better health risk assessment of the infected patients, it is essential to understand the transport behavior and the toxicity of the Omicron variant droplet deposition in the extrathoracic airways, which is missing in the literature. Therefore, this study aims to develop a numerical model for the Omicron droplet transport to the extrathoracic airways and to analyze that transport behavior. The finite volume method and ANSYS Fluent 2020 R2 solver were used for the numerical simulation. The Lagrangian approach, the discrete phase model, and the species transport model were employed to simulate the Omicron droplet transport and deposition. Different breathing rates, the mouth and nose inhalation methods were employed to analyze the viral toxicity at the airway wall. The results from this study indicated that there was a 33% of pressure drop for a flow rate at 30 l/min, while there was only a 3.5% of pressure drop for a 7.5 l/min. The nose inhalation of SARSCoV2 Omicron droplets is significantly more harmful than through the mouth due to a high deposition rate at the extrathoracic airways and high toxicity in the nasal cavities. The findings of this study would potentially improve knowledge of the health risk assessment of Omicroninfected patients.
How SARSCoV2 Omicron droplets transport and deposit in realistic extrathoracic airways
10.1063/5.0123213
Physics of Fluids
20221107T12:48:29Z
Mohammad S. Islam
Md. Mizanur Rahman
Akbar Arsalanloo
Hamidreza Mortazavy Beni
Puchanee Larpruenrudee
Nick S. Bennett
Richard Collins
Tevfik Gemci
Maureen Taylor
YuanTong Gu

Predictive method for flow condensation heat transfer in plain channels
https://aip.scitation.org/doi/10.1063/5.0121943?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The flow condensation heat transfer (FCHT) in plain channels has a variety of applications in many industrial sectors, and thus it is important to predict FCHT coefficients accurately. This paper compiled a large compound FCHT database containing 5607 data points and 30 fluids and presented a strategy for developing a new correlation of FCHT coefficients. The parameter range of the database is the hydraulic diameter D = 0.493–20 mm, vapor quality x = 0.003–0.992, mass flux G = 24–1533 kg/m2 s, heat flux q = 2.9–422 kW/m2, reduced pressure pr = 0.04–0.95, liquid Prandtl number Prl = 1.7–8.5, liquid Reynolds number Rel = 11.6–5.3 × 104, and gas Reynolds number Reg = 75.1–9.1 × 105. Based on the database and strategy, a new general correlation with substantially better accuracy was developed, which is applicable to plain channels of various sizes and a broad operational parameter range. It predicts the database with a mean absolute deviation (MAD) of 14.1%, while the best existing ΔTindependent correlation predicts the database with an MAD of 20.2%. The applicability of the new and 38 existing correlations to individual fluids was assessed. The new correlation performs best for 8 of the 14 fluids that have more than 50 data points in the entire database, while the most accurate existing one performs best only for 2 of them. The Fang number Fa plays an important role in the new correlation, implying that it relates to the fundamental mechanisms of both boiling and condensation heat transfer.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The flow condensation heat transfer (FCHT) in plain channels has a variety of applications in many industrial sectors, and thus it is important to predict FCHT coefficients accurately. This paper compiled a large compound FCHT database containing 5607 data points and 30 fluids and presented a strategy for developing a new correlation of FCHT coefficients. The parameter range of the database is the hydraulic diameter D = 0.493–20 mm, vapor quality x = 0.003–0.992, mass flux G = 24–1533 kg/m2 s, heat flux q = 2.9–422 kW/m2, reduced pressure pr = 0.04–0.95, liquid Prandtl number Prl = 1.7–8.5, liquid Reynolds number Rel = 11.6–5.3 × 104, and gas Reynolds number Reg = 75.1–9.1 × 105. Based on the database and strategy, a new general correlation with substantially better accuracy was developed, which is applicable to plain channels of various sizes and a broad operational parameter range. It predicts the database with a mean absolute deviation (MAD) of 14.1%, while the best existing ΔTindependent correlation predicts the database with an MAD of 20.2%. The applicability of the new and 38 existing correlations to individual fluids was assessed. The new correlation performs best for 8 of the 14 fluids that have more than 50 data points in the entire database, while the most accurate existing one performs best only for 2 of them. The Fang number Fa plays an important role in the new correlation, implying that it relates to the fundamental mechanisms of both boiling and condensation heat transfer.
Predictive method for flow condensation heat transfer in plain channels
10.1063/5.0121943
Physics of Fluids
20221107T12:49:34Z
© 2022 Author(s).

Numerical study of a hollow pileup yielded by deposition of successive hollow droplets
https://aip.scitation.org/doi/10.1063/5.0127450?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Understanding the pileup formation process of sequentially deposited droplets is vital in advancing dropletbased printing technologies. While pileups of simple droplets have been extensively studied, knowledge of the hollow pileup formation is inadequate. This paper presents a fully resolved numerical analysis of the pileup formed by successively depositing incoming hollow droplets on a presolidified (or base) droplet on a supercool surface. An axisymmetric fronttracking method is used to handle the simulations. The pileup height increases as the incoming droplets coalesce, while the hollow cores may or may not merge. The pileup shape and its hollow configuration depend on parameters such as the Stefan number, Peclet number, Weber number, Fourier number, and the size and number of hollow cores. Varying these parameters does not affect the peak formation at the top of the pile caused by volume expansion during phase change, although the Fourier number has a strong influence on the mean aspect ratio and solidification time of the pileup. Increasing the deposition rate enhances the coalescence of hollow cores and reduces the mean aspect ratio of the pileup. Reducing the Stefan number also promotes hollow core coalescence, which decreases the mean aspect ratio. However, the size of the hollow core and the Peclet and Weber numbers have almost no influence on the outer shape of the hollow pileup. The effect of the number of incoming droplets on the pileup formation is also revealed.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Understanding the pileup formation process of sequentially deposited droplets is vital in advancing dropletbased printing technologies. While pileups of simple droplets have been extensively studied, knowledge of the hollow pileup formation is inadequate. This paper presents a fully resolved numerical analysis of the pileup formed by successively depositing incoming hollow droplets on a presolidified (or base) droplet on a supercool surface. An axisymmetric fronttracking method is used to handle the simulations. The pileup height increases as the incoming droplets coalesce, while the hollow cores may or may not merge. The pileup shape and its hollow configuration depend on parameters such as the Stefan number, Peclet number, Weber number, Fourier number, and the size and number of hollow cores. Varying these parameters does not affect the peak formation at the top of the pile caused by volume expansion during phase change, although the Fourier number has a strong influence on the mean aspect ratio and solidification time of the pileup. Increasing the deposition rate enhances the coalescence of hollow cores and reduces the mean aspect ratio of the pileup. Reducing the Stefan number also promotes hollow core coalescence, which decreases the mean aspect ratio. However, the size of the hollow core and the Peclet and Weber numbers have almost no influence on the outer shape of the hollow pileup. The effect of the number of incoming droplets on the pileup formation is also revealed.
Numerical study of a hollow pileup yielded by deposition of successive hollow droplets
10.1063/5.0127450
Physics of Fluids
20221108T05:59:44Z
© 2022 Author(s).
Truong V. Vu
Nang X. Ho

Evidence of direct charge transfer in plasmonmediated photocatalytic water splitting: A timedependent density functional theory study
https://aip.scitation.org/doi/10.1063/5.0123366?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Photocatalytic water splitting is a promising route for hydrogen production and solar energy storage. Plasmonmediated water splitting has the potential to harvest photons with longer wavelengths compared with semiconductorbased photocatalysis. However, the mechanism of plasmoninduced charge transfer, the determining step of photochemistry, is not well understood. Here, we studied plasmonmediated water splitting at atomic length scale and femtosecond timescale. Linearresponse timedependent density functional theory calculations and Ehrenfest dynamics simulations were performed for a realistic H2O@Au6 model excited by the femtosecond laser. Wavelengthdependent charge transfer mechanisms were demonstrated. Especially, for the excitation of 2.25 eV that falls into the visible spectrum, evidence was presented for the dominant direct transfer of dorbital electrons from the gold cluster to the adsorbed water molecule. In this mechanism, the charge transfer leapfrogs the processes of excitation and thermalization within gold described in the classical theory. The results can assist the design of more energyefficient solar water splitting.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Photocatalytic water splitting is a promising route for hydrogen production and solar energy storage. Plasmonmediated water splitting has the potential to harvest photons with longer wavelengths compared with semiconductorbased photocatalysis. However, the mechanism of plasmoninduced charge transfer, the determining step of photochemistry, is not well understood. Here, we studied plasmonmediated water splitting at atomic length scale and femtosecond timescale. Linearresponse timedependent density functional theory calculations and Ehrenfest dynamics simulations were performed for a realistic H2O@Au6 model excited by the femtosecond laser. Wavelengthdependent charge transfer mechanisms were demonstrated. Especially, for the excitation of 2.25 eV that falls into the visible spectrum, evidence was presented for the dominant direct transfer of dorbital electrons from the gold cluster to the adsorbed water molecule. In this mechanism, the charge transfer leapfrogs the processes of excitation and thermalization within gold described in the classical theory. The results can assist the design of more energyefficient solar water splitting.
Evidence of direct charge transfer in plasmonmediated photocatalytic water splitting: A timedependent density functional theory study
10.1063/5.0123366
Physics of Fluids
20221109T12:03:15Z
© 2022 Author(s).

Numerical investigation of the segregation of turbulent emulsions
https://aip.scitation.org/doi/10.1063/5.0112565?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We study the segregation of emulsions in decaying turbulence using direct numerical simulations in combination with the volume of fluid method. To this end, we generate emulsions in forced homogeneous isotropic turbulence and then turn the forcing off and activate the gravitational acceleration. This allows us to study the segregation process in decaying turbulence and under gravity. We consider nonisodensity emulsions, where the dispersed phase is the lighter one. The segregation process is driven by both the minimization of the potential energy achieved by the sinking of the heavier phase as well as the minimization of the surface energy achieved by coalescence. To study these two processes and their impacts on the segregation progress in detail, we consider different buoyancy forces and surface tension coefficients in our investigation, resulting in five different configurations. The surface tension coefficient also alters the droplet size distribution of the emulsion. Using the threedimensional simulation results and the monitored data, we analyze the driving mechanisms and their impact on the segregation progress in detail. We propose a dimensionless number that reflects the energy release dominating the segregation. Moreover, we evaluate the time required for the rise of the lighter phase and study correlations with the varied parameters: gravitational acceleration and surface tension coefficient.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We study the segregation of emulsions in decaying turbulence using direct numerical simulations in combination with the volume of fluid method. To this end, we generate emulsions in forced homogeneous isotropic turbulence and then turn the forcing off and activate the gravitational acceleration. This allows us to study the segregation process in decaying turbulence and under gravity. We consider nonisodensity emulsions, where the dispersed phase is the lighter one. The segregation process is driven by both the minimization of the potential energy achieved by the sinking of the heavier phase as well as the minimization of the surface energy achieved by coalescence. To study these two processes and their impacts on the segregation progress in detail, we consider different buoyancy forces and surface tension coefficients in our investigation, resulting in five different configurations. The surface tension coefficient also alters the droplet size distribution of the emulsion. Using the threedimensional simulation results and the monitored data, we analyze the driving mechanisms and their impact on the segregation progress in detail. We propose a dimensionless number that reflects the energy release dominating the segregation. Moreover, we evaluate the time required for the rise of the lighter phase and study correlations with the varied parameters: gravitational acceleration and surface tension coefficient.
Numerical investigation of the segregation of turbulent emulsions
10.1063/5.0112565
Physics of Fluids
20221110T12:53:23Z
© 2022 Author(s).
T. Trummler
A. Begemann
E. Trautner
M. Klein

Experimental study on the transmission characteristics of nearfield detonation noise into water
https://aip.scitation.org/doi/10.1063/5.0119227?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>To study the transmission characteristics of nearfield detonation noise into water, the detonation noise transmission system is built on a laboratoryscale water tank using a detonation tube with a diameter of 30 mm. The interaction of the detonation gas jet with the air–water interface, the development of the cavity, and the growth of the liquid column are experimentally observed by a highspeed camera. The spectral distribution characteristics of detonation noise above and below the interface are recorded by a microphone, a hydrophone, and an underwater blast sensor. Analysis of the experimental images shows that the size of the cavity increases with increasing filling pressure and decreases with increasing nozzle height. By normalizing the evolution time of the cavity with the cavity lifetime, it is concluded that the time for the cavity to develop to the deepest is about 0.27, independent of the filling pressure. The pressure field data analysis results show that the main frequencies of the detonation sound waves are 100 and 400 Hz, and the frequency distribution has nothing to do with the filling pressure. Through the defined acoustic wave energy transmission coefficient, it is demonstrated that the detonation acoustic wave transmission coefficient decreases with the increase in the frequency, and the shock wave transmission coefficient decreases with the increase in the angle.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>To study the transmission characteristics of nearfield detonation noise into water, the detonation noise transmission system is built on a laboratoryscale water tank using a detonation tube with a diameter of 30 mm. The interaction of the detonation gas jet with the air–water interface, the development of the cavity, and the growth of the liquid column are experimentally observed by a highspeed camera. The spectral distribution characteristics of detonation noise above and below the interface are recorded by a microphone, a hydrophone, and an underwater blast sensor. Analysis of the experimental images shows that the size of the cavity increases with increasing filling pressure and decreases with increasing nozzle height. By normalizing the evolution time of the cavity with the cavity lifetime, it is concluded that the time for the cavity to develop to the deepest is about 0.27, independent of the filling pressure. The pressure field data analysis results show that the main frequencies of the detonation sound waves are 100 and 400 Hz, and the frequency distribution has nothing to do with the filling pressure. Through the defined acoustic wave energy transmission coefficient, it is demonstrated that the detonation acoustic wave transmission coefficient decreases with the increase in the frequency, and the shock wave transmission coefficient decreases with the increase in the angle.
Experimental study on the transmission characteristics of nearfield detonation noise into water
10.1063/5.0119227
Physics of Fluids
20221111T12:49:42Z
© 2022 Author(s).

Simulation of liquid jet atomization and droplet breakup via a VolumeofFluid Lagrangian–Eulerian strategy
https://aip.scitation.org/doi/10.1063/5.0122742?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The hybrid VolumeofFluid and Lagrangian–Eulerian (VoFLE) strategy is an attractive approach for reducing the computational cost of spray simulations while retaining a reasonable amount of fidelity. It is based on the concept of transitioning small liquid bodies or droplets to a Lagrangian–Eulerian (LE) representation, alleviating the burden of maintaining high resolution for small droplets. This hybrid VoFLE methodology is extended in the present work by incorporating a hydrodynamic breakup model based on maximum entropy formalism (MEF). This approach is particularly suitable for realistic spray conditions, such as highpressure fuel injectors, where adequate numerical resolution of the smallest droplets is extremely difficult. The first step in the present VoFLE treatment is the identification of unresolved liquid structures targeted for LE transition. This step is followed by the application of the MEF breakup model for those structures that are hydrodynamically unstable, resulting in the assignment of secondary drop sizes and velocities. The model is evaluated statistically and tested against experimental data from the Engine Combustion Network and the breakup of a water jet. Relatively favorable results are encountered in these tests.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The hybrid VolumeofFluid and Lagrangian–Eulerian (VoFLE) strategy is an attractive approach for reducing the computational cost of spray simulations while retaining a reasonable amount of fidelity. It is based on the concept of transitioning small liquid bodies or droplets to a Lagrangian–Eulerian (LE) representation, alleviating the burden of maintaining high resolution for small droplets. This hybrid VoFLE methodology is extended in the present work by incorporating a hydrodynamic breakup model based on maximum entropy formalism (MEF). This approach is particularly suitable for realistic spray conditions, such as highpressure fuel injectors, where adequate numerical resolution of the smallest droplets is extremely difficult. The first step in the present VoFLE treatment is the identification of unresolved liquid structures targeted for LE transition. This step is followed by the application of the MEF breakup model for those structures that are hydrodynamically unstable, resulting in the assignment of secondary drop sizes and velocities. The model is evaluated statistically and tested against experimental data from the Engine Combustion Network and the breakup of a water jet. Relatively favorable results are encountered in these tests.
Simulation of liquid jet atomization and droplet breakup via a VolumeofFluid Lagrangian–Eulerian strategy
10.1063/5.0122742
Physics of Fluids
20221111T12:50:59Z
© 2022 Author(s).
Mario F. Trujillo

Application of the Maxwell–Stefan theory in modeling gas diffusion experiments into isolated oil droplets by water
https://aip.scitation.org/doi/10.1063/5.0119766?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We have used the Maxwell–Stefan diffusion theory to model the mass transfer between tertiaryinjected gas and residual oil blocked by water, in order to predict the time required for the rupture of the water barrier due to oil swelling. We have also designed and conducted a set of visualization micromodel experiments on various pure and multicomponent oil–gas systems to measure the water rupture time in tertiary gas injection processes. The experimental results show that the initial pressure and dimensions of the system, the oil and gas composition, and the gas solubility in water control the oil swelling process. The experimentally measured rupture times are then employed to evaluate the reliability of the model and to compare its accuracy with that of a similar one using classical Fick's law. Our modeling results show that both models are able to estimate the water rupture time for pure systems with an acceptable precision. As for multicomponent mixtures, however, only the Maxwell–Stefan theory is capable of modeling the molecular diffusion process correctly and yields values close to reality, while the use of Fick's law would lead to erroneous results. Deficiency of the latter model becomes more acute when the diffusion direction in reality is contrary to what the model indicates, which leads to failure in calculating any value for rupture time at all for these cases.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We have used the Maxwell–Stefan diffusion theory to model the mass transfer between tertiaryinjected gas and residual oil blocked by water, in order to predict the time required for the rupture of the water barrier due to oil swelling. We have also designed and conducted a set of visualization micromodel experiments on various pure and multicomponent oil–gas systems to measure the water rupture time in tertiary gas injection processes. The experimental results show that the initial pressure and dimensions of the system, the oil and gas composition, and the gas solubility in water control the oil swelling process. The experimentally measured rupture times are then employed to evaluate the reliability of the model and to compare its accuracy with that of a similar one using classical Fick's law. Our modeling results show that both models are able to estimate the water rupture time for pure systems with an acceptable precision. As for multicomponent mixtures, however, only the Maxwell–Stefan theory is capable of modeling the molecular diffusion process correctly and yields values close to reality, while the use of Fick's law would lead to erroneous results. Deficiency of the latter model becomes more acute when the diffusion direction in reality is contrary to what the model indicates, which leads to failure in calculating any value for rupture time at all for these cases.
Application of the Maxwell–Stefan theory in modeling gas diffusion experiments into isolated oil droplets by water
10.1063/5.0119766
Physics of Fluids
20221116T10:41:51Z
© 2022 Author(s).
Seyedamir Mirazimi
Behzad Rostami
MohammadHossein Ghazanfari
Maryam Khosravi

An effective pseudopotential lattice Boltzmann model with extremely large density ratio and adjustable surface tension
https://aip.scitation.org/doi/10.1063/5.0123727?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The pseudopotential lattice Boltzmann (LB) model is versatile in modeling multiphase flows since the mesoscopic interaction potential enables it to directly describe the nonideal effect evading the tracking or integrating of phase interface. In this paper, we develop an effective pseudopotential lattice Boltzmann model to simultaneously realize the thermodynamic consistency, the extremely large density ratio, and the adjustable surface tension. Decoupling the mesh space from the momentum space by a scale factor, denser lattice nodes depict the transition region more accurately. The highprecision explicit finite difference method (EFM) further enhances the calculation accuracy of interaction force. The present model is validated to satisfy thermodynamic even at very low temperature, where the liquid–gas density ratio exceeds 1010. The spurious current can be suppressed to a very low level (<0.0007) despite the density ratio reaching tens of thousands. A modified pressure tension is introduced to tune the surface tension free from the influence of the density ratio. The numerical stability of multiphase simulations is significantly improved, and the droplet splashing is successfully reproduced at Reynolds number 25 000, while the density ratio is more than 10 000.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The pseudopotential lattice Boltzmann (LB) model is versatile in modeling multiphase flows since the mesoscopic interaction potential enables it to directly describe the nonideal effect evading the tracking or integrating of phase interface. In this paper, we develop an effective pseudopotential lattice Boltzmann model to simultaneously realize the thermodynamic consistency, the extremely large density ratio, and the adjustable surface tension. Decoupling the mesh space from the momentum space by a scale factor, denser lattice nodes depict the transition region more accurately. The highprecision explicit finite difference method (EFM) further enhances the calculation accuracy of interaction force. The present model is validated to satisfy thermodynamic even at very low temperature, where the liquid–gas density ratio exceeds 1010. The spurious current can be suppressed to a very low level (<0.0007) despite the density ratio reaching tens of thousands. A modified pressure tension is introduced to tune the surface tension free from the influence of the density ratio. The numerical stability of multiphase simulations is significantly improved, and the droplet splashing is successfully reproduced at Reynolds number 25 000, while the density ratio is more than 10 000.
An effective pseudopotential lattice Boltzmann model with extremely large density ratio and adjustable surface tension
10.1063/5.0123727
Physics of Fluids
20221116T10:41:40Z
© 2022 Author(s).

A Lagrangian analysis of partial cavitation growth and cavitation control mechanism
https://aip.scitation.org/doi/10.1063/5.0124388?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Partial cavitation has a strong unsteadiness, which will cause serious damage to the hydraulic machinery. The spanwise obstacle is nearly the most efficient method for controlling unsteady cavitation. In this study, numerical simulations of partial cavitating flows around NACA (National Advisory Committee for Aeronautics) 66 hydrofoils in two dimensions (2D) were carried out both with and without obstruction. The obstruction is placed at 0.37c, and its height is 0.1c. Utilizing the finitetime Lyapunov exponent, the Lagrangian coherent structures (LCSs) were developed to investigate the dynamic characteristics of the unsteady flow. By showing the dynamic evolution of the Lagrangian behaviors, the timedependent LCSs over the two different flows demonstrate the effectiveness of LCSs in explaining the evolution of the vortex during the partial cavitation process. With the use of LCSs, the vortex boundary and reentrant jet can be easily located, and the link between the vortexes can be readily seen. In the meantime, the vortex's origin and destination are shown by the stable and unstable manifolds, respectively. LCSs were then utilized to examine how the obstruction had an impact, and the following conclusions were reached. First, the obstruction can stop a portion of reentrant jets from passing through it. Second, the obstruction can curve the pathway of the reentrant jet, which has passed through it. Third, the obstruction prevents the cavity from flowing downstream. Finally, the obstruction continuously obliterates the expanding cavity across it. Simply said, the Lagrangian analysis based on LCSs provides a better understanding of the vortex dynamics than traditional visualization techniques, which is essential to understanding the great performance of the cavitationinduced unsteady flow.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Partial cavitation has a strong unsteadiness, which will cause serious damage to the hydraulic machinery. The spanwise obstacle is nearly the most efficient method for controlling unsteady cavitation. In this study, numerical simulations of partial cavitating flows around NACA (National Advisory Committee for Aeronautics) 66 hydrofoils in two dimensions (2D) were carried out both with and without obstruction. The obstruction is placed at 0.37c, and its height is 0.1c. Utilizing the finitetime Lyapunov exponent, the Lagrangian coherent structures (LCSs) were developed to investigate the dynamic characteristics of the unsteady flow. By showing the dynamic evolution of the Lagrangian behaviors, the timedependent LCSs over the two different flows demonstrate the effectiveness of LCSs in explaining the evolution of the vortex during the partial cavitation process. With the use of LCSs, the vortex boundary and reentrant jet can be easily located, and the link between the vortexes can be readily seen. In the meantime, the vortex's origin and destination are shown by the stable and unstable manifolds, respectively. LCSs were then utilized to examine how the obstruction had an impact, and the following conclusions were reached. First, the obstruction can stop a portion of reentrant jets from passing through it. Second, the obstruction can curve the pathway of the reentrant jet, which has passed through it. Third, the obstruction prevents the cavity from flowing downstream. Finally, the obstruction continuously obliterates the expanding cavity across it. Simply said, the Lagrangian analysis based on LCSs provides a better understanding of the vortex dynamics than traditional visualization techniques, which is essential to understanding the great performance of the cavitationinduced unsteady flow.
A Lagrangian analysis of partial cavitation growth and cavitation control mechanism
10.1063/5.0124388
Physics of Fluids
20221117T12:25:13Z
© 2022 Author(s).

On the velocity, size, and temperature of gaseous dendritic flames
https://aip.scitation.org/doi/10.1063/5.0118271?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Dendritic combustion in Hele–Shaw cells is investigated qualitatively using a simplified onedimensional thermodiffusive model. Formulas for the velocity, size, and temperature of the flamelets are derived. The temperature and velocity of the flames increase for small radii to allow for their survival regardless of the activation energy. In addition, the results obtained with very large activation energy were compared with experimental results, finding that additional tests are required due to the strong influence of gravity on the velocity and size estimations. Conditions for the existence of this anomalous propagation are investigated, confirming analytically that it can only happen for low Lewis numbers.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Dendritic combustion in Hele–Shaw cells is investigated qualitatively using a simplified onedimensional thermodiffusive model. Formulas for the velocity, size, and temperature of the flamelets are derived. The temperature and velocity of the flames increase for small radii to allow for their survival regardless of the activation energy. In addition, the results obtained with very large activation energy were compared with experimental results, finding that additional tests are required due to the strong influence of gravity on the velocity and size estimations. Conditions for the existence of this anomalous propagation are investigated, confirming analytically that it can only happen for low Lewis numbers.
On the velocity, size, and temperature of gaseous dendritic flames
10.1063/5.0118271
Physics of Fluids
20221101T11:30:04Z
© 2022 Author(s).
Jorge Yanez
Mike Kuznetsov
Fernando VeigaLópez

The nonlinear lift coefficient characteristics and active flow control of a symmetrical airfoil at a low Reynolds number
https://aip.scitation.org/doi/10.1063/5.0122875?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, the flow characteristics of a symmetrical airfoil at Re = 40 000 near a 0° angle of attack are investigated numerically, and the nonlinear lift coefficient phenomenon and different types of laminar separation bubble (LSB) structures are clearly observed. It is confirmed that the nonlinear characteristics of the lift coefficient originate from the differently shaped pressure distributions in the LSB. The causes of the different characteristics of the pressure distributions in different types of LSB are revealed by deriving the Reynoldsaveraged pressure gradient equation. It is determined that the viscous stress distribution near the wall is the key to the formation of different pressure distributions. Moreover, in order to suppress the disadvantages associated with the nonlinear lift coefficient of a symmetrical airfoil, an active flow control method based on local oscillation is adopted. By introducing an oscillation disturbance upstream of the separation bubble, the effect of Reynolds stress and convection on the wall is enhanced, the reattachment of the separation flow is promoted, and the formation of an LSB at the trailing edge is suppressed. Thus, the nonlinear characteristics of the lift coefficient due to the switching of the LSB structure are eliminated.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, the flow characteristics of a symmetrical airfoil at Re = 40 000 near a 0° angle of attack are investigated numerically, and the nonlinear lift coefficient phenomenon and different types of laminar separation bubble (LSB) structures are clearly observed. It is confirmed that the nonlinear characteristics of the lift coefficient originate from the differently shaped pressure distributions in the LSB. The causes of the different characteristics of the pressure distributions in different types of LSB are revealed by deriving the Reynoldsaveraged pressure gradient equation. It is determined that the viscous stress distribution near the wall is the key to the formation of different pressure distributions. Moreover, in order to suppress the disadvantages associated with the nonlinear lift coefficient of a symmetrical airfoil, an active flow control method based on local oscillation is adopted. By introducing an oscillation disturbance upstream of the separation bubble, the effect of Reynolds stress and convection on the wall is enhanced, the reattachment of the separation flow is promoted, and the formation of an LSB at the trailing edge is suppressed. Thus, the nonlinear characteristics of the lift coefficient due to the switching of the LSB structure are eliminated.
The nonlinear lift coefficient characteristics and active flow control of a symmetrical airfoil at a low Reynolds number
10.1063/5.0122875
Physics of Fluids
20221101T11:30:01Z
© 2022 Author(s).

Dataefficient deep reinforcement learning with expert demonstration for active flow control
https://aip.scitation.org/doi/10.1063/5.0120285?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Deep reinforcement learning (RL) is capable of identifying and modifying strategies for active flow control. However, the classic active formulation of deep RL requires lengthy active exploration. This paper describes the introduction of expert demonstration into a classic offpolicy RL algorithm, the soft actorcritic algorithm, for application to vortexinduced vibration problems. This combined onlinelearning framework is applied to an oscillator wake environment and a Navier–Stokes environment with expert demonstration obtained from the poleplacement method and surrogate model optimization. The results show that the soft actorcritic framework combined with expert demonstration enables rapid learning of active flow control strategies through a combination of prior demonstration data and online experience. This study develops a new dataefficient RL approach for discovering active flow control strategies for vortexinduced vibration, providing a more practical methodology for industrial applications.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Deep reinforcement learning (RL) is capable of identifying and modifying strategies for active flow control. However, the classic active formulation of deep RL requires lengthy active exploration. This paper describes the introduction of expert demonstration into a classic offpolicy RL algorithm, the soft actorcritic algorithm, for application to vortexinduced vibration problems. This combined onlinelearning framework is applied to an oscillator wake environment and a Navier–Stokes environment with expert demonstration obtained from the poleplacement method and surrogate model optimization. The results show that the soft actorcritic framework combined with expert demonstration enables rapid learning of active flow control strategies through a combination of prior demonstration data and online experience. This study develops a new dataefficient RL approach for discovering active flow control strategies for vortexinduced vibration, providing a more practical methodology for industrial applications.
Dataefficient deep reinforcement learning with expert demonstration for active flow control
10.1063/5.0120285
Physics of Fluids
20221102T02:43:26Z
© 2022 Author(s).

Comparative analysis of the flow control over a circular cylinder with detached flexible and rigid splitter plates
https://aip.scitation.org/doi/10.1063/5.0110398?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A comparative study is performed on a circular cylinder with both flexible and rigid splitter plates (SPs). This study has the novelty of using single and dual detached SPs located downstream of the cylinder. The dimensionless gap distance between the first splitter plate and the cylinder as well as the distance between the SPs are varied. The strain of flexible SPs can be used for energy harvesting from the flow. Therefore, a parametric study is performed to find the optimal design for placing piezoelectric polymers. The twodimensional fluid–structureinteraction analysis is performed based on the arbitrary Lagrangian–Eulerian scheme using COMSOL Multiphysics. Flow characteristics quantities, tip amplitude, and strain are evaluated at different arrangements of the SPs. The results reveal that wake control enhances effectively by doubling the number of SPs. Strain assessments indicate that the strain of dual SPs increases by more than 100% compared to the single plate case. In addition, the amplitude of the dual SPs increases by a remarkable ratio of 18.29 compared to the single plate. In the case of rigid and flexible SPs at a certain arrangement, dramatic reductions of 97.8% and 76.35% in the Strouhal number are obtained compared to a bare cylinder. In addition, 18% drag reduction compared to the bare cylinder is recorded for the rigid SPs. The presented passive method can be used as an attractive approach in flow control as well as energy harvesting from ocean waves and sea currents.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A comparative study is performed on a circular cylinder with both flexible and rigid splitter plates (SPs). This study has the novelty of using single and dual detached SPs located downstream of the cylinder. The dimensionless gap distance between the first splitter plate and the cylinder as well as the distance between the SPs are varied. The strain of flexible SPs can be used for energy harvesting from the flow. Therefore, a parametric study is performed to find the optimal design for placing piezoelectric polymers. The twodimensional fluid–structureinteraction analysis is performed based on the arbitrary Lagrangian–Eulerian scheme using COMSOL Multiphysics. Flow characteristics quantities, tip amplitude, and strain are evaluated at different arrangements of the SPs. The results reveal that wake control enhances effectively by doubling the number of SPs. Strain assessments indicate that the strain of dual SPs increases by more than 100% compared to the single plate case. In addition, the amplitude of the dual SPs increases by a remarkable ratio of 18.29 compared to the single plate. In the case of rigid and flexible SPs at a certain arrangement, dramatic reductions of 97.8% and 76.35% in the Strouhal number are obtained compared to a bare cylinder. In addition, 18% drag reduction compared to the bare cylinder is recorded for the rigid SPs. The presented passive method can be used as an attractive approach in flow control as well as energy harvesting from ocean waves and sea currents.
Comparative analysis of the flow control over a circular cylinder with detached flexible and rigid splitter plates
10.1063/5.0110398
Physics of Fluids
20221102T02:44:20Z
© 2022 Author(s).
Faezeh Eydi
Afsaneh Mojra
Rezvan Abdi

Heat transfer enhancement by a flexible inverted flag with an inclination angle
https://aip.scitation.org/doi/10.1063/5.0116970?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The heat transfer system, including an inclined inverted flag that plays a role of a vortex generator, is proposed in the present study. A twodimensional simulation is performed to analyze the effects of the inclination angle and the bending rigidity of the inverted flag on thermal performance. To consider the fluidflexible body–thermal interaction, an immersed boundary method is adopted. The four regimes are observed depending on the inclination angle and the bending rigidity, that is, largeamplitude flapping (LAF), smallamplitude flapping (SAF), deflected (D), and straight (S) modes. The SAF and LAF modes are observed to be favorable in terms of the heat transfer efficiency, which considers the heat flux and mechanical energy loss. A scaling analysis is performed to explain the correlation between the flapping kinematics and the thermal quantities. A scaling parameter is newly defined based on the momentum transfer to the inverted flag due to a vortical impulse and shows a proportional relation to the mean drag force with a slope of 0.166. The heat transfer efficiency is observed to be proportional and inversely proportional to the parameter in the SAF and LAF modes, respectively. The optimized heat transfer system is obtained at the angle of 12° and the bending rigidity of 0.7, where the efficiency is enhanced up to 112.8% over the baseline flow.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The heat transfer system, including an inclined inverted flag that plays a role of a vortex generator, is proposed in the present study. A twodimensional simulation is performed to analyze the effects of the inclination angle and the bending rigidity of the inverted flag on thermal performance. To consider the fluidflexible body–thermal interaction, an immersed boundary method is adopted. The four regimes are observed depending on the inclination angle and the bending rigidity, that is, largeamplitude flapping (LAF), smallamplitude flapping (SAF), deflected (D), and straight (S) modes. The SAF and LAF modes are observed to be favorable in terms of the heat transfer efficiency, which considers the heat flux and mechanical energy loss. A scaling analysis is performed to explain the correlation between the flapping kinematics and the thermal quantities. A scaling parameter is newly defined based on the momentum transfer to the inverted flag due to a vortical impulse and shows a proportional relation to the mean drag force with a slope of 0.166. The heat transfer efficiency is observed to be proportional and inversely proportional to the parameter in the SAF and LAF modes, respectively. The optimized heat transfer system is obtained at the angle of 12° and the bending rigidity of 0.7, where the efficiency is enhanced up to 112.8% over the baseline flow.
Heat transfer enhancement by a flexible inverted flag with an inclination angle
10.1063/5.0116970
Physics of Fluids
20221104T12:22:15Z
© 2022 Author(s).
Jae Won Shin
Sung Goon Park
Lian Shen

Flowinduced vibration of a circular cylinder with an attached elastic plate of high aspect ratio
https://aip.scitation.org/doi/10.1063/5.0120428?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The flowinduced transverse vibration of a cylinder (diameter [math]) with an attached flexible and elastic plate of high aspect ratio to its leeward side is investigated numerically at a low Reynolds number of 150 for a range of reduced velocities (Ur) using an inhouse developed fluid solver based on curvilinear immersed boundary method strongly coupled with an opensource finite elementbased structural solver. It was observed that an attached elastic plate of width [math] and length [math] suppresses large vibrations of the cylinder, but one with length L = 2, contrary to previous studies, amplifies vibrations up to five times of an isolated cylinder. Three regimes were observed: vortexinduced vibration (VIV), suppression, and galloping. In VIV regime for [math], lockin was observed where the vortex shedding frequency from the platecylinder system was seen to slightly increase relative to that of static cylinder–plate system to match with the natural frequency of the cylinder and the plate. In this regime, the deformations of the elastic plate were large (max. 91% of L) and in high modes (up to fifth mode), leading to new vortex patterns. The transverse displacement of the cylinder–plate system was found to reach nearly twice of an isolated cylinder in this regime. For [math], the cylinder–plate system was pushed into suppression regime, wherein its displacement was nullified because of lack of vorticity interaction and outofphase deformation. Beyond Ur = 9, the cylinder–plate system vibrated in the galloping regime, wherein it shed and generated forces as an asymmetric body creating an angle of attack with the incoming flow. The primary mode of deformation of the elastic plate progressively increased from second mode to third mode in galloping regime, and the transverse displacement of cylinder showed a linear increase with the increase in reduced velocity until Ur = 18. The vibration amplitude of the cylinder was higher in the galloping regime, but the vibrations of the plate were more intense (higher amplitude and mode) in the VIV regime. New vortex patterns were observed in the VIV and galloping regimes ranging from 2S mode till 2T mode including all the vortex pattern between them like 2S, 2P, 2Q, and P + T modes, which are reported for the first time.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The flowinduced transverse vibration of a cylinder (diameter [math]) with an attached flexible and elastic plate of high aspect ratio to its leeward side is investigated numerically at a low Reynolds number of 150 for a range of reduced velocities (Ur) using an inhouse developed fluid solver based on curvilinear immersed boundary method strongly coupled with an opensource finite elementbased structural solver. It was observed that an attached elastic plate of width [math] and length [math] suppresses large vibrations of the cylinder, but one with length L = 2, contrary to previous studies, amplifies vibrations up to five times of an isolated cylinder. Three regimes were observed: vortexinduced vibration (VIV), suppression, and galloping. In VIV regime for [math], lockin was observed where the vortex shedding frequency from the platecylinder system was seen to slightly increase relative to that of static cylinder–plate system to match with the natural frequency of the cylinder and the plate. In this regime, the deformations of the elastic plate were large (max. 91% of L) and in high modes (up to fifth mode), leading to new vortex patterns. The transverse displacement of the cylinder–plate system was found to reach nearly twice of an isolated cylinder in this regime. For [math], the cylinder–plate system was pushed into suppression regime, wherein its displacement was nullified because of lack of vorticity interaction and outofphase deformation. Beyond Ur = 9, the cylinder–plate system vibrated in the galloping regime, wherein it shed and generated forces as an asymmetric body creating an angle of attack with the incoming flow. The primary mode of deformation of the elastic plate progressively increased from second mode to third mode in galloping regime, and the transverse displacement of cylinder showed a linear increase with the increase in reduced velocity until Ur = 18. The vibration amplitude of the cylinder was higher in the galloping regime, but the vibrations of the plate were more intense (higher amplitude and mode) in the VIV regime. New vortex patterns were observed in the VIV and galloping regimes ranging from 2S mode till 2T mode including all the vortex pattern between them like 2S, 2P, 2Q, and P + T modes, which are reported for the first time.
Flowinduced vibration of a circular cylinder with an attached elastic plate of high aspect ratio
10.1063/5.0120428
Physics of Fluids
20221107T12:48:45Z
© 2022 Author(s).
Sujyesh Aanandh Manjunathan
Iman Borazjani

Classification of spatialtemporal flow patterns in a low Re wake based on the recurrent trajectory clustering
https://aip.scitation.org/doi/10.1063/5.0123627?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Coherent structures are ubiquitous in unsteady flows. They can be regarded as certain kinds of spatialtemporal patterns that interact with the neighboring field. Although they play a key role in convection and mixing, there is no consensus on how to define them, and their dynamics are complicated. In the past decades, many methods are developed to identify coherent structures based on instantaneous velocity fields (e.g., vortex identification) or longtime statistics (e.g., proper orthogonal decomposition), but the evolution process of individual structures is not well considered in the identification. In this paper, we propose a new method to classify coherent motions according to their evolution dynamics. Specifically, the evolutions are represented by trajectories in the phase space. We define a distance between two trajectories and use it to construct a network that characterizes all evolution patterns. Using spectrum clustering, we categorize these patterns into various groups. This method is applied to a low Reynolds number wake flow downstream of two cylindersintandem, where one of the cylinders oscillates in the transverse direction. The flow is quasiperiodic, and four types of recurrent spatialtemporal patterns can be identified. It is a useful tool to investigate low Reynolds number unsteady flows.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Coherent structures are ubiquitous in unsteady flows. They can be regarded as certain kinds of spatialtemporal patterns that interact with the neighboring field. Although they play a key role in convection and mixing, there is no consensus on how to define them, and their dynamics are complicated. In the past decades, many methods are developed to identify coherent structures based on instantaneous velocity fields (e.g., vortex identification) or longtime statistics (e.g., proper orthogonal decomposition), but the evolution process of individual structures is not well considered in the identification. In this paper, we propose a new method to classify coherent motions according to their evolution dynamics. Specifically, the evolutions are represented by trajectories in the phase space. We define a distance between two trajectories and use it to construct a network that characterizes all evolution patterns. Using spectrum clustering, we categorize these patterns into various groups. This method is applied to a low Reynolds number wake flow downstream of two cylindersintandem, where one of the cylinders oscillates in the transverse direction. The flow is quasiperiodic, and four types of recurrent spatialtemporal patterns can be identified. It is a useful tool to investigate low Reynolds number unsteady flows.
Classification of spatialtemporal flow patterns in a low Re wake based on the recurrent trajectory clustering
10.1063/5.0123627
Physics of Fluids
20221108T05:56:44Z
© 2022 Author(s).
Huixuan Wu
Meihua Zhang
Zhongquan Charlie Zheng

The configuration effect of flapping foils for energy harvesting
https://aip.scitation.org/doi/10.1063/5.0121283?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The configuration of multiple flapping foils is studied via numerical simulations. We comprehensively consider the effects of the streamwise distance, vertical spacing, and phase difference on the energy harvesting performance of flapping foils. We divide flapping foil configurations into three categories: tandem, unaligned, and parallel. The foil tandem configuration is optimal for multiple foils if using the existing efficiency formula. However, tandem configurations expand the diffusion range of the wake turbulence. Wake diffusion has a critical effect on multiple foil configurations, and the utilization of the vertical spacing has been neglected. Here, the effective angle of attack and the effective velocity are proposed based on multiple flapping foils, which can well predict various rules of the lift coefficient and guide studies on the optimal configuration of multiple foils. The optimal phase difference for the simple parallel configuration system is 135°, and the energy harvesting efficiency of parallel foils decreases with increasing vertical spacing. The stepwise configuration maximizes the utilization of the vertical spacing, making it optimal for practical engineering applications. The stepwise configuration not only recycles the wake of the upstream foils but also utilizes it to enhance energy harvesting from flapping turbines. In the multiple foil configurations, the energy harvesting efficiency of the downstream foil over the upstream foil is achieved for the first time using a stepwise configuration.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The configuration of multiple flapping foils is studied via numerical simulations. We comprehensively consider the effects of the streamwise distance, vertical spacing, and phase difference on the energy harvesting performance of flapping foils. We divide flapping foil configurations into three categories: tandem, unaligned, and parallel. The foil tandem configuration is optimal for multiple foils if using the existing efficiency formula. However, tandem configurations expand the diffusion range of the wake turbulence. Wake diffusion has a critical effect on multiple foil configurations, and the utilization of the vertical spacing has been neglected. Here, the effective angle of attack and the effective velocity are proposed based on multiple flapping foils, which can well predict various rules of the lift coefficient and guide studies on the optimal configuration of multiple foils. The optimal phase difference for the simple parallel configuration system is 135°, and the energy harvesting efficiency of parallel foils decreases with increasing vertical spacing. The stepwise configuration maximizes the utilization of the vertical spacing, making it optimal for practical engineering applications. The stepwise configuration not only recycles the wake of the upstream foils but also utilizes it to enhance energy harvesting from flapping turbines. In the multiple foil configurations, the energy harvesting efficiency of the downstream foil over the upstream foil is achieved for the first time using a stepwise configuration.
The configuration effect of flapping foils for energy harvesting
10.1063/5.0121283
Physics of Fluids
20221109T10:35:52Z
© 2022 Author(s).

Womersley's solution for the measurement of volume flow rates in transient laminar flow tubes
https://aip.scitation.org/doi/10.1063/5.0121232?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The characterization of transient flows within the Reynolds number range of Re = 10–100 and the Womersley number range α = 0.8–10 is required for the ongoing development of green monopropellant thrusters. However, at the ml/min scale flow rates of interest, these measurements are outside the capabilities of current commercial flow meters. It is proposed here that transient flows under the required dynamic conditions can be characterized via Womersley's solution for transient flow in a rigid tube. This solution method requires only the measured transient pressure gradient within a controlled laminar flow section and can be accomplished using existing commercial pressure measurement hardware. Experiments were performed where flow similarity was maintained with the ultimate thruster characterization application, but the radius of the flow passage was increased so that flows could be simultaneously characterized by both the proposed solution method and a commercial ultrasonic flow meter. It was shown that across the range of interest, applying Womersley's solution to a measured pressure gradient was an effective method of transient flow characterization. Additionally, it was shown that nonperiodic flows can be characterized except for the initial flow startup transient with solution convergence times approximating the analytical solution to starting flow in a pipe. While these results were expected due to an experimental design matching the assumptions required for Womersley's analytic solution, this work demonstrates that this method is practically feasible as novel instrumentation enabling previously unobtainable measurements.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The characterization of transient flows within the Reynolds number range of Re = 10–100 and the Womersley number range α = 0.8–10 is required for the ongoing development of green monopropellant thrusters. However, at the ml/min scale flow rates of interest, these measurements are outside the capabilities of current commercial flow meters. It is proposed here that transient flows under the required dynamic conditions can be characterized via Womersley's solution for transient flow in a rigid tube. This solution method requires only the measured transient pressure gradient within a controlled laminar flow section and can be accomplished using existing commercial pressure measurement hardware. Experiments were performed where flow similarity was maintained with the ultimate thruster characterization application, but the radius of the flow passage was increased so that flows could be simultaneously characterized by both the proposed solution method and a commercial ultrasonic flow meter. It was shown that across the range of interest, applying Womersley's solution to a measured pressure gradient was an effective method of transient flow characterization. Additionally, it was shown that nonperiodic flows can be characterized except for the initial flow startup transient with solution convergence times approximating the analytical solution to starting flow in a pipe. While these results were expected due to an experimental design matching the assumptions required for Womersley's analytic solution, this work demonstrates that this method is practically feasible as novel instrumentation enabling previously unobtainable measurements.
Womersley's solution for the measurement of volume flow rates in transient laminar flow tubes
10.1063/5.0121232
Physics of Fluids
20221114T11:51:31Z
© 2022 Author(s).
Matthew R. Gilpin
Niema M. Pahlevan

Simulation of flow field in silicon singlecrystal growth using physicsinformed neural network with spatial information
https://aip.scitation.org/doi/10.1063/5.0123811?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Melt convection plays a crucial role in the growth of silicon single crystals. In particular, melt flow transfers mass and heat, and it may strongly affect the crystal growth conditions. Understanding and controlling convection remains a significant challenge in industrial crystal production. Currently, numerical methods such as the finite element method and the finite volume method are mainly used to simulate melt convection in the crystal growth process. However, these methods are not suitable for most applications with realtime requirements. Physicsinformed neural networks (PINNs) have the advantages of fast calculation and wide application. They provide a new concept for the numerical solutions of nonlinear partial differential equations (PDEs). This paper proposes a PINN with spatial information to solve the silicon melt flow model, which does not depend on any simulation data. As the network depth (number of layers) increases, the derivative information in the PDE loss becomes weak, which reduces the expression of the original features in the loss function. Therefore, this study introduces spatial information into the hidden layer of the network, thereby enhancing the correlation between the network and the original input and improving the expression ability of the network. Specifically, silicon melt flow models under three rotating conditions are considered. Compared with other methods, the proposed algorithm can accurately capture regions with complex local morphology. The experimental results reveal the flow characteristics of the silicon melt and confirm the effectiveness of the proposed algorithm. All codes and data attached to this manuscript are publicly available on the following websites: https://github.com/callmedrcom/SIPINN.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Melt convection plays a crucial role in the growth of silicon single crystals. In particular, melt flow transfers mass and heat, and it may strongly affect the crystal growth conditions. Understanding and controlling convection remains a significant challenge in industrial crystal production. Currently, numerical methods such as the finite element method and the finite volume method are mainly used to simulate melt convection in the crystal growth process. However, these methods are not suitable for most applications with realtime requirements. Physicsinformed neural networks (PINNs) have the advantages of fast calculation and wide application. They provide a new concept for the numerical solutions of nonlinear partial differential equations (PDEs). This paper proposes a PINN with spatial information to solve the silicon melt flow model, which does not depend on any simulation data. As the network depth (number of layers) increases, the derivative information in the PDE loss becomes weak, which reduces the expression of the original features in the loss function. Therefore, this study introduces spatial information into the hidden layer of the network, thereby enhancing the correlation between the network and the original input and improving the expression ability of the network. Specifically, silicon melt flow models under three rotating conditions are considered. Compared with other methods, the proposed algorithm can accurately capture regions with complex local morphology. The experimental results reveal the flow characteristics of the silicon melt and confirm the effectiveness of the proposed algorithm. All codes and data attached to this manuscript are publicly available on the following websites: https://github.com/callmedrcom/SIPINN.
Simulation of flow field in silicon singlecrystal growth using physicsinformed neural network with spatial information
10.1063/5.0123811
Physics of Fluids
20221115T10:30:36Z
© 2022 Author(s).
Shuyan Shi
Ding Liu
Zhiran Huo

A novel forecast framework for unsteady flows based on a convolutional neural network
https://aip.scitation.org/doi/10.1063/5.0122271?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Fluid mechanics is an important area where deep learning produces excellent results and can bring about scientific innovation because of its high dimensionality, significant nonlinearity, and ability to process an enormous amount of data. Deep learning technology is currently being used to study fluid mechanics, and its application potential is gradually being demonstrated. We propose a novel multiresolution convolutional interaction network (MCIN), a hierarchical forecast framework based on a convolutional neural network. This structure can capture temporal dependencies at multiple temporal resolutions to enhance the forecasting performance of the original time series. The highdimensional data of the flow around a cylinder are projected into a lowdimensional subspace using a variational autoencoder (VAE) as a nonlinear orderreduction technique. Then, the data of the subspace are used as the input to MCIN to forecast future velocity fields. The proposed MCIN is compared to nonintrusive reducedorder models based on dynamic mode decomposition and long shortterm memory, combined with a VAE. The results demonstrate that MCIN has superior stability to other models in forecasting the evolution of complicated fluid flows and has the potential to forecast a greater number of future outcomes.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Fluid mechanics is an important area where deep learning produces excellent results and can bring about scientific innovation because of its high dimensionality, significant nonlinearity, and ability to process an enormous amount of data. Deep learning technology is currently being used to study fluid mechanics, and its application potential is gradually being demonstrated. We propose a novel multiresolution convolutional interaction network (MCIN), a hierarchical forecast framework based on a convolutional neural network. This structure can capture temporal dependencies at multiple temporal resolutions to enhance the forecasting performance of the original time series. The highdimensional data of the flow around a cylinder are projected into a lowdimensional subspace using a variational autoencoder (VAE) as a nonlinear orderreduction technique. Then, the data of the subspace are used as the input to MCIN to forecast future velocity fields. The proposed MCIN is compared to nonintrusive reducedorder models based on dynamic mode decomposition and long shortterm memory, combined with a VAE. The results demonstrate that MCIN has superior stability to other models in forecasting the evolution of complicated fluid flows and has the potential to forecast a greater number of future outcomes.
A novel forecast framework for unsteady flows based on a convolutional neural network
10.1063/5.0122271
Physics of Fluids
20221115T10:30:53Z
© 2022 Author(s).

Heat transfer and wakeinduced vibrations of heated tandem cylinders with two degrees of freedom: Effect of spacing ratio
https://aip.scitation.org/doi/10.1063/5.0124772?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The heat transfer and wakeinduced vibrations of a cylinder of circular cross section in the wake of another identical cylinder are numerically studied in this work at a Reynolds number (Re) = 100. The reduced velocities (Ur) are varied in the range of 2–14. The downstream cylinder is allowed to oscillate in two degrees of freedom, i.e., in the transverse as well as in the streamwise direction. The mass ratio (m*) is taken as 10, while the structural damping is ignored to get the maximum amplitude of vibration. The spacing ratio (L/D) between the cylinders is varied from 1.5 to 6, covering the major regimes, i.e., single body, reattachment, and coshedding. The coefficients of lift (CL) and drag (CD), vibrational amplitudes of the cylinder, the Nusselt number (Nu), the Strouhal number (St), and vortex shedding patterns are studied. The results are discussed with the help of liftdisplacement phase plots, cylinder trajectory plots, and vorticity and temperature contours. The lockin condition at Ur = 8 is observed for all values of L/D, whereas the lockin zone is the widest for the coshedding regime at L/D = 6. By increasing L/D from 1.5 to 2.5 at Ur = 8, the CL of the downstream cylinder increases by 43%, whereas the CL of the upstream cylinder decreases by 61%. The downstream cylinder experiences lower drag as compared to the upstream cylinder and stationary isolated cylinder. A maximum decrease in the average drag coefficient of 107%, as compared to the stationary isolated cylinder, was observed for the downstream cylinder at L/D = 1.5 and Ur = 2, leading to the negative drag. Mostly, the 2S and C(2S) vortex shedding pattern is observed, whereas a steady flow and chaotic pattern emerged in a few cases. The results reveal that with increasing L/D, the average Nu for both the upstream and downstream cylinders increases as the effect of each cylinder on the other diminishes.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The heat transfer and wakeinduced vibrations of a cylinder of circular cross section in the wake of another identical cylinder are numerically studied in this work at a Reynolds number (Re) = 100. The reduced velocities (Ur) are varied in the range of 2–14. The downstream cylinder is allowed to oscillate in two degrees of freedom, i.e., in the transverse as well as in the streamwise direction. The mass ratio (m*) is taken as 10, while the structural damping is ignored to get the maximum amplitude of vibration. The spacing ratio (L/D) between the cylinders is varied from 1.5 to 6, covering the major regimes, i.e., single body, reattachment, and coshedding. The coefficients of lift (CL) and drag (CD), vibrational amplitudes of the cylinder, the Nusselt number (Nu), the Strouhal number (St), and vortex shedding patterns are studied. The results are discussed with the help of liftdisplacement phase plots, cylinder trajectory plots, and vorticity and temperature contours. The lockin condition at Ur = 8 is observed for all values of L/D, whereas the lockin zone is the widest for the coshedding regime at L/D = 6. By increasing L/D from 1.5 to 2.5 at Ur = 8, the CL of the downstream cylinder increases by 43%, whereas the CL of the upstream cylinder decreases by 61%. The downstream cylinder experiences lower drag as compared to the upstream cylinder and stationary isolated cylinder. A maximum decrease in the average drag coefficient of 107%, as compared to the stationary isolated cylinder, was observed for the downstream cylinder at L/D = 1.5 and Ur = 2, leading to the negative drag. Mostly, the 2S and C(2S) vortex shedding pattern is observed, whereas a steady flow and chaotic pattern emerged in a few cases. The results reveal that with increasing L/D, the average Nu for both the upstream and downstream cylinders increases as the effect of each cylinder on the other diminishes.
Heat transfer and wakeinduced vibrations of heated tandem cylinders with two degrees of freedom: Effect of spacing ratio
10.1063/5.0124772
Physics of Fluids
20221116T10:41:36Z
© 2022 Author(s).
Ussama Ali
Md Islam
Isam Janajreh

Toward an asymptotic description of Prandtl–Batchelor flows with corners
https://aip.scitation.org/doi/10.1063/5.0124851?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The Prandtl–Batchelor theorem states that the vorticity in a steady laminar high Reynolds (Re) number flow containing closed streamlines should be constant; however, apart from the simple case of circular streamlines, very little is known about how to determine this constant (ω0). This paper revisits earlier work for flow driven by a surrounding smooth moving boundary, with a view to extending it to the case where the enclosing boundary has corners; for this purpose, a benchmark example from the literature for flow inside a semicircle is considered. However, the subsequent asymptotic analysis for [math] and numerical experimentation lead to an inconsistency: the asymptotic approach predicts boundarylayer separation, whereas a linearized asymptotic theory and computations of the full Navier–Stokes equations for [math] do not. Nevertheless, by considering a slightly modified problem instead, which does not suffer from this inconsistency, it is found that, when extrapolating the results of such highRe computations to infinite Re, the agreement for ω0 is around 5%, which is roughly in line with previous comparisons of this type. Possible future improvements of the asymptotic method are also discussed.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The Prandtl–Batchelor theorem states that the vorticity in a steady laminar high Reynolds (Re) number flow containing closed streamlines should be constant; however, apart from the simple case of circular streamlines, very little is known about how to determine this constant (ω0). This paper revisits earlier work for flow driven by a surrounding smooth moving boundary, with a view to extending it to the case where the enclosing boundary has corners; for this purpose, a benchmark example from the literature for flow inside a semicircle is considered. However, the subsequent asymptotic analysis for [math] and numerical experimentation lead to an inconsistency: the asymptotic approach predicts boundarylayer separation, whereas a linearized asymptotic theory and computations of the full Navier–Stokes equations for [math] do not. Nevertheless, by considering a slightly modified problem instead, which does not suffer from this inconsistency, it is found that, when extrapolating the results of such highRe computations to infinite Re, the agreement for ω0 is around 5%, which is roughly in line with previous comparisons of this type. Possible future improvements of the asymptotic method are also discussed.
Toward an asymptotic description of Prandtl–Batchelor flows with corners
10.1063/5.0124851
Physics of Fluids
20221117T12:25:40Z
© 2022 Author(s).
M. Vynnycky

Soret effect on the onset of viscous dissipation thermal instability for Poiseuille flows in binary mixtures
https://aip.scitation.org/doi/10.1063/5.0115663?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We investigate numerically the Soret effect on the linear instability properties in convection due to viscous dissipation in a horizontal channel filled with a binary fluid mixture. Two sets of boundary conditions of experimental interest are considered. Both have noslip boundaries for the velocity and no mass flux through them. The lower boundary is considered adiabatic, while the upper boundary is isothermal for case A and inversely for case B. As no external temperature or concentration difference is imposed on the layer, the cause of thermal instability is the flow rate through the volumetric heating induced by the viscous dissipation and the Soret effect inherent to binary mixtures. It is found that longitudinal rolls (LR) represent the preferred mode for the onset of convection. For case A, both oscillatory and steadystate LR may develop depending on the value of the separation ratio ψ, which represents the ratio between the mass contribution and the temperature contribution to buoyancy forces. The dependence of the instability thresholds on the separation ratio is discussed near and far from the codimensiontwo bifurcation point. For case B, the basic state remains stable for positive separation ratios, while it loses its stability via a stationary bifurcation with zero wave number for negative values of the separation ratio. The relevance of the theoretical results for the observability of such instability in real systems is discussed. Finally, we suggest a protocol to determine Soret coefficients by using the stability diagrams obtained in the current paper.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We investigate numerically the Soret effect on the linear instability properties in convection due to viscous dissipation in a horizontal channel filled with a binary fluid mixture. Two sets of boundary conditions of experimental interest are considered. Both have noslip boundaries for the velocity and no mass flux through them. The lower boundary is considered adiabatic, while the upper boundary is isothermal for case A and inversely for case B. As no external temperature or concentration difference is imposed on the layer, the cause of thermal instability is the flow rate through the volumetric heating induced by the viscous dissipation and the Soret effect inherent to binary mixtures. It is found that longitudinal rolls (LR) represent the preferred mode for the onset of convection. For case A, both oscillatory and steadystate LR may develop depending on the value of the separation ratio ψ, which represents the ratio between the mass contribution and the temperature contribution to buoyancy forces. The dependence of the instability thresholds on the separation ratio is discussed near and far from the codimensiontwo bifurcation point. For case B, the basic state remains stable for positive separation ratios, while it loses its stability via a stationary bifurcation with zero wave number for negative values of the separation ratio. The relevance of the theoretical results for the observability of such instability in real systems is discussed. Finally, we suggest a protocol to determine Soret coefficients by using the stability diagrams obtained in the current paper.
Soret effect on the onset of viscous dissipation thermal instability for Poiseuille flows in binary mixtures
10.1063/5.0115663
Physics of Fluids
20221101T01:01:54Z
© 2022 Author(s).
K. Ali Amar
S. C. Hirata
M. N. Ouarzazi

Linear stability of Poiseuille flow of viscoelastic fluid in a porous medium
https://aip.scitation.org/doi/10.1063/5.0117242?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We study the instability of plane Poiseuille flow of the viscoelastic secondorder fluid in a homogeneous porous medium. The viscoelastic fluid between two parallel plates is driven by the pressure gradient. The effects of elasticity number E (depends on fluid properties, geometry; E is defined below) and Darcy number Da (gives the permeability of porous medium; Da is defined below) on flow stability are analyzed through the energy method that provides qualitative behavior of flow stability, and the numerical solution of generalized eigenvalue problem that gives the precise upper bound for stability. The plane Poiseuille flow of secondorder fluid becomes unstable for increasing elasticity number while preserving Newtonian eigenspectrum up to a certain range of E. For large elasticity number, instability appears as a part of both wall and center modes for all Darcy numbers. We also noticed that along each neutral stability curve, the eigenfunctions are all antisymmetric with a single extremum near the channel walls. When E = 0.0011, we found an additional new elastic mode, which is unstable and also antisymmetric. For E < 0.0011, the neutral curves split into two lobes with different minima. The critical Reynolds number Rec is found to be decreasing (increasing) for higher (lower) values of fluid elasticity (Darcy number). Physical mechanisms are discussed in detail.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We study the instability of plane Poiseuille flow of the viscoelastic secondorder fluid in a homogeneous porous medium. The viscoelastic fluid between two parallel plates is driven by the pressure gradient. The effects of elasticity number E (depends on fluid properties, geometry; E is defined below) and Darcy number Da (gives the permeability of porous medium; Da is defined below) on flow stability are analyzed through the energy method that provides qualitative behavior of flow stability, and the numerical solution of generalized eigenvalue problem that gives the precise upper bound for stability. The plane Poiseuille flow of secondorder fluid becomes unstable for increasing elasticity number while preserving Newtonian eigenspectrum up to a certain range of E. For large elasticity number, instability appears as a part of both wall and center modes for all Darcy numbers. We also noticed that along each neutral stability curve, the eigenfunctions are all antisymmetric with a single extremum near the channel walls. When E = 0.0011, we found an additional new elastic mode, which is unstable and also antisymmetric. For E < 0.0011, the neutral curves split into two lobes with different minima. The critical Reynolds number Rec is found to be decreasing (increasing) for higher (lower) values of fluid elasticity (Darcy number). Physical mechanisms are discussed in detail.
Linear stability of Poiseuille flow of viscoelastic fluid in a porous medium
10.1063/5.0117242
Physics of Fluids
20221101T11:29:40Z
© 2022 Author(s).
Bharathi M. C.
Ramesh B. Kudenatti

Bluff body vortexinduced vibration control of floating wind turbines based on a novel intelligent robust control algorithm
https://aip.scitation.org/doi/10.1063/5.0121829?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Given the great potential of the offshore wind power generation in renewable energy sources, it will bring unprecedented significant development opportunities. Meanwhile, the installed capacity of floating wind turbines (FWTs) is huge. However, as one of the important parts of that, FWTs are always subjected to complex environmental loads during operation, which will critically affect the stability of wind power generation. Hence, it is urgent to analyze and control its stability for the safe operation of wind turbines. It is accepted that vortexinduced vibration (VIV) of a bluff body structure is the leading cause of structural damage to FWTs. For this reason, a radial basis function neural network sliding mode control (RBFNNSMC) is proposed to improve the modeling accuracy of bluff body VIV control. Then, the joint numerical analysis system was designed to achieve the completely coupled fluid structure vibration control of bluff body. The numerical results indicate that RBFNNSMC can better control the forward/crossflow vibration of bluff body. In addition, the controller is not responsive to changes in system parameters and has strong robustness.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Given the great potential of the offshore wind power generation in renewable energy sources, it will bring unprecedented significant development opportunities. Meanwhile, the installed capacity of floating wind turbines (FWTs) is huge. However, as one of the important parts of that, FWTs are always subjected to complex environmental loads during operation, which will critically affect the stability of wind power generation. Hence, it is urgent to analyze and control its stability for the safe operation of wind turbines. It is accepted that vortexinduced vibration (VIV) of a bluff body structure is the leading cause of structural damage to FWTs. For this reason, a radial basis function neural network sliding mode control (RBFNNSMC) is proposed to improve the modeling accuracy of bluff body VIV control. Then, the joint numerical analysis system was designed to achieve the completely coupled fluid structure vibration control of bluff body. The numerical results indicate that RBFNNSMC can better control the forward/crossflow vibration of bluff body. In addition, the controller is not responsive to changes in system parameters and has strong robustness.
Bluff body vortexinduced vibration control of floating wind turbines based on a novel intelligent robust control algorithm
10.1063/5.0121829
Physics of Fluids
20221102T02:43:56Z
© 2022 Author(s).

Optimizing control of stationary crossflow vortices excited by surface roughness
https://aip.scitation.org/doi/10.1063/5.0123935?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Stationary crossflow vortices are excited within the swept Hiemenz boundary layer via surface roughness and actively controlled using an optimally configured control device. Control is modeled using localized wall motion, but in practice the optimization strategy could be applied to other laminar flow control technologies. A sensorcontrol iterative procedure, based on solutions of the forward and adjoint linearized Navier–Stokes equations, is applied to both feedforward and feedback loop systems. The former strategy only allows the control settings to be configured once, while the latter approach permits the repeated reoptimization of the control device. Surface roughness establishes a stationary crossflow disturbance with a predefined set of flow conditions, but an unknown amplitude and phase. A sensor measures the local amplitude of the perturbation and relays the information to the control mechanism. Solutions of the adjoint linearized Navier–Stokes equations are coupled with the sensor measurements to configure and optimize the control mechanism, and establish an antiphase wave that brings about destructive wave interference. The amplitude of the stationary crossflow instability is reduced by an order 103 for the feedforward system, while amplitude reductions of the order 103 per iteration and 108 overall are realizable for the feedback modeling approach. Similar levels of flow control are realizable for a multiple controller configuration. However, stationary crossflow disturbances could not be eliminated indefinitely. Inevitably, the crossflow instability started to grow again, albeit at a considerably lower magnitude. The analysis is extended to include the effects of systematic error in the sensors measuring capability.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Stationary crossflow vortices are excited within the swept Hiemenz boundary layer via surface roughness and actively controlled using an optimally configured control device. Control is modeled using localized wall motion, but in practice the optimization strategy could be applied to other laminar flow control technologies. A sensorcontrol iterative procedure, based on solutions of the forward and adjoint linearized Navier–Stokes equations, is applied to both feedforward and feedback loop systems. The former strategy only allows the control settings to be configured once, while the latter approach permits the repeated reoptimization of the control device. Surface roughness establishes a stationary crossflow disturbance with a predefined set of flow conditions, but an unknown amplitude and phase. A sensor measures the local amplitude of the perturbation and relays the information to the control mechanism. Solutions of the adjoint linearized Navier–Stokes equations are coupled with the sensor measurements to configure and optimize the control mechanism, and establish an antiphase wave that brings about destructive wave interference. The amplitude of the stationary crossflow instability is reduced by an order 103 for the feedforward system, while amplitude reductions of the order 103 per iteration and 108 overall are realizable for the feedback modeling approach. Similar levels of flow control are realizable for a multiple controller configuration. However, stationary crossflow disturbances could not be eliminated indefinitely. Inevitably, the crossflow instability started to grow again, albeit at a considerably lower magnitude. The analysis is extended to include the effects of systematic error in the sensors measuring capability.
Optimizing control of stationary crossflow vortices excited by surface roughness
10.1063/5.0123935
Physics of Fluids
20221102T02:43:35Z
© 2022 Author(s).
Christian Thomas
Shahid Mughal

Hypersonic boundary layer transition on a concave wall induced by lowfrequency blowing and suction
https://aip.scitation.org/doi/10.1063/5.0113570?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Hypersonic boundary layer transitions caused by unsteady blowing and suction are investigated with linear stability analyses and direct numerical simulations (DNS). Three blowing–suction frequencies, i.e., 15, 30, and 45 kHz, are separately utilized to excite a pair of unsteady Görtler instability waves (the first two cases) or firstmode instability waves (the last case). These two primary instabilities, respectively, induce diamondshaped and Λshaped structures through selfinteractions. These structures are highly susceptible to highfrequency secondary instabilities, as is demonstrated by global Floquet analyses that take into account both temporal unsteadiness and spanwise spatial variations of the base flow. The secondary instability manifests as hairpin packets riding on the downstream end of the diamondshaped structures or reside in the outward sides of the two legs of the Λshaped structures. The theoretical results quantitatively agree with the DNS results. Energy analyses further reveal that the wallnormal productions dominate the energy transfer for the secondary instability of the unsteady Görtler vortices, while the spanwise productions are crucial to the secondary instabilities in the firstmode oblique breakdown. Quasisteady analyses based on the “frozen” base flow are also performed, whose results compare favorably with those from Floquet analyses in the lowestfrequency case.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Hypersonic boundary layer transitions caused by unsteady blowing and suction are investigated with linear stability analyses and direct numerical simulations (DNS). Three blowing–suction frequencies, i.e., 15, 30, and 45 kHz, are separately utilized to excite a pair of unsteady Görtler instability waves (the first two cases) or firstmode instability waves (the last case). These two primary instabilities, respectively, induce diamondshaped and Λshaped structures through selfinteractions. These structures are highly susceptible to highfrequency secondary instabilities, as is demonstrated by global Floquet analyses that take into account both temporal unsteadiness and spanwise spatial variations of the base flow. The secondary instability manifests as hairpin packets riding on the downstream end of the diamondshaped structures or reside in the outward sides of the two legs of the Λshaped structures. The theoretical results quantitatively agree with the DNS results. Energy analyses further reveal that the wallnormal productions dominate the energy transfer for the secondary instability of the unsteady Görtler vortices, while the spanwise productions are crucial to the secondary instabilities in the firstmode oblique breakdown. Quasisteady analyses based on the “frozen” base flow are also performed, whose results compare favorably with those from Floquet analyses in the lowestfrequency case.
Hypersonic boundary layer transition on a concave wall induced by lowfrequency blowing and suction
10.1063/5.0113570
Physics of Fluids
20221102T02:43:38Z
© 2022 Author(s).
Xi Chen
Jianqiang Chen
Xianxu Yuan

Nonlinear wave interactions in a transitional hypersonic boundary layer
https://aip.scitation.org/doi/10.1063/5.0120425?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The linear and nonlinear evolutions and breakdown of the second modes in hypersonic boundary layers (HBLs) on a flared cone are investigated using Rayleighscattering flow visualization and fastresponse pressure sensors. Based on two spatially separated pressure signals, crossbicoherence analysis that permits the distinction of sum and differenceinteractions is utilized to identify the nonlinear interactions. In addition, the visualization temporal and spatial resolution allows fine flow features to be captured to provide additional flow information. Amplitude correlation technique is used to estimate the nonlinear energy transfer between the modes. Our results show that nonlinear interactions between the second mode and the lowfrequency wave contribute to the growth of the lowfrequency wave, and the difference interactions between the second mode and its first harmonic play a dominant role in modulating the waves in the overall transition process. Amplitude correlation analysis reveals that the spectral energy is nonlinearly transferred from the second mode into its first harmonic and into lowfrequency wave, in agreement with the crossbicoherence analysis. The amplitude modulation of the second mode caused by the difference interaction between the second mode and its first harmonic will reduce the propagation speed of the second mode. However, at the final breakdown stage, this difference interaction vanishes, and the secondmode propagation velocity recovers quickly. Since the frequency of the second mode keeps almost unchanged over the entire transition process, a higher propagation velocity will result in a larger wavelength, indicating an elongation and deformation of the second mode. Eventually, the difference interaction between the second mode and the lowfrequency wave accompanying the energy transfer from the second mode to lowfrequency waves leads to the final breakdown of the HBLs into a turbulent state.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The linear and nonlinear evolutions and breakdown of the second modes in hypersonic boundary layers (HBLs) on a flared cone are investigated using Rayleighscattering flow visualization and fastresponse pressure sensors. Based on two spatially separated pressure signals, crossbicoherence analysis that permits the distinction of sum and differenceinteractions is utilized to identify the nonlinear interactions. In addition, the visualization temporal and spatial resolution allows fine flow features to be captured to provide additional flow information. Amplitude correlation technique is used to estimate the nonlinear energy transfer between the modes. Our results show that nonlinear interactions between the second mode and the lowfrequency wave contribute to the growth of the lowfrequency wave, and the difference interactions between the second mode and its first harmonic play a dominant role in modulating the waves in the overall transition process. Amplitude correlation analysis reveals that the spectral energy is nonlinearly transferred from the second mode into its first harmonic and into lowfrequency wave, in agreement with the crossbicoherence analysis. The amplitude modulation of the second mode caused by the difference interaction between the second mode and its first harmonic will reduce the propagation speed of the second mode. However, at the final breakdown stage, this difference interaction vanishes, and the secondmode propagation velocity recovers quickly. Since the frequency of the second mode keeps almost unchanged over the entire transition process, a higher propagation velocity will result in a larger wavelength, indicating an elongation and deformation of the second mode. Eventually, the difference interaction between the second mode and the lowfrequency wave accompanying the energy transfer from the second mode to lowfrequency waves leads to the final breakdown of the HBLs into a turbulent state.
Nonlinear wave interactions in a transitional hypersonic boundary layer
10.1063/5.0120425
Physics of Fluids
20221102T02:43:47Z
© 2022 Author(s).

Largeeddy simulation of unstable nonreactive flow in a swirler combustor
https://aip.scitation.org/doi/10.1063/5.0122462?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A comprehensive study on the influence of the unsteady nonreactive flow characteristics of turbulent flow in a threestage swirl combustion chamber using power spectral density methods was conducted using large eddy simulations. The results demonstrated that instabilities were observed owing to largescale vortex structures and periodic oscillations of the nonreactive flow. The boundary of the central recirculation zone (shear layers) enhanced the instability of the Helmholtz mode in the combustor. By considering the power spectral density of different monitoring points, the instability characteristics were accurately determined according to the oscillatory energy obtained in the nonreactive flow field. Largescale vortex structures and periodic oscillations were the main reasons for the unsteady characteristics of the nonreactive flow field. The large eddy simulation results were compared with the experimental data, and the average absolute relative deviation between the large eddy simulation and experimental velocity components in the combustor were less than 12.04%. The results provide valuable insights into the unstable nonreaction flow characteristics in the combustion chamber.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A comprehensive study on the influence of the unsteady nonreactive flow characteristics of turbulent flow in a threestage swirl combustion chamber using power spectral density methods was conducted using large eddy simulations. The results demonstrated that instabilities were observed owing to largescale vortex structures and periodic oscillations of the nonreactive flow. The boundary of the central recirculation zone (shear layers) enhanced the instability of the Helmholtz mode in the combustor. By considering the power spectral density of different monitoring points, the instability characteristics were accurately determined according to the oscillatory energy obtained in the nonreactive flow field. Largescale vortex structures and periodic oscillations were the main reasons for the unsteady characteristics of the nonreactive flow field. The large eddy simulation results were compared with the experimental data, and the average absolute relative deviation between the large eddy simulation and experimental velocity components in the combustor were less than 12.04%. The results provide valuable insights into the unstable nonreaction flow characteristics in the combustion chamber.
Largeeddy simulation of unstable nonreactive flow in a swirler combustor
10.1063/5.0122462
Physics of Fluids
20221103T12:34:58Z
© 2022 Author(s).

Convective instabilities in the Czochralski model with different radii ratios
https://aip.scitation.org/doi/10.1063/5.0117206?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this work, we explore the instability of the complex convection in the Czochralski model concerning the effects of the radii ratio, melt materials, and crystal rotation. Particularly, linear stability analysis is conducted based on the spectral element method for three groups of cases with the same interval for the variation of the radii ratio (Λ) but different material Prandtl number (Pr) and dimensionless crystal rotation velocity ωs. We observe that, for both ωs = 0 and ωs = 300, the mixed convection of silicon melt (Pr = 0.011) becomes less stable with the increase in radii ratio and the instability is of purely inertial mechanism. In contrast, as for the LiCaAlF6 melt (Pr = 1.4), a larger radii ratio would improve the stability and the instability is dominated by buoyancy mechanism for ωs = 300. Moreover, two times of critical wavenumber transitions occur in the critical stability curve for silicon melt (Pr = 0.011). Each transition associates with a convex turning point of the critical stability curve for ωs = 0, while only one turning point remains when ωs shifts to 300.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this work, we explore the instability of the complex convection in the Czochralski model concerning the effects of the radii ratio, melt materials, and crystal rotation. Particularly, linear stability analysis is conducted based on the spectral element method for three groups of cases with the same interval for the variation of the radii ratio (Λ) but different material Prandtl number (Pr) and dimensionless crystal rotation velocity ωs. We observe that, for both ωs = 0 and ωs = 300, the mixed convection of silicon melt (Pr = 0.011) becomes less stable with the increase in radii ratio and the instability is of purely inertial mechanism. In contrast, as for the LiCaAlF6 melt (Pr = 1.4), a larger radii ratio would improve the stability and the instability is dominated by buoyancy mechanism for ωs = 300. Moreover, two times of critical wavenumber transitions occur in the critical stability curve for silicon melt (Pr = 0.011). Each transition associates with a convex turning point of the critical stability curve for ωs = 0, while only one turning point remains when ωs shifts to 300.
Convective instabilities in the Czochralski model with different radii ratios
10.1063/5.0117206
Physics of Fluids
20221104T12:51:01Z
© 2022 Author(s).

Instability mechanisms of thermocapillary liquid bridges between disks of unequal radii
https://aip.scitation.org/doi/10.1063/5.0120825?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, we explore thermocapillary liquid bridges between two disks of unequal radii with Prandtl numbers Pr of 0.0258 (mercury) and 0.068 (gallium arsenide) to gain insight into the underlying instability mechanism. In the context of Legendre's spectral element method, we determine critical conditions via linear stability analysis and then identify the instability mechanism through energy analysis. For the mercury bridge (Pr = 0.0258), our analysis suggests that the flow instability undergoes an oscillatory bifurcation for radius ratios in the range of 0.5 ≤ Γr ≤ 0.66. In particular, we found three transitions between twodimensional steady axisymmetric flow and threedimensional stationary flow by further increasing the radius ratio to 0.73 ≤ Γr ≤ 0.76. For the gallium arsenide liquid bridge (Pr = 0.068), the instability is always an oscillatory bifurcation in the whole computational interval. Furthermore, our observations identify six instability modes with different mechanisms. All instability modes in the mercury bridge (Pr = 0.0258) are purely hydrodynamic, but the thermocapillary mechanism cannot be ignored in the gallium arsenide liquid bridge (Pr = 0.068) because of the enhanced Pr effect.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, we explore thermocapillary liquid bridges between two disks of unequal radii with Prandtl numbers Pr of 0.0258 (mercury) and 0.068 (gallium arsenide) to gain insight into the underlying instability mechanism. In the context of Legendre's spectral element method, we determine critical conditions via linear stability analysis and then identify the instability mechanism through energy analysis. For the mercury bridge (Pr = 0.0258), our analysis suggests that the flow instability undergoes an oscillatory bifurcation for radius ratios in the range of 0.5 ≤ Γr ≤ 0.66. In particular, we found three transitions between twodimensional steady axisymmetric flow and threedimensional stationary flow by further increasing the radius ratio to 0.73 ≤ Γr ≤ 0.76. For the gallium arsenide liquid bridge (Pr = 0.068), the instability is always an oscillatory bifurcation in the whole computational interval. Furthermore, our observations identify six instability modes with different mechanisms. All instability modes in the mercury bridge (Pr = 0.0258) are purely hydrodynamic, but the thermocapillary mechanism cannot be ignored in the gallium arsenide liquid bridge (Pr = 0.068) because of the enhanced Pr effect.
Instability mechanisms of thermocapillary liquid bridges between disks of unequal radii
10.1063/5.0120825
Physics of Fluids
20221104T12:22:03Z
© 2022 Author(s).

Dynamics of bypass transition behind roughness element subjected to pulses of freestream turbulence
https://aip.scitation.org/doi/10.1063/5.0120241?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This study explores the dynamics of bypass transition of a zero pressure gradient boundary layer transitioning under the combined influence of an isolated roughness element with pulses of freestream turbulence (FST). We consider a hemispherical roughness element placed over a flat plate, while the pulses of FST are introduced at the inlet, which is in contrast to continuous FST largely explored in the literature. For a fixed turbulence intensity and length scale, a series of eddyresolving simulations are carried out to examine the effect of varying the pulsing frequency of FST. The flow behind the roughness element remains stable in the absence of FST for the subcritical Reynolds number Rek = 400 considered in this study. We observe that with the pulses of FST, the transition is triggered due to the interaction of the FSTinduced Klebanoff streaks with the roughnessinduced streamwise vortices. With an increase in the frequency of FST pulses, the boundary layer has less time to relax to its unperturbed state resulting in an earlier onset of transition. The transition onset predicted is in favorable agreement with the correlations proposed in the literature. We analyze the growth of disturbance kinetic energy, the shape of secondary instabilities over the streaks, and their phase speeds in detail. The FST pulse convecting over the roughness element triggers the inner varicose modes in its nearwake region. The varicose modes decay rapidly further downstream and the wellknown sinuous instabilities (or the outer modes) trigger transition via transient growth associated with convective instabilities. Such clear identification of the sinuous and varicose instabilities is not usually observed in cases with continuous FST, highlighting the importance of our study in applications involving transition under intermittent turbulence.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This study explores the dynamics of bypass transition of a zero pressure gradient boundary layer transitioning under the combined influence of an isolated roughness element with pulses of freestream turbulence (FST). We consider a hemispherical roughness element placed over a flat plate, while the pulses of FST are introduced at the inlet, which is in contrast to continuous FST largely explored in the literature. For a fixed turbulence intensity and length scale, a series of eddyresolving simulations are carried out to examine the effect of varying the pulsing frequency of FST. The flow behind the roughness element remains stable in the absence of FST for the subcritical Reynolds number Rek = 400 considered in this study. We observe that with the pulses of FST, the transition is triggered due to the interaction of the FSTinduced Klebanoff streaks with the roughnessinduced streamwise vortices. With an increase in the frequency of FST pulses, the boundary layer has less time to relax to its unperturbed state resulting in an earlier onset of transition. The transition onset predicted is in favorable agreement with the correlations proposed in the literature. We analyze the growth of disturbance kinetic energy, the shape of secondary instabilities over the streaks, and their phase speeds in detail. The FST pulse convecting over the roughness element triggers the inner varicose modes in its nearwake region. The varicose modes decay rapidly further downstream and the wellknown sinuous instabilities (or the outer modes) trigger transition via transient growth associated with convective instabilities. Such clear identification of the sinuous and varicose instabilities is not usually observed in cases with continuous FST, highlighting the importance of our study in applications involving transition under intermittent turbulence.
Dynamics of bypass transition behind roughness element subjected to pulses of freestream turbulence
10.1063/5.0120241
Physics of Fluids
20221104T01:20:19Z
© 2022 Author(s).
Aditya Vaid
Nagabhushana Rao Vadlamani
Ananth Sivaramakrishnan Malathi
Vikrant Gupta

Phaselocking flows between orthogonally stretching parallel plates
https://aip.scitation.org/doi/10.1063/5.0124152?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, we explore the stability and dynamical relevance of a wide variety of steady, timeperiodic, quasiperiodic, and chaotic flows arising between orthogonally stretching parallel plates. We first explore the stability of all the steady flow solution families formerly identified by Ayats et al. [“Flows between orthogonally stretching parallel plates,” Phys. Fluids 33, 024103 (2021)], concluding that only the one that originates from the Stokesian approximation is actually stable. When both plates are shrinking at identical or nearly the same deceleration rates, this Stokesian flow exhibits a Hopf bifurcation that leads to stable timeperiodic regimes. The resulting timeperiodic orbits or flows are tracked for different Reynolds numbers and stretching rates while monitoring their Floquet exponents to identify secondary instabilities. It is found that these timeperiodic flows also exhibit Neimark–Sacker bifurcations, generating stable quasiperiodic flows (tori) that may sometimes give rise to chaotic dynamics through a Ruelle–Takens–Newhouse scenario. However, chaotic dynamics is unusually observed, as the quasiperiodic flows generally become phaselocked through a resonance mechanism before a strange attractor may arise, thus restoring the timeperiodicity of the flow. In this work, we have identified and tracked four different resonance regions, also known as Arnold tongues or horns. In particular, the 1 : 4 strong resonance region is explored in great detail, where the identified scenarios are in very good agreement with normal form theory.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, we explore the stability and dynamical relevance of a wide variety of steady, timeperiodic, quasiperiodic, and chaotic flows arising between orthogonally stretching parallel plates. We first explore the stability of all the steady flow solution families formerly identified by Ayats et al. [“Flows between orthogonally stretching parallel plates,” Phys. Fluids 33, 024103 (2021)], concluding that only the one that originates from the Stokesian approximation is actually stable. When both plates are shrinking at identical or nearly the same deceleration rates, this Stokesian flow exhibits a Hopf bifurcation that leads to stable timeperiodic regimes. The resulting timeperiodic orbits or flows are tracked for different Reynolds numbers and stretching rates while monitoring their Floquet exponents to identify secondary instabilities. It is found that these timeperiodic flows also exhibit Neimark–Sacker bifurcations, generating stable quasiperiodic flows (tori) that may sometimes give rise to chaotic dynamics through a Ruelle–Takens–Newhouse scenario. However, chaotic dynamics is unusually observed, as the quasiperiodic flows generally become phaselocked through a resonance mechanism before a strange attractor may arise, thus restoring the timeperiodicity of the flow. In this work, we have identified and tracked four different resonance regions, also known as Arnold tongues or horns. In particular, the 1 : 4 strong resonance region is explored in great detail, where the identified scenarios are in very good agreement with normal form theory.
Phaselocking flows between orthogonally stretching parallel plates
10.1063/5.0124152
Physics of Fluids
20221104T01:20:29Z
© 2022 Author(s).
B. Wang
R. Ayats
A. Meseguer
F. Marques

Bifurcations to quasiperiodicity of the torsional solutions of convection in rotating fluid spheres: Techniques and results
https://aip.scitation.org/doi/10.1063/5.0122146?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The linear stability of the periodic and axisymmetric solutions of the convection in rotating, internally heated, and selfgravitating fluid spheres is presented. The transition to quasiperiodic flows via Neimark–Sacker bifurcations of different azimuthal wave numbers, m, is studied using matrixfree continuation and Floquet theory. Several pairs of Ekman and Prandtl numbers are considered in the region where the first bifurcation from the conduction state gives rise to the axisymmetric solutions. It is shown that the azimuthal wave numbers m = 1 and m = 2 are preferred and that, for small Ekman and Prandtl numbers, the secondary bifurcations to different m accumulate close to the onset of convection. This study confirms some results previously found just by direct simulations. The methods presented can be applied to systems of parabolic partial differential equations with O(2) or SO(2) symmetry group, when a periodic orbit, invariant under the group, loses stability through a Neimark–Sacker bifurcation.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The linear stability of the periodic and axisymmetric solutions of the convection in rotating, internally heated, and selfgravitating fluid spheres is presented. The transition to quasiperiodic flows via Neimark–Sacker bifurcations of different azimuthal wave numbers, m, is studied using matrixfree continuation and Floquet theory. Several pairs of Ekman and Prandtl numbers are considered in the region where the first bifurcation from the conduction state gives rise to the axisymmetric solutions. It is shown that the azimuthal wave numbers m = 1 and m = 2 are preferred and that, for small Ekman and Prandtl numbers, the secondary bifurcations to different m accumulate close to the onset of convection. This study confirms some results previously found just by direct simulations. The methods presented can be applied to systems of parabolic partial differential equations with O(2) or SO(2) symmetry group, when a periodic orbit, invariant under the group, loses stability through a Neimark–Sacker bifurcation.
Bifurcations to quasiperiodicity of the torsional solutions of convection in rotating fluid spheres: Techniques and results
10.1063/5.0122146
Physics of Fluids
20221107T12:48:30Z
© 2022 Author(s).
J. Sánchez Umbría
M. Net

Effect of the surface pattern on the drag property of the superhydrophobic surface
https://aip.scitation.org/doi/10.1063/5.0113964?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Superhydrophobic surfaces with a slip effect have been proven to be effective to achieve surface drag reduction. In this paper, we fabricated superhydrophobic disks via laser ablation and chemical modification, and their dragreduction performance is tested by a rheometer. However, the superhydrophobic disk achieves a limited dragreduction effect (33.5% in maximum) with poor consistency (decay rapidly as rotating speed increases). To enhance the dragreduction performance of the superhydrophobic surfaces, surfaces with tunable patterns consisting of superhydrophobic and hydrophobic surfaces are proposed. The difference in wettability and slip length between different regions is employed to control the flow direction. The effect of different patterns is investigated both experimentally and numerically, and the results indicate that the pattern distribution performs a great impact on the dragreduction effect. The pattern consistent with the primary flow can not only enhance the effect (60.3% in maximum) but also the consistency (maintained at a higher rotating speed) of dragreduction. On the contrary, the pattern perpendicular to the flow direction is harmful to drag reduction and can even increase the drag.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Superhydrophobic surfaces with a slip effect have been proven to be effective to achieve surface drag reduction. In this paper, we fabricated superhydrophobic disks via laser ablation and chemical modification, and their dragreduction performance is tested by a rheometer. However, the superhydrophobic disk achieves a limited dragreduction effect (33.5% in maximum) with poor consistency (decay rapidly as rotating speed increases). To enhance the dragreduction performance of the superhydrophobic surfaces, surfaces with tunable patterns consisting of superhydrophobic and hydrophobic surfaces are proposed. The difference in wettability and slip length between different regions is employed to control the flow direction. The effect of different patterns is investigated both experimentally and numerically, and the results indicate that the pattern distribution performs a great impact on the dragreduction effect. The pattern consistent with the primary flow can not only enhance the effect (60.3% in maximum) but also the consistency (maintained at a higher rotating speed) of dragreduction. On the contrary, the pattern perpendicular to the flow direction is harmful to drag reduction and can even increase the drag.
Effect of the surface pattern on the drag property of the superhydrophobic surface
10.1063/5.0113964
Physics of Fluids
20221107T12:49:37Z
© 2022 Author(s).

The roles of rigid splitter plates in flowinduced vibration of a circular cylinder
https://aip.scitation.org/doi/10.1063/5.0126867?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>While it is known that rigid splitter plates play significant roles in flow control, the exact roles of them in flowinduced vibration (FIV) have not been systematically investigated. This has motivated the present work to experimentally investigate the FIV of a cylinder equipped with an upstream rigid splitter plate (USP), a downstream plate (DSP), and symmetrically arranged splitter plates in a water tunnel with Reynolds number of 1100–7700. The length of the plate is in a range of [math] = 0–3.6 ([math], L is the plate length, D is the cylinder diameter). The response characteristics, vortex evolution, fluid force, and pressure fields are thoroughly analyzed. Both USP and DSP can succeed in oscillation mitigation and drag reduction. However, dramatic galloping is observed for DSP with [math] = 0.4–3.2. The lowpressure region forms near the downstream plate is beneficial to trigger galloping. For USP, only vortexinduced vibration is found, and the transition of response branches corresponds to the variation in oscillation frequency and phase jumps in total transverse force and vortex force. However, the vortex mode transition from 2S to 2P disappears with long plate length. Flow visualization reveals that the upstream vortex induced by USP alters the downstream vortex shedding. Furthermore, a highpressure region forms near the tip of USP, yielding an obstructive force that suppresses the growth of oscillation. With the combination of USP and DSP, weak galloping is excited in a narrow range of [math] = 1.0–1.8, and the linear increase is also broken due to the existence of USP.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>While it is known that rigid splitter plates play significant roles in flow control, the exact roles of them in flowinduced vibration (FIV) have not been systematically investigated. This has motivated the present work to experimentally investigate the FIV of a cylinder equipped with an upstream rigid splitter plate (USP), a downstream plate (DSP), and symmetrically arranged splitter plates in a water tunnel with Reynolds number of 1100–7700. The length of the plate is in a range of [math] = 0–3.6 ([math], L is the plate length, D is the cylinder diameter). The response characteristics, vortex evolution, fluid force, and pressure fields are thoroughly analyzed. Both USP and DSP can succeed in oscillation mitigation and drag reduction. However, dramatic galloping is observed for DSP with [math] = 0.4–3.2. The lowpressure region forms near the downstream plate is beneficial to trigger galloping. For USP, only vortexinduced vibration is found, and the transition of response branches corresponds to the variation in oscillation frequency and phase jumps in total transverse force and vortex force. However, the vortex mode transition from 2S to 2P disappears with long plate length. Flow visualization reveals that the upstream vortex induced by USP alters the downstream vortex shedding. Furthermore, a highpressure region forms near the tip of USP, yielding an obstructive force that suppresses the growth of oscillation. With the combination of USP and DSP, weak galloping is excited in a narrow range of [math] = 1.0–1.8, and the linear increase is also broken due to the existence of USP.
The roles of rigid splitter plates in flowinduced vibration of a circular cylinder
10.1063/5.0126867
Physics of Fluids
20221108T05:48:12Z
© 2022 Author(s).

Unsteady disturbances in a swept wing boundary layer due to plasma forcing
https://aip.scitation.org/doi/10.1063/5.0124818?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This work investigates the response of a transitional boundary layer to spanwiseinvariant dielectric barrier discharge plasma actuator (PA) forcing on a [math] swept wing at a chord Reynolds number of [math]. Two important parameters of the PA operation are scrutinized, namely, the forcing frequency and the streamwise location of forcing. An array of passive discrete roughness elements is installed near the leading edge to promote and condition a set of critical stationary crossflow (CF) instability modes. Numerical solutions of the boundary layer equations and linear stability theory are used in combination with the experimental pressure distribution to provide predictions of critical stationary and traveling CF instabilities. The laminar–turbulent transition front is visualized and quantified by means of infrared thermography. Measurements of velocity fields are performed using hotwire anemometry scans at specific chordwise locations. The results demonstrate the inherent introduction of unsteady velocity disturbances by the plasma forcing. It is shown that, depending on actuator frequency and location, these disturbances can evolve into typical CF instabilities. Positive traveling lowfrequency type III modes are generally amplified by PA in all tested cases, while the occurrence of negative traveling highfrequency type I secondary modes is favored when PA is operating at high frequency and at relatively downstream locations, with respect to the leading edge.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This work investigates the response of a transitional boundary layer to spanwiseinvariant dielectric barrier discharge plasma actuator (PA) forcing on a [math] swept wing at a chord Reynolds number of [math]. Two important parameters of the PA operation are scrutinized, namely, the forcing frequency and the streamwise location of forcing. An array of passive discrete roughness elements is installed near the leading edge to promote and condition a set of critical stationary crossflow (CF) instability modes. Numerical solutions of the boundary layer equations and linear stability theory are used in combination with the experimental pressure distribution to provide predictions of critical stationary and traveling CF instabilities. The laminar–turbulent transition front is visualized and quantified by means of infrared thermography. Measurements of velocity fields are performed using hotwire anemometry scans at specific chordwise locations. The results demonstrate the inherent introduction of unsteady velocity disturbances by the plasma forcing. It is shown that, depending on actuator frequency and location, these disturbances can evolve into typical CF instabilities. Positive traveling lowfrequency type III modes are generally amplified by PA in all tested cases, while the occurrence of negative traveling highfrequency type I secondary modes is favored when PA is operating at high frequency and at relatively downstream locations, with respect to the leading edge.
Unsteady disturbances in a swept wing boundary layer due to plasma forcing
10.1063/5.0124818
Physics of Fluids
20221108T05:49:48Z
© 2022 Author(s).
F. Avallone
M. Kotsonis

Twodimensional Darcy–Bénard convection evolving in Fourier space
https://aip.scitation.org/doi/10.1063/5.0122215?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Nonlinear transient convection in a porous rectangle heated from below is studied by an analytically based method. Fourier series for the temperature and streamfunction are applied, where each Fourier coefficient evolves in time according to a coupled set of ordinary differential equations. The mathematical method can be considered as a recursive nonlinear mapping in Fourier space from a given state at a time t to an updated state at time t + dt, with an infinitesimal time increment. This nonlinear evolution in Fourier space requires normalmode compatible boundary conditions along the entire boundary. Each Fourier coefficient gets three contributions during its updating in time: (i) one decay term due to thermal diffusion, governed by linear theory; (ii) one growth term due to buoyancy, governed by linear theory; and (iii) quadratic nonlinearities from the convection term in the heat equation, involving all pairwise interactions between the Fourier modes. We present numerical computations with a standard Runge–Kutta method, with Rayleigh numbers up to ten times the wellknown critical value [math]. Our plots are produced with Fourier series truncated to include 15 normal modes in both the horizontal and vertical directions. For validation of our method, we present tables for the Nusselt number of steady convection, with a higher number (up to 20) of normal modes included in the truncated system of equations. Our computations of transient nonlinear convection lead to steady states. A final steady state is not unique for a given geometry, but depends on the initial state and the Rayleigh number. This ambiguity of steady states is depicted by a hysteresis loop. The Malkus hypothesis of maximal heat transfer is put into perspective. This hypothesis does not pick a preferred cell width, but it nevertheless constrains the hysteresis loop.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Nonlinear transient convection in a porous rectangle heated from below is studied by an analytically based method. Fourier series for the temperature and streamfunction are applied, where each Fourier coefficient evolves in time according to a coupled set of ordinary differential equations. The mathematical method can be considered as a recursive nonlinear mapping in Fourier space from a given state at a time t to an updated state at time t + dt, with an infinitesimal time increment. This nonlinear evolution in Fourier space requires normalmode compatible boundary conditions along the entire boundary. Each Fourier coefficient gets three contributions during its updating in time: (i) one decay term due to thermal diffusion, governed by linear theory; (ii) one growth term due to buoyancy, governed by linear theory; and (iii) quadratic nonlinearities from the convection term in the heat equation, involving all pairwise interactions between the Fourier modes. We present numerical computations with a standard Runge–Kutta method, with Rayleigh numbers up to ten times the wellknown critical value [math]. Our plots are produced with Fourier series truncated to include 15 normal modes in both the horizontal and vertical directions. For validation of our method, we present tables for the Nusselt number of steady convection, with a higher number (up to 20) of normal modes included in the truncated system of equations. Our computations of transient nonlinear convection lead to steady states. A final steady state is not unique for a given geometry, but depends on the initial state and the Rayleigh number. This ambiguity of steady states is depicted by a hysteresis loop. The Malkus hypothesis of maximal heat transfer is put into perspective. This hypothesis does not pick a preferred cell width, but it nevertheless constrains the hysteresis loop.
Twodimensional Darcy–Bénard convection evolving in Fourier space
10.1063/5.0122215
Physics of Fluids
20221109T12:03:28Z
© 2022 Author(s).
Peder A. Tyvand
Lars Molstad

Control and suppression of viscous fingering displacing nonNewtonian fluid with timedependent injection strategies
https://aip.scitation.org/doi/10.1063/5.0124066?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We explore the stabilization mechanism of the fluid–fluid interface in the radial Hele–Shaw cell, displacing a nonNewtonian fluid. It is possible to stabilize the interface following a nonlinear injection rate, [math], which is related to the displaced fluid rheology ([math] powerlaw index). This suggests the absence of fingering at constant injection when [math]. We propose a quantitative criterion to control the pattern formation and suppress fingering, through the dimensionless parameter J as a function of the physical and operating parameters, which is applicable for a generalized shear thinning fluid. The parameter J is related to the capillary number in the context of the powerlaw fluid, relating to the viscous and interfacial forces. The fingering morphology at higher order modes is affected by nonlinear effects. The results are nonintuitive, and we have shown a feasible approach toward long term fingering stabilization.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We explore the stabilization mechanism of the fluid–fluid interface in the radial Hele–Shaw cell, displacing a nonNewtonian fluid. It is possible to stabilize the interface following a nonlinear injection rate, [math], which is related to the displaced fluid rheology ([math] powerlaw index). This suggests the absence of fingering at constant injection when [math]. We propose a quantitative criterion to control the pattern formation and suppress fingering, through the dimensionless parameter J as a function of the physical and operating parameters, which is applicable for a generalized shear thinning fluid. The parameter J is related to the capillary number in the context of the powerlaw fluid, relating to the viscous and interfacial forces. The fingering morphology at higher order modes is affected by nonlinear effects. The results are nonintuitive, and we have shown a feasible approach toward long term fingering stabilization.
Control and suppression of viscous fingering displacing nonNewtonian fluid with timedependent injection strategies
10.1063/5.0124066
Physics of Fluids
20221110T12:52:25Z
© 2022 Author(s).
Pooja Singh
Sourav Mondal

Study of the process of metal droplets with high surface tension impinging on wall
https://aip.scitation.org/doi/10.1063/5.0123982?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The behavior of aluminum droplets impacting a wall critically affects the operation of solid rocket motors. In the present study, the flow and rebound of aluminum droplets with a high surface tension are studied experimentally and numerically. In addition, the impact of aluminum droplets at different inclined angles is monitored experimentally. When the droplet is about to rebound away from the wall, it is stretched to its maximum length. An empirical correlation formula is proposed to predict the maximum length when a droplet is about to bounce off a flat plane. The velocity and pressure distributions of a droplet flowing over a flat plane and an inclined plane are compared by using the volume of fluid method. Furthermore, the restitution coefficient of the droplet is discussed in detail. When normal Weber number [math] ranges from 0 to 20, normal restitution coefficient [math] ranges from 0.3 to 0.6. When tangential Weber number [math] <10, tangential restitution coefficient [math] is subject to great uncertainty. When 10 < [math] < 80, [math] is maintained at 0.75. The formulas for the total restitution coefficient, normal restitution coefficient, and tangential restitution coefficient of the aluminum droplets are also presented. Based on the principle of conservation of energy, we calculate the remaining energy of the aluminum droplets impinging on a wall and provide the relationship between the restitution coefficient and the dissipated energy. The results elucidate the mechanisms at work when aluminum droplets collide with a wall.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The behavior of aluminum droplets impacting a wall critically affects the operation of solid rocket motors. In the present study, the flow and rebound of aluminum droplets with a high surface tension are studied experimentally and numerically. In addition, the impact of aluminum droplets at different inclined angles is monitored experimentally. When the droplet is about to rebound away from the wall, it is stretched to its maximum length. An empirical correlation formula is proposed to predict the maximum length when a droplet is about to bounce off a flat plane. The velocity and pressure distributions of a droplet flowing over a flat plane and an inclined plane are compared by using the volume of fluid method. Furthermore, the restitution coefficient of the droplet is discussed in detail. When normal Weber number [math] ranges from 0 to 20, normal restitution coefficient [math] ranges from 0.3 to 0.6. When tangential Weber number [math] <10, tangential restitution coefficient [math] is subject to great uncertainty. When 10 < [math] < 80, [math] is maintained at 0.75. The formulas for the total restitution coefficient, normal restitution coefficient, and tangential restitution coefficient of the aluminum droplets are also presented. Based on the principle of conservation of energy, we calculate the remaining energy of the aluminum droplets impinging on a wall and provide the relationship between the restitution coefficient and the dissipated energy. The results elucidate the mechanisms at work when aluminum droplets collide with a wall.
Study of the process of metal droplets with high surface tension impinging on wall
10.1063/5.0123982
Physics of Fluids
20221111T12:51:02Z
© 2022 Author(s).

Linear temporal stability analysis on the inviscid sheared convective boundary layer flow
https://aip.scitation.org/doi/10.1063/5.0123044?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A linear temporal stability analysis is conducted for inviscid sheared convective boundary layer flow, in which the sheared instability with stable stratification coexists with and caps over the thermal instability with unstable stratification. The classic Taylor–Goldstein equation is applied with different stratification factors Js and Jb in the Brunt–Väisälä frequency, respectively. Two shearthermal hybrid instabilities, the hybrid shear stratified (HSS) and hybrid Rayleigh–Bénard (HRB) modes, are obtained by solving the eigenvalue problems. It is found that the temporal growth rates of the HSS and HRB modes vary differently with increased Jb in two distinct wavenumber ([math]) regions defined by the intersection point between the stability boundaries of the HSS and HRB modes. Based on [math] where the temporal growth rate of the HSS and HRB are equal, a map of the unique critical boundary, which separates the effective regions of the HSS and HRB modes, is constructed and found to be dependent on Js, Jb, and [math]. The examinations of the subordinate eigenfunctions indicate that the shear instability is well developed in the HSS mode, in which the large vortex structures may prevail and suppress the formation of convective rolls; the shear instability in the HRB mode is either “partly developed” when [math] or “undeveloped” when [math], thus only plays a secondary role to modify the dominant convective rolls, and as Jb increases, the eigenfunctions of the HSS mode exhibit different transitional behaviors in the two regions, signifying the “shear enhancement” and “shear sheltering” of the entrainment of buoyancy flux.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A linear temporal stability analysis is conducted for inviscid sheared convective boundary layer flow, in which the sheared instability with stable stratification coexists with and caps over the thermal instability with unstable stratification. The classic Taylor–Goldstein equation is applied with different stratification factors Js and Jb in the Brunt–Väisälä frequency, respectively. Two shearthermal hybrid instabilities, the hybrid shear stratified (HSS) and hybrid Rayleigh–Bénard (HRB) modes, are obtained by solving the eigenvalue problems. It is found that the temporal growth rates of the HSS and HRB modes vary differently with increased Jb in two distinct wavenumber ([math]) regions defined by the intersection point between the stability boundaries of the HSS and HRB modes. Based on [math] where the temporal growth rate of the HSS and HRB are equal, a map of the unique critical boundary, which separates the effective regions of the HSS and HRB modes, is constructed and found to be dependent on Js, Jb, and [math]. The examinations of the subordinate eigenfunctions indicate that the shear instability is well developed in the HSS mode, in which the large vortex structures may prevail and suppress the formation of convective rolls; the shear instability in the HRB mode is either “partly developed” when [math] or “undeveloped” when [math], thus only plays a secondary role to modify the dominant convective rolls, and as Jb increases, the eigenfunctions of the HSS mode exhibit different transitional behaviors in the two regions, signifying the “shear enhancement” and “shear sheltering” of the entrainment of buoyancy flux.
Linear temporal stability analysis on the inviscid sheared convective boundary layer flow
10.1063/5.0123044
Physics of Fluids
20221111T12:53:20Z
© 2022 Author(s).

Linear analysis of magnetohydrodynamic Richtmyer–Meshkov instability in cylindrical geometry for double interfaces in the presence of an azimuthal magnetic field
https://aip.scitation.org/doi/10.1063/5.0108684?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Richtmyer–Meshkov instability (RMI) occurs when a shock wave impulsively accelerates a perturbed density interface between different fluids. The present work investigates the suppression of RMI of double interfaces in terms of linear analysis in cylindrical geometry. An exponential increase/decrease in a growth rate is related to the Rayleigh–Taylor instability that occurs without a magnetic field as the lighter fluid penetrates the heavier one. The research program of inertial confinement fusion is one of the advanced applications where fluid mixing is the main mechanize of producing energy. The investigations represent the effects of different Atwood numbers or magnetic strengths on the suppression of the instabilities. Three different cases are considered with the hydrodynamics and magnetohydrodynamics (MHD). In the MHD case, the instability's growth rate reduces proportion to the Atwood ratios or the strength of the magnetic field. Two waves are interfering and running parallel and antiparallel to the interfaces and transport the generated vorticity at the interfaces, causing the perturbed interfaces' growth rate to oscillate in time, which is the essential suppression mechanism.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Richtmyer–Meshkov instability (RMI) occurs when a shock wave impulsively accelerates a perturbed density interface between different fluids. The present work investigates the suppression of RMI of double interfaces in terms of linear analysis in cylindrical geometry. An exponential increase/decrease in a growth rate is related to the Rayleigh–Taylor instability that occurs without a magnetic field as the lighter fluid penetrates the heavier one. The research program of inertial confinement fusion is one of the advanced applications where fluid mixing is the main mechanize of producing energy. The investigations represent the effects of different Atwood numbers or magnetic strengths on the suppression of the instabilities. Three different cases are considered with the hydrodynamics and magnetohydrodynamics (MHD). In the MHD case, the instability's growth rate reduces proportion to the Atwood ratios or the strength of the magnetic field. Two waves are interfering and running parallel and antiparallel to the interfaces and transport the generated vorticity at the interfaces, causing the perturbed interfaces' growth rate to oscillate in time, which is the essential suppression mechanism.
Linear analysis of magnetohydrodynamic Richtmyer–Meshkov instability in cylindrical geometry for double interfaces in the presence of an azimuthal magnetic field
10.1063/5.0108684
Physics of Fluids
20221114T11:51:37Z
© 2022 Author(s).
A. Bakhsh

Sustainable highpressure lightdriven water pump with a spiral tube structure and Büttiker–Landauer ratchet
https://aip.scitation.org/doi/10.1063/5.0121728?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Developing sustainable water transportation technology is essential for solving water shortage problems. In this study, we proposed a sustainable highpressure lightdriven water pump that used a spiral tube structure (where light hit one side and shadowed the other) with a Büttiker–Landauer (BL) ratchet. Moreover, we examined the performance of this water pump. By using a polyurethane tube with a diameter of 2.5 mm and a length of 5 m, we demonstrated that the lightdriven BL pump lifted water from a water source against the force of gravity, transported it horizontally along the spiral tube, and removed it from the tube against the surface tension. In particular, by the height scaleup design, we observed ∼800 times larger actual pressure difference ([math] Pa) than the previous lightdriven BL pump along with the pumping flow velocity up to 2.4 mm/s. In addition, by proposing a model that considers the effect of the transportation of heat energy from the hot region to the cold region, we explain the experimentally observed selfadjustment phenomenon for a flow velocity. Since our pump can work under sunlight without using electricity or fossil fuels, it is more sustainable than other pumps. Our findings should contribute to practical sustainable water transportation.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Developing sustainable water transportation technology is essential for solving water shortage problems. In this study, we proposed a sustainable highpressure lightdriven water pump that used a spiral tube structure (where light hit one side and shadowed the other) with a Büttiker–Landauer (BL) ratchet. Moreover, we examined the performance of this water pump. By using a polyurethane tube with a diameter of 2.5 mm and a length of 5 m, we demonstrated that the lightdriven BL pump lifted water from a water source against the force of gravity, transported it horizontally along the spiral tube, and removed it from the tube against the surface tension. In particular, by the height scaleup design, we observed ∼800 times larger actual pressure difference ([math] Pa) than the previous lightdriven BL pump along with the pumping flow velocity up to 2.4 mm/s. In addition, by proposing a model that considers the effect of the transportation of heat energy from the hot region to the cold region, we explain the experimentally observed selfadjustment phenomenon for a flow velocity. Since our pump can work under sunlight without using electricity or fossil fuels, it is more sustainable than other pumps. Our findings should contribute to practical sustainable water transportation.
Sustainable highpressure lightdriven water pump with a spiral tube structure and Büttiker–Landauer ratchet
10.1063/5.0121728
Physics of Fluids
20221114T11:52:01Z
© 2022 Author(s).

An experimental study of the inception of largescale structures in laminar flow through two parallel fins
https://aip.scitation.org/doi/10.1063/5.0123549?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This article presents an experimental investigation aimed to determine the onset conditions for the formation of largescale structures in flows through two parallel fins. The flow visualization results confirmed the presence of the coherent structures, determined the critical Reynolds number for their formation, and correlated it to the local flow parameters. The value of this critical Reynolds number, when based on local parameters, seems to be constant for all studied gap sizes and equal to 6.48. The study also documented the structures' axial spacing and convective speed, as a function of geometrical (gap width) and dynamical (Reynolds number) parameters. The structures' streamwise spacing, for a fixed gap width, asymptotically approaches a constant value when the Reynolds number is increased. This constant value increases linearly with the gap width. The structures' convective speed varies linearly with the Reynolds number. Distinct curves for each gap size are obtained when bulk quantities are employed. However, when normalized by the local velocity, its variation with the local Reynolds number seems to follow a single linear curve independently of the gap widths. The results showed that the coherent structures are local phenomena, best characterized by local parameters of the interfin flow region.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This article presents an experimental investigation aimed to determine the onset conditions for the formation of largescale structures in flows through two parallel fins. The flow visualization results confirmed the presence of the coherent structures, determined the critical Reynolds number for their formation, and correlated it to the local flow parameters. The value of this critical Reynolds number, when based on local parameters, seems to be constant for all studied gap sizes and equal to 6.48. The study also documented the structures' axial spacing and convective speed, as a function of geometrical (gap width) and dynamical (Reynolds number) parameters. The structures' streamwise spacing, for a fixed gap width, asymptotically approaches a constant value when the Reynolds number is increased. This constant value increases linearly with the gap width. The structures' convective speed varies linearly with the Reynolds number. Distinct curves for each gap size are obtained when bulk quantities are employed. However, when normalized by the local velocity, its variation with the local Reynolds number seems to follow a single linear curve independently of the gap widths. The results showed that the coherent structures are local phenomena, best characterized by local parameters of the interfin flow region.
An experimental study of the inception of largescale structures in laminar flow through two parallel fins
10.1063/5.0123549
Physics of Fluids
20221115T12:22:41Z
© 2022 Author(s).
Mohamed Sadok Guellouz

Effect of the odd viscosity on Faraday wave instability
https://aip.scitation.org/doi/10.1063/5.0124790?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Faraday waves arise in fluid systems with free surfaces subject to vertical oscillations of sufficient strength due to parametric resonance. The odd viscosity is a peculiar part of the viscosity stress tensor that does not result in dissipation and is allowed when parity symmetry is broken spontaneously or due to external magnetic fields or rotations. The effect of the odd viscosity on the classic Faraday instability of thin liquid films in infinite horizontal plates is investigated by utilizing both linear Floquet theory and nonlinear lubrication theory based on the weighted residual model. This work derives the nonlinear evolution equations about the flow rate and free surface height, and linear stability analysis is performed to achieve the damped Mathieu equation. The results show that the neutral stability curves derived from the Mathieu equation agree well with those obtained from the linear Floquet analysis, especially for lower viscosity ratios μ. The nonlinear numerical results simulated by the method of lines indicate interesting results where the odd viscosity gives rise to a “sliding” of the wave configuration parallel to the wall, and the interface wave then translates into a traveling wave.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Faraday waves arise in fluid systems with free surfaces subject to vertical oscillations of sufficient strength due to parametric resonance. The odd viscosity is a peculiar part of the viscosity stress tensor that does not result in dissipation and is allowed when parity symmetry is broken spontaneously or due to external magnetic fields or rotations. The effect of the odd viscosity on the classic Faraday instability of thin liquid films in infinite horizontal plates is investigated by utilizing both linear Floquet theory and nonlinear lubrication theory based on the weighted residual model. This work derives the nonlinear evolution equations about the flow rate and free surface height, and linear stability analysis is performed to achieve the damped Mathieu equation. The results show that the neutral stability curves derived from the Mathieu equation agree well with those obtained from the linear Floquet analysis, especially for lower viscosity ratios μ. The nonlinear numerical results simulated by the method of lines indicate interesting results where the odd viscosity gives rise to a “sliding” of the wave configuration parallel to the wall, and the interface wave then translates into a traveling wave.
Effect of the odd viscosity on Faraday wave instability
10.1063/5.0124790
Physics of Fluids
20221115T10:30:57Z
© 2022 Author(s).

Shearimposed falling thin Newtonian film over a porous slippery surface
https://aip.scitation.org/doi/10.1063/5.0120882?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The stability of a Newtonian thin film flow over a porous slippery wall approximated by Darcy's law is investigated. The modified Orr–Sommerfeld system is derived for the frequencydependent linear stability analysis and energybudget analysis. Moreover, in the longwave regime, both linear and weakly nonlinear stability analyses are conducted for small aspect ratios. In addition, the multiple scale approach is performed directly in the nonlinear deformation equation of the free surface to predict the extraordinary behavior of the amplitude and speed of the nonlinear disturbance in the subcritical and supercritical regimes. The study finds that the larger slipvelocity and externally imposed shear on the thin film increase the total kinetic energy of the infinitesimal perturbations. In a longwave regime, the critical conditions of the primary instability are described as a function of imposed shear stress that destabilizes the film flow for low critical Reynolds number. Furthermore, in the supercritical stable zone, both the nonlinear wave amplitude and phase speed increase with an increase in induced shear in the flow direction and velocity slip, and a reverse trend is observed in applying the imposed shear in the opposite flow direction. On the other hand, the nonlinear wave amplitude in the subcritical unstable zone increases and decreases, corresponding to the larger values of imposed shear and slip parameters, respectively.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The stability of a Newtonian thin film flow over a porous slippery wall approximated by Darcy's law is investigated. The modified Orr–Sommerfeld system is derived for the frequencydependent linear stability analysis and energybudget analysis. Moreover, in the longwave regime, both linear and weakly nonlinear stability analyses are conducted for small aspect ratios. In addition, the multiple scale approach is performed directly in the nonlinear deformation equation of the free surface to predict the extraordinary behavior of the amplitude and speed of the nonlinear disturbance in the subcritical and supercritical regimes. The study finds that the larger slipvelocity and externally imposed shear on the thin film increase the total kinetic energy of the infinitesimal perturbations. In a longwave regime, the critical conditions of the primary instability are described as a function of imposed shear stress that destabilizes the film flow for low critical Reynolds number. Furthermore, in the supercritical stable zone, both the nonlinear wave amplitude and phase speed increase with an increase in induced shear in the flow direction and velocity slip, and a reverse trend is observed in applying the imposed shear in the opposite flow direction. On the other hand, the nonlinear wave amplitude in the subcritical unstable zone increases and decreases, corresponding to the larger values of imposed shear and slip parameters, respectively.
Shearimposed falling thin Newtonian film over a porous slippery surface
10.1063/5.0120882
Physics of Fluids
20221116T10:41:15Z
© 2022 Author(s).
Md. Mouzakkir Hossain
Harekrushna Behera

Phototactic isotropic scattering bioconvection with oblique irradiation
https://aip.scitation.org/doi/10.1063/5.0127681?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The phenomenon of collective movements of microorganisms is referred to as “bioconvection.” Negative phototaxis refers to motions directed away from the source of illumination, and positive phototaxis refers to motions directed in that direction. In this study, numerical analysis is performed on both the steadystate and linear stability solutions of the isotropic scattering suspension with oblique collimated irradiation. The bottom boundary is taken to be rigid and the top is either stressfree or rigid. The governing equations are solved using a fourthorder Newton–Raphson–Kantorovich iterationbased finitedifference accurate method. Through variation in the angle of incidence, we found two different types of nature for lower and higher scattering albedo in the basic state concentration profile. In the case of a rigid (or stressfree) upper boundary, the bioconvection solutions are generally oscillatory (or stationary) and more stable (or unstable). The scattering model coincides with the upswimming model at higher wavenumbers.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The phenomenon of collective movements of microorganisms is referred to as “bioconvection.” Negative phototaxis refers to motions directed away from the source of illumination, and positive phototaxis refers to motions directed in that direction. In this study, numerical analysis is performed on both the steadystate and linear stability solutions of the isotropic scattering suspension with oblique collimated irradiation. The bottom boundary is taken to be rigid and the top is either stressfree or rigid. The governing equations are solved using a fourthorder Newton–Raphson–Kantorovich iterationbased finitedifference accurate method. Through variation in the angle of incidence, we found two different types of nature for lower and higher scattering albedo in the basic state concentration profile. In the case of a rigid (or stressfree) upper boundary, the bioconvection solutions are generally oscillatory (or stationary) and more stable (or unstable). The scattering model coincides with the upswimming model at higher wavenumbers.
Phototactic isotropic scattering bioconvection with oblique irradiation
10.1063/5.0127681
Physics of Fluids
20221117T12:25:52Z
© 2022 Author(s).
Sandeep Kumar

Effect of nonuniform stiffness distribution on the dynamics of inverted plates in a uniform flow
https://aip.scitation.org/doi/10.1063/5.0122657?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The stability of the inverted flexible plate with nonuniform stiffness distribution in a free stream is studied by numerical simulation and mathematical theory. In our study, the bending stiffness distribution is expressed as the function of the leading edge's bending stiffness [math] and the polynomial of the plate's coordinate. Based on the former theoretical work on the stability of inverted plates with uniform stiffness distribution, we derive the upper limit value of [math] at which the zerodeflection equilibrium loses its stability for the plate with nonuniform stiffness distribution. The critical [math] derived from the mathematical theory agrees well with that obtained from the numerical simulation. An effective bending stiffness is defined, which can be used to unify the regimes of the motion modes between uniform plates and nonuniform plates. Moreover, three orders of mass ratio [[math], and [math]] are investigated, and the underlying mechanism for large amplitude flapping is clarified for the inverted plate with different mass ratios. An appropriate bending stiffness distribution can greatly improve the deformation of the plate. The findings shed some light on the energy harvesting of the inverted plate.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The stability of the inverted flexible plate with nonuniform stiffness distribution in a free stream is studied by numerical simulation and mathematical theory. In our study, the bending stiffness distribution is expressed as the function of the leading edge's bending stiffness [math] and the polynomial of the plate's coordinate. Based on the former theoretical work on the stability of inverted plates with uniform stiffness distribution, we derive the upper limit value of [math] at which the zerodeflection equilibrium loses its stability for the plate with nonuniform stiffness distribution. The critical [math] derived from the mathematical theory agrees well with that obtained from the numerical simulation. An effective bending stiffness is defined, which can be used to unify the regimes of the motion modes between uniform plates and nonuniform plates. Moreover, three orders of mass ratio [[math], and [math]] are investigated, and the underlying mechanism for large amplitude flapping is clarified for the inverted plate with different mass ratios. An appropriate bending stiffness distribution can greatly improve the deformation of the plate. The findings shed some light on the energy harvesting of the inverted plate.
Effect of nonuniform stiffness distribution on the dynamics of inverted plates in a uniform flow
10.1063/5.0122657
Physics of Fluids
20221118T11:14:36Z
© 2022 Author(s).

Studies on flow field, and instabilities in a large diameter opposed jet burner
https://aip.scitation.org/doi/10.1063/5.0120564?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This work reports studies on flow fields, and instabilities exhibited by opposed jets at equal momenta, for Reynolds numbers (Re) ranging from 178 to 5000 using a large diameter counterflow jet setup. Flow instability investigations were conducted over a range of aspect ratios (α) too. This study identifies the regions of bistability and those of oscillatory stagnation plane offsets identified by Re. Experiments on flow field were carried out using particle image velocimetry technique, and the paper presents the axial and radial velocity profiles at various locations as well as their gradients. A decreasing trend in stagnation plane displacements with the Reynolds number was observed. The experiments in comparison with the past literature suggest a possible dependence of the stagnation plane displacements on a nozzleexit diameter. The trends in maximum stagnation plane displacements (δmax), as well as the critical Reynolds numbers (Recr) with aspect ratios (α), are analyzed and compared. The flow field studies reveal the need for two dimensional axisymmetric simulations with realistic velocity boundary conditions to predict opposed jet flow phenomena accurately. Reacting flow instability studies were also carried out for equal momenta using methaneair and ethylene–air flames at various aspect ratios. The results show an enhanced bistability for ethylene–air flames over methane–air flames.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This work reports studies on flow fields, and instabilities exhibited by opposed jets at equal momenta, for Reynolds numbers (Re) ranging from 178 to 5000 using a large diameter counterflow jet setup. Flow instability investigations were conducted over a range of aspect ratios (α) too. This study identifies the regions of bistability and those of oscillatory stagnation plane offsets identified by Re. Experiments on flow field were carried out using particle image velocimetry technique, and the paper presents the axial and radial velocity profiles at various locations as well as their gradients. A decreasing trend in stagnation plane displacements with the Reynolds number was observed. The experiments in comparison with the past literature suggest a possible dependence of the stagnation plane displacements on a nozzleexit diameter. The trends in maximum stagnation plane displacements (δmax), as well as the critical Reynolds numbers (Recr) with aspect ratios (α), are analyzed and compared. The flow field studies reveal the need for two dimensional axisymmetric simulations with realistic velocity boundary conditions to predict opposed jet flow phenomena accurately. Reacting flow instability studies were also carried out for equal momenta using methaneair and ethylene–air flames at various aspect ratios. The results show an enhanced bistability for ethylene–air flames over methane–air flames.
Studies on flow field, and instabilities in a large diameter opposed jet burner
10.1063/5.0120564
Physics of Fluids
20221118T11:32:44Z
© 2022 Author(s).
P Navaneethakrishnan
Krishna Sesha Giri

Predicting aerodynamic pressure on a square cylinder from wake velocity field by masked gated recurrent unit model
https://aip.scitation.org/doi/10.1063/5.0110491?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A masked gated recurrent unit (GRU) model is proposed to establish the mapping relationship between surface pressures on a square cylinder and wake velocities, which can be used to predict statistical and instantaneous aerodynamic pressure fields on a square cylinder from its wakefield. A novel mask net is proposed to figure out one or two wake points where the velocities contribute dominantly to the surface pressure field. A threedimensional unsteady largeeddy simulation of flow around a square cylinder is performed at Re = 22 000 to generate data for training and validating the proposed models. Results show that local mean pressure coefficients can be well predicted from velocities at even one wake point, but the accuracies of predicting fluctuating pressure coefficients and timeseries of local pressure coefficients depend on both the model and the surface pressure location, with more satisfactory predictions achieved in the crossflow direction. High correlation coefficients of pressure coefficient distributions around a square cylinder between predicted and real distributions are achieved except for the masked GRU model with one wake point. Meanwhile, in terms of the temporal correlation coefficient, all models exhibit good prediction of timeseries of pressure coefficients on the side and back surfaces where they are strongly affected by vortex shedding and lower accuracy on the front surface where the pressure coefficients deviate somewhat randomly around the mean value. Large prediction error occurs at the corners of the square cylinder. This study has potential application to risk analysis of structures subject to flowinduced loads.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A masked gated recurrent unit (GRU) model is proposed to establish the mapping relationship between surface pressures on a square cylinder and wake velocities, which can be used to predict statistical and instantaneous aerodynamic pressure fields on a square cylinder from its wakefield. A novel mask net is proposed to figure out one or two wake points where the velocities contribute dominantly to the surface pressure field. A threedimensional unsteady largeeddy simulation of flow around a square cylinder is performed at Re = 22 000 to generate data for training and validating the proposed models. Results show that local mean pressure coefficients can be well predicted from velocities at even one wake point, but the accuracies of predicting fluctuating pressure coefficients and timeseries of local pressure coefficients depend on both the model and the surface pressure location, with more satisfactory predictions achieved in the crossflow direction. High correlation coefficients of pressure coefficient distributions around a square cylinder between predicted and real distributions are achieved except for the masked GRU model with one wake point. Meanwhile, in terms of the temporal correlation coefficient, all models exhibit good prediction of timeseries of pressure coefficients on the side and back surfaces where they are strongly affected by vortex shedding and lower accuracy on the front surface where the pressure coefficients deviate somewhat randomly around the mean value. Large prediction error occurs at the corners of the square cylinder. This study has potential application to risk analysis of structures subject to flowinduced loads.
Predicting aerodynamic pressure on a square cylinder from wake velocity field by masked gated recurrent unit model
10.1063/5.0110491
Physics of Fluids
20221101T11:29:52Z
© 2022 Author(s).
Mengtao Yan
Zhiming Zhang
Shangce Gao
Shuyang Cao

Frozen propagation of Reynolds force vector from highfidelity data into Reynoldsaveraged simulations of secondary flows
https://aip.scitation.org/doi/10.1063/5.0123231?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Successful propagation of information from highfidelity sources (i.e., direct numerical simulations and largeeddy simulations) into Reynoldsaveraged Navier–Stokes (RANS) equations plays an important role in the emerging field of datadriven RANS modeling. Small errors carried in highfidelity data can propagate amplified errors into the mean flow field, and higher Reynolds numbers worsen the error propagation. In this study, we compare a series of propagation methods for two cases of Prandtl's secondary flows of the second kind: squareduct flow at a low Reynolds number and roughnessinduced secondary flow at a very high Reynolds number. We show that frozen treatments result in less error propagation than the implicit treatment of Reynolds stress tensor (RST), and for cases with very high Reynolds numbers, explicit and implicit treatments are not recommended. Inspired by the obtained results, we introduce the frozen treatment to the propagation of the Reynolds force vector (RFV), which leads to less error propagation. Specifically, for both cases at low and high Reynolds numbers, the propagation of RFV results in one order of magnitude lower error compared to the RST propagation. In the frozen treatment method, three different eddyviscosity models are used to evaluate the effect of turbulent diffusion on error propagation. We show that, regardless of the baseline model, the frozen treatment of RFV results in less error propagation. We combined one extra correction term for turbulent kinetic energy with the frozen treatment of RFV, which makes our propagation technique capable of reproducing both velocity and turbulent kinetic energy fields similar to highfidelity data.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Successful propagation of information from highfidelity sources (i.e., direct numerical simulations and largeeddy simulations) into Reynoldsaveraged Navier–Stokes (RANS) equations plays an important role in the emerging field of datadriven RANS modeling. Small errors carried in highfidelity data can propagate amplified errors into the mean flow field, and higher Reynolds numbers worsen the error propagation. In this study, we compare a series of propagation methods for two cases of Prandtl's secondary flows of the second kind: squareduct flow at a low Reynolds number and roughnessinduced secondary flow at a very high Reynolds number. We show that frozen treatments result in less error propagation than the implicit treatment of Reynolds stress tensor (RST), and for cases with very high Reynolds numbers, explicit and implicit treatments are not recommended. Inspired by the obtained results, we introduce the frozen treatment to the propagation of the Reynolds force vector (RFV), which leads to less error propagation. Specifically, for both cases at low and high Reynolds numbers, the propagation of RFV results in one order of magnitude lower error compared to the RST propagation. In the frozen treatment method, three different eddyviscosity models are used to evaluate the effect of turbulent diffusion on error propagation. We show that, regardless of the baseline model, the frozen treatment of RFV results in less error propagation. We combined one extra correction term for turbulent kinetic energy with the frozen treatment of RFV, which makes our propagation technique capable of reproducing both velocity and turbulent kinetic energy fields similar to highfidelity data.
Frozen propagation of Reynolds force vector from highfidelity data into Reynoldsaveraged simulations of secondary flows
10.1063/5.0123231
Physics of Fluids
20221101T11:29:10Z
© 2022 Author(s).
Ali Amarloo
Pourya Forooghi
Mahdi Abkar

New insights into the breathing physiology from transient respiratory nasal simulation
https://aip.scitation.org/doi/10.1063/5.0112223?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The flow characteristics and heat transfer during nasal breathing in the complete human upper airway were investigated through the respiratory cycle using transient numerical simulations. We postulate that the complete airway from the nasal cavity to the trachea most accurately represents dynamic airflow patterns during inhalation and exhalation as they are likely to be affected by downstream anatomical structures. A 3D model was constructed from a healthy adult computed tomography scan. Computational fluid dynamics simulations were performed with Ansys Fluent software [ANSYS Fluent, R1 User's Guide (ANSYS, Inc., 2020)] using the stressblended eddy simulation turbulence model looking at airflow patterns, velocity, mucosal temperature, and humidity (H2O fraction). One and a half breathing cycles were simulated for a total of 5.65 s, where the first inhalation cycle was discarded to avoid startup effects. The results demonstrated that airway geometry structures, including the turbinates, the soft palate, and the glottic region, affect the flow patterns differently during inspiration and expiration. It also demonstrated phenomena not seen in steady flow simulations or in those without the lower respiratory tract geometry, including the nasopharyngeal temperature imprint during inhalation, the nasopharyngeal jet during exhalation, and the flow structures of the larynx and laryngeal jet. The inclusion of the exhalation phase demonstrates airflow preconditioning before inhalation, which we postulate contributes to achieving alveolar conditions. Alveolar temperature and humidity conditions are not achieved by the nasal cavity alone, and we demonstrate the contribution of the nasopharynx and larynx to air conditioning. Including the complete airway with realistic anatomy and using transient airflow modeling provided new insights into the physiology of the respiratory cycle.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The flow characteristics and heat transfer during nasal breathing in the complete human upper airway were investigated through the respiratory cycle using transient numerical simulations. We postulate that the complete airway from the nasal cavity to the trachea most accurately represents dynamic airflow patterns during inhalation and exhalation as they are likely to be affected by downstream anatomical structures. A 3D model was constructed from a healthy adult computed tomography scan. Computational fluid dynamics simulations were performed with Ansys Fluent software [ANSYS Fluent, R1 User's Guide (ANSYS, Inc., 2020)] using the stressblended eddy simulation turbulence model looking at airflow patterns, velocity, mucosal temperature, and humidity (H2O fraction). One and a half breathing cycles were simulated for a total of 5.65 s, where the first inhalation cycle was discarded to avoid startup effects. The results demonstrated that airway geometry structures, including the turbinates, the soft palate, and the glottic region, affect the flow patterns differently during inspiration and expiration. It also demonstrated phenomena not seen in steady flow simulations or in those without the lower respiratory tract geometry, including the nasopharyngeal temperature imprint during inhalation, the nasopharyngeal jet during exhalation, and the flow structures of the larynx and laryngeal jet. The inclusion of the exhalation phase demonstrates airflow preconditioning before inhalation, which we postulate contributes to achieving alveolar conditions. Alveolar temperature and humidity conditions are not achieved by the nasal cavity alone, and we demonstrate the contribution of the nasopharynx and larynx to air conditioning. Including the complete airway with realistic anatomy and using transient airflow modeling provided new insights into the physiology of the respiratory cycle.
New insights into the breathing physiology from transient respiratory nasal simulation
10.1063/5.0112223
Physics of Fluids
20221101T11:29:24Z
© 2022 Author(s).
Kimberley Bradshaw
Patrick WarfieldMcAlpine
Sara Vahaji
Jake Emmerling
Hana Salati
Ray Sacks
David F. Fletcher
Narinder Singh
Kiao Inthavong

Phenomenon and analysis of direct initiation of detonation using multiple turbulent flame jets
https://aip.scitation.org/doi/10.1063/5.0122191?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This work reports experimental investigations on the direct initiation of detonation using multiple turbulent flame jets, with a special focus on the arrangement schemes and fundamental physics in the initiation processes. Results show that the direct initiation of detonation can be achieved using turbulent jets even when the jet tube diameter is much smaller than the empirical critical tube diameter due to flame–shock–wall interactions. Conspicuous evidence has been shown that the probability of the direct initiation increases significantly near the detonatability limit using multijets compared to a single jet. These results are found to be closely related to several new phenomena observed when using multiple jets to initiate the detonation. They are: (1) unexpected rapid promotion of the finalstage flame acceleration in ignition tubes by multiple jets, which is attributed to the fact that the expanding precursor shock waves propagate back into the adjacent tube and interact with the flame; (2) enhancement of hot spot generation by multiple jets due to the precursor shock intersection and the formation of an induction zone; (3) obvious velocity loss of impinging jets initiation as a result of induced hot spots propagation in the burned gases.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This work reports experimental investigations on the direct initiation of detonation using multiple turbulent flame jets, with a special focus on the arrangement schemes and fundamental physics in the initiation processes. Results show that the direct initiation of detonation can be achieved using turbulent jets even when the jet tube diameter is much smaller than the empirical critical tube diameter due to flame–shock–wall interactions. Conspicuous evidence has been shown that the probability of the direct initiation increases significantly near the detonatability limit using multijets compared to a single jet. These results are found to be closely related to several new phenomena observed when using multiple jets to initiate the detonation. They are: (1) unexpected rapid promotion of the finalstage flame acceleration in ignition tubes by multiple jets, which is attributed to the fact that the expanding precursor shock waves propagate back into the adjacent tube and interact with the flame; (2) enhancement of hot spot generation by multiple jets due to the precursor shock intersection and the formation of an induction zone; (3) obvious velocity loss of impinging jets initiation as a result of induced hot spots propagation in the burned gases.
Phenomenon and analysis of direct initiation of detonation using multiple turbulent flame jets
10.1063/5.0122191
Physics of Fluids
20221102T02:44:40Z
© 2022 Author(s).

Influence of nonuniform thermal boundary on flow and heat transfer characteristics in rectangular channel
https://aip.scitation.org/doi/10.1063/5.0118139?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, the flow behavior and heat transfer characteristic in a rectangular channel are numerically investigated. The nonuniform thermal boundary condition is arranged along the streamwise direction at the bottom of the rectangular channel. Furthermore, based on the flow field parameters obtained with numerical simulation, the dynamic modal decomposition (DMD) is carried out for viscous layer, buffer layer, and logarithmic region, respectively. The numerical results show that the hot bands of nonuniform thermal boundary affect the interaction of the velocity streaks along the streamwise direction, which reduces the vorticity of the buffer layer and the fluctuation of the velocity gradient vector. In the terms of entropy analysis, it can be found that the hot bands of nonuniform thermal boundary play a similar role of “riblets” and block the selfsustainment of the turbulent coherent structures. Moreover, the results of DMD manifest that the hot bands of nonuniform thermal boundary can improve the stability of viscous layer and buffer layer. The development of turbulent boundary layer is delayed by affecting the fluid characteristics in buffer layer. Compared to the channel without nonuniform thermal boundary condition, the maximum drag reduction rate of 8.35% can be achieved in considered cases, while a reduction in heat transfer performance of 2.74% occurs. In addition, the comprehensive performance coefficient increases slightly to 1.0013.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, the flow behavior and heat transfer characteristic in a rectangular channel are numerically investigated. The nonuniform thermal boundary condition is arranged along the streamwise direction at the bottom of the rectangular channel. Furthermore, based on the flow field parameters obtained with numerical simulation, the dynamic modal decomposition (DMD) is carried out for viscous layer, buffer layer, and logarithmic region, respectively. The numerical results show that the hot bands of nonuniform thermal boundary affect the interaction of the velocity streaks along the streamwise direction, which reduces the vorticity of the buffer layer and the fluctuation of the velocity gradient vector. In the terms of entropy analysis, it can be found that the hot bands of nonuniform thermal boundary play a similar role of “riblets” and block the selfsustainment of the turbulent coherent structures. Moreover, the results of DMD manifest that the hot bands of nonuniform thermal boundary can improve the stability of viscous layer and buffer layer. The development of turbulent boundary layer is delayed by affecting the fluid characteristics in buffer layer. Compared to the channel without nonuniform thermal boundary condition, the maximum drag reduction rate of 8.35% can be achieved in considered cases, while a reduction in heat transfer performance of 2.74% occurs. In addition, the comprehensive performance coefficient increases slightly to 1.0013.
Influence of nonuniform thermal boundary on flow and heat transfer characteristics in rectangular channel
10.1063/5.0118139
Physics of Fluids
20221102T02:44:32Z
© 2022 Author(s).

On acoustically modulated jet shear layers and the Nyquist–Shannon sampling theorem
https://aip.scitation.org/doi/10.1063/5.0118025?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The goal of this paper is to present the behavior of a jet shear layer in response to acoustic excitation from a signal processing perspective. The main idea is that the vortices that rollup in the jet shear layer are similar to the discrete samples of a digital control system, and, hence, that the Nyquist–Shannon sampling theorem should apply. We further hypothesize that the strength of a vortex is determined by the mean amplitude of the excitation waveform during its creation. We also argue that, at least in some cases, demodulation occurs as a result of the vorticity signal generated by the convection of discrete vortices past a point in the shear layer. This vorticity signal is related to the amplitude modulation (AM) excitation waveform by a halfwave rectification operation, a common implementation of an AM demodulator. To investigate these ideas, a free, round jet that is excited upstream of the nozzle is studied using particle image velocimetry. Experiments are conducted that confirm that the sampling theorem applies, and an aliased response is observed when the Nyquist limit is exceeded. Previous authors have attributed demodulation to a vortex merging mechanism, but we demonstrate that merging is not always required for demodulation and suggest that it is one of two mechanisms at play.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The goal of this paper is to present the behavior of a jet shear layer in response to acoustic excitation from a signal processing perspective. The main idea is that the vortices that rollup in the jet shear layer are similar to the discrete samples of a digital control system, and, hence, that the Nyquist–Shannon sampling theorem should apply. We further hypothesize that the strength of a vortex is determined by the mean amplitude of the excitation waveform during its creation. We also argue that, at least in some cases, demodulation occurs as a result of the vorticity signal generated by the convection of discrete vortices past a point in the shear layer. This vorticity signal is related to the amplitude modulation (AM) excitation waveform by a halfwave rectification operation, a common implementation of an AM demodulator. To investigate these ideas, a free, round jet that is excited upstream of the nozzle is studied using particle image velocimetry. Experiments are conducted that confirm that the sampling theorem applies, and an aliased response is observed when the Nyquist limit is exceeded. Previous authors have attributed demodulation to a vortex merging mechanism, but we demonstrate that merging is not always required for demodulation and suggest that it is one of two mechanisms at play.
On acoustically modulated jet shear layers and the Nyquist–Shannon sampling theorem
10.1063/5.0118025
Physics of Fluids
20221102T02:43:30Z
© 2022 Author(s).
C. J. Nicholls
K. Chakravarthy
B. M. T. Tang
B. A. O. Williams
M. Bacic

Numerical study of flow characteristics in compound meandering channels with vegetated floodplains
https://aip.scitation.org/doi/10.1063/5.0122089?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Large eddy simulations were conducted to simulate the flow in compound meandering channels whose main channel sinuosity was 1.381. Then, the floodplain vegetation was generalized using the momentum equation coupled with the drag force formula. The mean flow pattern, secondary flow, coherent structure, turbulence characteristics, and lateral mass and momentum transport with and without floodplain vegetation with relative depths (Dr) of 0.3–0.5 were studied. Results showed that the floodplain vegetation enabled the flow of the main channel to be more concentrated. The maximum average velocity in the cross section of the main channel increased by 100% and 30% when the relative depth was 0.3 and 0.5. Under the influence of floodplain vegetation, the secondary flow cell transformed greatly with the change in relative depth. When Dr < 0.3, the vegetation caused the vortex center of the secondary flow to move closer to the concave bank side, and the secondary flow distribution presents a flow pattern not flooding the floodplain. When Dr > 0.3, the spatial change in the secondary flow was not obvious. In addition, the floodplain vegetation did not change the largescale vortex that was separated from the boundary layer of the convex bank side. Meanwhile, the floodplain vegetation increased the overall turbulence intensity, turbulent kinetic energy, and Reynolds stress of the main channel, and it increased the range of lateral mass exchange of the inbank flow and the mean and turbulent transport flux of each cross section.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Large eddy simulations were conducted to simulate the flow in compound meandering channels whose main channel sinuosity was 1.381. Then, the floodplain vegetation was generalized using the momentum equation coupled with the drag force formula. The mean flow pattern, secondary flow, coherent structure, turbulence characteristics, and lateral mass and momentum transport with and without floodplain vegetation with relative depths (Dr) of 0.3–0.5 were studied. Results showed that the floodplain vegetation enabled the flow of the main channel to be more concentrated. The maximum average velocity in the cross section of the main channel increased by 100% and 30% when the relative depth was 0.3 and 0.5. Under the influence of floodplain vegetation, the secondary flow cell transformed greatly with the change in relative depth. When Dr < 0.3, the vegetation caused the vortex center of the secondary flow to move closer to the concave bank side, and the secondary flow distribution presents a flow pattern not flooding the floodplain. When Dr > 0.3, the spatial change in the secondary flow was not obvious. In addition, the floodplain vegetation did not change the largescale vortex that was separated from the boundary layer of the convex bank side. Meanwhile, the floodplain vegetation increased the overall turbulence intensity, turbulent kinetic energy, and Reynolds stress of the main channel, and it increased the range of lateral mass exchange of the inbank flow and the mean and turbulent transport flux of each cross section.
Numerical study of flow characteristics in compound meandering channels with vegetated floodplains
10.1063/5.0122089
Physics of Fluids
20221102T02:43:32Z
© 2022 Author(s).

Smoothed particle hydrodynamics physically reconsidered: The relation to explicit large eddy simulation and the issue of particle duality
https://aip.scitation.org/doi/10.1063/5.0105104?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this work, we will identify a novel relation between Smoothed Particle Hydrodynamics (SPH) and explicit large eddy simulation using a coarsegraining method from nonequilibrium molecular dynamics. While the current literature points at the conclusion that characteristic SPH issues become restrictive for subsonic turbulent flows, we see the potential to mitigate these SPH issues by explicit subfilter stress modeling. We verify our theory by various simulations of homogeneous, isotropic turbulence at [math] and compare the results to a direct numerical simulation [T. Dairay et al., “Numerical dissipation vs subgridscale modelling for large eddy simulation,” J. Comput. Phys. 337, 252–274 (2017)]. Although the simulations substantiate our theory, we see another issue arising, which is conceptually rooted in the particle itself, termed as particle duality. Finally, we conclude our work by acknowledging SPH as a coarsegraining method for turbulent flows, highlighting its capabilities and limitations.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this work, we will identify a novel relation between Smoothed Particle Hydrodynamics (SPH) and explicit large eddy simulation using a coarsegraining method from nonequilibrium molecular dynamics. While the current literature points at the conclusion that characteristic SPH issues become restrictive for subsonic turbulent flows, we see the potential to mitigate these SPH issues by explicit subfilter stress modeling. We verify our theory by various simulations of homogeneous, isotropic turbulence at [math] and compare the results to a direct numerical simulation [T. Dairay et al., “Numerical dissipation vs subgridscale modelling for large eddy simulation,” J. Comput. Phys. 337, 252–274 (2017)]. Although the simulations substantiate our theory, we see another issue arising, which is conceptually rooted in the particle itself, termed as particle duality. Finally, we conclude our work by acknowledging SPH as a coarsegraining method for turbulent flows, highlighting its capabilities and limitations.
Smoothed particle hydrodynamics physically reconsidered: The relation to explicit large eddy simulation and the issue of particle duality
10.1063/5.0105104
Physics of Fluids
20221103T12:35:02Z
© 2022 Author(s).
M. Okraschevski
N. Buerkle
R. Koch
H.J. Bauer

Passive fluidic control on aerooptics of transonic flow over turrets with rough walls
https://aip.scitation.org/doi/10.1063/5.0109309?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In the transonic flow over a hemisphereoncylinder turret, strong aerooptical effects can be caused by local shock/boundarylayer interactions and separation shear layers at the turret's zenith. The effects of an annular rough wall on the passive control of fluid and aerooptics are investigated by experimental measurements and numerical simulations. The local shock/boundarylayer interaction and separated shear layer at the zenith of the turret are recorded using shadowing and Mach–Zehnder interferometer measurements. The aerooptics are measured using a Shack–Hartmann wavefront sensor. The experimental results show that the annular rough wall on the turret weakens the local shock wave, moves the flow separation point forward, and reduces the wavefront distortion at the zenith. The rough wall functions for the shear stress transport (SST) kω turbulence model proposed by B. Aupoix [“Roughness corrections for the k–ω shear stress transport model: Status and proposals,” J. Fluids Eng. 137, 021202 (2014)] and C.H. Lee [“Rough boundary treatment method for the shearstress transport k–ω model,” Eng. Appl. Comput. Fluid 12, 261–269 (2018)] are used to further study the control effect of different roughnesses. Numerical simulations based on both rough wall functions show good agreement with the experimental measurements. For various transonic flows, the steady wavefront distortions at the zenith with the rough wall at roughness [math] are 21%–50% smaller than those with smooth walls. The smaller the supersonic region, the more effective the rough wall is in reducing wavefront distortion.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In the transonic flow over a hemisphereoncylinder turret, strong aerooptical effects can be caused by local shock/boundarylayer interactions and separation shear layers at the turret's zenith. The effects of an annular rough wall on the passive control of fluid and aerooptics are investigated by experimental measurements and numerical simulations. The local shock/boundarylayer interaction and separated shear layer at the zenith of the turret are recorded using shadowing and Mach–Zehnder interferometer measurements. The aerooptics are measured using a Shack–Hartmann wavefront sensor. The experimental results show that the annular rough wall on the turret weakens the local shock wave, moves the flow separation point forward, and reduces the wavefront distortion at the zenith. The rough wall functions for the shear stress transport (SST) kω turbulence model proposed by B. Aupoix [“Roughness corrections for the k–ω shear stress transport model: Status and proposals,” J. Fluids Eng. 137, 021202 (2014)] and C.H. Lee [“Rough boundary treatment method for the shearstress transport k–ω model,” Eng. Appl. Comput. Fluid 12, 261–269 (2018)] are used to further study the control effect of different roughnesses. Numerical simulations based on both rough wall functions show good agreement with the experimental measurements. For various transonic flows, the steady wavefront distortions at the zenith with the rough wall at roughness [math] are 21%–50% smaller than those with smooth walls. The smaller the supersonic region, the more effective the rough wall is in reducing wavefront distortion.
Passive fluidic control on aerooptics of transonic flow over turrets with rough walls
10.1063/5.0109309
Physics of Fluids
20221103T12:35:06Z
© 2022 Author(s).

Validation and parameterization of a novel physicsconstrained neural dynamics model applied to turbulent fluid flow
https://aip.scitation.org/doi/10.1063/5.0122115?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In fluid physics, datadriven models to enhance or accelerate time to solution are becoming increasingly popular for many application domains, such as alternatives to turbulence closures, system surrogates, or for new physics discovery. In the context of reduced order models of highdimensional timedependent fluid systems, machine learning methods grant the benefit of automated learning from data, but the burden of a model lies on its reducedorder representation of both the fluid state and physical dynamics. In this work, we build a physicsconstrained, datadriven reduced order model for Navier–Stokes equations to approximate spatiotemporal fluid dynamics in the canonical case of isotropic turbulence in a triply periodic box. The model design choices mimic numerical and physical constraints by, for example, implicitly enforcing the incompressibility constraint and utilizing continuous neural ordinary differential equations for tracking the evolution of the governing differential equation. We demonstrate this technique on a threedimensional, moderate Reynolds number turbulent fluid flow. In assessing the statistical quality and characteristics of the machinelearned model through rigorous diagnostic tests, we find that our model is capable of reconstructing the dynamics of the flow over large integral timescales, favoring accuracy at the larger length scales. More significantly, comprehensive diagnostics suggest that physically interpretable model parameters, corresponding to the representations of the fluid state and dynamics, have attributable and quantifiable impact on the quality of the model predictions and computational complexity.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In fluid physics, datadriven models to enhance or accelerate time to solution are becoming increasingly popular for many application domains, such as alternatives to turbulence closures, system surrogates, or for new physics discovery. In the context of reduced order models of highdimensional timedependent fluid systems, machine learning methods grant the benefit of automated learning from data, but the burden of a model lies on its reducedorder representation of both the fluid state and physical dynamics. In this work, we build a physicsconstrained, datadriven reduced order model for Navier–Stokes equations to approximate spatiotemporal fluid dynamics in the canonical case of isotropic turbulence in a triply periodic box. The model design choices mimic numerical and physical constraints by, for example, implicitly enforcing the incompressibility constraint and utilizing continuous neural ordinary differential equations for tracking the evolution of the governing differential equation. We demonstrate this technique on a threedimensional, moderate Reynolds number turbulent fluid flow. In assessing the statistical quality and characteristics of the machinelearned model through rigorous diagnostic tests, we find that our model is capable of reconstructing the dynamics of the flow over large integral timescales, favoring accuracy at the larger length scales. More significantly, comprehensive diagnostics suggest that physically interpretable model parameters, corresponding to the representations of the fluid state and dynamics, have attributable and quantifiable impact on the quality of the model predictions and computational complexity.
Validation and parameterization of a novel physicsconstrained neural dynamics model applied to turbulent fluid flow
10.1063/5.0122115
Physics of Fluids
20221104T12:50:56Z
© 2022 Author(s).
Varun Shankar
Gavin D. Portwood
Arvind T. Mohan
Peetak P. Mitra
Dilip Krishnamurthy
Christopher Rackauckas
Lucas A. Wilson
David P. Schmidt
Venkatasubramanian Viswanathan

Investigation of hemocompatibility and vortical structures for a centrifugal blood pump based on largeeddy simulation
https://aip.scitation.org/doi/10.1063/5.0117492?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The evolution of complex flow structures has a large impact on the hemocompatibility of the centrifugal blood pump. In this study, the hemodynamic performance and the hemocompatibility of a centrifugal blood pump are investigated based on largeeddy simulation (LES). Comparisons are conducted between the LES results and the results predicted by the renormalization group (RNG) [math] model and delayed detached eddy simulation (DDES) methods. The local trace criterion is utilized to analyze the vortical structures within the blood pump. Results show that the tip leakage vortex, the Taylor–Couette flow, and the flow separation are the most important flow structures in the blood pump. These structures have a significant influence on the hemodynamic performance and hemocompatibility. Quantitative comparison between the hemodynamic performance and the hemocompatibility is conducted between DDES, RNG [math], and LES results. Little difference is shown between DDES and LES results, while the RNG [math] model tends to underestimate the pressure and hemolysis due to adopting the steadystate approach, and the assumption of isotropy and equilibrium turbulence transport. In detail, the accuracy of RANS in predicting the strength of the main vortical structures is insufficient, which tends to underestimate the leakage vortex strength and overestimate the Taylor vortex strength. Furthermore, an analysis of the relationship between hemocompatibility and vortical structures indicates that the interaction between the boundary layer and the vortical structures, such as leakage vortex and Taylor vortex, induces more blood damage, while the blood damage caused by vortical structures in the mainstream is limited.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The evolution of complex flow structures has a large impact on the hemocompatibility of the centrifugal blood pump. In this study, the hemodynamic performance and the hemocompatibility of a centrifugal blood pump are investigated based on largeeddy simulation (LES). Comparisons are conducted between the LES results and the results predicted by the renormalization group (RNG) [math] model and delayed detached eddy simulation (DDES) methods. The local trace criterion is utilized to analyze the vortical structures within the blood pump. Results show that the tip leakage vortex, the Taylor–Couette flow, and the flow separation are the most important flow structures in the blood pump. These structures have a significant influence on the hemodynamic performance and hemocompatibility. Quantitative comparison between the hemodynamic performance and the hemocompatibility is conducted between DDES, RNG [math], and LES results. Little difference is shown between DDES and LES results, while the RNG [math] model tends to underestimate the pressure and hemolysis due to adopting the steadystate approach, and the assumption of isotropy and equilibrium turbulence transport. In detail, the accuracy of RANS in predicting the strength of the main vortical structures is insufficient, which tends to underestimate the leakage vortex strength and overestimate the Taylor vortex strength. Furthermore, an analysis of the relationship between hemocompatibility and vortical structures indicates that the interaction between the boundary layer and the vortical structures, such as leakage vortex and Taylor vortex, induces more blood damage, while the blood damage caused by vortical structures in the mainstream is limited.
Investigation of hemocompatibility and vortical structures for a centrifugal blood pump based on largeeddy simulation
10.1063/5.0117492
Physics of Fluids
20221104T01:20:11Z
© 2022 Author(s).
Yangwei Liu
Nan Xie
Yumeng Tang
Yan Zhang

A dynamic version of the improved delayed detachededdy simulation based on the differential Reynoldsstress model
https://aip.scitation.org/doi/10.1063/5.0119552?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A dynamic version of the improved delayed detachededdy simulation (IDDES) based on the differential Reynoldsstress model (RSM), referred to as the RSMDynIDDES, is developed by applying the dynamic Smagorinsky subgrid model to the large eddy simulation (LES) branch of the IDDES. The RSMDynIDDES simulates the periodic hills flow after a basic numerical validation for the decaying isotropic turbulence simulation. Wellpredicted velocity profiles and R eynolds stress distributions are obtained by the RSMDynIDDES in the periodic hills flow. The simulation results indicate that the RSMDynIDDES can capture more smallscale vortex structures in the LES region away from the wall than the original RSMbased IDDES (RSMIDDES). The RSMDynIDDES is also employed in simulating the transonic buffeting of a launch vehicle with a payload fairing. The numerical results have been compared with that of the RSMIDDES. It is found that the RSMDynIDDES can improve turbulence resolution in the offwall region while retaining the advantages of the original RSMIDDES in simulating the instability process of the free shear layer.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A dynamic version of the improved delayed detachededdy simulation (IDDES) based on the differential Reynoldsstress model (RSM), referred to as the RSMDynIDDES, is developed by applying the dynamic Smagorinsky subgrid model to the large eddy simulation (LES) branch of the IDDES. The RSMDynIDDES simulates the periodic hills flow after a basic numerical validation for the decaying isotropic turbulence simulation. Wellpredicted velocity profiles and R eynolds stress distributions are obtained by the RSMDynIDDES in the periodic hills flow. The simulation results indicate that the RSMDynIDDES can capture more smallscale vortex structures in the LES region away from the wall than the original RSMbased IDDES (RSMIDDES). The RSMDynIDDES is also employed in simulating the transonic buffeting of a launch vehicle with a payload fairing. The numerical results have been compared with that of the RSMIDDES. It is found that the RSMDynIDDES can improve turbulence resolution in the offwall region while retaining the advantages of the original RSMIDDES in simulating the instability process of the free shear layer.
A dynamic version of the improved delayed detachededdy simulation based on the differential Reynoldsstress model
10.1063/5.0119552
Physics of Fluids
20221104T01:20:15Z
© 2022 Author(s).

Air entrainment mechanism of chute bottom aerators in highspeed chute flows
https://aip.scitation.org/doi/10.1063/5.0121325?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In highhead dam projects, chute aerators are commonly applied as artificial aeration devices to prevent cavitation erosion damage in highspeed spillway and tunnel flows. These specific chute air–water flows are generated by jet lowersurface aeration above the bottom cavity and jet impact aeration on the bottom floor. These air entrainment processes determine the total air flux transport downstream of the chute flow. However, the differential air fluxes caused by the two different aeration mechanisms remain unclear. Based on physical model investigations, the two air entrained processes are observed by a highspeed camera, and detailed air flux data are measured in the aerator cavity and impact areas. The effects of the jet lowersurface disintegrating and then instantaneously reattaching to the chute floor are discussed. The measured air flux data indicate that the inertial jet impact entrainment dominates with increasing the Froude number, while the air flux proportion of the jet lowersurface aeration increases with the aerator height. A general prediction model for aerator cavity entrainment is proposed, considering the effects of the Froude number and jet lowersurface aeration. The scale effects of the coupling mechanism on chute aerator air entrainment highlight that aerator cavity air entrainment primarily depends on the inertial jet impact, and the main effects of aerator design on entrainment performance are manifested in the air cavity properties.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In highhead dam projects, chute aerators are commonly applied as artificial aeration devices to prevent cavitation erosion damage in highspeed spillway and tunnel flows. These specific chute air–water flows are generated by jet lowersurface aeration above the bottom cavity and jet impact aeration on the bottom floor. These air entrainment processes determine the total air flux transport downstream of the chute flow. However, the differential air fluxes caused by the two different aeration mechanisms remain unclear. Based on physical model investigations, the two air entrained processes are observed by a highspeed camera, and detailed air flux data are measured in the aerator cavity and impact areas. The effects of the jet lowersurface disintegrating and then instantaneously reattaching to the chute floor are discussed. The measured air flux data indicate that the inertial jet impact entrainment dominates with increasing the Froude number, while the air flux proportion of the jet lowersurface aeration increases with the aerator height. A general prediction model for aerator cavity entrainment is proposed, considering the effects of the Froude number and jet lowersurface aeration. The scale effects of the coupling mechanism on chute aerator air entrainment highlight that aerator cavity air entrainment primarily depends on the inertial jet impact, and the main effects of aerator design on entrainment performance are manifested in the air cavity properties.
Air entrainment mechanism of chute bottom aerators in highspeed chute flows
10.1063/5.0121325
Physics of Fluids
20221104T12:50:59Z
© 2022 Author(s).

Controlling flow separation over a curved ramp using vortex generator microjets
https://aip.scitation.org/doi/10.1063/5.0122831?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Introducing a fluid microjet into the boundary layer to increase fluid momentum and hence delay separation is a method for actively controlling a flow separation region. The present work numerically analyzed the control of a separation bubble behind a ramp. For this purpose, we first verified the steadystate numerical results for a flow (without a jet) over the ramp against reliable experimental studies from the literature. Next, the effects of introducing a microjet to the flow were also verified. A jet was then placed at three different distances above the ramp to study its impact on various parameters, including velocities, Reynolds stresses, pressure, vorticity, streamlines, and the separation bubble size. As the jet was moved further back, the jetinduced upwash region grew considerably. Finally, the effects of using three identical jets were studied and compared against those of a single jet. The results indicated that using a threejet array shrank the separation bubble. Using an array with d/D = 15 (distance between microjets over microjet diameter) can limit laterally the separation bubble about 2.75 times smaller than a single jet in the zdirection. Also, the employment of the jet managed to decrease the length of the separation zone in the xdirection up to 78%, in the case of Lx/L1 = 0.0143 (longitudinal distance of microjet from above the ramp over ramp length) and d/D = 10.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Introducing a fluid microjet into the boundary layer to increase fluid momentum and hence delay separation is a method for actively controlling a flow separation region. The present work numerically analyzed the control of a separation bubble behind a ramp. For this purpose, we first verified the steadystate numerical results for a flow (without a jet) over the ramp against reliable experimental studies from the literature. Next, the effects of introducing a microjet to the flow were also verified. A jet was then placed at three different distances above the ramp to study its impact on various parameters, including velocities, Reynolds stresses, pressure, vorticity, streamlines, and the separation bubble size. As the jet was moved further back, the jetinduced upwash region grew considerably. Finally, the effects of using three identical jets were studied and compared against those of a single jet. The results indicated that using a threejet array shrank the separation bubble. Using an array with d/D = 15 (distance between microjets over microjet diameter) can limit laterally the separation bubble about 2.75 times smaller than a single jet in the zdirection. Also, the employment of the jet managed to decrease the length of the separation zone in the xdirection up to 78%, in the case of Lx/L1 = 0.0143 (longitudinal distance of microjet from above the ramp over ramp length) and d/D = 10.
Controlling flow separation over a curved ramp using vortex generator microjets
10.1063/5.0122831
Physics of Fluids
20221104T12:22:22Z
© 2022 Author(s).
Mohammad Javad Pour Razzaghi
Yasin Masoumi
Seyed Mojtaba Rezaei Sani

Influence by the hub vortex on the instability of the tip vortices shed by propellers with and without winglets
https://aip.scitation.org/doi/10.1063/5.0122751?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Largeeddy simulations on a cylindrical grid consisting of 5 × 109 points are reported on both conventional and winglets propellers with and without a downstream shaft. Comparisons are focused on the influence by the hub vortex on the process of instability of the tip vortices. They demonstrate that in straight ahead conditions, this influence is actually quite limited for both propellers. The presence of the hub vortex at the wake core results in only a slight upstream shift of the instability of the tip vortices. Meanwhile, the development of the instability of the hub vortex is always delayed, compared to that of the tip vortices, and the former keeps coherent further downstream of their breakup. The results of this study highlight that the hub vortex is not a major source of instability of the tip vortices. Therefore, simplified configurations with no hub vortex, often adopted in the literature, can also provide a good approximation of the process of instability of the tip vortices shed by actual propellers. In contrast, the instability of the tip vortices could be the trigger of that of the hub vortex, whose development is slower. Therefore, experimental and computational studies aimed at analyzing the dynamics of the hub vortex should be designed accordingly, extending to further downstream distances.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Largeeddy simulations on a cylindrical grid consisting of 5 × 109 points are reported on both conventional and winglets propellers with and without a downstream shaft. Comparisons are focused on the influence by the hub vortex on the process of instability of the tip vortices. They demonstrate that in straight ahead conditions, this influence is actually quite limited for both propellers. The presence of the hub vortex at the wake core results in only a slight upstream shift of the instability of the tip vortices. Meanwhile, the development of the instability of the hub vortex is always delayed, compared to that of the tip vortices, and the former keeps coherent further downstream of their breakup. The results of this study highlight that the hub vortex is not a major source of instability of the tip vortices. Therefore, simplified configurations with no hub vortex, often adopted in the literature, can also provide a good approximation of the process of instability of the tip vortices shed by actual propellers. In contrast, the instability of the tip vortices could be the trigger of that of the hub vortex, whose development is slower. Therefore, experimental and computational studies aimed at analyzing the dynamics of the hub vortex should be designed accordingly, extending to further downstream distances.
Influence by the hub vortex on the instability of the tip vortices shed by propellers with and without winglets
10.1063/5.0122751
Physics of Fluids
20221104T12:50:40Z
© 2022 Author(s).
A. Posa
R. Broglia

Wallattached temperature structures in supersonic turbulent boundary layers
https://aip.scitation.org/doi/10.1063/5.0121900?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>It is well known that low and highspeed velocity streaks are statistically asymmetric. However, it is unclear how different the low and hightemperature structures (Tstructures) are even though they are strongly coupled with the streamwise velocity. Therefore, this paper identifies threedimensional wallattached temperature structures in supersonic turbulent boundary layers over cooled and heated walls (coming from direct numerical simulations) and separates them into positive and negative families. Wallattached Tstructures are selfsimilar; especially, the length and width of the positive family are linear functions of the height. The superposed temperature variance in both positive and negative families exhibits a logarithmic decay with the wall distance, while the superposed intensity of the wallnormal heat flux in the negative family shows a logarithmic growth. The modified strong Reynolds analogy proposed by Huang, Coleman, and Bradshaw [“Compressible turbulent channel flows: DNS results and modelling,” J. Fluid Mech. 305, 185–218 (1995)] is still valid in the negative family. The relative position between Tstructures of opposite signs depends on the wall temperature and that in the cooledwall case differs significantly from the relative position between low and highspeed streaks, especially those tall ones. In the cooledwall case, although positive temperature fluctuations below and above the maximum of the mean temperature can cluster to largescale wallattached structures, they are very likely dynamically unrelated.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>It is well known that low and highspeed velocity streaks are statistically asymmetric. However, it is unclear how different the low and hightemperature structures (Tstructures) are even though they are strongly coupled with the streamwise velocity. Therefore, this paper identifies threedimensional wallattached temperature structures in supersonic turbulent boundary layers over cooled and heated walls (coming from direct numerical simulations) and separates them into positive and negative families. Wallattached Tstructures are selfsimilar; especially, the length and width of the positive family are linear functions of the height. The superposed temperature variance in both positive and negative families exhibits a logarithmic decay with the wall distance, while the superposed intensity of the wallnormal heat flux in the negative family shows a logarithmic growth. The modified strong Reynolds analogy proposed by Huang, Coleman, and Bradshaw [“Compressible turbulent channel flows: DNS results and modelling,” J. Fluid Mech. 305, 185–218 (1995)] is still valid in the negative family. The relative position between Tstructures of opposite signs depends on the wall temperature and that in the cooledwall case differs significantly from the relative position between low and highspeed streaks, especially those tall ones. In the cooledwall case, although positive temperature fluctuations below and above the maximum of the mean temperature can cluster to largescale wallattached structures, they are very likely dynamically unrelated.
Wallattached temperature structures in supersonic turbulent boundary layers
10.1063/5.0121900
Physics of Fluids
20221107T12:49:20Z
© 2022 Author(s).

Large and smallscale characteristics in a temporally developing shearless turbulent mixing layer
https://aip.scitation.org/doi/10.1063/5.0121047?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Direct numerical simulation of a temporally developing shearless turbulent mixing layer is performed. Two quasihomogeneous isotropic turbulent (HIT) regions with different turbulent kinetic energies (TKEs) and a mixinglayer region temporally develop. The smallscale properties are analyzed with the velocity gradient tensor. The statistics on the velocity variances show that the development of the mixing layer is divided into two stages. In the first stage, grid turbulence in the largeTKE region has not fully developed, and the center of the mixing layer hardly moves. Largescale intermittency grows in the mixinglayer region at this stage. In the second stage, grid turbulence in the largeTKE region has fully developed, and the center of the mixing layer moves toward the smallTKE region. The smallscale intermittency is most significant in the mixinglayer region in both stages. The statistics on the velocity gradient tensor show that stronger vortex compression occurs more frequently in the mixinglayer region than in the quasiHIT regions at late times. In addition, the extensive and compressive eigenvalues of the rateofstrain tensor exhibit the strongest intermittency in the mixinglayer region at late times.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Direct numerical simulation of a temporally developing shearless turbulent mixing layer is performed. Two quasihomogeneous isotropic turbulent (HIT) regions with different turbulent kinetic energies (TKEs) and a mixinglayer region temporally develop. The smallscale properties are analyzed with the velocity gradient tensor. The statistics on the velocity variances show that the development of the mixing layer is divided into two stages. In the first stage, grid turbulence in the largeTKE region has not fully developed, and the center of the mixing layer hardly moves. Largescale intermittency grows in the mixinglayer region at this stage. In the second stage, grid turbulence in the largeTKE region has fully developed, and the center of the mixing layer moves toward the smallTKE region. The smallscale intermittency is most significant in the mixinglayer region in both stages. The statistics on the velocity gradient tensor show that stronger vortex compression occurs more frequently in the mixinglayer region than in the quasiHIT regions at late times. In addition, the extensive and compressive eigenvalues of the rateofstrain tensor exhibit the strongest intermittency in the mixinglayer region at late times.
Large and smallscale characteristics in a temporally developing shearless turbulent mixing layer
10.1063/5.0121047
Physics of Fluids
20221107T12:48:59Z
© 2022 Author(s).

Mean wake and aerodynamic forces for surfacemounted finiteheight square prisms of very small aspect ratio
https://aip.scitation.org/doi/10.1063/5.0123259?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The mean flow field, aerodynamic forces, bending moment and Strouhal number (St) were investigated for isolated surfacemounted finiteheight square prisms of very small aspect ratio (AR). The Reynolds number was Re [math] for the velocity measurements and [math] for the force, bending moment and St measurements. Prisms with AR = 0.5, 0.7 and 1 were considered, under two different boundary layer thicknesses of [math]–0.8 (thin) and [math] (thick). For both boundary layers, the mean drag force coefficient showed a sharper increase with AR compared with taller prisms, and the mean normal force coefficient increased smoothly, with a lower magnitude than pressurebased normal force coefficients. An approximately constant point of action of the drag force was found for AR < 1. While the thick boundary layer caused the spectral peaks to weaken and St to decrease, some periodicity was still found for all AR. These features were connected to the changes in the mean wake of the prisms with AR and [math]. A smaller AR and larger [math] had similar effects, causing the wake to shorten, the probability and type of reattachment of the flow on the free end to change, and the mean wake structure to transition from a streamwise wake vorticity pattern to an inner vorticity pattern. The prism with AR = 1 showed a dipole wake structure similar to that of taller prisms, while the unique wake topology of prisms with AR < 1 was found to be responsible for the different force and St trends identified in this range of AR.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The mean flow field, aerodynamic forces, bending moment and Strouhal number (St) were investigated for isolated surfacemounted finiteheight square prisms of very small aspect ratio (AR). The Reynolds number was Re [math] for the velocity measurements and [math] for the force, bending moment and St measurements. Prisms with AR = 0.5, 0.7 and 1 were considered, under two different boundary layer thicknesses of [math]–0.8 (thin) and [math] (thick). For both boundary layers, the mean drag force coefficient showed a sharper increase with AR compared with taller prisms, and the mean normal force coefficient increased smoothly, with a lower magnitude than pressurebased normal force coefficients. An approximately constant point of action of the drag force was found for AR < 1. While the thick boundary layer caused the spectral peaks to weaken and St to decrease, some periodicity was still found for all AR. These features were connected to the changes in the mean wake of the prisms with AR and [math]. A smaller AR and larger [math] had similar effects, causing the wake to shorten, the probability and type of reattachment of the flow on the free end to change, and the mean wake structure to transition from a streamwise wake vorticity pattern to an inner vorticity pattern. The prism with AR = 1 showed a dipole wake structure similar to that of taller prisms, while the unique wake topology of prisms with AR < 1 was found to be responsible for the different force and St trends identified in this range of AR.
Mean wake and aerodynamic forces for surfacemounted finiteheight square prisms of very small aspect ratio
10.1063/5.0123259
Physics of Fluids
20221107T12:48:36Z
© 2022 Author(s).
Barbara L. da Silva
Dylan G. H. Hahn
David Sumner
Donald J. Bergstrom

A new design for energysaving volutes in centrifugal pumps
https://aip.scitation.org/doi/10.1063/5.0122684?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Geometrical structures and operating performance of volutes used in centrifugal pumps are assessed in cross sections. By a volute, we mean the wetted part surrounding the impeller in a pump that converts the kinetic energy of water into potential energy. It is important to develop improved design principles and approaches for volutes to reduce hydraulic loss and increase the efficiency of pumps. However, when designing volutes, pump designers often neglect a key factor related to pump efficiency, namely, the energy loss due to the friction at the volute sidewall is directly proportional to the wetted perimeter of the cross section. In this paper, we show that the length of the wetted perimeter of the cross section is mathematically proportional to the friction loss in the volute. In addition, we present the design principles and calculation process to minimize the wetted perimeters of two kinds of cross section based on the velocity coefficient method using the same statistical data. The structures and shapes of the new cross sections are completely different from those of traditional volutes. Moreover, two different volutes with different cross sections are numerically investigated using a verified computational fluid dynamics technique. Both the head and the efficiency of the pump with the new volute are higher than those of a conventional pump under all working conditions. This paper provides a new design approach for the energysaving volutes of centrifugal pumps.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Geometrical structures and operating performance of volutes used in centrifugal pumps are assessed in cross sections. By a volute, we mean the wetted part surrounding the impeller in a pump that converts the kinetic energy of water into potential energy. It is important to develop improved design principles and approaches for volutes to reduce hydraulic loss and increase the efficiency of pumps. However, when designing volutes, pump designers often neglect a key factor related to pump efficiency, namely, the energy loss due to the friction at the volute sidewall is directly proportional to the wetted perimeter of the cross section. In this paper, we show that the length of the wetted perimeter of the cross section is mathematically proportional to the friction loss in the volute. In addition, we present the design principles and calculation process to minimize the wetted perimeters of two kinds of cross section based on the velocity coefficient method using the same statistical data. The structures and shapes of the new cross sections are completely different from those of traditional volutes. Moreover, two different volutes with different cross sections are numerically investigated using a verified computational fluid dynamics technique. Both the head and the efficiency of the pump with the new volute are higher than those of a conventional pump under all working conditions. This paper provides a new design approach for the energysaving volutes of centrifugal pumps.
A new design for energysaving volutes in centrifugal pumps
10.1063/5.0122684
Physics of Fluids
20221107T12:48:33Z
© 2022 Author(s).

Scaling patch analysis of planar turbulent mixing layers
https://aip.scitation.org/doi/10.1063/5.0122494?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Proper scales for the mean flow and Reynolds shear stress in planar turbulent mixing layers are determined from a scaling patch analysis of the mean continuity and momentum equations. By seeking an admissible scaling of the mean continuity equation, a proper scale for the mean transverse flow is determined as [math], where [math] is the growth rate of the mixing layer width and [math] is the difference between the velocity of the high speed stream Uh and the velocity of the low speed stream Ul. By seeking an admissible scaling for the mean momentum equation, a proper scale for the kinematic Reynolds shear stress is determined as [math], where [math] is the normalized velocity difference that emerges naturally in the admissible scaling of the mean momentum equation. Selfsimilar equations for the scaled mean transverse flow [math] and Reynolds shear stress [math] are derived from the mean continuity and mean momentum equations. Approximate equations for [math] and [math] are developed and found to agree well with experimental data.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Proper scales for the mean flow and Reynolds shear stress in planar turbulent mixing layers are determined from a scaling patch analysis of the mean continuity and momentum equations. By seeking an admissible scaling of the mean continuity equation, a proper scale for the mean transverse flow is determined as [math], where [math] is the growth rate of the mixing layer width and [math] is the difference between the velocity of the high speed stream Uh and the velocity of the low speed stream Ul. By seeking an admissible scaling for the mean momentum equation, a proper scale for the kinematic Reynolds shear stress is determined as [math], where [math] is the normalized velocity difference that emerges naturally in the admissible scaling of the mean momentum equation. Selfsimilar equations for the scaled mean transverse flow [math] and Reynolds shear stress [math] are derived from the mean continuity and mean momentum equations. Approximate equations for [math] and [math] are developed and found to agree well with experimental data.
Scaling patch analysis of planar turbulent mixing layers
10.1063/5.0122494
Physics of Fluids
20221107T12:49:02Z
© 2022 Author(s).
Tie Wei
Zhaorui Li
Daniel Livescu

Lagrangian wavelet analysis of turbulence modulation in particle–liquid mixing flows
https://aip.scitation.org/doi/10.1063/5.0127698?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A new experimental–theoretical framework has been developed to investigate turbulence and turbulence modulation in a twophase multicomponent particle–liquid flow in a mechanically agitated vessel. A discrete wavelet transform is used to decompose longterm threedimensional Lagrangian trajectories of flow phases, acquired by a technique of positron emission particle tracking, into their deterministic and stochastic subtrajectories. The subtrajectories are then used to construct the differentscale local velocity and turbulent kinetic energy (TKE) fields of the twophase flow. The effects of the particle size and size distribution mode (mono, binary, and polydisperse), particle concentration, impeller agitation speed, and pumping mode on turbulence intensity are investigated. Amongst these factors, the particle size, impeller pumping mode, and particle size distribution mode have a significant impact on liquid turbulence. The presence of large particles enhances liquid turbulence and broadens the region in the vessel characterized by high local TKE values. Results also show that a downpumping pitchedblade turbine generates significantly greater local maxima in the TKE field, which tend to be more localized in the impeller discharge stream. In addition, binary or polydisperse suspensions containing higher fractions of larger particles produce higher turbulence intensities in the carrier phase. The detailed information obtained on the turbulence intensity is crucial for better understanding of the dynamics of particle–liquid flows inside mixing vessels to aid the rational design of these units.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A new experimental–theoretical framework has been developed to investigate turbulence and turbulence modulation in a twophase multicomponent particle–liquid flow in a mechanically agitated vessel. A discrete wavelet transform is used to decompose longterm threedimensional Lagrangian trajectories of flow phases, acquired by a technique of positron emission particle tracking, into their deterministic and stochastic subtrajectories. The subtrajectories are then used to construct the differentscale local velocity and turbulent kinetic energy (TKE) fields of the twophase flow. The effects of the particle size and size distribution mode (mono, binary, and polydisperse), particle concentration, impeller agitation speed, and pumping mode on turbulence intensity are investigated. Amongst these factors, the particle size, impeller pumping mode, and particle size distribution mode have a significant impact on liquid turbulence. The presence of large particles enhances liquid turbulence and broadens the region in the vessel characterized by high local TKE values. Results also show that a downpumping pitchedblade turbine generates significantly greater local maxima in the TKE field, which tend to be more localized in the impeller discharge stream. In addition, binary or polydisperse suspensions containing higher fractions of larger particles produce higher turbulence intensities in the carrier phase. The detailed information obtained on the turbulence intensity is crucial for better understanding of the dynamics of particle–liquid flows inside mixing vessels to aid the rational design of these units.
Lagrangian wavelet analysis of turbulence modulation in particle–liquid mixing flows
10.1063/5.0127698
Physics of Fluids
20221107T12:48:39Z
© 2022 Author(s).
Chiya Savari
Mostafa Barigou

The local energy flux surrogate in turbulent openchannel flows
https://aip.scitation.org/doi/10.1063/5.0123888?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We present a local analysis of turbulence in openchannel flows, using timeseries velocity measurements. The method is based on a local form of the Kolmogorov “4/3law” of homogeneous turbulence for the thirdorder moment of velocity increments. Following the Duchon and Robert [“Inertial energy dissipation for weak solutions of incompressible Euler and Navier–Stokes equations,” Nonlinearity 13, 249 (2000)] idea, which envisions turbulence dissipation as a lack of smoothness of the Navier–Stokes solutions, we estimate the local energy flux in a laboratory experiment with natural bed flows. Taking advantage of onedimensional filtering techniques, under reasonable hypothesis, simple expressions of a surrogate of the energy flux are provided. The local energy flux surrogate reveals that, independently of the geometry, turbulence dissipation is highly intermittent. Among a variety of eddies that populate turbulence, dissipative singularities appear in sheetlike, tube, and filament structures, with large amplitude variations and rotations. This simplified technique can be applied to any measurement of hydrodynamic turbulence.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We present a local analysis of turbulence in openchannel flows, using timeseries velocity measurements. The method is based on a local form of the Kolmogorov “4/3law” of homogeneous turbulence for the thirdorder moment of velocity increments. Following the Duchon and Robert [“Inertial energy dissipation for weak solutions of incompressible Euler and Navier–Stokes equations,” Nonlinearity 13, 249 (2000)] idea, which envisions turbulence dissipation as a lack of smoothness of the Navier–Stokes solutions, we estimate the local energy flux in a laboratory experiment with natural bed flows. Taking advantage of onedimensional filtering techniques, under reasonable hypothesis, simple expressions of a surrogate of the energy flux are provided. The local energy flux surrogate reveals that, independently of the geometry, turbulence dissipation is highly intermittent. Among a variety of eddies that populate turbulence, dissipative singularities appear in sheetlike, tube, and filament structures, with large amplitude variations and rotations. This simplified technique can be applied to any measurement of hydrodynamic turbulence.
The local energy flux surrogate in turbulent openchannel flows
10.1063/5.0123888
Physics of Fluids
20221107T12:49:26Z
© 2022 Author(s).
S. Servidio
F. Coscarella
N. Penna
R. Gaudio

Numerical study on freespinning performance of oscillating water column Wells turbine in reciprocating airflows
https://aip.scitation.org/doi/10.1063/5.0122956?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The axialflow Wells turbine is one of the most widely used air turbines in oscillating water column wave energy converters. By Wells turbine, we mean a reaction air turbine developed by A. A. Wells of Queen's University Belfast in the late 1970s. A comprehensive understanding of its freespinning performance is crucial for determining control strategies for output power enhancement in practical engineering applications. In the present study, a threedimensional (3D) transient model was established on an ANSYSFluent® platform to simulate the timevarying flow field and motion state of the rotor during the freespinning process. After the model was validated with our experimental data, it was used to investigate the operation patterns in airflows with various profiles. The magnitude and phase features of the pressure difference and turbine torque were examined to identify the mechanism for overcoming the gradually ascending stage and maintaining dynamic equilibrium in the stable state. Additionally, the 3D flowfield details for several instants were demonstrated, including the severe vortex from the suction side in the poststall region, strong tip leakage vortex downstream of the rotor, downstream helical strip vortex, and cyclicasymmetric surface pressure distributions over the turbine. Furthermore, the effects of the cyclic volume flux on the freespinning performance were investigated.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The axialflow Wells turbine is one of the most widely used air turbines in oscillating water column wave energy converters. By Wells turbine, we mean a reaction air turbine developed by A. A. Wells of Queen's University Belfast in the late 1970s. A comprehensive understanding of its freespinning performance is crucial for determining control strategies for output power enhancement in practical engineering applications. In the present study, a threedimensional (3D) transient model was established on an ANSYSFluent® platform to simulate the timevarying flow field and motion state of the rotor during the freespinning process. After the model was validated with our experimental data, it was used to investigate the operation patterns in airflows with various profiles. The magnitude and phase features of the pressure difference and turbine torque were examined to identify the mechanism for overcoming the gradually ascending stage and maintaining dynamic equilibrium in the stable state. Additionally, the 3D flowfield details for several instants were demonstrated, including the severe vortex from the suction side in the poststall region, strong tip leakage vortex downstream of the rotor, downstream helical strip vortex, and cyclicasymmetric surface pressure distributions over the turbine. Furthermore, the effects of the cyclic volume flux on the freespinning performance were investigated.
Numerical study on freespinning performance of oscillating water column Wells turbine in reciprocating airflows
10.1063/5.0122956
Physics of Fluids
20221107T12:49:24Z
© 2022 Author(s).

Aspect ratio and combined installation in fluidic oscillatorcrossflow interactions
https://aip.scitation.org/doi/10.1063/5.0124033?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This paper describes a computational study of fluidic oscillatorcrossflow interactions. Fluidic oscillators are high potential active flow control devices, which offer promise for cooling and flow separation. Previous experimental studies have investigated the internal/external flow fields of fluidic oscillators. This improved the understanding of threedimensional flow fields; however, this has been limited to single aspect ratio nozzles in single variable installations. Research has not analyzed the effects of varying both oscillator parameters and installation conditions. Therefore, the present aim is to enhance flow field understanding by examining three aspect ratio (AR) nozzles in default perpendicular condition and in a “skewed incline” configuration (β = 60°, α = 75°). An unsteady Reynolds averaged Navier Stokes model was validated with prior experimental data and then used to evaluate timeaveraged and timeaccurate flow fields at three different blowing ratios. Strouhal number analysis uncovers the effectiveness of parametric changes. Internal flow fields are presented, identifying key differences and similarities. External flow field analysis in regard to spreading angles 55°–65° observes greater wallnormal penetration for lower AR oscillators with improved spanwise penetration for larger AR's. The combined installation produces stronger primary vortices, improving penetration along the wall, while inclination keeps vortices close to the wall. Oscillator effectiveness is dependent on the application.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This paper describes a computational study of fluidic oscillatorcrossflow interactions. Fluidic oscillators are high potential active flow control devices, which offer promise for cooling and flow separation. Previous experimental studies have investigated the internal/external flow fields of fluidic oscillators. This improved the understanding of threedimensional flow fields; however, this has been limited to single aspect ratio nozzles in single variable installations. Research has not analyzed the effects of varying both oscillator parameters and installation conditions. Therefore, the present aim is to enhance flow field understanding by examining three aspect ratio (AR) nozzles in default perpendicular condition and in a “skewed incline” configuration (β = 60°, α = 75°). An unsteady Reynolds averaged Navier Stokes model was validated with prior experimental data and then used to evaluate timeaveraged and timeaccurate flow fields at three different blowing ratios. Strouhal number analysis uncovers the effectiveness of parametric changes. Internal flow fields are presented, identifying key differences and similarities. External flow field analysis in regard to spreading angles 55°–65° observes greater wallnormal penetration for lower AR oscillators with improved spanwise penetration for larger AR's. The combined installation produces stronger primary vortices, improving penetration along the wall, while inclination keeps vortices close to the wall. Oscillator effectiveness is dependent on the application.
Aspect ratio and combined installation in fluidic oscillatorcrossflow interactions
10.1063/5.0124033
Physics of Fluids
20221108T02:10:35Z
© 2022 Author(s).
Oludamola P. Ayeni
Bjorn Cleton

Gridpoint and timestep requirements for largeeddy simulation and Reynoldsaveraged Navier–Stokes of stratified wakes
https://aip.scitation.org/doi/10.1063/5.0127487?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Estimates of gridpoint and timestep requirements exist for many canonical flows but not for stratified wakes. The purpose of this work is to fill in this gap. We apply the basic meshing principles and estimate the gridpoint and timestep requirements for Reynoldsaveraged Navier–Stokes (RANS) and largeeddy simulation (LES) of stratified wake flows at high Reynolds numbers, as arise in many geophysical, aircraft, and undersea vehicle systems. Scales representative of a submarine operating in a stably stratified ocean environment are considered, and the quantitative conclusions reached here can be adapted accordingly for particular applications. For a submarine, typical wake conditions are [math] and [math], and wakes extend to Nt = 1000, where Re0 and Fr0 are the initial Reynolds number and the internal Froude number of the wake, respectively, and N is the buoyancy frequency. We consider both spatially developing and temporally evolving wakes. We show that the grid points required for LES and RANS do not depend on the Reynolds number. The ratio of the grid points needed for LES and RANS is proportional to [math], where [math] marks the end of the late wake and the end of a computational fluid dynamics calculation. According to the present conservative estimates, [math] and [math] grid points are needed for LES and RANS of a spatially developing wake. The numbers are [math] and [math] for LES and RANS of a temporally evolving wake.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Estimates of gridpoint and timestep requirements exist for many canonical flows but not for stratified wakes. The purpose of this work is to fill in this gap. We apply the basic meshing principles and estimate the gridpoint and timestep requirements for Reynoldsaveraged Navier–Stokes (RANS) and largeeddy simulation (LES) of stratified wake flows at high Reynolds numbers, as arise in many geophysical, aircraft, and undersea vehicle systems. Scales representative of a submarine operating in a stably stratified ocean environment are considered, and the quantitative conclusions reached here can be adapted accordingly for particular applications. For a submarine, typical wake conditions are [math] and [math], and wakes extend to Nt = 1000, where Re0 and Fr0 are the initial Reynolds number and the internal Froude number of the wake, respectively, and N is the buoyancy frequency. We consider both spatially developing and temporally evolving wakes. We show that the grid points required for LES and RANS do not depend on the Reynolds number. The ratio of the grid points needed for LES and RANS is proportional to [math], where [math] marks the end of the late wake and the end of a computational fluid dynamics calculation. According to the present conservative estimates, [math] and [math] grid points are needed for LES and RANS of a spatially developing wake. The numbers are [math] and [math] for LES and RANS of a temporally evolving wake.
Gridpoint and timestep requirements for largeeddy simulation and Reynoldsaveraged Navier–Stokes of stratified wakes
10.1063/5.0127487
Physics of Fluids
20221108T01:21:41Z
© 2022 Author(s).
Robert F. Kunz

Effect of free stream turbulence in critical Reynolds number regime (1.6 × 105−6.1 × 105) on flow around circular cylinder
https://aip.scitation.org/doi/10.1063/5.0116754?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The flow around a cylinder is a classical aerodynamic problem involving the effect of Reynolds number (Re). Thus far, the impact of turbulence has not been fully clarified despite its important practical value in engineering applications. This study mainly investigates the influence of turbulence in the critical Re regime on the smooth and turbulent flows around a cylinder. The foregoing is accomplished by conducting static pressure measurement model experiments in the Re range of 1.6 × 105–6.1 × 105 and turbulence intensity range (Iu) of 5%–13%. Consequently, a series of useful results for engineering wind resistance design is obtained. The effect of turbulence on each subflow regime varies. The structural response to turbulence is more dangerous under conditions such as when the wind pressure coefficients fluctuate strongly and nonGaussianity is strong. The foregoing effects do not necessarily increase linearly with the turbulence intensity. In addition, the influence of turbulence evidently depends on the Re and corresponding flow regime. Therefore, turbulence must be cautiously managed in scaled model experiments and actual wind resistance design.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The flow around a cylinder is a classical aerodynamic problem involving the effect of Reynolds number (Re). Thus far, the impact of turbulence has not been fully clarified despite its important practical value in engineering applications. This study mainly investigates the influence of turbulence in the critical Re regime on the smooth and turbulent flows around a cylinder. The foregoing is accomplished by conducting static pressure measurement model experiments in the Re range of 1.6 × 105–6.1 × 105 and turbulence intensity range (Iu) of 5%–13%. Consequently, a series of useful results for engineering wind resistance design is obtained. The effect of turbulence on each subflow regime varies. The structural response to turbulence is more dangerous under conditions such as when the wind pressure coefficients fluctuate strongly and nonGaussianity is strong. The foregoing effects do not necessarily increase linearly with the turbulence intensity. In addition, the influence of turbulence evidently depends on the Re and corresponding flow regime. Therefore, turbulence must be cautiously managed in scaled model experiments and actual wind resistance design.
Effect of free stream turbulence in critical Reynolds number regime (1.6 × 105−6.1 × 105) on flow around circular cylinder
10.1063/5.0116754
Physics of Fluids
20221108T01:21:59Z
© 2022 Author(s).

Effects of thermal expansion on moderately intense turbulence in premixed flames
https://aip.scitation.org/doi/10.1063/5.0123211?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This study aims at analytically and numerically exploring the influence of combustioninduced thermal expansion on turbulence in premixed flames. In the theoretical part, contributions of solenoidal and potential velocity fluctuations to the unclosed component of the advection term in the Reynoldsaveraged Navier–Stokes equations are compared, and a new criterion for assessing the importance of the thermal expansion effects is introduced. The criterion highlights a ratio of the dilatation in the laminar flame to the largescale gradient of root mean square (rms) velocity in the turbulent flame brush. To support the theoretical study, direct numerical simulation (DNS) data obtained earlier from two complexchemistry, lean H2–air flames are analyzed. In line with the new criterion, even at sufficiently high Karlovitz numbers, the results show significant influence of combustioninduced potential velocity fluctuations on the second moments of the turbulent velocity upstream of and within the flame brush. In particular, the DNS data demonstrate that (i) potential and solenoidal rms velocities are comparable in the unburnt gas close to the leading edge of the flame brush and (ii) potential and solenoidal rms velocities conditioned to unburnt gas are comparable within the entire flame brush. Moreover, combustioninduced thermal expansion affects not only the potential velocity but even the solenoidal one. The latter effects manifest themselves in a negative correlation between solenoidal velocity fluctuations and dilatation or in the countergradient behavior of the solenoidal scalar flux. Finally, a turbulenceinpremixedflame diagram is sketched to discuss the influence of combustioninduced thermal expansion on various ranges of turbulence spectrum.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This study aims at analytically and numerically exploring the influence of combustioninduced thermal expansion on turbulence in premixed flames. In the theoretical part, contributions of solenoidal and potential velocity fluctuations to the unclosed component of the advection term in the Reynoldsaveraged Navier–Stokes equations are compared, and a new criterion for assessing the importance of the thermal expansion effects is introduced. The criterion highlights a ratio of the dilatation in the laminar flame to the largescale gradient of root mean square (rms) velocity in the turbulent flame brush. To support the theoretical study, direct numerical simulation (DNS) data obtained earlier from two complexchemistry, lean H2–air flames are analyzed. In line with the new criterion, even at sufficiently high Karlovitz numbers, the results show significant influence of combustioninduced potential velocity fluctuations on the second moments of the turbulent velocity upstream of and within the flame brush. In particular, the DNS data demonstrate that (i) potential and solenoidal rms velocities are comparable in the unburnt gas close to the leading edge of the flame brush and (ii) potential and solenoidal rms velocities conditioned to unburnt gas are comparable within the entire flame brush. Moreover, combustioninduced thermal expansion affects not only the potential velocity but even the solenoidal one. The latter effects manifest themselves in a negative correlation between solenoidal velocity fluctuations and dilatation or in the countergradient behavior of the solenoidal scalar flux. Finally, a turbulenceinpremixedflame diagram is sketched to discuss the influence of combustioninduced thermal expansion on various ranges of turbulence spectrum.
Effects of thermal expansion on moderately intense turbulence in premixed flames
10.1063/5.0123211
Physics of Fluids
20221108T01:21:34Z
© 2022 Author(s).
Vladimir A. Sabelnikov
Andrei N. Lipatnikov
Nikolay V. Nikitin
Francisco E. HernándezPérez
Hong G. Im

Visualization of the flow behind a backwardfacing step using a schlieren technique and tetrafluoroethane
https://aip.scitation.org/doi/10.1063/5.0120852?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This paper explores the possibility of visualizing the lowspeed, nearwall flow using a schlieren imaging technique. In this investigation, wind tunnel testing is conducted on a backwardfacing step at step Reynolds numbers of 2940, 5140, and 6120. Tetrafluoroethane is injected through a rectangular slot mounted flush to the wind tunnel floor and is introduced to the downstream region of the step as refractive tracers. The visualization acquired at the midsection suggests that the mass flux of tetrafluoroethane should be approximately 5%–10% lower than that of the freestream air to ensure that the injection is less intrusive to the flow field while still achieving the visibility required to characterize the flow field. For longexposure photography, the reattachment length or the length scale behind the step can be estimated when the exposure time of a camera is 3.2–12 times longer than the timescale of vortex rollup. For highspeed photography, the sequence of vortex shedding can be adequately visualized when the frame rate of a camera is at least four times higher than the vortex shedding frequency.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This paper explores the possibility of visualizing the lowspeed, nearwall flow using a schlieren imaging technique. In this investigation, wind tunnel testing is conducted on a backwardfacing step at step Reynolds numbers of 2940, 5140, and 6120. Tetrafluoroethane is injected through a rectangular slot mounted flush to the wind tunnel floor and is introduced to the downstream region of the step as refractive tracers. The visualization acquired at the midsection suggests that the mass flux of tetrafluoroethane should be approximately 5%–10% lower than that of the freestream air to ensure that the injection is less intrusive to the flow field while still achieving the visibility required to characterize the flow field. For longexposure photography, the reattachment length or the length scale behind the step can be estimated when the exposure time of a camera is 3.2–12 times longer than the timescale of vortex rollup. For highspeed photography, the sequence of vortex shedding can be adequately visualized when the frame rate of a camera is at least four times higher than the vortex shedding frequency.
Visualization of the flow behind a backwardfacing step using a schlieren technique and tetrafluoroethane
10.1063/5.0120852
Physics of Fluids
20221108T01:21:55Z
© 2022 Author(s).
Pititat Itsariyapinyo

Mean flow data assimilation based on physicsinformed neural networks
https://aip.scitation.org/doi/10.1063/5.0116218?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Physicsinformed neural networks (PINNs) can be used to solve partial differential equations (PDEs) and identify hidden variables by incorporating the governing equations into neural network training. In this study, we apply PINNs to the assimilation of turbulent mean flow data and investigate the method's ability to identify inaccessible variables and closure terms from sparse data. Using highfidelity largeeddy simulation data and particle image velocimetry measured mean fields, we show that PINNs are suitable for simultaneously identifying multiple missing quantities in turbulent flows and providing continuous and differentiable mean fields consistent with the provided PDEs. In this way, consistent and complete mean states can be provided, which are essential for linearized mean field methods. The presented method does not require a grid or discretization scheme, is easy to implement, and can be used for a wide range of applications, making it a very promising tool for mean fieldbased methods in fluid mechanics.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Physicsinformed neural networks (PINNs) can be used to solve partial differential equations (PDEs) and identify hidden variables by incorporating the governing equations into neural network training. In this study, we apply PINNs to the assimilation of turbulent mean flow data and investigate the method's ability to identify inaccessible variables and closure terms from sparse data. Using highfidelity largeeddy simulation data and particle image velocimetry measured mean fields, we show that PINNs are suitable for simultaneously identifying multiple missing quantities in turbulent flows and providing continuous and differentiable mean fields consistent with the provided PDEs. In this way, consistent and complete mean states can be provided, which are essential for linearized mean field methods. The presented method does not require a grid or discretization scheme, is easy to implement, and can be used for a wide range of applications, making it a very promising tool for mean fieldbased methods in fluid mechanics.
Mean flow data assimilation based on physicsinformed neural networks
10.1063/5.0116218
Physics of Fluids
20221109T12:03:44Z
© 2022 Author(s).
Jakob G. R. von Saldern
Johann Moritz Reumschüssel
Thomas L. Kaiser
Moritz Sieber
Kilian Oberleithner

A nonintrusive reduced order model with transformer neural network and its application
https://aip.scitation.org/doi/10.1063/5.0123185?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, a novel method to construct nonintrusive reduced order model (ROM) is proposed. The method is based on proper orthogonal decomposition and transformer neural network. Proper orthogonal decomposition is used to generate the basis functions of the lowdimensional flow field, and the coefficients are taken as lowdimensional flow field features. Transformer network is used to extract temporal feature relationships from lowdimensional features. Compared with recurrent neural network and convolutional neural network, transformer network can better capture flow dynamics. At online stage, the input temporal flow sequences are calculated in parallel and can effectively reduce online calculation time. The model proposed in this paper has been verified in two scenarios: twodimensional flow past a cylinder and twodimensional flow past a building group. Experimental results show that our model can better capture the flowing change details and has higher accuracy. Compared with the ROM based on long shortterm memory and temporal convolutional network, the prediction error is reduced by 35% and 60%, and the time cost is reduced by 65% and 60%. Finally, we apply the ROMs to a practical threedimensional complicated scenario, flow past London South Bank University, and discuss future development of ROMs.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, a novel method to construct nonintrusive reduced order model (ROM) is proposed. The method is based on proper orthogonal decomposition and transformer neural network. Proper orthogonal decomposition is used to generate the basis functions of the lowdimensional flow field, and the coefficients are taken as lowdimensional flow field features. Transformer network is used to extract temporal feature relationships from lowdimensional features. Compared with recurrent neural network and convolutional neural network, transformer network can better capture flow dynamics. At online stage, the input temporal flow sequences are calculated in parallel and can effectively reduce online calculation time. The model proposed in this paper has been verified in two scenarios: twodimensional flow past a cylinder and twodimensional flow past a building group. Experimental results show that our model can better capture the flowing change details and has higher accuracy. Compared with the ROM based on long shortterm memory and temporal convolutional network, the prediction error is reduced by 35% and 60%, and the time cost is reduced by 65% and 60%. Finally, we apply the ROMs to a practical threedimensional complicated scenario, flow past London South Bank University, and discuss future development of ROMs.
A nonintrusive reduced order model with transformer neural network and its application
10.1063/5.0123185
Physics of Fluids
20221109T12:03:40Z
© 2022 Author(s).
Pin Wu
Feng Qiu
Weibing Feng
Fangxing Fang
Christopher Pain

A novel algorithm for visualizing and quantifying vortices in complex 3D flows based on marching and converging vortex atoms
https://aip.scitation.org/doi/10.1063/5.0128611?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Numerous investigations aiming to reveal the underlying physics behind complex flows highlighted the important roles of vortices. This article proposes an integrative algorithm for visualizing and quantifying the vortices in threedimensional flows. The algorithm not only extracts the vortex centerlines but also returns the vortex radii and circulations varying along the centerlines. The novel aspect of this algorithm is to represent the vortex field as a collection of discrete vortex atoms. By iteratively updating the positions of these vortex atoms, the algorithm manipulates them into marching toward the underlying vortex centerlines. The radii and circulations varying along the centerlines are estimated based on the vortex atoms converged on the vortex centerlines. The accuracy and robustness of the algorithm are first accessed by numerical tests based on a synthetic vortex ring. Subsequently, the algorithm is employed to investigate the complex vortices in a turbulent boundary layer, validating the scaling law of the vortices reported in the literature. At last, the algorithm is applied to the threedimensional experimental data of the wake flow behind a wallmounted hemisphere. It concludes that the algorithm can be used as an effective tool for analyzing vortices in complex flows.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Numerous investigations aiming to reveal the underlying physics behind complex flows highlighted the important roles of vortices. This article proposes an integrative algorithm for visualizing and quantifying the vortices in threedimensional flows. The algorithm not only extracts the vortex centerlines but also returns the vortex radii and circulations varying along the centerlines. The novel aspect of this algorithm is to represent the vortex field as a collection of discrete vortex atoms. By iteratively updating the positions of these vortex atoms, the algorithm manipulates them into marching toward the underlying vortex centerlines. The radii and circulations varying along the centerlines are estimated based on the vortex atoms converged on the vortex centerlines. The accuracy and robustness of the algorithm are first accessed by numerical tests based on a synthetic vortex ring. Subsequently, the algorithm is employed to investigate the complex vortices in a turbulent boundary layer, validating the scaling law of the vortices reported in the literature. At last, the algorithm is applied to the threedimensional experimental data of the wake flow behind a wallmounted hemisphere. It concludes that the algorithm can be used as an effective tool for analyzing vortices in complex flows.
A novel algorithm for visualizing and quantifying vortices in complex 3D flows based on marching and converging vortex atoms
10.1063/5.0128611
Physics of Fluids
20221109T12:03:18Z
© 2022 Author(s).

Pfaffian, breather, and hybrid solutions for a (2 + 1)dimensional generalized nonlinear system in fluid mechanics and plasma physics
https://aip.scitation.org/doi/10.1063/5.0119516?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Fluid mechanics is seen as the study on the underlying mechanisms of liquids, gases and plasmas, and the forces on them. In this paper, we investigate a (2 + 1)dimensional generalized nonlinear system in fluid mechanics and plasma physics. By virtue of the Pfaffian technique, the Nthorder Pfaffian solutions are derived and proved, where N is a positive integer. Based on the Nthorder Pfaffian solutions, the first and secondorder breather solutions are obtained. In addition, Ytype and Xtype breather solutions are constructed. Furthermore, we investigate the influence of the coefficients in the system on those breathers as follows: The locations and periods of those breathers are related to δ1, δ2, δ3, δ4, and δ5, where δc's [math] are the constant coefficients in the system. Moreover, hybrid solutions composed of the breathers and solitons are derived. Interactions between the Y/Xtype breather and Ytype soliton are illustrated graphically, respectively. Then, we show the influence of the coefficients in the system on the interactions between the Y/Xtype breather and Ytype soliton.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Fluid mechanics is seen as the study on the underlying mechanisms of liquids, gases and plasmas, and the forces on them. In this paper, we investigate a (2 + 1)dimensional generalized nonlinear system in fluid mechanics and plasma physics. By virtue of the Pfaffian technique, the Nthorder Pfaffian solutions are derived and proved, where N is a positive integer. Based on the Nthorder Pfaffian solutions, the first and secondorder breather solutions are obtained. In addition, Ytype and Xtype breather solutions are constructed. Furthermore, we investigate the influence of the coefficients in the system on those breathers as follows: The locations and periods of those breathers are related to δ1, δ2, δ3, δ4, and δ5, where δc's [math] are the constant coefficients in the system. Moreover, hybrid solutions composed of the breathers and solitons are derived. Interactions between the Y/Xtype breather and Ytype soliton are illustrated graphically, respectively. Then, we show the influence of the coefficients in the system on the interactions between the Y/Xtype breather and Ytype soliton.
Pfaffian, breather, and hybrid solutions for a (2 + 1)dimensional generalized nonlinear system in fluid mechanics and plasma physics
10.1063/5.0119516
Physics of Fluids
20221109T12:03:12Z
© 2022 Author(s).

Thermal analysis of baffle jetting in fuel rod assembly
https://aip.scitation.org/doi/10.1063/5.0109255?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Baffle jetting plays a significant role when it comes to safe operation of nuclear power plants. The baffle jetting phenomenon is the generation of horizontal flow impingement on fuel/control rods during the outward flow of the primary coolant into a nuclear reactor. To understand the flow and heat transfer characteristics under the baffle jetting conditions, large eddy simulations (LES) of flow around a [math] fuel rod assembly were conducted. Three Reynolds numbers based on jet width and inlet velocity were considered [math], [math], and [math]. A temperature difference of [math]°C between the inlet fluid and the heated rods was considered to analyze the heat transfer characteristics within the assembly under baffle jetting. Various flow parameters were computed such as pressure coefficients along different rods, mean and fluctuating forces, Strouhal number, local and averaged Nusselt numbers. LES results were validated against experimental measurements and other numerical data. It was observed that the effect of the baffle jet was more significant on the first streamwise row of rods with the stagnation points at the lower part of these rods. Furthermore, the averaged Nusselt number was found to be higher on rods in the streamwise direction of the jet, rather than at other locations.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Baffle jetting plays a significant role when it comes to safe operation of nuclear power plants. The baffle jetting phenomenon is the generation of horizontal flow impingement on fuel/control rods during the outward flow of the primary coolant into a nuclear reactor. To understand the flow and heat transfer characteristics under the baffle jetting conditions, large eddy simulations (LES) of flow around a [math] fuel rod assembly were conducted. Three Reynolds numbers based on jet width and inlet velocity were considered [math], [math], and [math]. A temperature difference of [math]°C between the inlet fluid and the heated rods was considered to analyze the heat transfer characteristics within the assembly under baffle jetting. Various flow parameters were computed such as pressure coefficients along different rods, mean and fluctuating forces, Strouhal number, local and averaged Nusselt numbers. LES results were validated against experimental measurements and other numerical data. It was observed that the effect of the baffle jet was more significant on the first streamwise row of rods with the stagnation points at the lower part of these rods. Furthermore, the averaged Nusselt number was found to be higher on rods in the streamwise direction of the jet, rather than at other locations.
Thermal analysis of baffle jetting in fuel rod assembly
10.1063/5.0109255
Physics of Fluids
20221110T12:18:13Z
© 2022 Author(s).
Mohamed Ali
Ahmed K. Alkaabi
Saeed A. Alameri
Imran Afgan

The effect of surface roughness on the Lagrangian coherent structures in turbulent Rayleigh–Bénard convection
https://aip.scitation.org/doi/10.1063/5.0103755?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We perform direct numerical simulations of turbulent Rayleigh–Bénard (RB) convection in a closed square cell with roughness plates at Rayleigh number fixed at [math] and the Prandtl number fixed at Pr = 1. To gain insight into the effect of surface roughness on material transport in turbulent Rayleigh–Bénard convection, the Lagrangian coherent structures (LCSs) are extracted using the finitetime Lyapunov exponent method in the cases of different roughness heights. First, we find that lobe structures are widely present in RB convection and we elucidate how they play a part in transporting heat from conerflow rolls to largescale circulation. Then, we quantify the heat flux along the LCSs, which contributes to 80% of the total flux. This implies that the LCSs play an important role in heat transport regardless of the roughness height. Furthermore, two different mechanisms of heat transport in RB convection induced by roughness heights are explained in the Lagrangian perspective: the decrease in Nu number in the cases of [math] is caused by the LCSs between the roughness elements which hinders the exchange of material between the fluid in the cavity and the bulk region; whereas, the increase in Nu number in the case of [math] is produced by the enhanced mixing events of the convection that enhance the contribution of heat transport in the bulk region.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We perform direct numerical simulations of turbulent Rayleigh–Bénard (RB) convection in a closed square cell with roughness plates at Rayleigh number fixed at [math] and the Prandtl number fixed at Pr = 1. To gain insight into the effect of surface roughness on material transport in turbulent Rayleigh–Bénard convection, the Lagrangian coherent structures (LCSs) are extracted using the finitetime Lyapunov exponent method in the cases of different roughness heights. First, we find that lobe structures are widely present in RB convection and we elucidate how they play a part in transporting heat from conerflow rolls to largescale circulation. Then, we quantify the heat flux along the LCSs, which contributes to 80% of the total flux. This implies that the LCSs play an important role in heat transport regardless of the roughness height. Furthermore, two different mechanisms of heat transport in RB convection induced by roughness heights are explained in the Lagrangian perspective: the decrease in Nu number in the cases of [math] is caused by the LCSs between the roughness elements which hinders the exchange of material between the fluid in the cavity and the bulk region; whereas, the increase in Nu number in the case of [math] is produced by the enhanced mixing events of the convection that enhance the contribution of heat transport in the bulk region.
The effect of surface roughness on the Lagrangian coherent structures in turbulent Rayleigh–Bénard convection
10.1063/5.0103755
Physics of Fluids
20221110T12:18:10Z
© 2022 Author(s).
Hang Cheng
Hao Jiang
Kai Leong Chong
Quan Zhou
Yulu Liu
Zhiming Lu

An efficient model for subgridscale velocity enrichment for largeeddy simulations of turbulent flows
https://aip.scitation.org/doi/10.1063/5.0127231?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In some applications of largeeddy simulation (LES), in addition to providing a closure model for the subgridscale stress tensor, it is necessary to also provide means to approximate the subgridscale velocity field. In this work, we derive a new model for the subgridscale velocity that can be used in such LES applications. The model consists in solving a linearized form of the momentum equation for the subgridscale velocity using a truncated Fourierseries approach. Solving within a structured grid of statistically homogeneous subdomains enables the treatment of inhomogeneous problems. It is shown that the generated subgridscale velocity emulates key properties of turbulent flows, such as the right kinetic energy spectrum, realistic strain–rotation relations, and intermittency. The model is also shown to predict the correct inhomogeneous and anisotropic velocity statistics in unbounded flows. The computational costs of the model are still of the same order as the costs of the LES.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In some applications of largeeddy simulation (LES), in addition to providing a closure model for the subgridscale stress tensor, it is necessary to also provide means to approximate the subgridscale velocity field. In this work, we derive a new model for the subgridscale velocity that can be used in such LES applications. The model consists in solving a linearized form of the momentum equation for the subgridscale velocity using a truncated Fourierseries approach. Solving within a structured grid of statistically homogeneous subdomains enables the treatment of inhomogeneous problems. It is shown that the generated subgridscale velocity emulates key properties of turbulent flows, such as the right kinetic energy spectrum, realistic strain–rotation relations, and intermittency. The model is also shown to predict the correct inhomogeneous and anisotropic velocity statistics in unbounded flows. The computational costs of the model are still of the same order as the costs of the LES.
An efficient model for subgridscale velocity enrichment for largeeddy simulations of turbulent flows
10.1063/5.0127231
Physics of Fluids
20221110T12:17:59Z
© 2022 Author(s).
M. Hausmann
F. Evrard
B. van Wachem

The dynamics of cylinderwake/boundarylayer interaction revealed by turbulent transports
https://aip.scitation.org/doi/10.1063/5.0111483?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The flow past a cylinder near a plane wall for small gap ratios ([math], and 0.9) and fixed ReD = 1000 is numerically studied. The fundamental flow features are characterized by the instantaneous and mean fields. Then, the dynamics of cylinderwake/boundarylayer interaction are revealed by the turbulent momentum transport and kinetic energy production. The turbulent fluctuations caused by the secondary vortex (SV) (at [math], 0.9) and the novel tertiary vortex (TV) (at [math]) can be observed in the distributions of Reynolds stresses. For [math] and [math], the wake/boundarylayer interaction is dominated by ejection and sweep events, which are related to the generation of the hairpin vortex. These two bursting events lead to the momentum transport between the high and lowspeed sides. For [math], the ejection event is not found in the interaction region because the head of the hairpin vortex is entrained into the wake. The upper roller (RU) helps to transport highmomentum fluid toward the wall in this case, although it does not take part in the interaction directly. The shedding of RU, the lower roller (RL), SV (at [math] and 0.9), and KH (Kelvin–Helmholtz) vortex (at [math]) and the generation of the hairpin vortex are crucial to turbulent kinetic energy (TKE) production. The RU, KH vortex, and SV transfer [math] out to [math] and [math] resulting redistribution of the TKE. While RL, surviving for a shorter time, transfers [math] out to [math] and [math], helping explain why it disappears quickly, TV only transfers out [math] out to [math], and its TKE comes from other terms rather than the production term. The redistribution of TKE due to the generation of the hairpin vortex can result in the slower growth rate of the secondary disturbance growth stage, promoting the wall boundary layer transition.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The flow past a cylinder near a plane wall for small gap ratios ([math], and 0.9) and fixed ReD = 1000 is numerically studied. The fundamental flow features are characterized by the instantaneous and mean fields. Then, the dynamics of cylinderwake/boundarylayer interaction are revealed by the turbulent momentum transport and kinetic energy production. The turbulent fluctuations caused by the secondary vortex (SV) (at [math], 0.9) and the novel tertiary vortex (TV) (at [math]) can be observed in the distributions of Reynolds stresses. For [math] and [math], the wake/boundarylayer interaction is dominated by ejection and sweep events, which are related to the generation of the hairpin vortex. These two bursting events lead to the momentum transport between the high and lowspeed sides. For [math], the ejection event is not found in the interaction region because the head of the hairpin vortex is entrained into the wake. The upper roller (RU) helps to transport highmomentum fluid toward the wall in this case, although it does not take part in the interaction directly. The shedding of RU, the lower roller (RL), SV (at [math] and 0.9), and KH (Kelvin–Helmholtz) vortex (at [math]) and the generation of the hairpin vortex are crucial to turbulent kinetic energy (TKE) production. The RU, KH vortex, and SV transfer [math] out to [math] and [math] resulting redistribution of the TKE. While RL, surviving for a shorter time, transfers [math] out to [math] and [math], helping explain why it disappears quickly, TV only transfers out [math] out to [math], and its TKE comes from other terms rather than the production term. The redistribution of TKE due to the generation of the hairpin vortex can result in the slower growth rate of the secondary disturbance growth stage, promoting the wall boundary layer transition.
The dynamics of cylinderwake/boundarylayer interaction revealed by turbulent transports
10.1063/5.0111483
Physics of Fluids
20221111T12:53:49Z
© 2022 Author(s).

Detection of an internal solitary wave by the underwater vehicle based on machine learning
https://aip.scitation.org/doi/10.1063/5.0123365?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A new hydrodynamic artificial intelligence detection method is proposed to realize the accurate detection of internal solitary waves (ISWs) by the underwater vehicle. Two deep convolution neural network structures are established to predict the relative position between the underwater vehicle and ISW and the flow field around the underwater vehicle. By combining field observation data and the computational fluid dynamics method, accurate numerical simulation of the motion of the underwater vehicle in a real ISW environment is achieved. The training process for the neural network is implemented by building a dataset from the above results. It is shown that the position prediction accuracy of the network for ISW is larger than 95%. For the prediction of the flow field around the underwater vehicle, it is found that the addition of the convolutional block attention module can increase the prediction accuracy. Moreover, the reduction of the number of sensors by the dynamic mode decomposition method and kmeans clustering method is realized. The accuracy can still reach 92% even when the number of sensors is reduced. This study is the first to use hydrodynamic signals for the detection of ISW, which can enhance the navigation safety of underwater vehicles.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A new hydrodynamic artificial intelligence detection method is proposed to realize the accurate detection of internal solitary waves (ISWs) by the underwater vehicle. Two deep convolution neural network structures are established to predict the relative position between the underwater vehicle and ISW and the flow field around the underwater vehicle. By combining field observation data and the computational fluid dynamics method, accurate numerical simulation of the motion of the underwater vehicle in a real ISW environment is achieved. The training process for the neural network is implemented by building a dataset from the above results. It is shown that the position prediction accuracy of the network for ISW is larger than 95%. For the prediction of the flow field around the underwater vehicle, it is found that the addition of the convolutional block attention module can increase the prediction accuracy. Moreover, the reduction of the number of sensors by the dynamic mode decomposition method and kmeans clustering method is realized. The accuracy can still reach 92% even when the number of sensors is reduced. This study is the first to use hydrodynamic signals for the detection of ISW, which can enhance the navigation safety of underwater vehicles.
Detection of an internal solitary wave by the underwater vehicle based on machine learning
10.1063/5.0123365
Physics of Fluids
20221111T12:55:23Z
© 2022 Author(s).

Unsupervised deep learning of spatial organizations of coherent structures in a turbulent channel flow
https://aip.scitation.org/doi/10.1063/5.0123555?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We examined the capability of an unsupervised deep learning network to capture the spatial organizations of largescale structures in a crossstream plane of a fully developed turbulent channel flow at [math]. For this purpose, a generative adversarial network (GAN) is trained using the instantaneous flow fields in the crossstream plane obtained by a direct numerical simulation (DNS) to generate similar flow fields. Then, these flow fields are examined by focusing on the turbulent statistics and the spatial organizations of coherent structures. We extracted the intense regions of the streamwise velocity fluctuations (u) and the vortical structures in the crossstream plane. Comparing the DNS and GAN flow fields, it is revealed that the network not only presents the onepoint and twopoint statistics quite accurately but also successfully predicts the structural characteristics hidden in the training dataset. We further explored the meandering motions of largescale u structures by measuring their waviness in the crossstream plane. It is shown that as the size of the u structures increases, they exhibit more aggressive waviness behavior which in turn increases the average number of vortical structures surrounding the lowmomentum structures. The success of GAN in this study suggests its potential to predict similar information at a high Reynolds number and, thus, be utilized as an inflow turbulence generator to provide instantaneous boundary conditions for more complicated problems, such as turbulent boundary layers. This has the potential to greatly reduce the computational costs of DNS related to a required large computational domain at high Reynolds numbers.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We examined the capability of an unsupervised deep learning network to capture the spatial organizations of largescale structures in a crossstream plane of a fully developed turbulent channel flow at [math]. For this purpose, a generative adversarial network (GAN) is trained using the instantaneous flow fields in the crossstream plane obtained by a direct numerical simulation (DNS) to generate similar flow fields. Then, these flow fields are examined by focusing on the turbulent statistics and the spatial organizations of coherent structures. We extracted the intense regions of the streamwise velocity fluctuations (u) and the vortical structures in the crossstream plane. Comparing the DNS and GAN flow fields, it is revealed that the network not only presents the onepoint and twopoint statistics quite accurately but also successfully predicts the structural characteristics hidden in the training dataset. We further explored the meandering motions of largescale u structures by measuring their waviness in the crossstream plane. It is shown that as the size of the u structures increases, they exhibit more aggressive waviness behavior which in turn increases the average number of vortical structures surrounding the lowmomentum structures. The success of GAN in this study suggests its potential to predict similar information at a high Reynolds number and, thus, be utilized as an inflow turbulence generator to provide instantaneous boundary conditions for more complicated problems, such as turbulent boundary layers. This has the potential to greatly reduce the computational costs of DNS related to a required large computational domain at high Reynolds numbers.
Unsupervised deep learning of spatial organizations of coherent structures in a turbulent channel flow
10.1063/5.0123555
Physics of Fluids
20221111T12:55:20Z
© 2022 Author(s).
Mohammad Javad Sayyari

On the effect of flow regime and pore structure on the flow signatures in porous media
https://aip.scitation.org/doi/10.1063/5.0120201?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this study, lattice Boltzmann method (LBM) is utilized for threedimensional simulation of fluid flow through two porous structures, consisting of grains with the same diameter: (i) a homogeneous porous domain, in which the grains are placed with a simple cubic packing configuration, and (ii) a randomly packed porous domain. An ultrafine mesh size is considered to perform the simulations in three orders of magnitude of Reynolds number ([math]), covering laminar to turbulent flow regimes, and capture different flow signatures. Pore velocity fields are derived, and their sample probability density functions (PDF) are analyzed vs time to investigate the dynamics of the flow. The analysis of the PDFs clearly shows that stagnant zones play a significant role in the formation of the pore flow fields, manifested by multimodal PDFs, and the distribution of the velocities in porous media at various [math] cannot be characterized by a single PDF model regardless of the pore structure. While the velocities at the stagnant regions and in the vicinity of the solid boundaries are primarily affected by the viscous forces and exhibit a powerlaw PDF at different [math], the velocities in the main (preferential) flow pathways away from the boundaries are shown to be influenced by the inertial forces, hence having an exponential PDF when [math] is low. At high [math], however, depending on the tortuosity of the porous structure, the velocities may exhibit an exponential or even Laplace PDF.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this study, lattice Boltzmann method (LBM) is utilized for threedimensional simulation of fluid flow through two porous structures, consisting of grains with the same diameter: (i) a homogeneous porous domain, in which the grains are placed with a simple cubic packing configuration, and (ii) a randomly packed porous domain. An ultrafine mesh size is considered to perform the simulations in three orders of magnitude of Reynolds number ([math]), covering laminar to turbulent flow regimes, and capture different flow signatures. Pore velocity fields are derived, and their sample probability density functions (PDF) are analyzed vs time to investigate the dynamics of the flow. The analysis of the PDFs clearly shows that stagnant zones play a significant role in the formation of the pore flow fields, manifested by multimodal PDFs, and the distribution of the velocities in porous media at various [math] cannot be characterized by a single PDF model regardless of the pore structure. While the velocities at the stagnant regions and in the vicinity of the solid boundaries are primarily affected by the viscous forces and exhibit a powerlaw PDF at different [math], the velocities in the main (preferential) flow pathways away from the boundaries are shown to be influenced by the inertial forces, hence having an exponential PDF when [math] is low. At high [math], however, depending on the tortuosity of the porous structure, the velocities may exhibit an exponential or even Laplace PDF.
On the effect of flow regime and pore structure on the flow signatures in porous media
10.1063/5.0120201
Physics of Fluids
20221111T12:57:39Z
© 2022 Author(s).
Mehrdad Vasheghani Farahani
Mohaddeseh Mousavi Nezhad

Generation of turbulent inflow data from realistic approximations of the covariance tensor
https://aip.scitation.org/doi/10.1063/5.0106664?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This study presents a novel synthetic inflow generator capable of producing a random field matching a realistic set of twopoint statistics with minimal input. The method is based on two main elements. The first element is a procedure to infer realistic twopoint covariance tensors from readily available data (e.g., freestream velocity, boundary layer thickness, and turbulence intensity) by a preliminary Reynoldsaveraged Navier–Stokes simulation with an explicit algebraic Reynolds stress model closure. The second element is an efficient eigendecomposition step of the twopoint correlation tensor, which determines a set of modes. The modal decomposition guarantees the spatial correlation in transversal directions, while the temporal correlation/streamwise spatial correlation is obtained by digital filters based on longitudinal and transversal spectra of a realistic shape and Taylor's hypothesis. The instantaneous inlet flow field is obtained by a linear combination of the modes via uncorrelated random weights with unit variance. The modes are generated in a computationally inexpensive preprocessing step. Compared to existing inflow generation methods that try to match given twopoint statistics, the proposed method relieves the burden of obtaining data from direct numerical simulation (DNS) or experiments, while the complexity of the eigenvalue problem that needs to be solved is reduced. The proposed method is shown to produce a realistic turbulent channel flow and a realistic turbulent boundary layer by the largeeddy simulation, which contains statistics that are in good agreement with results from DNS. The proposed inflow generator features, costeffectiveness, robustness, and potential for generalization to complex geometries.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This study presents a novel synthetic inflow generator capable of producing a random field matching a realistic set of twopoint statistics with minimal input. The method is based on two main elements. The first element is a procedure to infer realistic twopoint covariance tensors from readily available data (e.g., freestream velocity, boundary layer thickness, and turbulence intensity) by a preliminary Reynoldsaveraged Navier–Stokes simulation with an explicit algebraic Reynolds stress model closure. The second element is an efficient eigendecomposition step of the twopoint correlation tensor, which determines a set of modes. The modal decomposition guarantees the spatial correlation in transversal directions, while the temporal correlation/streamwise spatial correlation is obtained by digital filters based on longitudinal and transversal spectra of a realistic shape and Taylor's hypothesis. The instantaneous inlet flow field is obtained by a linear combination of the modes via uncorrelated random weights with unit variance. The modes are generated in a computationally inexpensive preprocessing step. Compared to existing inflow generation methods that try to match given twopoint statistics, the proposed method relieves the burden of obtaining data from direct numerical simulation (DNS) or experiments, while the complexity of the eigenvalue problem that needs to be solved is reduced. The proposed method is shown to produce a realistic turbulent channel flow and a realistic turbulent boundary layer by the largeeddy simulation, which contains statistics that are in good agreement with results from DNS. The proposed inflow generator features, costeffectiveness, robustness, and potential for generalization to complex geometries.
Generation of turbulent inflow data from realistic approximations of the covariance tensor
10.1063/5.0106664
Physics of Fluids
20221115T10:30:49Z
© 2022 Author(s).
Joshua HopeCollins
Luca di Mare

Flow characteristics in nonconventional combustion chamber configurations
https://aip.scitation.org/doi/10.1063/5.0122770?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Flow characteristics in combustion chambers of several noncircular crosssectional configurations were numerically investigated. Different crosssectional configurations, including annular and multisector annulus configurations with different fillets, were proposed, examined, and compared to the conventional circular arrangement. A numerical model was established and validated against other findings. The model was then used to study the effects of different design parameters, including convex and fillet radii, as well as some operational parameters, such as the angular velocity on the total pressure drop and the secondary flow characteristics like the swirl number, secondary vortex intensity, and helicity. The results revealed the optimum number of annulus sectors to be six, at which the highest hydraulic diameter was achieved for the same crosssectional area. Optimum values of the inner and outer corner fillets were also obtained. Several valve arrangements were also investigated to achieve the highest possible valve area ratio for the proposed configurations. One of the proposed configurations, which incorporates six annular chambers with filleted corners with a hydraulic diameter ratio of 0.991 compared to the conventional circular design, denoted AFC4, achieved a more compact design while maintaining very comparable flow characteristics to those of the circular design. Although it needs more thorough investigations, it could present a viable alternative design option for the combustion chambers.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Flow characteristics in combustion chambers of several noncircular crosssectional configurations were numerically investigated. Different crosssectional configurations, including annular and multisector annulus configurations with different fillets, were proposed, examined, and compared to the conventional circular arrangement. A numerical model was established and validated against other findings. The model was then used to study the effects of different design parameters, including convex and fillet radii, as well as some operational parameters, such as the angular velocity on the total pressure drop and the secondary flow characteristics like the swirl number, secondary vortex intensity, and helicity. The results revealed the optimum number of annulus sectors to be six, at which the highest hydraulic diameter was achieved for the same crosssectional area. Optimum values of the inner and outer corner fillets were also obtained. Several valve arrangements were also investigated to achieve the highest possible valve area ratio for the proposed configurations. One of the proposed configurations, which incorporates six annular chambers with filleted corners with a hydraulic diameter ratio of 0.991 compared to the conventional circular design, denoted AFC4, achieved a more compact design while maintaining very comparable flow characteristics to those of the circular design. Although it needs more thorough investigations, it could present a viable alternative design option for the combustion chambers.
Flow characteristics in nonconventional combustion chamber configurations
10.1063/5.0122770
Physics of Fluids
20221115T12:22:36Z
© 2022 Author(s).
Moustafa M. Amer
Mahmoud A. Shouman
Khairy F. Megalaa
Mohamed S. Salem

Autoencoderassisted analysis of amplitude and wavelength modulation of nearwall turbulence by outer largescale structures in channel flow at friction Reynolds number of 5200
https://aip.scitation.org/doi/10.1063/5.0123119?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This paper reports a novel methodology that allows the intensity of, and the underlying mechanism for, the amplitude and lengthscale modulation (amplification or attenuation) of turbulent stresses in the inner layer of a channel flow at [math] to be clarified. A unique aspect of the present framework is the use of an autoencoder algorithm to separate fullvolume extremely large direct numerical simulation (DNS) fields into largescale and smallscale motions. This approach is adopted in preference to the empirical mode decomposition (EMD) previously used by the present authors at the lower Reynolds number, [math], because resource requirements posed by the EMD quickly become untenable due to the extremely large DNS dataset and the large solution box needed to capture the wide spectrum of scales at the present Reynolds number. A second original element is a formalism that derived the modulation, conditional on largescale fluctuations, from continuous statistical quantities represented as multivariablejoint probabilitydensity functions, thus obviating the need for any discrete representation or binning beyond that imposed by the discrete DNS solution. A third novel aspect is the use of the lengthscalewise derivative of the secondorder structure function to quantify the modulation (increase or decrease) in the length scale, again conditional on largescale structures. Apart from illuminating the modulation itself, the study examined the validity of the quasisteady hypothesis that proposes that the nearwall turbulence is universal when scaled by the spatially and temporally varying largescale wall shear stress rather than its time average.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This paper reports a novel methodology that allows the intensity of, and the underlying mechanism for, the amplitude and lengthscale modulation (amplification or attenuation) of turbulent stresses in the inner layer of a channel flow at [math] to be clarified. A unique aspect of the present framework is the use of an autoencoder algorithm to separate fullvolume extremely large direct numerical simulation (DNS) fields into largescale and smallscale motions. This approach is adopted in preference to the empirical mode decomposition (EMD) previously used by the present authors at the lower Reynolds number, [math], because resource requirements posed by the EMD quickly become untenable due to the extremely large DNS dataset and the large solution box needed to capture the wide spectrum of scales at the present Reynolds number. A second original element is a formalism that derived the modulation, conditional on largescale fluctuations, from continuous statistical quantities represented as multivariablejoint probabilitydensity functions, thus obviating the need for any discrete representation or binning beyond that imposed by the discrete DNS solution. A third novel aspect is the use of the lengthscalewise derivative of the secondorder structure function to quantify the modulation (increase or decrease) in the length scale, again conditional on largescale structures. Apart from illuminating the modulation itself, the study examined the validity of the quasisteady hypothesis that proposes that the nearwall turbulence is universal when scaled by the spatially and temporally varying largescale wall shear stress rather than its time average.
Autoencoderassisted analysis of amplitude and wavelength modulation of nearwall turbulence by outer largescale structures in channel flow at friction Reynolds number of 5200
10.1063/5.0123119
Physics of Fluids
20221115T10:30:40Z
© 2022 Author(s).
L. Agostini
M. Leschziner

Numerical investigation and parametric analysis of an attached eddy model applied to inlet condition
https://aip.scitation.org/doi/10.1063/5.0122737?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Generating a realistic turbulent field at the inflow is of great importance as well as a complex challenge for largeeddy simulation. As a new synthetic turbulence method, the attached eddy model (AEM) was initially proposed by Townsend, where the velocity field is induced by a hierarchy of randomly distributed Λshape eddies by using the Biot–Savart law. Although extensive research has theoretically proved the existence and effectiveness of AEM, there have been a few numerical investigations on its practical applications. In this paper, the AEM method is implemented in an opensource software code_saturne to generate inlet conditions. The AEM generation process is detailed and described by defining various parameters. The new model is then applied to turbulent channel flows with [math] = 180, 395, and 590, respectively. The results are compared with the direct numerical simulation to validate its ability to accurately predict the velocity and turbulent kinetic energy profiles. It is also compared with the simulation by using the synthetic eddy method to assess its potential to improve the drop process of the wall shear stress. AEM is shown to be efficient in developing turbulent kinetic energy in the nearwall region. Furthermore, a parametric analysis of the present model is carried out to discuss in detail the specific effect of each factor. This parametric analysis shows the impact of different model settings on the velocity fluctuation. These results are expected to inspire future practical applications of AEM.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Generating a realistic turbulent field at the inflow is of great importance as well as a complex challenge for largeeddy simulation. As a new synthetic turbulence method, the attached eddy model (AEM) was initially proposed by Townsend, where the velocity field is induced by a hierarchy of randomly distributed Λshape eddies by using the Biot–Savart law. Although extensive research has theoretically proved the existence and effectiveness of AEM, there have been a few numerical investigations on its practical applications. In this paper, the AEM method is implemented in an opensource software code_saturne to generate inlet conditions. The AEM generation process is detailed and described by defining various parameters. The new model is then applied to turbulent channel flows with [math] = 180, 395, and 590, respectively. The results are compared with the direct numerical simulation to validate its ability to accurately predict the velocity and turbulent kinetic energy profiles. It is also compared with the simulation by using the synthetic eddy method to assess its potential to improve the drop process of the wall shear stress. AEM is shown to be efficient in developing turbulent kinetic energy in the nearwall region. Furthermore, a parametric analysis of the present model is carried out to discuss in detail the specific effect of each factor. This parametric analysis shows the impact of different model settings on the velocity fluctuation. These results are expected to inspire future practical applications of AEM.
Numerical investigation and parametric analysis of an attached eddy model applied to inlet condition
10.1063/5.0122737
Physics of Fluids
20221115T10:30:46Z
© 2022 Author(s).

Investigation on the lubrication characteristics of lowviscosity lubricated microgrooved bearings considering turbulence and misalignment
https://aip.scitation.org/doi/10.1063/5.0127398?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This paper numerically investigates the effect of turbulence and journal misalignment on the lubrication characteristics of microgrooved bearings with lowviscosity lubricant. The generalized average Reynolds equation satisfying the mass conservation cavitation algorithm is developed by integrating the average flow model proposed by Patir and Cheng, the Ng–Pan turbulent model, and the P[math] model proposed by Elrod and Adams. With this model, the finite difference method is used in the numerical procedure. Moreover, the mathematical models of microgrooves with different bottom shapes, that is, rectangle, isosceles triangle, left triangle, and right triangle, are given. The validity of the proposed model is verified by the comparisons with the published literature. Based on numerical simulation, the minimum film thickness, eccentricity ratio, attitude angle, maximum film pressure, friction torque, misalignment moment, film thickness, and pressure distributions under different external loads, rotational speeds, radial clearances, misalignment angles, and microgroove parameters between models with and without turbulence and misalignment are comparatively analyzed. The numerical results reveal that turbulence may occur under heavy external load, high rotational speed, and large radius clearance. Concurrently, turbulence increases the minimum fluid film thickness and attitude angle, decreases the eccentricity ratio and friction torque, and enhances the bearing capacity. Furthermore, the larger misalignment angle results in the smaller minimum film thickness, eccentricity ratio and attitude angle, and the larger maximum film pressure, misalignment moment, and axial tilt of film pressure. Numerical simulations can provide theoretical guidance for the optimization of the geometrical parameters of microgrooved bearings.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This paper numerically investigates the effect of turbulence and journal misalignment on the lubrication characteristics of microgrooved bearings with lowviscosity lubricant. The generalized average Reynolds equation satisfying the mass conservation cavitation algorithm is developed by integrating the average flow model proposed by Patir and Cheng, the Ng–Pan turbulent model, and the P[math] model proposed by Elrod and Adams. With this model, the finite difference method is used in the numerical procedure. Moreover, the mathematical models of microgrooves with different bottom shapes, that is, rectangle, isosceles triangle, left triangle, and right triangle, are given. The validity of the proposed model is verified by the comparisons with the published literature. Based on numerical simulation, the minimum film thickness, eccentricity ratio, attitude angle, maximum film pressure, friction torque, misalignment moment, film thickness, and pressure distributions under different external loads, rotational speeds, radial clearances, misalignment angles, and microgroove parameters between models with and without turbulence and misalignment are comparatively analyzed. The numerical results reveal that turbulence may occur under heavy external load, high rotational speed, and large radius clearance. Concurrently, turbulence increases the minimum fluid film thickness and attitude angle, decreases the eccentricity ratio and friction torque, and enhances the bearing capacity. Furthermore, the larger misalignment angle results in the smaller minimum film thickness, eccentricity ratio and attitude angle, and the larger maximum film pressure, misalignment moment, and axial tilt of film pressure. Numerical simulations can provide theoretical guidance for the optimization of the geometrical parameters of microgrooved bearings.
Investigation on the lubrication characteristics of lowviscosity lubricated microgrooved bearings considering turbulence and misalignment
10.1063/5.0127398
Physics of Fluids
20221116T10:41:22Z
© 2022 Author(s).

Mechanistic study on modification of convective internal boundary layer by spanwise surface motion
https://aip.scitation.org/doi/10.1063/5.0124832?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Modification of a convective internal boundary layer (IBL) by spanwise motion of a warm surface is investigated by imposing different surface moving speeds in the present study. Our analysis shows that the spanwise surface motion reduces the Reynolds shear stress right after the increase in the surface temperature in the convective IBL. The maximum decreasing rate of the Reynolds shear stress is found to be approximately 75% at the largest moving speed of the warm surface considered in the manuscript. Due to the reduction of the Reynolds shear stress, the vertical momentum transport is fundamentally altered, and the mean flow accelerates immediately after the increase in the surface temperature. By scrutinizing the instantaneous and conditional averaged flow fields as well as the premultiplied energy spectra, we have attributed the reduction of the Reynolds shear stress to the suppression of the nearsurface velocity streaks and quasistreamwise vortices, and the delayed growth of the convective structures, such as thermal plumes. Our investigation suggests that the developments of the convective IBL can be influenced by a strong spanwise motion of the warm surface, which should be taken into consideration in the prediction model for practical applications.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Modification of a convective internal boundary layer (IBL) by spanwise motion of a warm surface is investigated by imposing different surface moving speeds in the present study. Our analysis shows that the spanwise surface motion reduces the Reynolds shear stress right after the increase in the surface temperature in the convective IBL. The maximum decreasing rate of the Reynolds shear stress is found to be approximately 75% at the largest moving speed of the warm surface considered in the manuscript. Due to the reduction of the Reynolds shear stress, the vertical momentum transport is fundamentally altered, and the mean flow accelerates immediately after the increase in the surface temperature. By scrutinizing the instantaneous and conditional averaged flow fields as well as the premultiplied energy spectra, we have attributed the reduction of the Reynolds shear stress to the suppression of the nearsurface velocity streaks and quasistreamwise vortices, and the delayed growth of the convective structures, such as thermal plumes. Our investigation suggests that the developments of the convective IBL can be influenced by a strong spanwise motion of the warm surface, which should be taken into consideration in the prediction model for practical applications.
Mechanistic study on modification of convective internal boundary layer by spanwise surface motion
10.1063/5.0124832
Physics of Fluids
20221116T10:41:32Z
© 2022 Author(s).

Drag reduction of blowingbased active control in a turbulent boundary layer
https://aip.scitation.org/doi/10.1063/5.0123451?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Direct numerical simulations are conducted to gain insight into the blowingbased active control in a spatially developing turbulent boundary layer at a low Reynolds number. The drag reduction properties and mechanisms of different blowing velocity distribution forms under constant wallnormal mass flux are comparatively studied, including uniform blowing and blowingonly opposition control (BOOC). After the application of blowing control, the selfsimilarity of the Reynolds shear stress is influenced. The property of drag reduction and control gain of the blowingbased active control schemes in the turbulent boundary layer is similar to that in turbulent channel flow, i.e., the BOOC scheme can achieve higher drag reduction than uniform blowing, but the control gain reduces. Due to the coexistence of the opposition effect and the induction effect, the negative wallnormal velocity fluctuations accompanied by the sweep motion are induced to form smallscale flow structures in the nearwall region. The decomposition of the skinfriction drag coefficient shows that the changes of each contribution term are basically the same for different blowing schemes, except that the BOOC scheme has a more substantial influence on mean convection and spatial development. According to the property that the drag reduction of the BOOC scheme with additional threshold limitation is equivalent to that without the restriction, it can be determined that the effect of blowingbased active control is mainly based on the temporal and spatial averaging effects of blowing, including the opposition effect and the induction effect.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Direct numerical simulations are conducted to gain insight into the blowingbased active control in a spatially developing turbulent boundary layer at a low Reynolds number. The drag reduction properties and mechanisms of different blowing velocity distribution forms under constant wallnormal mass flux are comparatively studied, including uniform blowing and blowingonly opposition control (BOOC). After the application of blowing control, the selfsimilarity of the Reynolds shear stress is influenced. The property of drag reduction and control gain of the blowingbased active control schemes in the turbulent boundary layer is similar to that in turbulent channel flow, i.e., the BOOC scheme can achieve higher drag reduction than uniform blowing, but the control gain reduces. Due to the coexistence of the opposition effect and the induction effect, the negative wallnormal velocity fluctuations accompanied by the sweep motion are induced to form smallscale flow structures in the nearwall region. The decomposition of the skinfriction drag coefficient shows that the changes of each contribution term are basically the same for different blowing schemes, except that the BOOC scheme has a more substantial influence on mean convection and spatial development. According to the property that the drag reduction of the BOOC scheme with additional threshold limitation is equivalent to that without the restriction, it can be determined that the effect of blowingbased active control is mainly based on the temporal and spatial averaging effects of blowing, including the opposition effect and the induction effect.
Drag reduction of blowingbased active control in a turbulent boundary layer
10.1063/5.0123451
Physics of Fluids
20221116T10:41:19Z
© 2022 Author(s).

Investigation on corner stall prediction and flow control by blended blade and end wall technology in a compressor cascade based on modified Spalart–Allmaras model
https://aip.scitation.org/doi/10.1063/5.0123788?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Corner stall has a significant impact on the performance of compressor cascades, but it is difficult to predict precisely using conventional Reynoldsaveraged Navier–Stokes models. In view of this, first, the Spalart–Allmaras (SA) turbulence model modified with helicity is recalibrated to predict corner stall accurately. The internal reasons why the modified SA model does not overestimate the extent and intensity of corner stall as the original SA model is further explored through the analysis of turbulence transport nature. The investigation of corner stall control in a modified National Advisory Committee for Aeronautics 65 cascade by the blended blade and end wall (BBEW) technology is then carried out using the recalibrated MSA model. The numerical results indicate that the BBEW technology can eliminate the separation vortex on the end wall and change the flow field from corner stall to corner separation. The best BBEW scheme reduces the total pressure loss coefficient by 14.13%. The BBEW technology can most significantly enhance the aerodynamic performance of the compressor cascade when the maximum BBEW thickness is close to the trailing edge. When the maximum BBEW thickness is in the same position, the control effect rises first and subsequently falls as the maximum BBEW thickness grows. These research results serve as a guide for choosing turbulence models and designing the BBEW schemes.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Corner stall has a significant impact on the performance of compressor cascades, but it is difficult to predict precisely using conventional Reynoldsaveraged Navier–Stokes models. In view of this, first, the Spalart–Allmaras (SA) turbulence model modified with helicity is recalibrated to predict corner stall accurately. The internal reasons why the modified SA model does not overestimate the extent and intensity of corner stall as the original SA model is further explored through the analysis of turbulence transport nature. The investigation of corner stall control in a modified National Advisory Committee for Aeronautics 65 cascade by the blended blade and end wall (BBEW) technology is then carried out using the recalibrated MSA model. The numerical results indicate that the BBEW technology can eliminate the separation vortex on the end wall and change the flow field from corner stall to corner separation. The best BBEW scheme reduces the total pressure loss coefficient by 14.13%. The BBEW technology can most significantly enhance the aerodynamic performance of the compressor cascade when the maximum BBEW thickness is close to the trailing edge. When the maximum BBEW thickness is in the same position, the control effect rises first and subsequently falls as the maximum BBEW thickness grows. These research results serve as a guide for choosing turbulence models and designing the BBEW schemes.
Investigation on corner stall prediction and flow control by blended blade and end wall technology in a compressor cascade based on modified Spalart–Allmaras model
10.1063/5.0123788
Physics of Fluids
20221116T10:41:43Z
© 2022 Author(s).

Investigation on the hump region generation mechanism of pump mode in lowhead pumped hydrostorage unit
https://aip.scitation.org/doi/10.1063/5.0130836?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The pump mode of the lowhead pumped hydrostorage unit (pumpturbine) may operate in the hump region under extreme conditions due to the influence of water level variation, and the resulting energy conversion instability will seriously threaten the safety of the unit. However, the generation mechanism of the hump region is still not sufficiently understood, which is mainly due to two reasons: the dominant unstable flow structures that induce the formation of the hump region have not been uniformly recognized, and the influence of the dominant unstable flow structures on the impeller's working capacity has not been effectively revealed. In this study, experiments and numerical simulations were carried out on the lowhead pumped hydrostorage unit in the pump mode, and the following results were obtained. It is found that the dominant unstable flow structures that induce the formation of the hump region are the leading edge backflow on the blade inlet shroud side and the hornlike vortex on the blade outlet hub side. The leading edge backflow reduces the blade loading and limits the impeller's working capacity, and the hornlike vortex increases the blade loading and increases the impeller's working capacity. The analysis revealed that the formation of the hump region is the result of the mutual restriction of the hornlike vortex and the leading edge backflow.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The pump mode of the lowhead pumped hydrostorage unit (pumpturbine) may operate in the hump region under extreme conditions due to the influence of water level variation, and the resulting energy conversion instability will seriously threaten the safety of the unit. However, the generation mechanism of the hump region is still not sufficiently understood, which is mainly due to two reasons: the dominant unstable flow structures that induce the formation of the hump region have not been uniformly recognized, and the influence of the dominant unstable flow structures on the impeller's working capacity has not been effectively revealed. In this study, experiments and numerical simulations were carried out on the lowhead pumped hydrostorage unit in the pump mode, and the following results were obtained. It is found that the dominant unstable flow structures that induce the formation of the hump region are the leading edge backflow on the blade inlet shroud side and the hornlike vortex on the blade outlet hub side. The leading edge backflow reduces the blade loading and limits the impeller's working capacity, and the hornlike vortex increases the blade loading and increases the impeller's working capacity. The analysis revealed that the formation of the hump region is the result of the mutual restriction of the hornlike vortex and the leading edge backflow.
Investigation on the hump region generation mechanism of pump mode in lowhead pumped hydrostorage unit
10.1063/5.0130836
Physics of Fluids
20221117T12:25:45Z
© 2022 Author(s).

Insight into the flow dynamics of a high shear injector equipped with centerbody: Suppression of precessing vortex core oscillations
https://aip.scitation.org/doi/10.1063/5.0131385?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A precessing vortex core (PVC) is a selfexcited helical instability that results from the precession of the vortex core around the flow axis in the upstream region of a vortex breakdown bubble. PVC oscillation in a swirl flowbased combustor aids the thermoacoustic instability that results in hardware damage and poor emission characteristics of the engine. The PVC oscillation can be suppressed intermittently or absolutely in the high shear injectorbased combustor with proper design and placement of the fuel nozzle in the injector. A high shear injector is an arrangement of two radial swirlers in general, namely, primary and secondary swirlers, equipped with a fuel nozzle at its center to deliver the fuel. In this study, we examine the impact of the placement of the fuel nozzle/centerbody and its design over the dynamics of PVC oscillations in a nonreacting flow in a counterrotating swirler/high shear injector. Timeresolved highspeed (@ 5 kHz) stereoscopic particle image velocimetry measurements are conducted to elucidate the dynamics of PVC and other coherent structures. Spectral proper orthogonal decomposition of the velocity field data shows that fuel nozzle flushing with the base of the primary swirler has the most robust PVC oscillation that subsequently gets intermittent or suppressed by placing the centerbody of diameters, Dc = 7, 9, and 11 mm at constant upstream mass flow rate. The results show that the centerbody with the end face flushing to the exit plane would be helpful to avoid PVC with proper selection of the centerbody diameter.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A precessing vortex core (PVC) is a selfexcited helical instability that results from the precession of the vortex core around the flow axis in the upstream region of a vortex breakdown bubble. PVC oscillation in a swirl flowbased combustor aids the thermoacoustic instability that results in hardware damage and poor emission characteristics of the engine. The PVC oscillation can be suppressed intermittently or absolutely in the high shear injectorbased combustor with proper design and placement of the fuel nozzle in the injector. A high shear injector is an arrangement of two radial swirlers in general, namely, primary and secondary swirlers, equipped with a fuel nozzle at its center to deliver the fuel. In this study, we examine the impact of the placement of the fuel nozzle/centerbody and its design over the dynamics of PVC oscillations in a nonreacting flow in a counterrotating swirler/high shear injector. Timeresolved highspeed (@ 5 kHz) stereoscopic particle image velocimetry measurements are conducted to elucidate the dynamics of PVC and other coherent structures. Spectral proper orthogonal decomposition of the velocity field data shows that fuel nozzle flushing with the base of the primary swirler has the most robust PVC oscillation that subsequently gets intermittent or suppressed by placing the centerbody of diameters, Dc = 7, 9, and 11 mm at constant upstream mass flow rate. The results show that the centerbody with the end face flushing to the exit plane would be helpful to avoid PVC with proper selection of the centerbody diameter.
Insight into the flow dynamics of a high shear injector equipped with centerbody: Suppression of precessing vortex core oscillations
10.1063/5.0131385
Physics of Fluids
20221117T12:25:32Z
© 2022 Author(s).
Sonu Kumar
Saptarshi Basu

On the turbulent boundary layer over a flat plate at moderate Reynolds numbers
https://aip.scitation.org/doi/10.1063/5.0124498?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Two separate experimental campaigns of a spatially developing turbulent boundary layer under approximately zeropressuregradient at moderate Reynolds numbers ([math]) are conducted with stereoscopic Particle Image Velocimetry (PIV) and one component Hot Wire Anemometry. This range of Reynolds numbers is found to be of particular interest for turbulent boundary layer control investigations. The motivations behind this work rely on the lack of recent studies that provide a rigorous experimental database on a flat plate turbulent boundary layer, openly available online. This is critical as, in most of the cases, the modification of the statistics resulting from turbulent boundary layer control strategies are compared with a smooth baseline reference. The statistics of the velocity fields, obtained with the two techniques, show a good match with the direct numerical simulation in literature results. We focused on the skin friction evaluation by means of Clauser's chart technique. The near wall turbulence activity and the associated coherent structures are investigated by means of the Variable Interval Time Averaging technique using the hot wire signal. The influence of the acquisition and algorithm parameters as well as the effect of the Reynolds number are reported. The logarithmic and outer structures are investigated by applying the Uniform Momentum Zones technique to the PIV dataset. The hierarchical distribution of the uniform momentum zones as a function of the wall distance as well as their variation with the Reynolds number confirm the validity of the attached eddy model even at the moderate Reynolds numbers of the current investigation.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Two separate experimental campaigns of a spatially developing turbulent boundary layer under approximately zeropressuregradient at moderate Reynolds numbers ([math]) are conducted with stereoscopic Particle Image Velocimetry (PIV) and one component Hot Wire Anemometry. This range of Reynolds numbers is found to be of particular interest for turbulent boundary layer control investigations. The motivations behind this work rely on the lack of recent studies that provide a rigorous experimental database on a flat plate turbulent boundary layer, openly available online. This is critical as, in most of the cases, the modification of the statistics resulting from turbulent boundary layer control strategies are compared with a smooth baseline reference. The statistics of the velocity fields, obtained with the two techniques, show a good match with the direct numerical simulation in literature results. We focused on the skin friction evaluation by means of Clauser's chart technique. The near wall turbulence activity and the associated coherent structures are investigated by means of the Variable Interval Time Averaging technique using the hot wire signal. The influence of the acquisition and algorithm parameters as well as the effect of the Reynolds number are reported. The logarithmic and outer structures are investigated by applying the Uniform Momentum Zones technique to the PIV dataset. The hierarchical distribution of the uniform momentum zones as a function of the wall distance as well as their variation with the Reynolds number confirm the validity of the attached eddy model even at the moderate Reynolds numbers of the current investigation.
On the turbulent boundary layer over a flat plate at moderate Reynolds numbers
10.1063/5.0124498
Physics of Fluids
20221118T11:14:32Z
© 2022 Author(s).
Francesco Scarano
Marc C. Jacob
Xavier Carbonneau
Erwin R. Gowree

An efficient discrete unified gaskinetic scheme for compressible turbulence
https://aip.scitation.org/doi/10.1063/5.0120490?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, we develop an efficient Boltzmannequationbased mesoscopic approach to simulate threedimensional (3D) compressible turbulence, using reduced Gauss–Hermite quadrature (GHQ) orders by redefining the second distribution in terms of the total energy in the double distribution function approach. This allows the use of two sets of 3D offlattice discrete particle velocity models, namely, a 27 discrete velocity model of the seventhorder GHQ accuracy (D3V27A7) combined with a 13 discrete velocity model of the fifthorder GHQ accuracy (D3V13A5), to achieve full consistency with the Navier–Stokes–Fourier system. The source terms in the Boltzmann–Bhatnagar–Gross–Krook system are designed to adjust both the Prandtl number and bulktoshear viscosity ratio. Compressible decaying homogeneous isotropic turbulence (DHIT) is simulated at low and moderate turbulent Mach numbers to validate our code. It is observed that the simulation results are in good agreement with those in the existing literatures. Furthermore, the terms in the transport equation of turbulent kinetic energy are analyzed in detail, to illustrate four different transient stages from the initial random flow field to the developed DHIT. It is shown that the transient pressuredilatation transfer happens rapidly, while the smallscale vortical structures take a longer time to establish physically. Compared to the existing literatures, our approach represents the most efficient mesoscopic scheme for compressible turbulence under the double distribution function formulation.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, we develop an efficient Boltzmannequationbased mesoscopic approach to simulate threedimensional (3D) compressible turbulence, using reduced Gauss–Hermite quadrature (GHQ) orders by redefining the second distribution in terms of the total energy in the double distribution function approach. This allows the use of two sets of 3D offlattice discrete particle velocity models, namely, a 27 discrete velocity model of the seventhorder GHQ accuracy (D3V27A7) combined with a 13 discrete velocity model of the fifthorder GHQ accuracy (D3V13A5), to achieve full consistency with the Navier–Stokes–Fourier system. The source terms in the Boltzmann–Bhatnagar–Gross–Krook system are designed to adjust both the Prandtl number and bulktoshear viscosity ratio. Compressible decaying homogeneous isotropic turbulence (DHIT) is simulated at low and moderate turbulent Mach numbers to validate our code. It is observed that the simulation results are in good agreement with those in the existing literatures. Furthermore, the terms in the transport equation of turbulent kinetic energy are analyzed in detail, to illustrate four different transient stages from the initial random flow field to the developed DHIT. It is shown that the transient pressuredilatation transfer happens rapidly, while the smallscale vortical structures take a longer time to establish physically. Compared to the existing literatures, our approach represents the most efficient mesoscopic scheme for compressible turbulence under the double distribution function formulation.
An efficient discrete unified gaskinetic scheme for compressible turbulence
10.1063/5.0120490
Physics of Fluids
20221101T11:30:21Z
© 2022 Author(s).

A partitioncoupled Eulerian–Lagrangian method for largedeformation simulation of compressible fluid
https://aip.scitation.org/doi/10.1063/5.0118978?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We present a partitioncoupled Eulerian–Lagrangian method (PCELM) for accurately tracking a free interface and a contact discontinuity of the compressible fluid with large deformation. This method tracks the interface by arranging splittable Lagrangian particles on an Eulerian grid and adopts a partitionweighted bidirectional mapping between particles and grids using a cubic Bspline as interpolation function. PCELM suppresses oscillation of the discontinuous surface by this partitionweighted remapping method and solves the problem of numerical fracture by a particle splitting method. A virtual particle method is also proposed to deal with discontinuity of particle flow at the boundary and to maintain interpolation accuracy at the boundary. The conservation of mass, momentum, and energy of PCELM is proved by conservation analysis. Accuracy tests and simulations of discontinuous surfaces and free interfaces are performed to verify the accuracy and stability of PCELM. The results show that PCELM has strong energy conservation and low energy dissipation and that it is not only better at suppressing oscillations than the original method, but can also simulate a compressible fluid with large deformation more accurately than weighted essentially nonoscillatory schemes.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We present a partitioncoupled Eulerian–Lagrangian method (PCELM) for accurately tracking a free interface and a contact discontinuity of the compressible fluid with large deformation. This method tracks the interface by arranging splittable Lagrangian particles on an Eulerian grid and adopts a partitionweighted bidirectional mapping between particles and grids using a cubic Bspline as interpolation function. PCELM suppresses oscillation of the discontinuous surface by this partitionweighted remapping method and solves the problem of numerical fracture by a particle splitting method. A virtual particle method is also proposed to deal with discontinuity of particle flow at the boundary and to maintain interpolation accuracy at the boundary. The conservation of mass, momentum, and energy of PCELM is proved by conservation analysis. Accuracy tests and simulations of discontinuous surfaces and free interfaces are performed to verify the accuracy and stability of PCELM. The results show that PCELM has strong energy conservation and low energy dissipation and that it is not only better at suppressing oscillations than the original method, but can also simulate a compressible fluid with large deformation more accurately than weighted essentially nonoscillatory schemes.
A partitioncoupled Eulerian–Lagrangian method for largedeformation simulation of compressible fluid
10.1063/5.0118978
Physics of Fluids
20221101T11:30:24Z
© 2022 Author(s).

Numerical study on wave configuration of wedgeinduced oblique detonation wave: Reactive boundary layer effect
https://aip.scitation.org/doi/10.1063/5.0118194?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A numerical simulation solving the Reynoldsaveraged Navier–Stokes equation is presented to investigate the initiation and evolution of the wedgeinduced oblique detonation wave (ODW) with emphasis on the effects of the burning boundary layer. The nondimensional activation energy (Ea) is selected as the bifurcate parameter, which varies from 30 to 50. The largest induction ignition length behind the oblique shock/detonation wave is shown to be proportional to the Ea. The initiation of ODW can be attributed to the collision and diffraction of reactive waves. The wave configuration, a series of compression waves (or shock wave), is observed at the conjunction point of the burning boundary layer and combustion wave, which intensifies the pressure jump as increasing the Ea. The polar line analysis demonstrates that the pressure jump triggers the transition from regular reflection to Mach reflection near triplepoint. The oscillations of the ODW wave structures, for example, the triplepoint and Mach stem, can be attributed to the Rayleigh–Taylor instabilities developed on the reaction front of the boundary layer, which shall be appropriately suppressed to remain the steadiness of the ODW and flow configuration.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A numerical simulation solving the Reynoldsaveraged Navier–Stokes equation is presented to investigate the initiation and evolution of the wedgeinduced oblique detonation wave (ODW) with emphasis on the effects of the burning boundary layer. The nondimensional activation energy (Ea) is selected as the bifurcate parameter, which varies from 30 to 50. The largest induction ignition length behind the oblique shock/detonation wave is shown to be proportional to the Ea. The initiation of ODW can be attributed to the collision and diffraction of reactive waves. The wave configuration, a series of compression waves (or shock wave), is observed at the conjunction point of the burning boundary layer and combustion wave, which intensifies the pressure jump as increasing the Ea. The polar line analysis demonstrates that the pressure jump triggers the transition from regular reflection to Mach reflection near triplepoint. The oscillations of the ODW wave structures, for example, the triplepoint and Mach stem, can be attributed to the Rayleigh–Taylor instabilities developed on the reaction front of the boundary layer, which shall be appropriately suppressed to remain the steadiness of the ODW and flow configuration.
Numerical study on wave configuration of wedgeinduced oblique detonation wave: Reactive boundary layer effect
10.1063/5.0118194
Physics of Fluids
20221101T11:30:05Z
© 2022 Author(s).

Effects of competitive adsorption on production capacity during CO2 displacement of CH4 in shale
https://aip.scitation.org/doi/10.1063/5.0122802?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>During CO2 displacement of CH4 in shale, competitive adsorption results in reduced pore space used for gas flow in shale, which is closely associated with the production capacity of shalegas reservoirs. Thus, the present work investigates the effects of CO2–CH4 competitive adsorption on production capacity. Herein, a slit–pore model is developed in terms of gas storage (CO2 and CH4) and graphene pores using molecular dynamics and implemented via largescale atomic/molecular massively parallel simulator. The effects of CO2 injection pressure, temperature, and velocity and of pore size on CO2–CH4 displacement and competitive adsorption properties are simulated and examined. Hence, the displacement efficiency of CH4 and the adsorption layer thickness of the CO2–CH4 binary mixture are determined. Moreover, based on a basic seepage model of planar linear flooding, the effect of CO2–CH4 competitive adsorption on production capacity is analytically investigated. Results demonstrate that the production capacity with consideration of adsorption layer thickness is less than that without consideration of adsorption layer thickness, illustrating that CO2–CH4 competitive adsorption behaviors are closely connected with permeability, flow rate, and production capacity of shalegas reservoirs, especially for shalegas reservoirs containing large numbers of pores and slits.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>During CO2 displacement of CH4 in shale, competitive adsorption results in reduced pore space used for gas flow in shale, which is closely associated with the production capacity of shalegas reservoirs. Thus, the present work investigates the effects of CO2–CH4 competitive adsorption on production capacity. Herein, a slit–pore model is developed in terms of gas storage (CO2 and CH4) and graphene pores using molecular dynamics and implemented via largescale atomic/molecular massively parallel simulator. The effects of CO2 injection pressure, temperature, and velocity and of pore size on CO2–CH4 displacement and competitive adsorption properties are simulated and examined. Hence, the displacement efficiency of CH4 and the adsorption layer thickness of the CO2–CH4 binary mixture are determined. Moreover, based on a basic seepage model of planar linear flooding, the effect of CO2–CH4 competitive adsorption on production capacity is analytically investigated. Results demonstrate that the production capacity with consideration of adsorption layer thickness is less than that without consideration of adsorption layer thickness, illustrating that CO2–CH4 competitive adsorption behaviors are closely connected with permeability, flow rate, and production capacity of shalegas reservoirs, especially for shalegas reservoirs containing large numbers of pores and slits.
Effects of competitive adsorption on production capacity during CO2 displacement of CH4 in shale
10.1063/5.0122802
Physics of Fluids
20221101T11:29:14Z
© 2022 Author(s).

Flame acceleration and transition to detonation in a pre/mainchamber combustion system
https://aip.scitation.org/doi/10.1063/5.0122240?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Numerical simulations are performed to study the mechanism of deflagration to detonation transition (DDT) in a pre/mainchamber combustion system with a stoichiometric ethylene–oxygen mixture. A Godunov algorithm, fifthorder in space, and thirdorder in time, is used to solve the fully compressible Navier–Stokes equations on a dynamically adapting mesh. A singlestep, calibrated chemical diffusive model described by Arrhenius kinetics is used for energy release and conservation between the fuel and the product. The twodimensional simulation shows that a laminar flame grows in the prechamber and then develops into a jet flame as it passes through the orifice. A strong shock forms immediately ahead of the flame, reflecting off the walls and interacting with the flame front. The shock–flame interactions are crucial for the development of flame instabilities, which trigger the subsequent flame development. The DDT arises due to a shockfocusing mechanism, where multiple shocks collide at the flame front. A chemical explosive mode analysis (CEMA) criterion is developed to study the DDT ignition mode. Preliminary onedimensional computations for a laminar propagating flame, a fast flame deflagration, and a Chapman–Jouguet detonation are conducted to demonstrate the validity of CEMA on the chemicaldiffusive model, as well as to determine the proper conditioning value for CEMA diagnostic. The twodimensional analysis with CEMA indicates that the DDT initiated by the shockfocusing mechanism can form a strong thermal expansion region at the flame front that features large positive eigenvalues for the chemical explosive mode and dominance of the local autoignition mode. Thus, the CEMA criterion proposed in this study provides a robust diagnostic for identifying autoignitionsupported DDT, of which the emergence of excessive local autoignition mode is found to be a precursor. The effect of grid size, initial temperature, and orifice size are then evaluated, and results show that although the closechamber DDT is highly stochastic, the detonation initiation mechanism remains robust.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Numerical simulations are performed to study the mechanism of deflagration to detonation transition (DDT) in a pre/mainchamber combustion system with a stoichiometric ethylene–oxygen mixture. A Godunov algorithm, fifthorder in space, and thirdorder in time, is used to solve the fully compressible Navier–Stokes equations on a dynamically adapting mesh. A singlestep, calibrated chemical diffusive model described by Arrhenius kinetics is used for energy release and conservation between the fuel and the product. The twodimensional simulation shows that a laminar flame grows in the prechamber and then develops into a jet flame as it passes through the orifice. A strong shock forms immediately ahead of the flame, reflecting off the walls and interacting with the flame front. The shock–flame interactions are crucial for the development of flame instabilities, which trigger the subsequent flame development. The DDT arises due to a shockfocusing mechanism, where multiple shocks collide at the flame front. A chemical explosive mode analysis (CEMA) criterion is developed to study the DDT ignition mode. Preliminary onedimensional computations for a laminar propagating flame, a fast flame deflagration, and a Chapman–Jouguet detonation are conducted to demonstrate the validity of CEMA on the chemicaldiffusive model, as well as to determine the proper conditioning value for CEMA diagnostic. The twodimensional analysis with CEMA indicates that the DDT initiated by the shockfocusing mechanism can form a strong thermal expansion region at the flame front that features large positive eigenvalues for the chemical explosive mode and dominance of the local autoignition mode. Thus, the CEMA criterion proposed in this study provides a robust diagnostic for identifying autoignitionsupported DDT, of which the emergence of excessive local autoignition mode is found to be a precursor. The effect of grid size, initial temperature, and orifice size are then evaluated, and results show that although the closechamber DDT is highly stochastic, the detonation initiation mechanism remains robust.
Flame acceleration and transition to detonation in a pre/mainchamber combustion system
10.1063/5.0122240
Physics of Fluids
20221102T02:43:20Z
© 2022 Author(s).
M. Davy

Shock train/glancing shock/boundary layer interaction in a curved isolator with sidewall contraction
https://aip.scitation.org/doi/10.1063/5.0120400?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Herein, a curved isolator with sidewall contraction of a hypersonic scramjet is extracted and simplified. The flow characteristics in the isolator with an entrance Mach number of 3.46 are studied. The results show that the lateral contraction of the sidewall generates a strong shock and its subsequent reflected shock, which induce glancing shock wave/boundary layer interaction (GSWBLI), rolling up largescale vortices adjacent to the symmetry plane. As the downstream backpressure increases, the shock train propagates upstream and interacts with the glancing shock and vortices inevitably, forming a typical shock train/glancing shock/boundary layer interaction (STGSBLI) phenomenon. Different from the shock train in a straight isolator, it can be divided into two distinct parts, i.e., the center part and the side part. For the center part, it behaves in a quasitwodimensional manner, which is primarily associated with STGSBLI, forcing the lowmomentum subsonic flow to principally accumulate near the symmetry plane and weakening the spanwise pressure gradient in the center part. For the side part, it locates behind the glancing and reflected shocks and is much shorter than the center part, resulting in a streamwise extension of the supersonic flow region near the sidewall. As the shock train moves upstream, the quasitwodimensional region enlarges and extends to the sidewall. It is demonstrated that the aforementioned two parts of the shock train can also be discovered in curved sidewallcontraction isolators with different centerlines. Therefore, the STGSBLI is the dominant and universal physical phenomenon in isolators with sidewall contraction.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Herein, a curved isolator with sidewall contraction of a hypersonic scramjet is extracted and simplified. The flow characteristics in the isolator with an entrance Mach number of 3.46 are studied. The results show that the lateral contraction of the sidewall generates a strong shock and its subsequent reflected shock, which induce glancing shock wave/boundary layer interaction (GSWBLI), rolling up largescale vortices adjacent to the symmetry plane. As the downstream backpressure increases, the shock train propagates upstream and interacts with the glancing shock and vortices inevitably, forming a typical shock train/glancing shock/boundary layer interaction (STGSBLI) phenomenon. Different from the shock train in a straight isolator, it can be divided into two distinct parts, i.e., the center part and the side part. For the center part, it behaves in a quasitwodimensional manner, which is primarily associated with STGSBLI, forcing the lowmomentum subsonic flow to principally accumulate near the symmetry plane and weakening the spanwise pressure gradient in the center part. For the side part, it locates behind the glancing and reflected shocks and is much shorter than the center part, resulting in a streamwise extension of the supersonic flow region near the sidewall. As the shock train moves upstream, the quasitwodimensional region enlarges and extends to the sidewall. It is demonstrated that the aforementioned two parts of the shock train can also be discovered in curved sidewallcontraction isolators with different centerlines. Therefore, the STGSBLI is the dominant and universal physical phenomenon in isolators with sidewall contraction.
Shock train/glancing shock/boundary layer interaction in a curved isolator with sidewall contraction
10.1063/5.0120400
Physics of Fluids
20221102T02:43:44Z
© 2022 Author(s).

Dynamics study of shock wave intersection under highfrequency sine oscillation incoming flow
https://aip.scitation.org/doi/10.1063/5.0110802?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This paper numerically studies the dynamics of symmetrical wedge shock intersection under sinusoidal supersonic oscillation conditions. The 15° symmetrical wedges are used as the shock generator, and the sinusoidal oscillation is used as the inflow condition. Two forms are considered: (I) The fluctuation amplitude is kept constant (A = 1.4), and the influence of the fluctuation frequency from 4 kHz with a step of 2–10 kHz in the shock wave system is considered. (II) Keeping the frequency constant (f = 10 kHz), the effect of three amplitudes (A = 1.0, 1.4, 1.8) on shock waves is considered. A detailed analysis of unsteady flow features, including the Mach stem growth, the swing of slip lines, pressure evolution, and peculiar pressure wave phenomenon are presented with a focus on the bidirectional regular intersection (RI ↔ MI) Mach intersection transition process. The study found that: RI ↔ MI always occurs near the von Neumann solution, and there are premature transformation and hysteresis. The higher the frequency, the more noticeable the hysteresis and premature transformation are, the more obvious the swing of slip lines is. The lower the frequency, the longer the bidirectional transition time of the RI ↔ MI, the greater the maximum height of the Mach stem, the more frequent the triple points' pressure fluctuation. In addition, the oscillating flow will cause the propagation of pressure waves in the slip line channel and the transition from transverse waves to longitudinal waves. Under the condition of different amplitudes, the greater the amplitude is, the greater the height of the Mach stem is. When the amplitude is maximum, the Mach number of partial incoming flow is less than the minimum Mach number of the attached oblique shock wave. The evolution of the detached shock wave will lead to the complexity of the system. As the amplitude increases, the greater the pressure difference of the triple points, the greater the curvature of the incident shocks. The research of the unsteady shock wave intersection under the oscillating flow is useful to the study of supersonic flow, loss control, and heat and mass transfer of detonation engines and intake ducts.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This paper numerically studies the dynamics of symmetrical wedge shock intersection under sinusoidal supersonic oscillation conditions. The 15° symmetrical wedges are used as the shock generator, and the sinusoidal oscillation is used as the inflow condition. Two forms are considered: (I) The fluctuation amplitude is kept constant (A = 1.4), and the influence of the fluctuation frequency from 4 kHz with a step of 2–10 kHz in the shock wave system is considered. (II) Keeping the frequency constant (f = 10 kHz), the effect of three amplitudes (A = 1.0, 1.4, 1.8) on shock waves is considered. A detailed analysis of unsteady flow features, including the Mach stem growth, the swing of slip lines, pressure evolution, and peculiar pressure wave phenomenon are presented with a focus on the bidirectional regular intersection (RI ↔ MI) Mach intersection transition process. The study found that: RI ↔ MI always occurs near the von Neumann solution, and there are premature transformation and hysteresis. The higher the frequency, the more noticeable the hysteresis and premature transformation are, the more obvious the swing of slip lines is. The lower the frequency, the longer the bidirectional transition time of the RI ↔ MI, the greater the maximum height of the Mach stem, the more frequent the triple points' pressure fluctuation. In addition, the oscillating flow will cause the propagation of pressure waves in the slip line channel and the transition from transverse waves to longitudinal waves. Under the condition of different amplitudes, the greater the amplitude is, the greater the height of the Mach stem is. When the amplitude is maximum, the Mach number of partial incoming flow is less than the minimum Mach number of the attached oblique shock wave. The evolution of the detached shock wave will lead to the complexity of the system. As the amplitude increases, the greater the pressure difference of the triple points, the greater the curvature of the incident shocks. The research of the unsteady shock wave intersection under the oscillating flow is useful to the study of supersonic flow, loss control, and heat and mass transfer of detonation engines and intake ducts.
Dynamics study of shock wave intersection under highfrequency sine oscillation incoming flow
10.1063/5.0110802
Physics of Fluids
20221102T02:44:03Z
© 2022 Author(s).

Finite difference alternative unequalsized weighted essentially nonoscillatory schemes for hyperbolic conservation laws
https://aip.scitation.org/doi/10.1063/5.0123597?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, two unequalsized weighted essentially nonoscillatory (USWENO) schemes are proposed for solving hyperbolic conservation laws. First, an alternative USWENO (AUSWENO) scheme based directly on the values of conserved variables at the grid points is designed. This scheme can inherit all the advantages of the original USWENO scheme [J. Zhu and J. Qiu, “A new fifth order finite difference WENO scheme for solving hyperbolic conservation laws,” J. Comput. Phys. 318, 110–121 (2016).], such as the arbitrariness of the linear weights. Moreover, this presented AUSWENO scheme enables any monotone fluxes applicable to this framework, whereas the original USWENO scheme is only suitable for the more dissipative smooth flux splitting. Therefore, the method in this paper has a smaller L1 and [math] numerical errors than the original scheme under the same conditions. Second, in order to further improve the computational efficiency of the above AUSWENO scheme, a hybrid AUSWENO scheme is proposed by combining a hybrid strategy. This strategy identifies the discontinuous regions directly based on the extreme points of the reconstruction polynomial corresponding to the fivepoint stencil, which brings the important advantage that it does not depend on the specific problem and does not contain any artificial adjustable parameters. Finally, the performance of the above two AUSWENO schemes in terms of low dissipation, shock capture capability, discontinuity detection capability, and computational efficiency is verified by some benchmark one and twodimensional numerical examples.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, two unequalsized weighted essentially nonoscillatory (USWENO) schemes are proposed for solving hyperbolic conservation laws. First, an alternative USWENO (AUSWENO) scheme based directly on the values of conserved variables at the grid points is designed. This scheme can inherit all the advantages of the original USWENO scheme [J. Zhu and J. Qiu, “A new fifth order finite difference WENO scheme for solving hyperbolic conservation laws,” J. Comput. Phys. 318, 110–121 (2016).], such as the arbitrariness of the linear weights. Moreover, this presented AUSWENO scheme enables any monotone fluxes applicable to this framework, whereas the original USWENO scheme is only suitable for the more dissipative smooth flux splitting. Therefore, the method in this paper has a smaller L1 and [math] numerical errors than the original scheme under the same conditions. Second, in order to further improve the computational efficiency of the above AUSWENO scheme, a hybrid AUSWENO scheme is proposed by combining a hybrid strategy. This strategy identifies the discontinuous regions directly based on the extreme points of the reconstruction polynomial corresponding to the fivepoint stencil, which brings the important advantage that it does not depend on the specific problem and does not contain any artificial adjustable parameters. Finally, the performance of the above two AUSWENO schemes in terms of low dissipation, shock capture capability, discontinuity detection capability, and computational efficiency is verified by some benchmark one and twodimensional numerical examples.
Finite difference alternative unequalsized weighted essentially nonoscillatory schemes for hyperbolic conservation laws
10.1063/5.0123597
Physics of Fluids
20221103T12:34:57Z
© 2022 Author(s).

A study of the acoustic effect inside cylindrical bubble produced by underwater electrical discharge
https://aip.scitation.org/doi/10.1063/5.0116125?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The evolution of a cylindrical gaseous bubble produced by an underwater electrical discharge is considered in the present study. Both the gas flow inside and the water flow around the bubble are theoretically analyzed in a cylindrical coordinate system. By using the potential flow theory and multiple scale expansion method, governing equations of both flows and the bubble surface are formulated. The radial oscillation of the bubble surface is composed of a slowchanging equilibrium part and a fastchanging displacement. The former corresponds to a quiescent water domain and a uniform gas column, and the latter corresponds to acoustic waves in gas and water flows. The axial gas wave can evolve into a stable standing wave if the bubble length is multiples of half a wavelength. The internal acoustic standing wave then causes a synchronous smallamplitude oscillation of the bubble surface when the frequency of the acoustic wave is close to the natural frequency of the bubble surface. An underwater discharge experiment is implemented to validate our theory. Finally, a novel method to estimate the plasma pressure is proposed based on our theory.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The evolution of a cylindrical gaseous bubble produced by an underwater electrical discharge is considered in the present study. Both the gas flow inside and the water flow around the bubble are theoretically analyzed in a cylindrical coordinate system. By using the potential flow theory and multiple scale expansion method, governing equations of both flows and the bubble surface are formulated. The radial oscillation of the bubble surface is composed of a slowchanging equilibrium part and a fastchanging displacement. The former corresponds to a quiescent water domain and a uniform gas column, and the latter corresponds to acoustic waves in gas and water flows. The axial gas wave can evolve into a stable standing wave if the bubble length is multiples of half a wavelength. The internal acoustic standing wave then causes a synchronous smallamplitude oscillation of the bubble surface when the frequency of the acoustic wave is close to the natural frequency of the bubble surface. An underwater discharge experiment is implemented to validate our theory. Finally, a novel method to estimate the plasma pressure is proposed based on our theory.
A study of the acoustic effect inside cylindrical bubble produced by underwater electrical discharge
10.1063/5.0116125
Physics of Fluids
20221103T12:35:17Z
© 2022 Author(s).

Direct numerical simulation of shock wave/turbulent boundary layer interaction in a swept compression ramp at Mach 6
https://aip.scitation.org/doi/10.1063/5.0118578?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Swept compression ramps widely exist in supersonic/hypersonic vehicles and have become a typical standard model for studying threedimensional (3D) shock wave/turbulent boundary layer interactions (STBLIs). In this paper, we conduct a direct numerical simulation of swept compression ramp STBLI with a 34° compression angle and a 45° sweep angle at Mach 6 using a heterogeneous parallel finite difference solver. Benefitting from the powerful computing performance of the graphics processing unit, the computational grid number exceeds 5 × 106 with the spatiotemporal evolution data of hypersonic 3D STBLI obtained. The results show that the flow of the hypersonic swept compression ramp follows the quasiconical symmetry. A supersonic crossflow with helical motion appears in the interaction region, and its velocity increases along the spanwise direction. Fluids from the highenergydensity region pass through the bow shock at the head of the main shock and crash into the wall downstream of the reattachment, resulting in the peaks in skin friction and heat flux. The peak friction and heating increase along the spanwise direction because of the spanwise variation in the shock wave inclination. In the interaction region, the unsteadiness is dominated by the midfrequency motion, whereas the lowfrequency largescale motion is nearly absent. Two reasons for the lack of lowfrequency unsteadiness are given: (1) The separation shock is significantly weaker than the reattachment shock and main shock; and (2) because of the supersonic crossflow, the perturbations propagating at the sound speed are not selfsustaining but flow along the rdirection and toward the spanwise boundary.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Swept compression ramps widely exist in supersonic/hypersonic vehicles and have become a typical standard model for studying threedimensional (3D) shock wave/turbulent boundary layer interactions (STBLIs). In this paper, we conduct a direct numerical simulation of swept compression ramp STBLI with a 34° compression angle and a 45° sweep angle at Mach 6 using a heterogeneous parallel finite difference solver. Benefitting from the powerful computing performance of the graphics processing unit, the computational grid number exceeds 5 × 106 with the spatiotemporal evolution data of hypersonic 3D STBLI obtained. The results show that the flow of the hypersonic swept compression ramp follows the quasiconical symmetry. A supersonic crossflow with helical motion appears in the interaction region, and its velocity increases along the spanwise direction. Fluids from the highenergydensity region pass through the bow shock at the head of the main shock and crash into the wall downstream of the reattachment, resulting in the peaks in skin friction and heat flux. The peak friction and heating increase along the spanwise direction because of the spanwise variation in the shock wave inclination. In the interaction region, the unsteadiness is dominated by the midfrequency motion, whereas the lowfrequency largescale motion is nearly absent. Two reasons for the lack of lowfrequency unsteadiness are given: (1) The separation shock is significantly weaker than the reattachment shock and main shock; and (2) because of the supersonic crossflow, the perturbations propagating at the sound speed are not selfsustaining but flow along the rdirection and toward the spanwise boundary.
Direct numerical simulation of shock wave/turbulent boundary layer interaction in a swept compression ramp at Mach 6
10.1063/5.0118578
Physics of Fluids
20221103T12:35:49Z
© 2022 Author(s).

Improvement to a general methodology for freestream preservation on curvilinear grids
https://aip.scitation.org/doi/10.1063/5.0120313?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Exact freestream preservation is an important property for finitedifference schemes designed on curvilinear grids. Geometrically induced errors from a nonpreserved freestream will cause severe computational instability and numerical inaccuracy. We improve a general numerical strategy to eliminate the geometrically induced errors of finitedifference schemes on both stationary and dynamic curvilinear grids. The main idea is that, instead of using the standard Euler equations in the generalized coordinate system as the governing equations, we solve full forms of the transformed equations and modify the flux functions to share the Jacobian and metrics on the grid point where the flux derivative is located. The metrics and Jacobian are gridrelated geometric parameters that appear in the transformed equations as derivatives. Properly handling these is critical for freestream preservation. The results of our numerical tests show the excellent freestream and vortex preservation properties and robust shockcapturing properties of the new strategy compared with those of the standard method.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Exact freestream preservation is an important property for finitedifference schemes designed on curvilinear grids. Geometrically induced errors from a nonpreserved freestream will cause severe computational instability and numerical inaccuracy. We improve a general numerical strategy to eliminate the geometrically induced errors of finitedifference schemes on both stationary and dynamic curvilinear grids. The main idea is that, instead of using the standard Euler equations in the generalized coordinate system as the governing equations, we solve full forms of the transformed equations and modify the flux functions to share the Jacobian and metrics on the grid point where the flux derivative is located. The metrics and Jacobian are gridrelated geometric parameters that appear in the transformed equations as derivatives. Properly handling these is critical for freestream preservation. The results of our numerical tests show the excellent freestream and vortex preservation properties and robust shockcapturing properties of the new strategy compared with those of the standard method.
Improvement to a general methodology for freestream preservation on curvilinear grids
10.1063/5.0120313
Physics of Fluids
20221103T12:35:44Z
© 2022 Author(s).

Interaction mechanism between the tip leakage flow and inlet boundary layer in a highly loaded compressor cascade based on scaleadaptive simulation
https://aip.scitation.org/doi/10.1063/5.0123485?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Based on the scaleadaptive simulation, the interaction mechanism between the tip leakage flow (TLF) and the inlet boundary layer (IBL) and its effects on the tip flow field and aerodynamic performance of the compressor cascade were investigated. The timeaveraged results show that the IBL reduces the blade tip load near the leading edge region, decreases the axial momentum of the TLF, and inhibits the development of the TLF to a certain extent. On the other hand, the IBL promotes the coupling of the tip leakage vortex, secondary vortex, and separation vortex and induces breakdown, resulting in a large area of severe flow separation in the corner region, greatly reducing the diffusion capacity and significantly increasing the flow loss in the tip region. The analysis of the unsteady transient flow fields indicates that the unsteady fluctuation in the tip region is mainly caused by the tip leakage vortex and flow separation near the blade trailing edge. The former is suppressed under the influence of the IBL, while the latter is amplified. The highintensity oscillation due to the breakdown and decomposition of the tip vortex structures plays a critical role in the fluctuation of the cascade performance. By means of proper orthogonal decomposition, it is found that the IBL enhances the fluctuation of smallscale vortex structures related to flow separation and leakage flow and makes the stability of the tip flow field worse.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Based on the scaleadaptive simulation, the interaction mechanism between the tip leakage flow (TLF) and the inlet boundary layer (IBL) and its effects on the tip flow field and aerodynamic performance of the compressor cascade were investigated. The timeaveraged results show that the IBL reduces the blade tip load near the leading edge region, decreases the axial momentum of the TLF, and inhibits the development of the TLF to a certain extent. On the other hand, the IBL promotes the coupling of the tip leakage vortex, secondary vortex, and separation vortex and induces breakdown, resulting in a large area of severe flow separation in the corner region, greatly reducing the diffusion capacity and significantly increasing the flow loss in the tip region. The analysis of the unsteady transient flow fields indicates that the unsteady fluctuation in the tip region is mainly caused by the tip leakage vortex and flow separation near the blade trailing edge. The former is suppressed under the influence of the IBL, while the latter is amplified. The highintensity oscillation due to the breakdown and decomposition of the tip vortex structures plays a critical role in the fluctuation of the cascade performance. By means of proper orthogonal decomposition, it is found that the IBL enhances the fluctuation of smallscale vortex structures related to flow separation and leakage flow and makes the stability of the tip flow field worse.
Interaction mechanism between the tip leakage flow and inlet boundary layer in a highly loaded compressor cascade based on scaleadaptive simulation
10.1063/5.0123485
Physics of Fluids
20221104T12:22:12Z
© 2022 Author(s).

Field synergy principle for compressible laminar flow and the application for drag reduction in microchannel
https://aip.scitation.org/doi/10.1063/5.0110710?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Considering the density variation of compressible fluid, the field synergy principle for compressible laminar flow is presented based on the incompressible flow field synergy principle. The flow resistance is related to the synergy of velocity field and density logarithmic gradient field. Based on the principle of minimum mechanical energy dissipation, the compressible laminar flow field synergy equation is derived. The field synergy principle is verified by an example of the microchannel with expansion cross section, and the structure is optimized. The results indicate that the viscous dissipation value in the entire flow domain of the optimized structure can be reduced by 13.5%, and the angle between the velocity vector and the gradient field of logarithm of density increases. In fluid flows, it will reduce the fluid flow drag to decrease the synergy angle between the velocity vector and the gradient field of logarithm of density.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Considering the density variation of compressible fluid, the field synergy principle for compressible laminar flow is presented based on the incompressible flow field synergy principle. The flow resistance is related to the synergy of velocity field and density logarithmic gradient field. Based on the principle of minimum mechanical energy dissipation, the compressible laminar flow field synergy equation is derived. The field synergy principle is verified by an example of the microchannel with expansion cross section, and the structure is optimized. The results indicate that the viscous dissipation value in the entire flow domain of the optimized structure can be reduced by 13.5%, and the angle between the velocity vector and the gradient field of logarithm of density increases. In fluid flows, it will reduce the fluid flow drag to decrease the synergy angle between the velocity vector and the gradient field of logarithm of density.
Field synergy principle for compressible laminar flow and the application for drag reduction in microchannel
10.1063/5.0110710
Physics of Fluids
20221104T12:22:26Z
© 2022 Author(s).

A modified walladapting local eddyviscosity model for largeeddy simulation of compressible wallbounded flow
https://aip.scitation.org/doi/10.1063/5.0119413?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The walladapting local eddyviscosity (WALE) model in largeeddy simulation can well predict wallbounded flows but it is also well known for excessive dissipation. In this study, we apply the minimumdissipation model to constrain the WALE model in compressible flows and obtain the coefficient of the WALE model. Through this process, the dissipation of WALE model can be lower while it still maintains strong stability. In the modified WALE model, the isotropic part of the subgridscale (SGS) stress is also reconstructed. In the filtered total energy equation, all of the extra SGS unclosed terms (besides SGS stress and SGS heat flux) are modeled instead of neglecting some SGS terms, such as the SGS viscous diffusion. The modified WALE model is tested in a compressible turbulent channel flow and a supersonic turbulent boundary layer over a compression corner. The new model can well predict the mean velocity, the mean temperature, the Reynolds stress, and the separation bubble.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The walladapting local eddyviscosity (WALE) model in largeeddy simulation can well predict wallbounded flows but it is also well known for excessive dissipation. In this study, we apply the minimumdissipation model to constrain the WALE model in compressible flows and obtain the coefficient of the WALE model. Through this process, the dissipation of WALE model can be lower while it still maintains strong stability. In the modified WALE model, the isotropic part of the subgridscale (SGS) stress is also reconstructed. In the filtered total energy equation, all of the extra SGS unclosed terms (besides SGS stress and SGS heat flux) are modeled instead of neglecting some SGS terms, such as the SGS viscous diffusion. The modified WALE model is tested in a compressible turbulent channel flow and a supersonic turbulent boundary layer over a compression corner. The new model can well predict the mean velocity, the mean temperature, the Reynolds stress, and the separation bubble.
A modified walladapting local eddyviscosity model for largeeddy simulation of compressible wallbounded flow
10.1063/5.0119413
Physics of Fluids
20221107T12:48:55Z
© 2022 Author(s).

Study of the streamwise location of a micro vortex generator for a separationcontrol mechanism in supersonic flow
https://aip.scitation.org/doi/10.1063/5.0123541?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A shock wave/boundary layer interaction is a common phenomenon in supersonic (hypersonic) flows, and it usually occurs in an airbreathing propulsion system. It induces a large separation bubble and a local peak heat flux, and means of controlling it have attracted much attention. In this paper, threedimensional Reynoldsaveraged Navier–Stokes equations and the shear stress transfer k–ω model are employed to study the flow control mechanism of a micro vortex generator in a supersonic flow with a freestream at a Mach number of 2.9; the influence of the streamwise location is taken into consideration. At the same time, due to the size of the separation bubble induced by the shock wave/boundary layer interaction, the total pressure recovery coefficient and the wall heat flux density are used to evaluate the control performance. The results show that the size of the separation bubble is greatly reduced, the area of the separation bubble is reduced by 29.63%, and its volume is reduced by 63.27%. However, this entails a total pressure loss and a large peak heat flux, and this should be dealt with through multiobjective design optimization approaches.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A shock wave/boundary layer interaction is a common phenomenon in supersonic (hypersonic) flows, and it usually occurs in an airbreathing propulsion system. It induces a large separation bubble and a local peak heat flux, and means of controlling it have attracted much attention. In this paper, threedimensional Reynoldsaveraged Navier–Stokes equations and the shear stress transfer k–ω model are employed to study the flow control mechanism of a micro vortex generator in a supersonic flow with a freestream at a Mach number of 2.9; the influence of the streamwise location is taken into consideration. At the same time, due to the size of the separation bubble induced by the shock wave/boundary layer interaction, the total pressure recovery coefficient and the wall heat flux density are used to evaluate the control performance. The results show that the size of the separation bubble is greatly reduced, the area of the separation bubble is reduced by 29.63%, and its volume is reduced by 63.27%. However, this entails a total pressure loss and a large peak heat flux, and this should be dealt with through multiobjective design optimization approaches.
Study of the streamwise location of a micro vortex generator for a separationcontrol mechanism in supersonic flow
10.1063/5.0123541
Physics of Fluids
20221107T12:49:06Z
© 2022 Author(s).

Multidomain physicsinformed neural network for solving forward and inverse problems of steadystate heat conduction in multilayer media
https://aip.scitation.org/doi/10.1063/5.0116038?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, a novel deep learning technique, called multidomain physicsinformed neural network (MPINN), is presented to solve forward and inverse problems of steadystate heat conduction in multilayer media. By adopting the domain decomposition technique, the multilayer media is first divided into several subdomains. Then, the fully connected neural network is employed to approximate the temperature field on each subdomain. Finally, a large total network framework is formed by combining subnetworks of all the mediums and using continuity conditions on interfaces. By training the total network, we can obtain the temperature distribution over the whole computational domain, including the interface between every two mediums. In the proposed method, the boundary conditions are introduced into the loss function, and the governing equation is used as a constrain item, which ensures the accuracy and stability of numerical approximation. As a meshless collocation technology, the MPINN does not require tedious procedures such as meshing and numerical integration, and can freely address forward and inverse problems of thin body and coating structure. Several numerical examples are given to illustrate the efficiency and performance of the new method. Results indicate that the Swish and the Sigmoid functions are two better activation functions for such problems. As the number of nodes increases, the number of hidden layers does not need to be increased. Even for the thin film at nanoscale, the MPINN still obtains accurate results. Moreover, the proposed scheme shows better performance than the traditional boundary element method in solving nonlinear heat conduction problems.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, a novel deep learning technique, called multidomain physicsinformed neural network (MPINN), is presented to solve forward and inverse problems of steadystate heat conduction in multilayer media. By adopting the domain decomposition technique, the multilayer media is first divided into several subdomains. Then, the fully connected neural network is employed to approximate the temperature field on each subdomain. Finally, a large total network framework is formed by combining subnetworks of all the mediums and using continuity conditions on interfaces. By training the total network, we can obtain the temperature distribution over the whole computational domain, including the interface between every two mediums. In the proposed method, the boundary conditions are introduced into the loss function, and the governing equation is used as a constrain item, which ensures the accuracy and stability of numerical approximation. As a meshless collocation technology, the MPINN does not require tedious procedures such as meshing and numerical integration, and can freely address forward and inverse problems of thin body and coating structure. Several numerical examples are given to illustrate the efficiency and performance of the new method. Results indicate that the Swish and the Sigmoid functions are two better activation functions for such problems. As the number of nodes increases, the number of hidden layers does not need to be increased. Even for the thin film at nanoscale, the MPINN still obtains accurate results. Moreover, the proposed scheme shows better performance than the traditional boundary element method in solving nonlinear heat conduction problems.
Multidomain physicsinformed neural network for solving forward and inverse problems of steadystate heat conduction in multilayer media
10.1063/5.0116038
Physics of Fluids
20221107T12:49:43Z
© 2022 Author(s).

On the prediction of noise generated by urban air mobility (UAM) vehicles. I. Integration of fundamental acoustic metrics
https://aip.scitation.org/doi/10.1063/5.0124134?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This work identifies and explores several aeroacoustic metrics that allow for urban air mobility (UAM) vehicle noise prediction. An increase in production and use of UAM and distributed electric propulsion vehicles within populated civilian areas stands behind the need to minimize the noise produced by these vehicles. The Federal Aviation Administration's (FAA's) strict noise regulations on UAM aircraft compels designers to place a significant emphasis, early in the design phase, on the characterization and analysis of the external noise generated by these vehicles, namely, to ensure their design viability. To accomplish this, the present study focuses on the analysis and interpretation of predicted noise signals using a set of characteristic metrics that can be instrumental at guiding the design process. Following a thorough review of metrics standardized by the International Civil Aviation Organization as well as the FAA, seven general metrics are identified, evaluated, and discussed in the context of UAM noise prediction. When used in conjunction with a modern surfacevorticity panel code, these metrics are shown to provide an effective assortment of tools to concisely describe UAMbased acoustic signal properties.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This work identifies and explores several aeroacoustic metrics that allow for urban air mobility (UAM) vehicle noise prediction. An increase in production and use of UAM and distributed electric propulsion vehicles within populated civilian areas stands behind the need to minimize the noise produced by these vehicles. The Federal Aviation Administration's (FAA's) strict noise regulations on UAM aircraft compels designers to place a significant emphasis, early in the design phase, on the characterization and analysis of the external noise generated by these vehicles, namely, to ensure their design viability. To accomplish this, the present study focuses on the analysis and interpretation of predicted noise signals using a set of characteristic metrics that can be instrumental at guiding the design process. Following a thorough review of metrics standardized by the International Civil Aviation Organization as well as the FAA, seven general metrics are identified, evaluated, and discussed in the context of UAM noise prediction. When used in conjunction with a modern surfacevorticity panel code, these metrics are shown to provide an effective assortment of tools to concisely describe UAMbased acoustic signal properties.
On the prediction of noise generated by urban air mobility (UAM) vehicles. I. Integration of fundamental acoustic metrics
10.1063/5.0124134
Physics of Fluids
20221107T12:49:17Z
© 2022 Author(s).
Daniel S. Little
Joseph Majdalani
Roy J. Hartfield
Vivek Ahuja

On the prediction of noise generated by urban air mobility (UAM) vehicles. II. Implementation of the Farassat F1A formulation into a modern surfacevorticity panel solver
https://aip.scitation.org/doi/10.1063/5.0105002?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This study focuses on the integration of established acoustic prediction techniques directly into a surfacevorticity solver. The main objective is to enhance an aircraft designer's ability to characterize the acoustic signatures generated by urban air mobility (UAM) vehicles, in general, and distributed electric propulsion (DEP) concepts, in particular, including unmanned aerial vehicles. Our solver consists of a reliable, surfacevorticity panel code that incorporates viscous boundarylayer corrections. Thus, it offers a computationally efficient commercial tool for conceptual design and preliminary aerodynamic analysis. By implementing the Farassat F1A acoustics formulation directly into the solver, a new intuitive capability is achieved, which is both conversive with modern engineering tools and efficient in setup and speed of execution. In addition to its application to the X57 highlift propeller and the Revolutionary Vertical Lift Technology Tiltwing electric Vertical TakeOff and Landing (eVTOL) vehicle by the National Aeronautics and Space Administration, this capability is systematically demonstrated using three particular case studies. These consist of both single and sixpropeller Joby S4 eVTOL as well as a small eightpropeller Kittyhawk KHH1 DEP vehicle. Although the details of this tool and underlying equations are showcased in this article, the acoustic metrics that can be effectively used to characterize the noise level generated by a UAM in flight are described in a companion article. By embedding this assortment of insightful metrics into a simple and userfriendly flow solver, a much improved flowacoustic analysis capability is thereby provided to support the design of future aircraft.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This study focuses on the integration of established acoustic prediction techniques directly into a surfacevorticity solver. The main objective is to enhance an aircraft designer's ability to characterize the acoustic signatures generated by urban air mobility (UAM) vehicles, in general, and distributed electric propulsion (DEP) concepts, in particular, including unmanned aerial vehicles. Our solver consists of a reliable, surfacevorticity panel code that incorporates viscous boundarylayer corrections. Thus, it offers a computationally efficient commercial tool for conceptual design and preliminary aerodynamic analysis. By implementing the Farassat F1A acoustics formulation directly into the solver, a new intuitive capability is achieved, which is both conversive with modern engineering tools and efficient in setup and speed of execution. In addition to its application to the X57 highlift propeller and the Revolutionary Vertical Lift Technology Tiltwing electric Vertical TakeOff and Landing (eVTOL) vehicle by the National Aeronautics and Space Administration, this capability is systematically demonstrated using three particular case studies. These consist of both single and sixpropeller Joby S4 eVTOL as well as a small eightpropeller Kittyhawk KHH1 DEP vehicle. Although the details of this tool and underlying equations are showcased in this article, the acoustic metrics that can be effectively used to characterize the noise level generated by a UAM in flight are described in a companion article. By embedding this assortment of insightful metrics into a simple and userfriendly flow solver, a much improved flowacoustic analysis capability is thereby provided to support the design of future aircraft.
On the prediction of noise generated by urban air mobility (UAM) vehicles. II. Implementation of the Farassat F1A formulation into a modern surfacevorticity panel solver
10.1063/5.0105002
Physics of Fluids
20221107T12:48:51Z
© 2022 Author(s).
Vivek Ahuja
Daniel S. Little
Joseph Majdalani
Roy J. Hartfield

Largeeddy simulation study of rotating detonation supersonic turbine nozzle generated by the method of characteristics under oscillating incoming flow
https://aip.scitation.org/doi/10.1063/5.0111900?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Rotating detonation turbine engine is receiving considerable attention due to its' high cycle efficiency, outstanding thrust characteristics, selfpressurization, and energysaving attributes. Conventional turbines are inefficient (30%) under rotating detonation inflow conditions. In order to obtain the turbine operating efficiently under the condition of rotating detonation inflow, this paper uses the method of characteristics and Bessel parameterization to design the blade profile of the rotating detonation supersonic turbine. The Large Eddy Simulation is used to numerically study the flow field characteristics of the supersonic turbine blade designed by the method of characteristics. The study found that the rotating detonation supersonic turbine guide vane can effectively reduce the pressure oscillation amplitude of the incoming flow to 25% of the original amplitude, and the main frequency (10 kHz) of the incoming flow occupies the main part of the flow field frequency. Second, the morphological evolution of the shock waves attenuates the adverse pressure gradient on the suction surface. The separation area of the suction surface slowly oscillates and attenuates, and is eventually confined to a small region. The wake accelerates and dissipates under the squeezing jet of the dovetail wave and the intense shearing action, forming a small wake area. The attenuation of largescale separation gradually reduces the separation loss and wake loss, and the convergence and interaction of shock waves and the wake vortex significantly enhance the proportion of entropy production in the shock region. From the pressure coefficient and is entropic Mach number distributions, it is found that the blade load is mainly concentrated in the tail, and is minimized when the flow field becomes stable. These features provide a reference for the design of rotating detonation supersonic turbines and a deeper understanding of the flow field characteristics of rotating detonation turbine engines.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Rotating detonation turbine engine is receiving considerable attention due to its' high cycle efficiency, outstanding thrust characteristics, selfpressurization, and energysaving attributes. Conventional turbines are inefficient (30%) under rotating detonation inflow conditions. In order to obtain the turbine operating efficiently under the condition of rotating detonation inflow, this paper uses the method of characteristics and Bessel parameterization to design the blade profile of the rotating detonation supersonic turbine. The Large Eddy Simulation is used to numerically study the flow field characteristics of the supersonic turbine blade designed by the method of characteristics. The study found that the rotating detonation supersonic turbine guide vane can effectively reduce the pressure oscillation amplitude of the incoming flow to 25% of the original amplitude, and the main frequency (10 kHz) of the incoming flow occupies the main part of the flow field frequency. Second, the morphological evolution of the shock waves attenuates the adverse pressure gradient on the suction surface. The separation area of the suction surface slowly oscillates and attenuates, and is eventually confined to a small region. The wake accelerates and dissipates under the squeezing jet of the dovetail wave and the intense shearing action, forming a small wake area. The attenuation of largescale separation gradually reduces the separation loss and wake loss, and the convergence and interaction of shock waves and the wake vortex significantly enhance the proportion of entropy production in the shock region. From the pressure coefficient and is entropic Mach number distributions, it is found that the blade load is mainly concentrated in the tail, and is minimized when the flow field becomes stable. These features provide a reference for the design of rotating detonation supersonic turbines and a deeper understanding of the flow field characteristics of rotating detonation turbine engines.
Largeeddy simulation study of rotating detonation supersonic turbine nozzle generated by the method of characteristics under oscillating incoming flow
10.1063/5.0111900
Physics of Fluids
20221108T01:22:01Z
© 2022 Author(s).

Relationship between physical parameters of supercritical fluids and normal shock characteristics
https://aip.scitation.org/doi/10.1063/5.0122905?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Physical parameters of supercritical fluids change drastically near the critical region, which makes it difficult to predict and analyze the supercritical fluid flow parameters behind the normal shock wave. In this paper, in combination with supercritical fluid physical parameters database, we employed an iterative algorithm to solve the flow parameters behind normal shock by deriving shock equations. The change of normal shock parameters of six supercritical fluids with inflow state was studied by the controlled variable method and a correlation analysis. The results show that when the inflow Mach number is fixed, the normal shock parameters, such as density ratio and pressure ratio, change rapidly in the Widom zone as a result of the dramatic changes of the physical parameters. When the inflow state is the same, the normal shock pressure ratio of NH3 is the highest, whereas that of C8H24O2Si3 and C10H22 are pretty low. The normal shock intensity of supercritical fluid is better reflected by the pressure ratio rather than Mach number. According to the correlation analysis, the compressibility factor and the sound speed of inflow are the main physical factors that affect the normal shock density ratio and pressure ratio, respectively. Based on the main physical factors, empirical equations for predicting the change trend of normal shock pressure ratio and density ratio are obtained.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Physical parameters of supercritical fluids change drastically near the critical region, which makes it difficult to predict and analyze the supercritical fluid flow parameters behind the normal shock wave. In this paper, in combination with supercritical fluid physical parameters database, we employed an iterative algorithm to solve the flow parameters behind normal shock by deriving shock equations. The change of normal shock parameters of six supercritical fluids with inflow state was studied by the controlled variable method and a correlation analysis. The results show that when the inflow Mach number is fixed, the normal shock parameters, such as density ratio and pressure ratio, change rapidly in the Widom zone as a result of the dramatic changes of the physical parameters. When the inflow state is the same, the normal shock pressure ratio of NH3 is the highest, whereas that of C8H24O2Si3 and C10H22 are pretty low. The normal shock intensity of supercritical fluid is better reflected by the pressure ratio rather than Mach number. According to the correlation analysis, the compressibility factor and the sound speed of inflow are the main physical factors that affect the normal shock density ratio and pressure ratio, respectively. Based on the main physical factors, empirical equations for predicting the change trend of normal shock pressure ratio and density ratio are obtained.
Relationship between physical parameters of supercritical fluids and normal shock characteristics
10.1063/5.0122905
Physics of Fluids
20221108T01:21:56Z
© 2022 Author(s).

Neuralnetworkbased Riemann solver for real fluids and high explosives; application to computational fluid dynamics
https://aip.scitation.org/doi/10.1063/5.0123466?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The Riemann problem is fundamental to most computational fluid dynamics (CFD) codes for simulating compressible flows. The time to obtain the exact solution to this problem for real fluids is high because of the complexity of the fluid model, which includes the equation of state; as a result, approximate Riemann solvers are used in lieu of the exact ones, even for ideal gases. We used fully connected feedforward neural networks to find the solution to the Riemann problem for calorically imperfect gases, supercritical fluids, and high explosives and then embedded these network into a onedimensional finite volume CFD code. We showed that for real fluids, the neural networks can be more than five orders of magnitude faster than the exact solver, with prediction errors below 0.8%. The same neural networks embedded in a CFD code yields very good agreement with the overall exact solution, with a speedup of three orders of magnitude with respect to the same CFD code that use the exact Riemann solver to resolve the flux at the interfaces. Compared to the Rusanov flux reconstruction method, the neural network is half as fast but yields a higher accuracy and is able to converge to the exact solution with a coarser grid.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The Riemann problem is fundamental to most computational fluid dynamics (CFD) codes for simulating compressible flows. The time to obtain the exact solution to this problem for real fluids is high because of the complexity of the fluid model, which includes the equation of state; as a result, approximate Riemann solvers are used in lieu of the exact ones, even for ideal gases. We used fully connected feedforward neural networks to find the solution to the Riemann problem for calorically imperfect gases, supercritical fluids, and high explosives and then embedded these network into a onedimensional finite volume CFD code. We showed that for real fluids, the neural networks can be more than five orders of magnitude faster than the exact solver, with prediction errors below 0.8%. The same neural networks embedded in a CFD code yields very good agreement with the overall exact solution, with a speedup of three orders of magnitude with respect to the same CFD code that use the exact Riemann solver to resolve the flux at the interfaces. Compared to the Rusanov flux reconstruction method, the neural network is half as fast but yields a higher accuracy and is able to converge to the exact solution with a coarser grid.
Neuralnetworkbased Riemann solver for real fluids and high explosives; application to computational fluid dynamics
10.1063/5.0123466
Physics of Fluids
20221108T02:10:32Z
© 2022 Author(s).
Matteo Ruggeri
Indradip Roy
Michael J. Mueterthies
Tom Gruenwald
Carlo Scalo

Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide. I. Argon dilution from 2200 to 8700 K
https://aip.scitation.org/doi/10.1063/5.0109109?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This work investigates the hightemperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in argon (Ar) to target NO–Ar and NO–NO interactions and to augment the subsequent inference of rates for NO diluted in nitrogen (N2). [J. W. Streicher et al., “Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide. II. Nitrogen dilution from 1900 to 8200 K,” Phys. Fluids (submitted)]. In both Part I and Part II, two continuouswave ultraviolet laser diagnostics were used to probe quantumstatespecific timehistories of NO behind reflected shocks in hightemperature shocktube experiments, enabling inferences of multiple vibrational relaxation times and reaction rate constants for NO decomposition reactions. These diagnostics both probed absorbance (α) in the ground vibrational state of NO but in multiple rotational states utilizing light at 224.8150 and 226.1025 nm. The absorbance was subsequently used to infer quantumstatespecific timehistories for translational/rotational temperature (Ttr) via the absorbance ratio and number density of NO (nNO) via α, Ttr, and the absorbance cross sections (σ). The experiments for Ar dilution probed mixtures of 2% NO/Ar, 1% NO/Ar, and 0.4% NO/Ar for initial postreflectedshock conditions from 2200–8700 K and 0.12–0.97 atm. Further analysis of the absorbance, temperature, and number density timehistories yielded two vibrational relaxation times ([math] and [math]) and four rate coefficients for multiple NO decomposition reactions ([math], [math], [math], and [math])—each of which is extended to higher temperatures than any previous study and with reduced scatter and uncertainty. Generally, these rate data are consistent with data from the literature, although [math] and [math] are observed to differ strongly from both the Millikan and White correlation and Park twotemperature model.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This work investigates the hightemperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in argon (Ar) to target NO–Ar and NO–NO interactions and to augment the subsequent inference of rates for NO diluted in nitrogen (N2). [J. W. Streicher et al., “Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide. II. Nitrogen dilution from 1900 to 8200 K,” Phys. Fluids (submitted)]. In both Part I and Part II, two continuouswave ultraviolet laser diagnostics were used to probe quantumstatespecific timehistories of NO behind reflected shocks in hightemperature shocktube experiments, enabling inferences of multiple vibrational relaxation times and reaction rate constants for NO decomposition reactions. These diagnostics both probed absorbance (α) in the ground vibrational state of NO but in multiple rotational states utilizing light at 224.8150 and 226.1025 nm. The absorbance was subsequently used to infer quantumstatespecific timehistories for translational/rotational temperature (Ttr) via the absorbance ratio and number density of NO (nNO) via α, Ttr, and the absorbance cross sections (σ). The experiments for Ar dilution probed mixtures of 2% NO/Ar, 1% NO/Ar, and 0.4% NO/Ar for initial postreflectedshock conditions from 2200–8700 K and 0.12–0.97 atm. Further analysis of the absorbance, temperature, and number density timehistories yielded two vibrational relaxation times ([math] and [math]) and four rate coefficients for multiple NO decomposition reactions ([math], [math], [math], and [math])—each of which is extended to higher temperatures than any previous study and with reduced scatter and uncertainty. Generally, these rate data are consistent with data from the literature, although [math] and [math] are observed to differ strongly from both the Millikan and White correlation and Park twotemperature model.
Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide. I. Argon dilution from 2200 to 8700 K
10.1063/5.0109109
Physics of Fluids
20221108T01:24:01Z
© 2022 Author(s).
Jesse W. Streicher
Ajay Krish
Ronald K. Hanson

Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide: II. Nitrogen dilution from 1900 to 8200 K
https://aip.scitation.org/doi/10.1063/5.0122787?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This work investigates the hightemperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in nitrogen (N2) to target the NO–N2 rates relevant to hightemperature air, thereby building off the argon (Ar) experiments investigated in Part I. [J. W. Streicher et al., “Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide. I. Argon dilution from 2200 to 8700 K,” Phys. Fluids 34, 116122 (2022)] Again, two continuouswave ultraviolet laser diagnostics were used to obtain quantumstatespecific time histories of NO in hightemperature shocktube experiments, including absorbance (α) in the ground vibrational state of NO, translational/rotational temperature (Ttr), and number density of NO (nNO). The experiments probed mixtures of 2% and 0.4% NO diluted in either pure N2 (NO/N2) or an equal parts N2/Ar mixture (NO/N2/Ar). The NO/N2 experiments spanned initial postreflectedshock conditions from 1900–7000 K and 0.05–1.14 atm, while the NO/N2/Ar experiments spanned from 1900–8200 K and 0.11–1.52 atm. This work leveraged two vibrational relaxation times from Part I ([math] and [math]) and extended measurements to include the vibrational–translational and vibrational–vibrational relaxation times with N2 ([math] and [math]). Similarly, this work leveraged the four rate coefficients from Part I ([math], [math], [math], and [math]) and extended measurements to include NO dissociation with N2 ([math]). A few studies have directly inferred these rates from experiments, and the current data differ from common model values. In particular, [math] differs slightly from the Millikan and White correlation, [math] is four times slower than Taylor et al.'s inference, and [math] is four times slower than the Park twotemperature model. The unique experimental measurements and dilution in N2 in this study significantly improve the understanding of the vibrational relaxation and decomposition of NO in hightemperature air.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This work investigates the hightemperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in nitrogen (N2) to target the NO–N2 rates relevant to hightemperature air, thereby building off the argon (Ar) experiments investigated in Part I. [J. W. Streicher et al., “Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide. I. Argon dilution from 2200 to 8700 K,” Phys. Fluids 34, 116122 (2022)] Again, two continuouswave ultraviolet laser diagnostics were used to obtain quantumstatespecific time histories of NO in hightemperature shocktube experiments, including absorbance (α) in the ground vibrational state of NO, translational/rotational temperature (Ttr), and number density of NO (nNO). The experiments probed mixtures of 2% and 0.4% NO diluted in either pure N2 (NO/N2) or an equal parts N2/Ar mixture (NO/N2/Ar). The NO/N2 experiments spanned initial postreflectedshock conditions from 1900–7000 K and 0.05–1.14 atm, while the NO/N2/Ar experiments spanned from 1900–8200 K and 0.11–1.52 atm. This work leveraged two vibrational relaxation times from Part I ([math] and [math]) and extended measurements to include the vibrational–translational and vibrational–vibrational relaxation times with N2 ([math] and [math]). Similarly, this work leveraged the four rate coefficients from Part I ([math], [math], [math], and [math]) and extended measurements to include NO dissociation with N2 ([math]). A few studies have directly inferred these rates from experiments, and the current data differ from common model values. In particular, [math] differs slightly from the Millikan and White correlation, [math] is four times slower than Taylor et al.'s inference, and [math] is four times slower than the Park twotemperature model. The unique experimental measurements and dilution in N2 in this study significantly improve the understanding of the vibrational relaxation and decomposition of NO in hightemperature air.
Hightemperature vibrational relaxation and decomposition of shockheated nitric oxide: II. Nitrogen dilution from 1900 to 8200 K
10.1063/5.0122787
Physics of Fluids
20221108T01:24:07Z
© 2022 Author(s).
Jesse W. Streicher
Ajay Krish
Ronald K. Hanson

Effect of corrugation on limiting arctab mixing performance
https://aip.scitation.org/doi/10.1063/5.0127031?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The effect of corrugation on the mixing promoting performance of a limiting arctab has been assessed by studying the decay of a Mach 1.76 jet emerging from a convergentdivergent nozzle, with a limiting arctab, with and without corrugation, running along a diameter at the nozzle exit, operated at nozzle pressure ratios of 4.5 and 8. Uncontrolled jet was also studied for comparison. The physics behind the difference between the mixing caused by the uncorrugated and corrugated tabs in the presence of adverse and favorable pressure gradient is addressed using the shadowgraph images of the jet core as the main source and the centerline pitot pressure decay as the supporting material. The results show that the mixing promotion caused by the tabs leads to the shortening of the third and fourth shock cells in the jet core. This can be regarded as an advantage from the screech suppression point of view.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The effect of corrugation on the mixing promoting performance of a limiting arctab has been assessed by studying the decay of a Mach 1.76 jet emerging from a convergentdivergent nozzle, with a limiting arctab, with and without corrugation, running along a diameter at the nozzle exit, operated at nozzle pressure ratios of 4.5 and 8. Uncontrolled jet was also studied for comparison. The physics behind the difference between the mixing caused by the uncorrugated and corrugated tabs in the presence of adverse and favorable pressure gradient is addressed using the shadowgraph images of the jet core as the main source and the centerline pitot pressure decay as the supporting material. The results show that the mixing promotion caused by the tabs leads to the shortening of the third and fourth shock cells in the jet core. This can be regarded as an advantage from the screech suppression point of view.
Effect of corrugation on limiting arctab mixing performance
10.1063/5.0127031
Physics of Fluids
20221110T12:18:14Z
© 2022 Author(s).
Ethirajan Rathakrishnan

Onset conditions for Mach disk formation in underexpanded jet flows
https://aip.scitation.org/doi/10.1063/5.0122861?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this study, the formation conditions of the Mach disk in an underexpanded jet flow were numerically and theoretically investigated under sonic injection conditions and the assumption of an axisymmetric flow. The numerical results demonstrated that the threshold nozzlepressure ratio (NPR) at which the Mach disk occurred was situated between 3.03 and 3.12, which is lower than those reported in the previous studies. Since the oscillation frequency of the Mach disk was approximately constant over a wide range of NPRs and the amplitude was weak, it was regarded as a steady shock. In addition, the Mach disk was found to form when the total pressure loss reached approximately 40%. To predict the appearance of the Mach disk, we proposed a theoretical model based on a quasionedimensional flow by considering the Mach disk as a normal shock wave on the axis. Based only on injection and ambient conditions, the totalpressure loss ratio derived from the model was in good agreement with that obtained from the numerical simulations, indicating that the proposed model provides useful knowledge for Mach disk occurrence prediction.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this study, the formation conditions of the Mach disk in an underexpanded jet flow were numerically and theoretically investigated under sonic injection conditions and the assumption of an axisymmetric flow. The numerical results demonstrated that the threshold nozzlepressure ratio (NPR) at which the Mach disk occurred was situated between 3.03 and 3.12, which is lower than those reported in the previous studies. Since the oscillation frequency of the Mach disk was approximately constant over a wide range of NPRs and the amplitude was weak, it was regarded as a steady shock. In addition, the Mach disk was found to form when the total pressure loss reached approximately 40%. To predict the appearance of the Mach disk, we proposed a theoretical model based on a quasionedimensional flow by considering the Mach disk as a normal shock wave on the axis. Based only on injection and ambient conditions, the totalpressure loss ratio derived from the model was in good agreement with that obtained from the numerical simulations, indicating that the proposed model provides useful knowledge for Mach disk occurrence prediction.
Onset conditions for Mach disk formation in underexpanded jet flows
10.1063/5.0122861
Physics of Fluids
20221110T12:17:24Z
© 2022 Author(s).
Ryota Muraoka
Toshihiko Hiejima

Thermodynamic modeling for numerical simulations based on the generalized cubic equation of state
https://aip.scitation.org/doi/10.1063/5.0122277?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We further elaborate on the generalized formulation for cubic equation of state proposed by Cismondi and Mollerup [Fluid Phase Equilib. 232, 74–89 (2005)]. With this formulation, all wellknown cubic equations of state can be described with a certain pair of values, which allow for a generic implementation of different equations of state. Based on this generalized formulation, we derive a complete thermodynamic model for computational fluid dynamics simulations by providing the resulting correlations for all required thermodynamic properties. For the transport properties, we employ the Chung correlations. Our generic implementation includes the often used equations of state Soave–Redlich–Kwong and Peng–Robinson and the Redlich–Kwong–Peng–Robinson equation of state. The first two assume a universal critical compressibility factor and are, therefore, only suitable for fluids with a matching critical compressibility. The Redlich–Kwong–Peng–Robinson overcomes this limitation by considering the equation of state parameter as a function of the critical compressibility. We compare the resulting thermodynamic modeling for the three equations of state for selected fluids with each other and CoolProp reference data. Additionally, we provide a Python tool called real gas thermodynamic python library (realtpl). This tool can be used to evaluate and compare the results for a wide range of different fluids. We also provide an implementation of the generalized form in OpenFOAM.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We further elaborate on the generalized formulation for cubic equation of state proposed by Cismondi and Mollerup [Fluid Phase Equilib. 232, 74–89 (2005)]. With this formulation, all wellknown cubic equations of state can be described with a certain pair of values, which allow for a generic implementation of different equations of state. Based on this generalized formulation, we derive a complete thermodynamic model for computational fluid dynamics simulations by providing the resulting correlations for all required thermodynamic properties. For the transport properties, we employ the Chung correlations. Our generic implementation includes the often used equations of state Soave–Redlich–Kwong and Peng–Robinson and the Redlich–Kwong–Peng–Robinson equation of state. The first two assume a universal critical compressibility factor and are, therefore, only suitable for fluids with a matching critical compressibility. The Redlich–Kwong–Peng–Robinson overcomes this limitation by considering the equation of state parameter as a function of the critical compressibility. We compare the resulting thermodynamic modeling for the three equations of state for selected fluids with each other and CoolProp reference data. Additionally, we provide a Python tool called real gas thermodynamic python library (realtpl). This tool can be used to evaluate and compare the results for a wide range of different fluids. We also provide an implementation of the generalized form in OpenFOAM.
Thermodynamic modeling for numerical simulations based on the generalized cubic equation of state
10.1063/5.0122277
Physics of Fluids
20221116T01:36:37Z
© 2022 Author(s).
T. Trummler
M. Glatzle
A. Doehring
N. Urban
M. Klein

Densitydriven exchange flow propagating over an array of densified obstacles
https://aip.scitation.org/doi/10.1063/5.0120342?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The evolution of bottompropagating gravity currents with the presence of an array of densified obstacles submerged in a channel is investigated using largeeddy simulations. Our attention is particularly focused on the flow transition of gravity currents over rough surfaces with extra resistance that provokes significant dissipative processes. Two geometric parameters of the roughness elements, namely, the submergence ratio of the obstacle D/H and the gapspacing ratio [math] between obstacles, govern their kinematic and dynamic effects on the propagation of gravity currents. Physically, D/H plays a significant role in the control of the current diversion, and [math] regulates the flow pathway of gravity current propagation. The integrated measures show that two distinct flow morphologies are identified. For a low submergence ratio ([math]), an overtopping flow is formed in which the gravity current travels on the top of the array and undergoes an inconspicuous loss of buoyancy, subject to minimal vertical convective instability interacting with the underlying ambient fluid within the gap regions. For a sufficiently high submergence ratio ([math]) and a certain gap spacing ([math]), an overrunning flow is formed in which the current rapidly decelerates to a buoyancy–inertia state and then transitions to a dragdominated state with a gain in excessive drag, in which the front velocity is proportional to [math]. However, the simulation results show a turning point toward an increase in the gap spacing as [math], that the maximum drag acting on the gravity current is measured when it impinges on the second obstacle of an array, and that the drag coefficient goes up by [math], depending on D/H. The propagation of the gravity current does not show a higher sensitivity to the retarding effect instead. Meanwhile, the promotion of energy conversion occurs because of the gravity current encountering the continuous climbing and plunging flow behavior between two adjacent obstacles in regular motions.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The evolution of bottompropagating gravity currents with the presence of an array of densified obstacles submerged in a channel is investigated using largeeddy simulations. Our attention is particularly focused on the flow transition of gravity currents over rough surfaces with extra resistance that provokes significant dissipative processes. Two geometric parameters of the roughness elements, namely, the submergence ratio of the obstacle D/H and the gapspacing ratio [math] between obstacles, govern their kinematic and dynamic effects on the propagation of gravity currents. Physically, D/H plays a significant role in the control of the current diversion, and [math] regulates the flow pathway of gravity current propagation. The integrated measures show that two distinct flow morphologies are identified. For a low submergence ratio ([math]), an overtopping flow is formed in which the gravity current travels on the top of the array and undergoes an inconspicuous loss of buoyancy, subject to minimal vertical convective instability interacting with the underlying ambient fluid within the gap regions. For a sufficiently high submergence ratio ([math]) and a certain gap spacing ([math]), an overrunning flow is formed in which the current rapidly decelerates to a buoyancy–inertia state and then transitions to a dragdominated state with a gain in excessive drag, in which the front velocity is proportional to [math]. However, the simulation results show a turning point toward an increase in the gap spacing as [math], that the maximum drag acting on the gravity current is measured when it impinges on the second obstacle of an array, and that the drag coefficient goes up by [math], depending on D/H. The propagation of the gravity current does not show a higher sensitivity to the retarding effect instead. Meanwhile, the promotion of energy conversion occurs because of the gravity current encountering the continuous climbing and plunging flow behavior between two adjacent obstacles in regular motions.
Densitydriven exchange flow propagating over an array of densified obstacles
10.1063/5.0120342
Physics of Fluids
20221101T02:08:50Z
© 2022 Author(s).
ChingSen Wu

Energy and fluxbudget theory for surface layers in atmospheric convective turbulence
https://aip.scitation.org/doi/10.1063/5.0123401?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The energy and fluxbudget (EFB) theory developed previously for atmospheric stably stratified turbulence is extended to the surface layer in atmospheric convective turbulence. This theory is based on budget equations for turbulent energies and fluxes in the Boussinesq approximation. In the lower part of the surface layer in the atmospheric convective boundary layer, the rate of turbulence production of the turbulent kinetic energy (TKE) caused by the surface shear is much larger than that caused by the buoyancy, which results in threedimensional turbulence of very complex nature. In the upper part of the surface layer, the rate of turbulence production of TKE due to the shear is much smaller than that caused by the buoyancy, which causes unusual strongly anisotropic buoyancydriven turbulence. Considering the applications of the obtained results to the atmospheric convective boundarylayer turbulence, the theoretical relationships potentially useful in modeling applications have been derived. The developed EFB theory allows us to obtain a smooth transition between a stably stratified turbulence to a convective turbulence. The EFB theory for the surface layer in a convective turbulence provides an analytical expression for the entire surface layer including the transition range between the lower and upper parts of the surface layer, and it allows us to determine the vertical profiles for all turbulent characteristics, including TKE, the intensity of turbulent potential temperature fluctuations, the vertical turbulent fluxes of momentum and buoyancy (proportional to potential temperature), the integral turbulence scale, the turbulence anisotropy, the turbulent Prandtl number, and the flux Richardson number.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The energy and fluxbudget (EFB) theory developed previously for atmospheric stably stratified turbulence is extended to the surface layer in atmospheric convective turbulence. This theory is based on budget equations for turbulent energies and fluxes in the Boussinesq approximation. In the lower part of the surface layer in the atmospheric convective boundary layer, the rate of turbulence production of the turbulent kinetic energy (TKE) caused by the surface shear is much larger than that caused by the buoyancy, which results in threedimensional turbulence of very complex nature. In the upper part of the surface layer, the rate of turbulence production of TKE due to the shear is much smaller than that caused by the buoyancy, which causes unusual strongly anisotropic buoyancydriven turbulence. Considering the applications of the obtained results to the atmospheric convective boundarylayer turbulence, the theoretical relationships potentially useful in modeling applications have been derived. The developed EFB theory allows us to obtain a smooth transition between a stably stratified turbulence to a convective turbulence. The EFB theory for the surface layer in a convective turbulence provides an analytical expression for the entire surface layer including the transition range between the lower and upper parts of the surface layer, and it allows us to determine the vertical profiles for all turbulent characteristics, including TKE, the intensity of turbulent potential temperature fluctuations, the vertical turbulent fluxes of momentum and buoyancy (proportional to potential temperature), the integral turbulence scale, the turbulence anisotropy, the turbulent Prandtl number, and the flux Richardson number.
Energy and fluxbudget theory for surface layers in atmospheric convective turbulence
10.1063/5.0123401
Physics of Fluids
20221104T12:50:53Z
© 2022 Author(s).
I. Rogachevskii
N. Kleeorin
S. Zilitinkevich

Momentum and pseudomomentum in a shallow water equation
https://aip.scitation.org/doi/10.1063/5.0120645?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A basic shallow water system with variable topography is analyzed from the point of view of a Lagrangian derivation of momentum, energy, and pseudomomentum balances. A twodimensional action and associated momentum equation are derived. The latter is further manipulated to derive additional equations for energy and pseudomomentum. This revealed structure emphasizes broken symmetries in space and a reference configuration and preserved symmetry in time.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A basic shallow water system with variable topography is analyzed from the point of view of a Lagrangian derivation of momentum, energy, and pseudomomentum balances. A twodimensional action and associated momentum equation are derived. The latter is further manipulated to derive additional equations for energy and pseudomomentum. This revealed structure emphasizes broken symmetries in space and a reference configuration and preserved symmetry in time.
Momentum and pseudomomentum in a shallow water equation
10.1063/5.0120645
Physics of Fluids
20221104T12:22:09Z
© 2022 Author(s).
J. A. Hanna

Learning ocean circulation models with reservoir computing
https://aip.scitation.org/doi/10.1063/5.0119061?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Two elementary models of ocean circulation, the wellknown doublegyre stream function model and a singlelayer quasigeostrophic (QG) basin model, are used to generate flow data that sample a range of possible dynamical behavior for particular flow parameters. A reservoir computing (RC) machine learning algorithm then learns these models from the stream function time series. In the case of the QG model, a system of partial differential equations with three physically relevant dimensionless parameters is solved, including Munk and Stommeltype solutions. The effectiveness of a RC approach to learning these ocean circulation models is evident from its ability to capture the characteristics of these ocean circulation models with limited data including predictive forecasts. Further assessment of the accuracy and usefulness of the RC approach is conducted by evaluating the role of both physical and numerical parameters and by comparison with particle trajectories and with wellestablished quantitative assessments, including finitetime Lyapunov exponents and proper orthogonal decomposition. The results show the capability of the methods outlined in this article to be applied to key research problems on ocean transport, such as predictive modeling or control.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Two elementary models of ocean circulation, the wellknown doublegyre stream function model and a singlelayer quasigeostrophic (QG) basin model, are used to generate flow data that sample a range of possible dynamical behavior for particular flow parameters. A reservoir computing (RC) machine learning algorithm then learns these models from the stream function time series. In the case of the QG model, a system of partial differential equations with three physically relevant dimensionless parameters is solved, including Munk and Stommeltype solutions. The effectiveness of a RC approach to learning these ocean circulation models is evident from its ability to capture the characteristics of these ocean circulation models with limited data including predictive forecasts. Further assessment of the accuracy and usefulness of the RC approach is conducted by evaluating the role of both physical and numerical parameters and by comparison with particle trajectories and with wellestablished quantitative assessments, including finitetime Lyapunov exponents and proper orthogonal decomposition. The results show the capability of the methods outlined in this article to be applied to key research problems on ocean transport, such as predictive modeling or control.
Learning ocean circulation models with reservoir computing
10.1063/5.0119061
Physics of Fluids
20221109T12:03:37Z
© 2022 Author(s).
Kevin Yao
Eric Forgoston
Philip Yecko

Staircase formation in unstably stratified double diffusive finger convection
https://aip.scitation.org/doi/10.1063/5.0122882?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Double diffusive staircases are experimentally investigated in a fluid layer with a stabilizing temperature gradient and a destabilizing gradient of ion concentration. Gradients of temperature and ion concentration are maintained in a steady state within an electrochemical system. Staircases are observed even if the density stratification is unstable. None of the previously proposed mechanisms for staircase formation can be recognized in the experiments. Ion transport through fingers that are part of a staircase is not the same as the transport through fingers extending through the entire cell. Fingers cease to exist if the diffusive heat transport between neighboring fingers is insufficient.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Double diffusive staircases are experimentally investigated in a fluid layer with a stabilizing temperature gradient and a destabilizing gradient of ion concentration. Gradients of temperature and ion concentration are maintained in a steady state within an electrochemical system. Staircases are observed even if the density stratification is unstable. None of the previously proposed mechanisms for staircase formation can be recognized in the experiments. Ion transport through fingers that are part of a staircase is not the same as the transport through fingers extending through the entire cell. Fingers cease to exist if the diffusive heat transport between neighboring fingers is insufficient.
Staircase formation in unstably stratified double diffusive finger convection
10.1063/5.0122882
Physics of Fluids
20221110T12:17:27Z
© 2022 Author(s).
A. Rosenthal
K. Lüdemann
A. Tilgner

Triad resonance of flexural gravity waves in a twolayer fluid within the framework of blocking dynamics
https://aip.scitation.org/doi/10.1063/5.0117974?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The present study deals with the formation of triads in flexural gravity waves in a twolayer density stratified fluid having a flexible platecovered surface and an interface within the framework of blocking, which can be used as a mechanism for understanding the spectral distribution of wave energy. The physical model is considered in a twodimensional Cartesian coordinate system, and the formations of triads are discussed geometrically and validated analytically. The study demonstrates the formation of at most eight triads of three different classes for flexural gravity waves before the threshold of blocking, whereas a maximum of six triads of three different classes have been reported in the case of freesurface gravity waves. However, at least twenty triads are formed for any frequency within the blocking limits for the compressive force lying within the threshold of blocking and buckling limit, irrespective of water depth. On the other hand, 24 triads are formed for a certain frequency in the left neighborhood of the primary blocking point, while thirty triads occur when the frequency is considered in the right neighborhood of the secondary blocking point for specific values of compressive force and density ratio in the case of deep water/short flexural gravity waves in both the layers. In addition, a new class of triad, which is rarely found in the case of surface gravity waves, occurs for a certain frequency within the blocking limits for a suitable choice of compressive force and density ratio in the case of waves in deep/intermediate upper layer depth.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The present study deals with the formation of triads in flexural gravity waves in a twolayer density stratified fluid having a flexible platecovered surface and an interface within the framework of blocking, which can be used as a mechanism for understanding the spectral distribution of wave energy. The physical model is considered in a twodimensional Cartesian coordinate system, and the formations of triads are discussed geometrically and validated analytically. The study demonstrates the formation of at most eight triads of three different classes for flexural gravity waves before the threshold of blocking, whereas a maximum of six triads of three different classes have been reported in the case of freesurface gravity waves. However, at least twenty triads are formed for any frequency within the blocking limits for the compressive force lying within the threshold of blocking and buckling limit, irrespective of water depth. On the other hand, 24 triads are formed for a certain frequency in the left neighborhood of the primary blocking point, while thirty triads occur when the frequency is considered in the right neighborhood of the secondary blocking point for specific values of compressive force and density ratio in the case of deep water/short flexural gravity waves in both the layers. In addition, a new class of triad, which is rarely found in the case of surface gravity waves, occurs for a certain frequency within the blocking limits for a suitable choice of compressive force and density ratio in the case of waves in deep/intermediate upper layer depth.
Triad resonance of flexural gravity waves in a twolayer fluid within the framework of blocking dynamics
10.1063/5.0117974
Physics of Fluids
20221116T10:41:26Z
© 2022 Author(s).
N. Bisht
S. Boral
T. Sahoo
Michael. H. Meylan

An unconventional tsunami: 2022 Tonga event
https://aip.scitation.org/doi/10.1063/5.0122830?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>On January 15, 2022, a powerful eruption occurred at Honga TongaHunga Ha'apai volcano, Tonga, and a tsunami was generated to propagate across the Pacific Ocean. The recorded tsunami waves were both earlier and more destructive than predicted by the conventional tsunami models. In this study, we investigate the underlying mechanism of this tsunami event, which is confirmed to be a combination of the atmospheric forcing, the volcanic eruption and the local resonance. Our numerical results show that the atmospheric pressure variations induced by the volcano eruption generated early waves with small amplitudes of about 0.1 m, while the volcano eruption as a direct source, with a duration of 8 min and an ejected volume of 0.3 km3, triggered large waves propagating across the South Pacific Ocean with amplitudes of about 0.5 m. In addition, the local resonance effect resulted in extreme waves with amplitude of 0.8–1.7 m in the coastal regions. These results reasonably explain the observed facts, confirming that the 2022 Tonga tsunami was an unconventional event.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>On January 15, 2022, a powerful eruption occurred at Honga TongaHunga Ha'apai volcano, Tonga, and a tsunami was generated to propagate across the Pacific Ocean. The recorded tsunami waves were both earlier and more destructive than predicted by the conventional tsunami models. In this study, we investigate the underlying mechanism of this tsunami event, which is confirmed to be a combination of the atmospheric forcing, the volcanic eruption and the local resonance. Our numerical results show that the atmospheric pressure variations induced by the volcano eruption generated early waves with small amplitudes of about 0.1 m, while the volcano eruption as a direct source, with a duration of 8 min and an ejected volume of 0.3 km3, triggered large waves propagating across the South Pacific Ocean with amplitudes of about 0.5 m. In addition, the local resonance effect resulted in extreme waves with amplitude of 0.8–1.7 m in the coastal regions. These results reasonably explain the observed facts, confirming that the 2022 Tonga tsunami was an unconventional event.
An unconventional tsunami: 2022 Tonga event
10.1063/5.0122830
Physics of Fluids
20221116T10:41:33Z
© 2022 Author(s).
Peida Han
Xiping Yu

Inertiogravity Poincaré waves and the quantum relativistic Klein–Gordon equation, nearinertial waves and the nonrelativistic Schrödinger equation
https://aip.scitation.org/doi/10.1063/5.0120375?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Shallow water inertiogravity Poincaré waves in a rotating frame satisfy the Klein–Gordon equation, originally derived for relativistic, spinless quantum particles. Here, we compare these two superficially unrelated phenomena, suggesting a reason for them sharing the same equation. We discuss their energy conservation laws and the equivalency between the nonrelativistic limit of the Klein–Gordon equation, yielding the Schrödinger equation, and the nearinertial wave limit in the shallow water system.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Shallow water inertiogravity Poincaré waves in a rotating frame satisfy the Klein–Gordon equation, originally derived for relativistic, spinless quantum particles. Here, we compare these two superficially unrelated phenomena, suggesting a reason for them sharing the same equation. We discuss their energy conservation laws and the equivalency between the nonrelativistic limit of the Klein–Gordon equation, yielding the Schrödinger equation, and the nearinertial wave limit in the shallow water system.
Inertiogravity Poincaré waves and the quantum relativistic Klein–Gordon equation, nearinertial waves and the nonrelativistic Schrödinger equation
10.1063/5.0120375
Physics of Fluids
20221117T12:25:42Z
© 2022 Author(s).
E. Heifetz
Leo R. M. Maas
J. Mak
I. Pomerantz

Porescale study of mineral dissolution in heterogeneous structures and deep learning prediction of permeability
https://aip.scitation.org/doi/10.1063/5.0123966?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Reactive transport processes in porous media with dissolution of solid structures are widely encountered in scientific and engineering problems. In the present work, the reactive transport processes in heterogeneous porous structures generated by Monte Carlo stochastic movement are simulated by using the lattice Boltzmann method. Six dissolution patterns are identified under different Peclet and Damkohler numbers, including uniform pattern, hybrid pattern, compact pattern, conical pattern, dominant pattern, and ramified pattern. Particularly, when Peclet and Damkohler numbers are larger than 1, the increase in the heterogeneity rises the chance of preferential channel flow in the porous medium and thus intensifies the wormhole phenomena, leading to higher permeability. The porescale results also show that compared with the specific surface area, the permeability is more sensitive to the alteration of the structural heterogeneity, and it is challenging to propose a general formula between permeability and porosity under different reactive transport conditions and structural heterogeneity. Thus, deep neural network is employed to predict the permeability–porosity relationship. The average value of mean absolute percentage error of prediction of 12 additional permeability–porosity curves is 6.89%, indicating the promising potential of using deep learning for predicting the complicated variations of permeability in heterogeneous porous media with dissolution of solid structures.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Reactive transport processes in porous media with dissolution of solid structures are widely encountered in scientific and engineering problems. In the present work, the reactive transport processes in heterogeneous porous structures generated by Monte Carlo stochastic movement are simulated by using the lattice Boltzmann method. Six dissolution patterns are identified under different Peclet and Damkohler numbers, including uniform pattern, hybrid pattern, compact pattern, conical pattern, dominant pattern, and ramified pattern. Particularly, when Peclet and Damkohler numbers are larger than 1, the increase in the heterogeneity rises the chance of preferential channel flow in the porous medium and thus intensifies the wormhole phenomena, leading to higher permeability. The porescale results also show that compared with the specific surface area, the permeability is more sensitive to the alteration of the structural heterogeneity, and it is challenging to propose a general formula between permeability and porosity under different reactive transport conditions and structural heterogeneity. Thus, deep neural network is employed to predict the permeability–porosity relationship. The average value of mean absolute percentage error of prediction of 12 additional permeability–porosity curves is 6.89%, indicating the promising potential of using deep learning for predicting the complicated variations of permeability in heterogeneous porous media with dissolution of solid structures.
Porescale study of mineral dissolution in heterogeneous structures and deep learning prediction of permeability
10.1063/5.0123966
Physics of Fluids
20221117T12:25:51Z
© 2022 Author(s).

Simplified twodimensional model for global atmospheric dynamics
https://aip.scitation.org/doi/10.1063/5.0119855?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We present a simplified model of the atmosphere of a terrestrial planet as an open twodimensional system described by an ideal gas with velocity [math], density ρ, and temperature T fields. Starting with the Chern–Simons equations for a free inviscid fluid, the external effects of radiation and the exchange of matter with the strata, as well as diffusion and dissipation, are included. The resulting dynamics is governed by a set of nonlinear differential equations of the first order in time. This defines an initial value problem that can be integrated given the radiation balance of the planet. If the nonlinearities are neglected, the integration can be done in analytic form using standard Green function methods, with small nonlinearities incorporated as perturbative corrections in a consistent way. If the nonlinear approximation is not justified, the problem can be integrated numerically. The analytic expressions as well as the simulations of the linear regime for a continuous range of parameters in the equations are provided, which allows to explore the response of the model to changes of those parameters. In particular, it is observed that a 2.5% reduction in the emissivity of the atmosphere can lead to an increase of 7 °C of the average global temperature.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We present a simplified model of the atmosphere of a terrestrial planet as an open twodimensional system described by an ideal gas with velocity [math], density ρ, and temperature T fields. Starting with the Chern–Simons equations for a free inviscid fluid, the external effects of radiation and the exchange of matter with the strata, as well as diffusion and dissipation, are included. The resulting dynamics is governed by a set of nonlinear differential equations of the first order in time. This defines an initial value problem that can be integrated given the radiation balance of the planet. If the nonlinearities are neglected, the integration can be done in analytic form using standard Green function methods, with small nonlinearities incorporated as perturbative corrections in a consistent way. If the nonlinear approximation is not justified, the problem can be integrated numerically. The analytic expressions as well as the simulations of the linear regime for a continuous range of parameters in the equations are provided, which allows to explore the response of the model to changes of those parameters. In particular, it is observed that a 2.5% reduction in the emissivity of the atmosphere can lead to an increase of 7 °C of the average global temperature.
Simplified twodimensional model for global atmospheric dynamics
10.1063/5.0119855
Physics of Fluids
20221121T11:01:37Z
© 2022 Author(s).
Martín JacquesCoper
Valentina OrtizGuzmán
Jorge Zanelli

Computational investigation of natural ventilation induced by solar chimneys: Significance of building space on thermofluid behavior
https://aip.scitation.org/doi/10.1063/5.0115520?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A solar chimney is a typical device to harness naturally available energy resources to power ventilation inside buildings. A comparative computational fluid dynamics study of the flow and thermal mechanics inside solar chimneys is performed in this study. We utilize a variational multiscale formulation to model the combined turbulent/laminar flow regimes presented in the natural ventilation problem in the sense of large eddy simulation. Nitsche type weak enforcement of Dirichlet boundary conditions is integrated into the numerical framework to address the excessive mesh resolution requirement in flow and thermal boundary layers. Numerical methodology is verified and validated against experimental data in a model room with a solar chimney, and good agreement between the present results and the reference data is obtained. Finally, the thermofluid characteristics are investigated in a building equipped with different solar chimney designs. Particular emphases are placed on the effects of attached building spaces to the flows within the chimneys. The results indicated that the complex and realistic building space in this paper reduces the turbulence entering the solar chimney inlet and, thus, increases the air flow rate by up to 48.9% compared with the frequently used standalone chimney simulation paradigm. In addition, the thermal comfort indices are presented. With increased air flow rate in the chimney, the overall thermal comfort in the building is likely to be decreased, suggesting the necessity in the future studies to consider thermal comfort as an optimization objective.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A solar chimney is a typical device to harness naturally available energy resources to power ventilation inside buildings. A comparative computational fluid dynamics study of the flow and thermal mechanics inside solar chimneys is performed in this study. We utilize a variational multiscale formulation to model the combined turbulent/laminar flow regimes presented in the natural ventilation problem in the sense of large eddy simulation. Nitsche type weak enforcement of Dirichlet boundary conditions is integrated into the numerical framework to address the excessive mesh resolution requirement in flow and thermal boundary layers. Numerical methodology is verified and validated against experimental data in a model room with a solar chimney, and good agreement between the present results and the reference data is obtained. Finally, the thermofluid characteristics are investigated in a building equipped with different solar chimney designs. Particular emphases are placed on the effects of attached building spaces to the flows within the chimneys. The results indicated that the complex and realistic building space in this paper reduces the turbulence entering the solar chimney inlet and, thus, increases the air flow rate by up to 48.9% compared with the frequently used standalone chimney simulation paradigm. In addition, the thermal comfort indices are presented. With increased air flow rate in the chimney, the overall thermal comfort in the building is likely to be decreased, suggesting the necessity in the future studies to consider thermal comfort as an optimization objective.
Computational investigation of natural ventilation induced by solar chimneys: Significance of building space on thermofluid behavior
10.1063/5.0115520
Physics of Fluids
20221101T11:30:15Z
© 2022 Author(s).
Fei Xu
Songzhe Xu
Qingang Xiong

Hydrodynamic performance and energy absorption of multiple spherical absorbers along a straight coast
https://aip.scitation.org/doi/10.1063/5.0118052?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The development and utilization of wave energy have great potentiality to alleviate the urgent problem of global energy shortage. Spherical bodies can be used as point absorbers to extract wave energy, and much attention has been paid to the performance of spherical absorbers in an open water domain. This study focuses on the hydrodynamic performance and energy absorption of multiple spherical absorbers in front of a straight coast. The coast is assumed to be a fully reflecting vertical wall, and all the absorbers are restricted to only heave motion. An analytical solution based on linear potential flow theory is developed for the problem of wave diffraction and radiation by multiple absorbers. In the solution procedure, the hydrodynamic problem is transformed into an equivalent problem in an open water domain by applying the image principle. The velocity potential of the fluid motion is solved using the method of multipole expansions combined with the shift of local spherical coordinate systems. Then, the wave excitation force, added mass coefficient, radiation damping coefficient, and energy extraction performance of the absorbers are calculated. Case studies are presented to analyze the effects of the coastal reflection and hydrodynamic interaction among absorbers on the energy extraction performance of the wave energy converter (WEC) system. The effects of wave frequency, incident angle, spacing between the absorber and coast, submergence depth, absorber number, and plane layout are also clarified. The results suggest that the energy extraction performance of an isolated absorber is significantly improved when the motions of the waves and absorber are in resonance, and the coastal reflection can enhance the overall energy extraction performance for a WEC system with multiple absorbers. In addition, when the number of absorbers increases, the effects of the coastal reflection and hydrodynamic interaction become more complicated.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The development and utilization of wave energy have great potentiality to alleviate the urgent problem of global energy shortage. Spherical bodies can be used as point absorbers to extract wave energy, and much attention has been paid to the performance of spherical absorbers in an open water domain. This study focuses on the hydrodynamic performance and energy absorption of multiple spherical absorbers in front of a straight coast. The coast is assumed to be a fully reflecting vertical wall, and all the absorbers are restricted to only heave motion. An analytical solution based on linear potential flow theory is developed for the problem of wave diffraction and radiation by multiple absorbers. In the solution procedure, the hydrodynamic problem is transformed into an equivalent problem in an open water domain by applying the image principle. The velocity potential of the fluid motion is solved using the method of multipole expansions combined with the shift of local spherical coordinate systems. Then, the wave excitation force, added mass coefficient, radiation damping coefficient, and energy extraction performance of the absorbers are calculated. Case studies are presented to analyze the effects of the coastal reflection and hydrodynamic interaction among absorbers on the energy extraction performance of the wave energy converter (WEC) system. The effects of wave frequency, incident angle, spacing between the absorber and coast, submergence depth, absorber number, and plane layout are also clarified. The results suggest that the energy extraction performance of an isolated absorber is significantly improved when the motions of the waves and absorber are in resonance, and the coastal reflection can enhance the overall energy extraction performance for a WEC system with multiple absorbers. In addition, when the number of absorbers increases, the effects of the coastal reflection and hydrodynamic interaction become more complicated.
Hydrodynamic performance and energy absorption of multiple spherical absorbers along a straight coast
10.1063/5.0118052
Physics of Fluids
20221101T11:29:35Z
© 2022 Author(s).

The dynamics of a bubble in the internal fluid flow of a pipeline
https://aip.scitation.org/doi/10.1063/5.0112496?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In the aeronautical and marine engineering fields, bubbles are often carried in the pipelines of filling systems and marine risers. Under the action of internal flow, air bubbles seriously threaten device security. Therefore, to analyze the motion and deformation of a bubble in the internal fluid flow of a pipeline, we establish a corresponding boundary element numerical model based on the potential flow theory. A comparison of the numerical model results with the experimental results verifies the accuracy of the model. Subsequently, we simulate the dynamics of a bubble under the action of the internal flow, and the influence of the velocity, pipe radius, and surface tension on the bubble movement are discussed. When the dimensionless flow velocity exceeds 0.3, the bubbles will be seriously deformed. Different flow velocity directions cause different deformations of the annular bubbles in the later stages. Additionally, the channel limits bubble deformation. If the pipe radius is greater than 5, the effect of the pipe on the bubble is negligible. We also find that surface tension keeps the bubbles morphologically stable under internal flow. When the surface tension coefficient exceeds 2.45 × 10−4, it will not make the bubble toroidal.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In the aeronautical and marine engineering fields, bubbles are often carried in the pipelines of filling systems and marine risers. Under the action of internal flow, air bubbles seriously threaten device security. Therefore, to analyze the motion and deformation of a bubble in the internal fluid flow of a pipeline, we establish a corresponding boundary element numerical model based on the potential flow theory. A comparison of the numerical model results with the experimental results verifies the accuracy of the model. Subsequently, we simulate the dynamics of a bubble under the action of the internal flow, and the influence of the velocity, pipe radius, and surface tension on the bubble movement are discussed. When the dimensionless flow velocity exceeds 0.3, the bubbles will be seriously deformed. Different flow velocity directions cause different deformations of the annular bubbles in the later stages. Additionally, the channel limits bubble deformation. If the pipe radius is greater than 5, the effect of the pipe on the bubble is negligible. We also find that surface tension keeps the bubbles morphologically stable under internal flow. When the surface tension coefficient exceeds 2.45 × 10−4, it will not make the bubble toroidal.
The dynamics of a bubble in the internal fluid flow of a pipeline
10.1063/5.0112496
Physics of Fluids
20221102T02:44:23Z
© 2022 Author(s).

Dissolution characteristics of solutes with different shapes using the moving particle semiimplicit method
https://aip.scitation.org/doi/10.1063/5.0120966?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Dissolution characteristics of solutes with different shapes are studied. To simulate the process of dissolution, a diffusion and dissolution model based on the moving particle semiimplicit (MPS) method is proposed. First, the diffusion equation is introduced to the MPS method. Compared with the analytical solution, concentration diffusion can be accurately simulated with the model. Then, a coupling relationship between concentration, density, and viscosity is established. The relationship deals with the changes in physical parameters of the fluids caused by the diffusion, affecting the fluid flow. As the density change cannot be ignored in the mass conservation equation, the equation is rededuced in this paper. In addition, the dissolution model is introduced to the MPS method. The dissolution model is verified by the dissolution simulation of sessile droplets in water. Finally, the dissolution of solutes with different shapes in water is simulated using the proposed method. Five cases with different solute shapes are set to simulate five different drugs. Five cases with different solute shapes are set to simulate five different drugs. The solid solute shapes used are rectangle, capsule, heartshaped, and circle, and the liquid solute is a rectangle shape. The dissolution of the solute is comprehensively affected by the contact between the solute and water, the concentration difference, and the intensity of convection. The small concentration difference and the low convective velocity cause the existence of insoluble points in the heartshaped case, which decreases the dissolution rate. Dimensional analysis is carried out to address the relative importance of diffusion to convection. In the dissolution of solutes with different shapes, the effect of convective cannot be ignored when the nondimensional number is lower than 2.5 × 10−5.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Dissolution characteristics of solutes with different shapes are studied. To simulate the process of dissolution, a diffusion and dissolution model based on the moving particle semiimplicit (MPS) method is proposed. First, the diffusion equation is introduced to the MPS method. Compared with the analytical solution, concentration diffusion can be accurately simulated with the model. Then, a coupling relationship between concentration, density, and viscosity is established. The relationship deals with the changes in physical parameters of the fluids caused by the diffusion, affecting the fluid flow. As the density change cannot be ignored in the mass conservation equation, the equation is rededuced in this paper. In addition, the dissolution model is introduced to the MPS method. The dissolution model is verified by the dissolution simulation of sessile droplets in water. Finally, the dissolution of solutes with different shapes in water is simulated using the proposed method. Five cases with different solute shapes are set to simulate five different drugs. Five cases with different solute shapes are set to simulate five different drugs. The solid solute shapes used are rectangle, capsule, heartshaped, and circle, and the liquid solute is a rectangle shape. The dissolution of the solute is comprehensively affected by the contact between the solute and water, the concentration difference, and the intensity of convection. The small concentration difference and the low convective velocity cause the existence of insoluble points in the heartshaped case, which decreases the dissolution rate. Dimensional analysis is carried out to address the relative importance of diffusion to convection. In the dissolution of solutes with different shapes, the effect of convective cannot be ignored when the nondimensional number is lower than 2.5 × 10−5.
Dissolution characteristics of solutes with different shapes using the moving particle semiimplicit method
10.1063/5.0120966
Physics of Fluids
20221102T02:43:41Z
© 2022 Author(s).

Modelform uncertainty quantification of Reynoldsaveraged Navier–Stokes modeling of flows over a SD7003 airfoil
https://aip.scitation.org/doi/10.1063/5.0116282?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Reynoldsaveraged Navier–Stokes (RANS) models are known to be inaccurate in complex flows, for instance, laminarturbulent transition, and RANS uncertainty quantification (UQ) is essential to estimate the uncertainty in their predictions. In this study, a recent physicsbased UQ framework that introduces eigenvalue, eigenvector, and turbulence kinetic energy perturbations to the modeled Reynolds stress tensor has been used to estimate the uncertainty in the flow field. We introduce a regressionbased marker function that focuses on the turbulence kinetic energy perturbation for the simulation of laminarturbulent transitional flows over an Selig–Donovan 7003 airfoil. We observed a monotonic behavior of the magnitude of the predicted uncertainty bounds varying with the turbulence kinetic energy perturbation. Importantly, the predicted uncertainty bounds show a synergy behavior that dramatically increases the size of uncertainty bounds and can successfully encompass the reference data when the eigenvalue perturbations are augmented with the marker function.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Reynoldsaveraged Navier–Stokes (RANS) models are known to be inaccurate in complex flows, for instance, laminarturbulent transition, and RANS uncertainty quantification (UQ) is essential to estimate the uncertainty in their predictions. In this study, a recent physicsbased UQ framework that introduces eigenvalue, eigenvector, and turbulence kinetic energy perturbations to the modeled Reynolds stress tensor has been used to estimate the uncertainty in the flow field. We introduce a regressionbased marker function that focuses on the turbulence kinetic energy perturbation for the simulation of laminarturbulent transitional flows over an Selig–Donovan 7003 airfoil. We observed a monotonic behavior of the magnitude of the predicted uncertainty bounds varying with the turbulence kinetic energy perturbation. Importantly, the predicted uncertainty bounds show a synergy behavior that dramatically increases the size of uncertainty bounds and can successfully encompass the reference data when the eigenvalue perturbations are augmented with the marker function.
Modelform uncertainty quantification of Reynoldsaveraged Navier–Stokes modeling of flows over a SD7003 airfoil
10.1063/5.0116282
Physics of Fluids
20221102T02:43:15Z
© 2022 Author(s).
Minghan Chu
Xiaohua Wu
David E. Rival

Numerical simulation on the effect of inclination on rectangular buoyancydriven, turbulent diffusion flame
https://aip.scitation.org/doi/10.1063/5.0123891?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The presence of an inclined wall can significantly alter the flow dynamics of a buoyancydriven turbulent flame. Although flame structure on an inclined wall has been widely investigated, fluid flow has not been widely investigated. A large eddy simulation is performed to study the effect of inclination on the flow dynamics of a flame. The results show that at the initial stage of combustion, a vortex forms at the left side of the flame due to its expansion and the adverse pressure gradient downward. The vortex expands and lifts with the combustion flow, splitting the flame by stretching its left edge. During the continuous combustion stage, the flame is inclined on the inclined wall. Air entrainment on the left side of the flame flows parallel to the inclined wall, while air entrainment on the right side is significantly reduced due to the upward movement of the combustion flow. The flame inclination angle increases with increasing inclination angle and attaches the inclined wall when the inclination angle is 30°, leading to a higher mean temperature and velocity near the inclined wall.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The presence of an inclined wall can significantly alter the flow dynamics of a buoyancydriven turbulent flame. Although flame structure on an inclined wall has been widely investigated, fluid flow has not been widely investigated. A large eddy simulation is performed to study the effect of inclination on the flow dynamics of a flame. The results show that at the initial stage of combustion, a vortex forms at the left side of the flame due to its expansion and the adverse pressure gradient downward. The vortex expands and lifts with the combustion flow, splitting the flame by stretching its left edge. During the continuous combustion stage, the flame is inclined on the inclined wall. Air entrainment on the left side of the flame flows parallel to the inclined wall, while air entrainment on the right side is significantly reduced due to the upward movement of the combustion flow. The flame inclination angle increases with increasing inclination angle and attaches the inclined wall when the inclination angle is 30°, leading to a higher mean temperature and velocity near the inclined wall.
Numerical simulation on the effect of inclination on rectangular buoyancydriven, turbulent diffusion flame
10.1063/5.0123891
Physics of Fluids
20221102T02:43:23Z
© 2022 Author(s).

Airfoil trailingedge noise and drag reduction at a moderate Reynolds number using wavy geometries
https://aip.scitation.org/doi/10.1063/5.0120124?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This study utilizes a hybrid aeroacoustic model to investigate how airfoils with spanwise wavy geometries can be used to reduce trailingedge noise alongside improving the aerodynamic performance. A smooth airfoil is compared to four variants, which have spanwise surface waves of different wavelengths, at a Reynolds number of Re = 64 000 and an angleofattack of 1°. The first three variants have a geometry modified by a single wavelength, whereas the fourth has a surface composed of two wavelengths, which creates a more irregular surface variation. The results show that modest noise reductions of around 4 dB are achieved for the first three variants, but a much larger reduction of 17.7 dB is achieved for the fourth variant. The mechanisms behind the noise reduction are explored, and it is shown that the geometry reduces the spanwise correlation of the pressure fluctuations and also modifies the boundary layer dynamics, which contributes to the large reduction. It is further shown that a wavy geometry can reduce the drag force by reducing the shear stress over parts of the airfoil surface and by limiting the flow separation on the suction side. The fourth variant is also assessed across a wider range of angles ([math]) and is shown to produce less noise than the smooth wing across the entire range as well as a drag reduction for [math].
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This study utilizes a hybrid aeroacoustic model to investigate how airfoils with spanwise wavy geometries can be used to reduce trailingedge noise alongside improving the aerodynamic performance. A smooth airfoil is compared to four variants, which have spanwise surface waves of different wavelengths, at a Reynolds number of Re = 64 000 and an angleofattack of 1°. The first three variants have a geometry modified by a single wavelength, whereas the fourth has a surface composed of two wavelengths, which creates a more irregular surface variation. The results show that modest noise reductions of around 4 dB are achieved for the first three variants, but a much larger reduction of 17.7 dB is achieved for the fourth variant. The mechanisms behind the noise reduction are explored, and it is shown that the geometry reduces the spanwise correlation of the pressure fluctuations and also modifies the boundary layer dynamics, which contributes to the large reduction. It is further shown that a wavy geometry can reduce the drag force by reducing the shear stress over parts of the airfoil surface and by limiting the flow separation on the suction side. The fourth variant is also assessed across a wider range of angles ([math]) and is shown to produce less noise than the smooth wing across the entire range as well as a drag reduction for [math].
Airfoil trailingedge noise and drag reduction at a moderate Reynolds number using wavy geometries
10.1063/5.0120124
Physics of Fluids
20221102T02:43:18Z
© 2022 Author(s).
T. A. Smith
C. A. Klettner

Selfsimilar diffuse boundary method for phase boundary driven flow
https://aip.scitation.org/doi/10.1063/5.0107739?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Interactions between an evolving solid and inviscid flow can result in substantial computational complexity, particularly in circumstances involving varied boundary conditions between the solid and fluid phases. Examples of such interactions include melting, sublimation, and deflagration, all of which exhibit bidirectional coupling, mass/heat transfer, and topological change of the solid–fluid interface. The diffuse interface method is a powerful technique that has been used to describe a wide range of solidphase interfacedriven phenomena. The implicit treatment of the interface eliminates the need for cumbersome interface tracking, and advances in adaptive mesh refinement have provided a way to sufficiently resolve diffuse interfaces without excessive computational cost. However, the general scaleinvariant coupling of these techniques to flow solvers has been relatively unexplored. In this work, a robust method is presented for treating diffuse solid–fluid interfaces with arbitrary boundary conditions. Source terms defined over the diffuse region mimic boundary conditions at the solid–fluid interface, and it is demonstrated that the diffuse length scale has no adverse effects. To show the efficacy of the method, a onedimensional implementation is introduced and tested for three types of boundaries: mass flux through the boundary, a moving boundary, and passive interaction of the boundary with an incident acoustic wave. Twodimensional results are presented as well these demonstrate expected behavior in all cases. Convergence analysis is also performed and compared against the sharpinterface solution, and linear convergence is observed. This method lays the groundwork for the extension to viscous flow and the solution of problems involving timevarying massflux boundaries.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Interactions between an evolving solid and inviscid flow can result in substantial computational complexity, particularly in circumstances involving varied boundary conditions between the solid and fluid phases. Examples of such interactions include melting, sublimation, and deflagration, all of which exhibit bidirectional coupling, mass/heat transfer, and topological change of the solid–fluid interface. The diffuse interface method is a powerful technique that has been used to describe a wide range of solidphase interfacedriven phenomena. The implicit treatment of the interface eliminates the need for cumbersome interface tracking, and advances in adaptive mesh refinement have provided a way to sufficiently resolve diffuse interfaces without excessive computational cost. However, the general scaleinvariant coupling of these techniques to flow solvers has been relatively unexplored. In this work, a robust method is presented for treating diffuse solid–fluid interfaces with arbitrary boundary conditions. Source terms defined over the diffuse region mimic boundary conditions at the solid–fluid interface, and it is demonstrated that the diffuse length scale has no adverse effects. To show the efficacy of the method, a onedimensional implementation is introduced and tested for three types of boundaries: mass flux through the boundary, a moving boundary, and passive interaction of the boundary with an incident acoustic wave. Twodimensional results are presented as well these demonstrate expected behavior in all cases. Convergence analysis is also performed and compared against the sharpinterface solution, and linear convergence is observed. This method lays the groundwork for the extension to viscous flow and the solution of problems involving timevarying massflux boundaries.
Selfsimilar diffuse boundary method for phase boundary driven flow
10.1063/5.0107739
Physics of Fluids
20221102T02:44:12Z
© 2022 Author(s).
Emma M. Schmidt
J. Matt Quinlan
Brandon Runnels

Implicit block dataparallel relaxation scheme of Navier–Stokes equations using graphics processing units
https://aip.scitation.org/doi/10.1063/5.0119698?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The granularity of computational fluid dynamics (CFD) generally refers to the point granularity parallelization as a unit of the grid when graphics processing units (GPUs) are utilized as the computing carrier. In commonly deployed implicit time advancement schemes, the parallel dimensionality must be reduced, resulting in the time advancement procedure becoming the only highly timeconsuming step in the whole CFD computing procedures. In this paper, a block dataparallel lowerupper relaxation (BDPLUR) scheme based on Jacobi iteration and Roe's flux scheme is proposed and then implemented on a GPU. Numerical experiments are carried out and show that the convergence speed of the BDPLUR scheme, especially when implemented on a GPU, is approximately ten times higher than that of the original dataparallel lowerupper relaxation scheme and more than 100 times higher than that of the lowerupper symmetric Gauss–Seidel scheme. Moreover, the influence of different Courant–Friedrichs–Lewy numbers on the convergence time is discussed, and different viscous matrices are compared. Standard cases are adopted to verify the effectiveness of the BDPLUR scheme.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The granularity of computational fluid dynamics (CFD) generally refers to the point granularity parallelization as a unit of the grid when graphics processing units (GPUs) are utilized as the computing carrier. In commonly deployed implicit time advancement schemes, the parallel dimensionality must be reduced, resulting in the time advancement procedure becoming the only highly timeconsuming step in the whole CFD computing procedures. In this paper, a block dataparallel lowerupper relaxation (BDPLUR) scheme based on Jacobi iteration and Roe's flux scheme is proposed and then implemented on a GPU. Numerical experiments are carried out and show that the convergence speed of the BDPLUR scheme, especially when implemented on a GPU, is approximately ten times higher than that of the original dataparallel lowerupper relaxation scheme and more than 100 times higher than that of the lowerupper symmetric Gauss–Seidel scheme. Moreover, the influence of different Courant–Friedrichs–Lewy numbers on the convergence time is discussed, and different viscous matrices are compared. Standard cases are adopted to verify the effectiveness of the BDPLUR scheme.
Implicit block dataparallel relaxation scheme of Navier–Stokes equations using graphics processing units
10.1063/5.0119698
Physics of Fluids
20221103T02:42:51Z
© 2022 Author(s).

An optimized collisionaveraged variable soft sphere parameter set for air, carbon, and corresponding ionized species
https://aip.scitation.org/doi/10.1063/5.0118040?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A collisionaveraged parameter set for air, carbon, and the corresponding ionized species for the variable soft sphere collision model is suggested which is suitable for the earth's atmosphere or mars atmosphere, for example. The parameter set is generated through collision integral fits and a number of optimization steps so that individual subsets can also be used for, e.g., air or without ionized species. In addition, the parameter set can be extended by further species without having to carry out the complete optimization again, which is shown in the example of argon. The limitations of the collisionaverage model are discussed and in which cases the collisionspecific model or other models should be used. The model is compared with collision integrals from various publications.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A collisionaveraged parameter set for air, carbon, and the corresponding ionized species for the variable soft sphere collision model is suggested which is suitable for the earth's atmosphere or mars atmosphere, for example. The parameter set is generated through collision integral fits and a number of optimization steps so that individual subsets can also be used for, e.g., air or without ionized species. In addition, the parameter set can be extended by further species without having to carry out the complete optimization again, which is shown in the example of argon. The limitations of the collisionaverage model are discussed and in which cases the collisionspecific model or other models should be used. The model is compared with collision integrals from various publications.
An optimized collisionaveraged variable soft sphere parameter set for air, carbon, and corresponding ionized species
10.1063/5.0118040
Physics of Fluids
20221104T12:22:07Z
© 2022 Author(s).
M. Pfeiffer

A transient hydrodynamic model of screen channel liquid acquisition devices for inspace cryogenic propellant management
https://aip.scitation.org/doi/10.1063/5.0119031?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Screen channel liquid acquisition devices (LADs) will play a crucial role in future deep space travel. It is essential that vaporfree delivery of propellants during tanktotank transfer is ensured to maximize yield from storage tanks and prevent potential combustion instabilities. The screen channel LAD utilizes a fine screen wire mesh that can separate phases in a low Bond number (i.e., microgravity) environment using surface tension forces. This study presents the development and verification of a new model for transient screen compliance, one of the influential factors for screen channel LAD design. Screen compliance is crucial during LAD channel outflow transients because the slight deflection of the screen can provide needed mass to satisfy rapid outflow demands and reduce the pressure difference across the screen. The model is successfully verified against computational fluid dynamics simulations. In addition, the characteristic speed for the governing screen compliance equations is derived, which allows for numerical stability criteria to be established. As shown in this study, the transient maximum pressure difference across the screen can greatly exceed the steady state maximum pressure difference across the screen in many cases.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Screen channel liquid acquisition devices (LADs) will play a crucial role in future deep space travel. It is essential that vaporfree delivery of propellants during tanktotank transfer is ensured to maximize yield from storage tanks and prevent potential combustion instabilities. The screen channel LAD utilizes a fine screen wire mesh that can separate phases in a low Bond number (i.e., microgravity) environment using surface tension forces. This study presents the development and verification of a new model for transient screen compliance, one of the influential factors for screen channel LAD design. Screen compliance is crucial during LAD channel outflow transients because the slight deflection of the screen can provide needed mass to satisfy rapid outflow demands and reduce the pressure difference across the screen. The model is successfully verified against computational fluid dynamics simulations. In addition, the characteristic speed for the governing screen compliance equations is derived, which allows for numerical stability criteria to be established. As shown in this study, the transient maximum pressure difference across the screen can greatly exceed the steady state maximum pressure difference across the screen in many cases.
A transient hydrodynamic model of screen channel liquid acquisition devices for inspace cryogenic propellant management
10.1063/5.0119031
Physics of Fluids
20221104T12:21:54Z
© 2022 Author(s).
C. F. Camarotti
J. W. Hartwig
J. N. Chung

Hydrodynamic performance of dualchamber Oscillating Water Column array under oblique waves
https://aip.scitation.org/doi/10.1063/5.0118655?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>A multiple Oscillating Water Column (OWC) device may provide better wave absorption over a wider frequency bandwidth than a singlechamber OWC due to multiple resonances. The scattering and radiation of threedimensional oblique waves by an array of periodic dualchamber OWCs are considered here along a coastal cliff. A semianalytical model was developed based on potential flow theory and matching eigenfunction method to investigate the oblique wave interaction with a dualchamber OWC array system. The velocity singularity at the tip of a chamber wall is resolved by introducing the Galerkin technique to accelerate the convergence. The semianalytical solution is verified by the Haskind relation and energy conservation law. Hydrodynamics of the proposed system and the influence of wave and geometric parameters were investigated. Theoretical results indicate that a dualchamber OWC array has a broader capture bandwidth than a singlechamber OWC array for both normal and oblique waves. The presence of the alongshore and crossshore sloshing resonance is theoretically confirmed in each subchamber of OWC unit, which decreases the hydrodynamic efficiency and increases the wave reflection drastically. Although the wave loading on the chamber wall decreases with increasing incident wave angle θ, the wave loading on chamber/partition wall may increase sharply due to sloshing resonance at critical frequency kc. To our knowledge, this is the first attempt to investigate the hydrodynamics of dualchamber OWC array under oblique waves. The present theoretical results indicate the potential risks of structural damage and total wave reflection due to sloshing resonance, which should be an important design consideration.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>A multiple Oscillating Water Column (OWC) device may provide better wave absorption over a wider frequency bandwidth than a singlechamber OWC due to multiple resonances. The scattering and radiation of threedimensional oblique waves by an array of periodic dualchamber OWCs are considered here along a coastal cliff. A semianalytical model was developed based on potential flow theory and matching eigenfunction method to investigate the oblique wave interaction with a dualchamber OWC array system. The velocity singularity at the tip of a chamber wall is resolved by introducing the Galerkin technique to accelerate the convergence. The semianalytical solution is verified by the Haskind relation and energy conservation law. Hydrodynamics of the proposed system and the influence of wave and geometric parameters were investigated. Theoretical results indicate that a dualchamber OWC array has a broader capture bandwidth than a singlechamber OWC array for both normal and oblique waves. The presence of the alongshore and crossshore sloshing resonance is theoretically confirmed in each subchamber of OWC unit, which decreases the hydrodynamic efficiency and increases the wave reflection drastically. Although the wave loading on the chamber wall decreases with increasing incident wave angle θ, the wave loading on chamber/partition wall may increase sharply due to sloshing resonance at critical frequency kc. To our knowledge, this is the first attempt to investigate the hydrodynamics of dualchamber OWC array under oblique waves. The present theoretical results indicate the potential risks of structural damage and total wave reflection due to sloshing resonance, which should be an important design consideration.
Hydrodynamic performance of dualchamber Oscillating Water Column array under oblique waves
10.1063/5.0118655
Physics of Fluids
20221107T12:49:40Z
© 2022 Author(s).

Nonlocal conservation laws and dynamics of soliton solutions of (2 + 1)dimensional Boiti–Leon–Pempinelli system
https://aip.scitation.org/doi/10.1063/5.0123825?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this article, we obtain several new exact solutions of (2 + 1)dimensional Boiti–Leon–Pempinelli system of nonlinear partial differential equations (PDEs) which describes the evolution of horizontal velocity component of water waves propagating in two directions. We perform the Lie symmetry analysis to the given system and construct a onedimensional optimal subalgebra which involves some arbitrary functions of spatial variables. Symmetry group classifications of infinitedimensional Lie algebra for higherdimensional system of PDEs are very interesting and rare in the literature. Several new exact solutions are obtained by symmetry reduction using each of the optimal subalgebra and these solutions have not been reported earlier in the previous studies to the best of our knowledge. We then study the dynamical behavior of some exact solutions by numerical simulations and observed many interesting phenomena, such as traveling waves, kink and antikink type solitons, and singular kink type solitons. We construct several conservation laws of the system by using a multiplier method. As an application, we study the nonlocal conservation laws of the system by constructing potential systems and appending gauge constraints. In fact, determining nonlocal conservation laws for higherdimensional nonlinear system of PDEs arising from divergence type conservation laws is very rare in the literature and have huge consequences in the study of nonlocal symmetries.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this article, we obtain several new exact solutions of (2 + 1)dimensional Boiti–Leon–Pempinelli system of nonlinear partial differential equations (PDEs) which describes the evolution of horizontal velocity component of water waves propagating in two directions. We perform the Lie symmetry analysis to the given system and construct a onedimensional optimal subalgebra which involves some arbitrary functions of spatial variables. Symmetry group classifications of infinitedimensional Lie algebra for higherdimensional system of PDEs are very interesting and rare in the literature. Several new exact solutions are obtained by symmetry reduction using each of the optimal subalgebra and these solutions have not been reported earlier in the previous studies to the best of our knowledge. We then study the dynamical behavior of some exact solutions by numerical simulations and observed many interesting phenomena, such as traveling waves, kink and antikink type solitons, and singular kink type solitons. We construct several conservation laws of the system by using a multiplier method. As an application, we study the nonlocal conservation laws of the system by constructing potential systems and appending gauge constraints. In fact, determining nonlocal conservation laws for higherdimensional nonlinear system of PDEs arising from divergence type conservation laws is very rare in the literature and have huge consequences in the study of nonlocal symmetries.
Nonlocal conservation laws and dynamics of soliton solutions of (2 + 1)dimensional Boiti–Leon–Pempinelli system
10.1063/5.0123825
Physics of Fluids
20221108T01:21:44Z
© 2022 Author(s).
Subhankar Sil
T. Raja Sekhar

A study with the lattice Boltzmann method on the conversion efficiency of a packedbed reactor with different oriented packed beads configurations
https://aip.scitation.org/doi/10.1063/5.0124680?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>We consider packedbed reactors with dielectric beads in a twodimensional channel geometry, apply an electric field perpendicular to the walls, and explore numerically the sensitivity of reaction conversion efficiencies of a dissociation reaction on system parameters like shape, orientation, and size of the beads and porosity of packedbed systems. We have developed a lattice Boltzmann (LB) model that allows for simultaneous simulation of the flow field, the electric field within fluid and (solid) beads, and transport of (charged) species, such as ions and reagents. It solves Navier–Stokes for the fluid flow and the concentration field for neutral and charged species by the advection–diffusion and Nernst–Planck equation, respectively, formulated in the LB framework. The model allows to compute electric field strengths in the fluid and in the beads, by solving the Poisson equation. The method is suitable for arbitrary geometries of the flow domain and does not require bodyfitted meshes. Two important conclusions can be drawn. First, the proposed LB model enables simulation of a reactive electrokinetic fluid in a reactor with dielectric packed beads of arbitrary shape, size, and orientation. The LB method is based on Cartesian meshes irrespective of the shape of the beads and is highly parallelizable and can be extended to threedimensional packedbed reactors. Second, we show that reactor conversion efficiency is sensitive to shape, orientation, and size of the beads and the porosity of the packedbed reactor. Present observations will guide the parameter settings for the beads and packedbed reactor of more realistic threedimensional configurations.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>We consider packedbed reactors with dielectric beads in a twodimensional channel geometry, apply an electric field perpendicular to the walls, and explore numerically the sensitivity of reaction conversion efficiencies of a dissociation reaction on system parameters like shape, orientation, and size of the beads and porosity of packedbed systems. We have developed a lattice Boltzmann (LB) model that allows for simultaneous simulation of the flow field, the electric field within fluid and (solid) beads, and transport of (charged) species, such as ions and reagents. It solves Navier–Stokes for the fluid flow and the concentration field for neutral and charged species by the advection–diffusion and Nernst–Planck equation, respectively, formulated in the LB framework. The model allows to compute electric field strengths in the fluid and in the beads, by solving the Poisson equation. The method is suitable for arbitrary geometries of the flow domain and does not require bodyfitted meshes. Two important conclusions can be drawn. First, the proposed LB model enables simulation of a reactive electrokinetic fluid in a reactor with dielectric packed beads of arbitrary shape, size, and orientation. The LB method is based on Cartesian meshes irrespective of the shape of the beads and is highly parallelizable and can be extended to threedimensional packedbed reactors. Second, we show that reactor conversion efficiency is sensitive to shape, orientation, and size of the beads and the porosity of the packedbed reactor. Present observations will guide the parameter settings for the beads and packedbed reactor of more realistic threedimensional configurations.
A study with the lattice Boltzmann method on the conversion efficiency of a packedbed reactor with different oriented packed beads configurations
10.1063/5.0124680
Physics of Fluids
20221108T01:24:51Z
© 2022 Author(s).
Herman J. H. Clercx
Federico Toschi

Study of an air bubble curtain along a wall in water and radiated noise mitigation
https://aip.scitation.org/doi/10.1063/5.0121099?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The injection of air bubbles into the liquid phase of a freestream flow has several impacts on the flow structure, which depends on the volume and size of the bubbles. This work experimentally investigates the characteristics of air bubble injection into freestream flow using three different injector models. The effects of the bubble curtain on the sound wave attention are studied. A wide range of air injection rates from 2 to 50 standard liters per minute is injected into the freestream water at Froude numbers (Fr) of 50.5, 70.7, and 90.9. The injector model is placed on the sidewall, which is where the bubble curtain is generated. Highspeed cameras and an image processing technique are used to visualize and quantify the projected void fraction (PVF) of air bubbles. The sound measurement system consists of two hydrophones. The first hydrophone projects sound waves at discrete frequencies ranging from 17 to 50 kHz, and the second receives the transmitted sound waves. The bubble PVF is observed to increase with the air injection rate as the Froude number increases. There is no significant increment in the thickness of the bubbly curtain when the air injection rate or Froude number increases. The different air injector models produce various air bubble flow patterns, and model II provides the highest PVF. The bubble curtain greatly suppresses sound waves at all frequency ranges. However, the sound attenuation rate varies along the frequency range. The insertion loss increases depending on the PVF of the bubbles, freestream velocity, and frequency range. At a high Froude number of 90.9, the increased air injection rate does not affect the insertion loss. Model II has a relatively higher insertion loss rate at frequencies >26 kHz.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The injection of air bubbles into the liquid phase of a freestream flow has several impacts on the flow structure, which depends on the volume and size of the bubbles. This work experimentally investigates the characteristics of air bubble injection into freestream flow using three different injector models. The effects of the bubble curtain on the sound wave attention are studied. A wide range of air injection rates from 2 to 50 standard liters per minute is injected into the freestream water at Froude numbers (Fr) of 50.5, 70.7, and 90.9. The injector model is placed on the sidewall, which is where the bubble curtain is generated. Highspeed cameras and an image processing technique are used to visualize and quantify the projected void fraction (PVF) of air bubbles. The sound measurement system consists of two hydrophones. The first hydrophone projects sound waves at discrete frequencies ranging from 17 to 50 kHz, and the second receives the transmitted sound waves. The bubble PVF is observed to increase with the air injection rate as the Froude number increases. There is no significant increment in the thickness of the bubbly curtain when the air injection rate or Froude number increases. The different air injector models produce various air bubble flow patterns, and model II provides the highest PVF. The bubble curtain greatly suppresses sound waves at all frequency ranges. However, the sound attenuation rate varies along the frequency range. The insertion loss increases depending on the PVF of the bubbles, freestream velocity, and frequency range. At a high Froude number of 90.9, the increased air injection rate does not affect the insertion loss. Model II has a relatively higher insertion loss rate at frequencies >26 kHz.
Study of an air bubble curtain along a wall in water and radiated noise mitigation
10.1063/5.0121099
Physics of Fluids
20221110T12:17:29Z
© 2022 Author(s).
Ali Kareem Hilo
JiWoo Hong
KiSeong Kim
ByoungKwon Ahn
JaeHyuk Lee
Suyong Shin
IlSung Moon

Prediction and optimization of airfoil aerodynamic performance using deep neural network coupled Bayesian method
https://aip.scitation.org/doi/10.1063/5.0122595?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, we proposed an innovative Bayesian optimization (BO) coupled with deep learning for rapid airfoil shape optimization to maximize aerodynamic performance of airfoils. The proposed aerodynamic coefficient prediction model (ACPM) consists of a convolutional path and a fully connected path, which enables the reconstruction of the endtoend mapping between the Hicks–Henne (H–H) parameterized geometry and the aerodynamic coefficients of an airfoil. The computational fluid dynamics (CFD) model is first validated with the data in the literature, and the numerically simulated lift and drag coefficients were set as the ground truth to guide the model training and validate the network model based ACPM. The average accuracy of lift and drag coefficient predictions are both about 99%, and the determination coefficient R2 are more than 0.9970 and 0.9539, respectively. Coupled with the proposed ACPM, instead of the conventional expensive CFD simulator, the Bayesian method improved the ratio of lift and drag coefficients by more than 43%, where the optimized shape parameters of the airfoil coincide well with the results by the CFD. Furthermore, the whole optimization time is less than 2 min, two orders faster than the traditional BOCFD framework. The obtained results demonstrate the great potential of the BOACPM framework in fast and accurate airfoil shape optimization and design.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, we proposed an innovative Bayesian optimization (BO) coupled with deep learning for rapid airfoil shape optimization to maximize aerodynamic performance of airfoils. The proposed aerodynamic coefficient prediction model (ACPM) consists of a convolutional path and a fully connected path, which enables the reconstruction of the endtoend mapping between the Hicks–Henne (H–H) parameterized geometry and the aerodynamic coefficients of an airfoil. The computational fluid dynamics (CFD) model is first validated with the data in the literature, and the numerically simulated lift and drag coefficients were set as the ground truth to guide the model training and validate the network model based ACPM. The average accuracy of lift and drag coefficient predictions are both about 99%, and the determination coefficient R2 are more than 0.9970 and 0.9539, respectively. Coupled with the proposed ACPM, instead of the conventional expensive CFD simulator, the Bayesian method improved the ratio of lift and drag coefficients by more than 43%, where the optimized shape parameters of the airfoil coincide well with the results by the CFD. Furthermore, the whole optimization time is less than 2 min, two orders faster than the traditional BOCFD framework. The obtained results demonstrate the great potential of the BOACPM framework in fast and accurate airfoil shape optimization and design.
Prediction and optimization of airfoil aerodynamic performance using deep neural network coupled Bayesian method
10.1063/5.0122595
Physics of Fluids
20221110T12:18:08Z
© 2022 Author(s).
Nadine Aubry

Predicting the drop size passing through a superhydrophobic orifice
https://aip.scitation.org/doi/10.1063/5.0125906?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Superhydrophobic surfaces can be utilized in various applications, such as enhanced heat transfer, antiicing, selfcleaning, and viscous drag reduction. In this work, we investigated the water droplet size separation using superhydrophobic surfaces, which is relatively new and unexplored research field, but yet promising for pharmaceutical and medical applications. We developed a theoretical model for predicting the diameter of a droplet passing through a smaller superhydrophobic orifice by considering the balance of forces, geometrical characteristics, and the surface wettability. For verification of the model, experimental water droplet size separation was conducted using a thin superhydrophobic copper foil with a lasercut orifice with a diameter ranging from 1.2 to 2.1 mm. A comparison of the experimental and analytical results has shown that the error of the model is less than 20% within the model's validity range with the upper limit at the capillary length of the fluid. By moving away from this limit toward smaller droplet diameters, the accuracy of the model improves and reaches an error of less than 7% at the smallest used orifice diameter of 1.2 mm.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Superhydrophobic surfaces can be utilized in various applications, such as enhanced heat transfer, antiicing, selfcleaning, and viscous drag reduction. In this work, we investigated the water droplet size separation using superhydrophobic surfaces, which is relatively new and unexplored research field, but yet promising for pharmaceutical and medical applications. We developed a theoretical model for predicting the diameter of a droplet passing through a smaller superhydrophobic orifice by considering the balance of forces, geometrical characteristics, and the surface wettability. For verification of the model, experimental water droplet size separation was conducted using a thin superhydrophobic copper foil with a lasercut orifice with a diameter ranging from 1.2 to 2.1 mm. A comparison of the experimental and analytical results has shown that the error of the model is less than 20% within the model's validity range with the upper limit at the capillary length of the fluid. By moving away from this limit toward smaller droplet diameters, the accuracy of the model improves and reaches an error of less than 7% at the smallest used orifice diameter of 1.2 mm.
Predicting the drop size passing through a superhydrophobic orifice
10.1063/5.0125906
Physics of Fluids
20221111T12:57:45Z
© 2022 Author(s).
Samo Jereb
Matevž Zupančič
Matic Može
Iztok Golobič

Similarity solution for isothermal flow behind the magnetogasdynamic cylindrical shock wave in a rotating nonideal gas with the effect of the gravitational field
https://aip.scitation.org/doi/10.1063/5.0123031?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>In this paper, we investigate a system of quasilinear hyperbolic partial differential equations, which describes the propagation of cylindrical shock waves in a rotating nonideal gas with the effects of the gravitational field and the axial magnetic field. It is assumed that the flow is isothermal. The Lie group of transformations is used to generate the selfsimilar solutions of the considered problem in the medium of uniform density. The axial and azimuthal components of fluid velocity and magnetic field are supposed to be varying. We find the generators of the Lie group of transformations by employing the invariant surface criteria. We discovered four alternative solutions by selecting the arbitrary constants indicated in the generators' phrase. Only in three out of these four cases, the selfsimilar solutions exist. Two types of shock paths appear while solving the above cases. The powerlaw shock path appears in the first and third cases, while the exponentiallaw shock path appears in the second case. To find selfsimilar solutions, these cases have been solved numerically. We graphically show the distributions of flow variables behind the shock wave so that we can observe the effect on flow variables of the various values of the nonideal parameter, Alfvén Mach number, adiabatic exponent, gravitational parameter, and ambient azimuthal velocity exponent. For the computational task, we used “MATLAB” software.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>In this paper, we investigate a system of quasilinear hyperbolic partial differential equations, which describes the propagation of cylindrical shock waves in a rotating nonideal gas with the effects of the gravitational field and the axial magnetic field. It is assumed that the flow is isothermal. The Lie group of transformations is used to generate the selfsimilar solutions of the considered problem in the medium of uniform density. The axial and azimuthal components of fluid velocity and magnetic field are supposed to be varying. We find the generators of the Lie group of transformations by employing the invariant surface criteria. We discovered four alternative solutions by selecting the arbitrary constants indicated in the generators' phrase. Only in three out of these four cases, the selfsimilar solutions exist. Two types of shock paths appear while solving the above cases. The powerlaw shock path appears in the first and third cases, while the exponentiallaw shock path appears in the second case. To find selfsimilar solutions, these cases have been solved numerically. We graphically show the distributions of flow variables behind the shock wave so that we can observe the effect on flow variables of the various values of the nonideal parameter, Alfvén Mach number, adiabatic exponent, gravitational parameter, and ambient azimuthal velocity exponent. For the computational task, we used “MATLAB” software.
Similarity solution for isothermal flow behind the magnetogasdynamic cylindrical shock wave in a rotating nonideal gas with the effect of the gravitational field
10.1063/5.0123031
Physics of Fluids
20221111T12:57:42Z
© 2022 Author(s).
Swati Chauhan
Deepika Singh
Rajan Arora

Comparing different nonlinear dimensionality reduction techniques for datadriven unsteady fluid flow modeling
https://aip.scitation.org/doi/10.1063/5.0127284?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>Computational fluid dynamics (CFD) is known for producing highdimensional spatiotemporal data. Recent advances in machine learning (ML) have introduced a myriad of techniques for extracting physical information from CFD. Identifying an optimal set of coordinates for representing the data in a lowdimensional embedding is a crucial first step toward datadriven reducedorder modeling and other ML tasks. This is usually done via principal component analysis (PCA), which gives an optimal linear approximation. However, fluid flows are often complex and have nonlinear structures, which cannot be discovered or efficiently represented by PCA. Several unsupervised ML algorithms have been developed in other branches of science for nonlinear dimensionality reduction (NDR), but have not been extensively used for fluid flows. Here, four manifold learning and two deep learning (autoencoder)based NDR methods are investigated and compared to PCA. These are tested on two canonical fluid flow problems (laminar and turbulent) and two biomedical flows in brain aneurysms. The data reconstruction capabilities of these methods are compared, and the challenges are discussed. The temporal vs spatial arrangement of data and its influence on NDR mode extraction is investigated. Finally, the modes are qualitatively compared. The results suggest that using NDR methods would be beneficial for building more efficient reducedorder models of fluid flows. All NDR techniques resulted in smaller reconstruction errors for spatial reduction. Temporal reduction was a harder task; nevertheless, it resulted in physically interpretable modes. Our work is one of the first comprehensive comparisons of various NDR methods in unsteady flows.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>Computational fluid dynamics (CFD) is known for producing highdimensional spatiotemporal data. Recent advances in machine learning (ML) have introduced a myriad of techniques for extracting physical information from CFD. Identifying an optimal set of coordinates for representing the data in a lowdimensional embedding is a crucial first step toward datadriven reducedorder modeling and other ML tasks. This is usually done via principal component analysis (PCA), which gives an optimal linear approximation. However, fluid flows are often complex and have nonlinear structures, which cannot be discovered or efficiently represented by PCA. Several unsupervised ML algorithms have been developed in other branches of science for nonlinear dimensionality reduction (NDR), but have not been extensively used for fluid flows. Here, four manifold learning and two deep learning (autoencoder)based NDR methods are investigated and compared to PCA. These are tested on two canonical fluid flow problems (laminar and turbulent) and two biomedical flows in brain aneurysms. The data reconstruction capabilities of these methods are compared, and the challenges are discussed. The temporal vs spatial arrangement of data and its influence on NDR mode extraction is investigated. Finally, the modes are qualitatively compared. The results suggest that using NDR methods would be beneficial for building more efficient reducedorder models of fluid flows. All NDR techniques resulted in smaller reconstruction errors for spatial reduction. Temporal reduction was a harder task; nevertheless, it resulted in physically interpretable modes. Our work is one of the first comprehensive comparisons of various NDR methods in unsteady flows.
Comparing different nonlinear dimensionality reduction techniques for datadriven unsteady fluid flow modeling
10.1063/5.0127284
Physics of Fluids
20221116T10:41:54Z
© 2022 Author(s).
Hunor Csala
Scott T. M. Dawson
Amirhossein Arzani

Dynamic characteristics of moving droplets impacting sessile droplets with different Reynolds numbers
https://aip.scitation.org/doi/10.1063/5.0109293?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The collision of moving droplets with sessile droplets is a common occurrence in fields of industry, including power generation, chemical engineering, and aerospace, among others. In this paper, the collision of propylene glycol, glycerol, and deionized water droplets is studied for given collision speeds and different volume ratios of moving and sessile droplets using highspeed photography. It is found that droplet collision at a speed of about 0.25 m/s leads to compression deformation, whereas collision at about 1.10 m/s typically produces features, such as a nonsplashing liquid crown and a central liquid jet. In this paper, the main characteristics of the above phenomena are quantified, the influence of the Reynolds number at different volume ratios is studied, and the main phenomena are explained from the perspective of an energy analysis. The findings reported here are significant for the solution of practical engineering problems and improving the stability of equipment operation.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The collision of moving droplets with sessile droplets is a common occurrence in fields of industry, including power generation, chemical engineering, and aerospace, among others. In this paper, the collision of propylene glycol, glycerol, and deionized water droplets is studied for given collision speeds and different volume ratios of moving and sessile droplets using highspeed photography. It is found that droplet collision at a speed of about 0.25 m/s leads to compression deformation, whereas collision at about 1.10 m/s typically produces features, such as a nonsplashing liquid crown and a central liquid jet. In this paper, the main characteristics of the above phenomena are quantified, the influence of the Reynolds number at different volume ratios is studied, and the main phenomena are explained from the perspective of an energy analysis. The findings reported here are significant for the solution of practical engineering problems and improving the stability of equipment operation.
Dynamic characteristics of moving droplets impacting sessile droplets with different Reynolds numbers
10.1063/5.0109293
Physics of Fluids
20221116T10:41:56Z
© 2022 Author(s).

Application of a variational hybrid quantumclassical algorithm to heat conduction equation and analysis of time complexity
https://aip.scitation.org/doi/10.1063/5.0121778?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>The prosperous development of both hardware and algorithms for quantum computing (QC) potentially prompts a paradigm shift in scientific computing in various fields. As an increasingly active topic in QC, the variational quantum algorithm leads a promising tool for solving partial differential equations on noisy intermediate scale quantum devices. Although a clear perspective on the advantages of QC over classical computing techniques for specific mathematical and physical problems exists, applications of QC in computational fluid dynamics to solve practical flow problems, though promising, are still at the early stage of development. To explore QC in practical simulation of flow problems, this work applies a variational hybrid quantumclassical algorithm, namely the variational quantum linear solver (VQLS), to resolve the heat conduction equation through finite difference discretization of the Laplacian operator. Details of the VQLS implementation are discussed by various test instances of linear systems. The effect of the number of shots on the accuracy is studied, which reveals a logarithmic relationship. Furthermore, the heuristic scaling of the VQLS with the precision ε, the number of qubits n and the condition number k validates its time complexity reported in the literature. In addition, the successful state vector simulations of the heat conduction equation in one and two dimensions demonstrate the validity of the present VQLSbased algorithm by proofofconcept results. Finally, the heuristic scaling for the heat conduction problem indicates that the time complexity of the present approach is logarithmically dependent on the precision ε and linearly dependent on the number of qubits n.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>The prosperous development of both hardware and algorithms for quantum computing (QC) potentially prompts a paradigm shift in scientific computing in various fields. As an increasingly active topic in QC, the variational quantum algorithm leads a promising tool for solving partial differential equations on noisy intermediate scale quantum devices. Although a clear perspective on the advantages of QC over classical computing techniques for specific mathematical and physical problems exists, applications of QC in computational fluid dynamics to solve practical flow problems, though promising, are still at the early stage of development. To explore QC in practical simulation of flow problems, this work applies a variational hybrid quantumclassical algorithm, namely the variational quantum linear solver (VQLS), to resolve the heat conduction equation through finite difference discretization of the Laplacian operator. Details of the VQLS implementation are discussed by various test instances of linear systems. The effect of the number of shots on the accuracy is studied, which reveals a logarithmic relationship. Furthermore, the heuristic scaling of the VQLS with the precision ε, the number of qubits n and the condition number k validates its time complexity reported in the literature. In addition, the successful state vector simulations of the heat conduction equation in one and two dimensions demonstrate the validity of the present VQLSbased algorithm by proofofconcept results. Finally, the heuristic scaling for the heat conduction problem indicates that the time complexity of the present approach is logarithmically dependent on the precision ε and linearly dependent on the number of qubits n.
Application of a variational hybrid quantumclassical algorithm to heat conduction equation and analysis of time complexity
10.1063/5.0121778
Physics of Fluids
20221117T12:25:23Z
© 2022 Author(s).
S. C. Chew
B. C. Khoo

Development of a scattering model for diatomic gas–solid surface interactions by an unsupervised machine learning approach
https://aip.scitation.org/doi/10.1063/5.0110117?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This work proposes a new stochastic gas–solid scattering model for diatomic gas molecules constructed based on the collisional data obtained from molecular dynamics (MD) simulations. The Gaussian mixture (GM) approach, which is an unsupervised machine learning approach, is applied to H2 and N2 gases interacting with Ni surfaces in a twoparallel wall system under rarefied conditions. The main advantage of this approach is that the entire translational and rotational velocity components of the gas molecules before and after colliding with the surface can be utilized for training the GM model. This creates the possibility to study also highly nonequilibrium systems and accurately capture the energy exchange between the different molecular modes that cannot be captured by the classical scattering kernels. Considering the MD results as the reference solutions, the performance of the GMdriven scattering model is assessed in comparison with the Cercignani–Lampis–Lord (CLL) scattering model in different benchmarking systems: the Fourier thermal problem, the Couette flow problem, and a combined Fourier–Couette flow problem. This assessment is performed in terms of the distribution of the velocity components and energy modes, as well as accommodation coefficients. It is shown that the predicted results by the GM model are in better agreement with the original MD data. Especially, for H2 gas the GM model outperforms the CLL model. The results for N2 molecules are relatively less affected by changing the thermal and flow properties of the system, which is caused by the presence of a stronger adsorption layer.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This work proposes a new stochastic gas–solid scattering model for diatomic gas molecules constructed based on the collisional data obtained from molecular dynamics (MD) simulations. The Gaussian mixture (GM) approach, which is an unsupervised machine learning approach, is applied to H2 and N2 gases interacting with Ni surfaces in a twoparallel wall system under rarefied conditions. The main advantage of this approach is that the entire translational and rotational velocity components of the gas molecules before and after colliding with the surface can be utilized for training the GM model. This creates the possibility to study also highly nonequilibrium systems and accurately capture the energy exchange between the different molecular modes that cannot be captured by the classical scattering kernels. Considering the MD results as the reference solutions, the performance of the GMdriven scattering model is assessed in comparison with the Cercignani–Lampis–Lord (CLL) scattering model in different benchmarking systems: the Fourier thermal problem, the Couette flow problem, and a combined Fourier–Couette flow problem. This assessment is performed in terms of the distribution of the velocity components and energy modes, as well as accommodation coefficients. It is shown that the predicted results by the GM model are in better agreement with the original MD data. Especially, for H2 gas the GM model outperforms the CLL model. The results for N2 molecules are relatively less affected by changing the thermal and flow properties of the system, which is caused by the presence of a stronger adsorption layer.
Development of a scattering model for diatomic gas–solid surface interactions by an unsupervised machine learning approach
10.1063/5.0110117
Physics of Fluids
20221117T12:25:58Z
© 2022 Author(s).
Shahin Mohammad Nejad
Silvia Nedea
Arjan Frijns
David Smeulders

Experimental study of the influence of structural parameters on pressure fluctuations of cone–cylinder–hemisphere models
https://aip.scitation.org/doi/10.1063/5.0125915?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>When a viscous fluid flows over the surface of an object, different regions of the wall form different boundary layers. Fluctuating pressure in this boundary layer acts on the surface of the structure, causing it to vibrate and radiate noise; simultaneously, structural deformations will also have an impact on the flow field, and boundary layer pressure fluctuation is the most important component of flow noise. The characteristic parameters of a model, such as its wall thickness and the length of its parallel body section, will affect the pressure fluctuations it experiences. However, most studies treat the structure of the model as a rigid body. Therefore, this paper conducted experiments to examine the influences of the wall thickness and the parallel body length of a model on the pressure fluctuations it experiences. It was found that the fluctuating pressure at a given measuring position increases with decreasing wall thickness, and it decreases with increasing parallel body length. Then, this study demonstrated through comparative experiments that elastic and scale effects are important factors that cannot be ignored in calculations and experiments relating to pressure fluctuations. In addition, according to the characteristics of pressure fluctuation test values in different regions, the pressurefluctuation prediction empirical formulas for different regions of the boundary layer were established or improved on the basis of previous research on pressure fluctuation in different regions of the boundary layer. Finally, by pasting a flow exciter at the transition position in the boundary layer of the model can keep its flow noise down, the experimental results show that rough particles can split the large vortex into smaller vortices and reduce flow noise by more than 5 dB. These results and empirical formulas provide references for numerical and experimental research examining pressure fluctuations.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>When a viscous fluid flows over the surface of an object, different regions of the wall form different boundary layers. Fluctuating pressure in this boundary layer acts on the surface of the structure, causing it to vibrate and radiate noise; simultaneously, structural deformations will also have an impact on the flow field, and boundary layer pressure fluctuation is the most important component of flow noise. The characteristic parameters of a model, such as its wall thickness and the length of its parallel body section, will affect the pressure fluctuations it experiences. However, most studies treat the structure of the model as a rigid body. Therefore, this paper conducted experiments to examine the influences of the wall thickness and the parallel body length of a model on the pressure fluctuations it experiences. It was found that the fluctuating pressure at a given measuring position increases with decreasing wall thickness, and it decreases with increasing parallel body length. Then, this study demonstrated through comparative experiments that elastic and scale effects are important factors that cannot be ignored in calculations and experiments relating to pressure fluctuations. In addition, according to the characteristics of pressure fluctuation test values in different regions, the pressurefluctuation prediction empirical formulas for different regions of the boundary layer were established or improved on the basis of previous research on pressure fluctuation in different regions of the boundary layer. Finally, by pasting a flow exciter at the transition position in the boundary layer of the model can keep its flow noise down, the experimental results show that rough particles can split the large vortex into smaller vortices and reduce flow noise by more than 5 dB. These results and empirical formulas provide references for numerical and experimental research examining pressure fluctuations.
Experimental study of the influence of structural parameters on pressure fluctuations of cone–cylinder–hemisphere models
10.1063/5.0125915
Physics of Fluids
20221117T12:25:20Z
© 2022 Author(s).
Yuhui Li
Xuhong Miao
Jingping Xiao
Fuzhen Pang
Hongfu Wang

A numerical study of the rapid deflagrationtodetonation transition
https://aip.scitation.org/doi/10.1063/5.0127197?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/11">Volume 34, Issue 11</a>, November 2022. <br/>This paper describes numerically the rapid deflagrationtodetonation transition (DDT) in detail in a highfrequency pulse detonation rocket engine. Different from traditional DDT, reactants are injected into the chamber from near the open end and travel toward the closed end. Previous experiments have implied that the gasdynamic shock by injecting in a confined space and the intensive turbulence generated by the highspeed jet play important roles in the detonation initiation, but explanations of how, when, and where the detonation is generated were not presented clearly due to the limitation of experimental observation. In this work, highresolution twodimensional simulations are performed to investigate this process employing a physical model similar to the experimental configuration. A new mechanism manifesting itself as a complicated vortex–flame interaction is found for the flame transition from a laminar to compressible or choking regime. It is discovered that the gasdynamic shock, after reflecting from the end wall, triggers the detonation through the gradient of reactivity with the hot spot formed by the collision of the shock and the flame. A dimensionless criterion defined by the ratio of the acoustic speed to the inverse gradient of the ignition delay time is applied to further describe the spontaneous wave propagation from the perspective of chemphysical dynamics. This criterion quantitatively gives a good prediction of the propagating mode from the subsonic deflagration to a developing detonation, even in such a complex scenario as encountered in this work.
Physics of Fluids, Volume 34, Issue 11, November 2022. <br/>This paper describes numerically the rapid deflagrationtodetonation transition (DDT) in detail in a highfrequency pulse detonation rocket engine. Different from traditional DDT, reactants are injected into the chamber from near the open end and travel toward the closed end. Previous experiments have implied that the gasdynamic shock by injecting in a confined space and the intensive turbulence generated by the highspeed jet play important roles in the detonation initiation, but explanations of how, when, and where the detonation is generated were not presented clearly due to the limitation of experimental observation. In this work, highresolution twodimensional simulations are performed to investigate this process employing a physical model similar to the experimental configuration. A new mechanism manifesting itself as a complicated vortex–flame interaction is found for the flame transition from a laminar to compressible or choking regime. It is discovered that the gasdynamic shock, after reflecting from the end wall, triggers the detonation through the gradient of reactivity with the hot spot formed by the collision of the shock and the flame. A dimensionless criterion defined by the ratio of the acoustic speed to the inverse gradient of the ignition delay time is applied to further describe the spontaneous wave propagation from the perspective of chemphysical dynamics. This criterion quantitatively gives a good prediction of the propagating mode from the subsonic deflagration to a developing detonation, even in such a complex scenario as encountered in this work.
A numerical study of the rapid deflagrationtodetonation transition
10.1063/5.0127197
Physics of Fluids
20221121T11:01:40Z
© 2022 Author(s).
Ralf Deiterding