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.
https://aip.scitation.org/loi/phf?af=R&feed=mostrecent
American Institute of Physics: Physics of Fluids: Table of Contents
American Institute of Physics
enUS
Physics of Fluids
https://aip.scitation.org/na101/home/literatum/publisher/aip/journals/covergifs/phf/cover.jpg
https://aip.scitation.org/loi/phf?af=R&feed=mostrecent

Tsunami generation by a seabed deformation in the presence of a viscoelastic mud
https://aip.scitation.org/doi/10.1063/5.0132230%40phf.2023.TSUN2022.issue1?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/TSUN2022/1">Volume TSUN2022, Issue 1</a>, March 2023. <br/>In this work, under the assumption of linear water waves, we study tsunamis generated by a seabed deformation in the presence of viscoelastic mud. We divide the total control volume under study into a water layer, which is assumed to be an irrotational and inviscid flow, and a mud layer with viscoelastic properties that obeys a linear Maxwell rheological model. Considering that fluid layer thicknesses are of the same order of magnitude and that they are much smaller than the characteristic horizontal length of the seabed deformation, we obtain a semianalytical solution that models the evolution of the free surface elevation. For the above limits, the fluid motion in the water layer is essentially horizontal. Passive and active tsunami generation cases are analyzed. The seabed deformation is modeled as a Heaviside step function. For an active generation case, when the mud layer thickness increases, the tsunami's maximum amplitude decreases. For the passive generation case, the tsunami's maximum amplitude remains constant in a finite time interval of the same order of magnitude as the characteristic time; this phenomenon does not occur for the active generation case.
Physics of Fluids, Volume TSUN2022, Issue 1, March 2023. <br/>In this work, under the assumption of linear water waves, we study tsunamis generated by a seabed deformation in the presence of viscoelastic mud. We divide the total control volume under study into a water layer, which is assumed to be an irrotational and inviscid flow, and a mud layer with viscoelastic properties that obeys a linear Maxwell rheological model. Considering that fluid layer thicknesses are of the same order of magnitude and that they are much smaller than the characteristic horizontal length of the seabed deformation, we obtain a semianalytical solution that models the evolution of the free surface elevation. For the above limits, the fluid motion in the water layer is essentially horizontal. Passive and active tsunami generation cases are analyzed. The seabed deformation is modeled as a Heaviside step function. For an active generation case, when the mud layer thickness increases, the tsunami's maximum amplitude decreases. For the passive generation case, the tsunami's maximum amplitude remains constant in a finite time interval of the same order of magnitude as the characteristic time; this phenomenon does not occur for the active generation case.
Tsunami generation by a seabed deformation in the presence of a viscoelastic mud
10.1063/5.0132230@phf.2023.TSUN2022.issue1
Physics of Fluids
20230119T11:11:53Z
© 2023 Author(s).
S. BahenaJimenez
E. Bautista
F. Méndez

Experimental study of the surge and boreinduced impact pressure on a vertical wall and its foundation
https://aip.scitation.org/doi/10.1063/5.0128668%40phf.2023.TSUN2022.issue1?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/TSUN2022/1">Volume TSUN2022, Issue 1</a>, March 2023. <br/>Both surge and bore impacts could lead to the failure of coastal structures. Nevertheless, differences between the surge and boreinduced hydrodynamic impact processes on a vertical wall are still unclear. Meanwhile, investigation of the bed pressure features during the wall impact is also rare. In this study, a series of dambreak experiments were conducted to specify the hydrodynamic characteristics of the surge and bore impact pressure on a vertical wall and its foundation. In the experiment, same initial water head was applied with five different initial downstream water levels (IDWLs). Temporal variations of the surge/bore impact pressures at four elevations on the wall and four positions along the bed were recorded. The surge induced maximum water height on the wall is larger than the boreinduced one, which decreases with the increase in the IDWL. With the increase in the IDWL, the initial peak impact pressure gradually decays owing to the slowing down of flow velocity and the significant air entrainment at the bore front. Regarding the initial peak pressure and its rise time, it is confirmed that the initial surge impact pressure is sensitive to the wall elevation, whereas it is relatively uniform along the wall bottom region for the bore impact pressure. As for the measured bed pressure, the initial impact zone induced by the secondary flow near the wall and the falling impact zone caused by the falling down of water mass from the splashups away from the wall are identified, showing different hydrodynamic features.
Physics of Fluids, Volume TSUN2022, Issue 1, March 2023. <br/>Both surge and bore impacts could lead to the failure of coastal structures. Nevertheless, differences between the surge and boreinduced hydrodynamic impact processes on a vertical wall are still unclear. Meanwhile, investigation of the bed pressure features during the wall impact is also rare. In this study, a series of dambreak experiments were conducted to specify the hydrodynamic characteristics of the surge and bore impact pressure on a vertical wall and its foundation. In the experiment, same initial water head was applied with five different initial downstream water levels (IDWLs). Temporal variations of the surge/bore impact pressures at four elevations on the wall and four positions along the bed were recorded. The surge induced maximum water height on the wall is larger than the boreinduced one, which decreases with the increase in the IDWL. With the increase in the IDWL, the initial peak impact pressure gradually decays owing to the slowing down of flow velocity and the significant air entrainment at the bore front. Regarding the initial peak pressure and its rise time, it is confirmed that the initial surge impact pressure is sensitive to the wall elevation, whereas it is relatively uniform along the wall bottom region for the bore impact pressure. As for the measured bed pressure, the initial impact zone induced by the secondary flow near the wall and the falling impact zone caused by the falling down of water mass from the splashups away from the wall are identified, showing different hydrodynamic features.
Experimental study of the surge and boreinduced impact pressure on a vertical wall and its foundation
10.1063/5.0128668@phf.2023.TSUN2022.issue1
Physics of Fluids
20230103T01:55:29Z
© 2023 Author(s).

Experimental study on flow kinematics of dambreak induced surge impacting onto a vertical wall
https://aip.scitation.org/doi/10.1063/5.0137475%40phf.2023.TSUN2022.issue1?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/TSUN2022/1">Volume TSUN2022, Issue 1</a>, March 2023. <br/>Tsunami surges are frequently simulated by dambreak flows over dry beds. The purpose of this study is to quantitatively investigate the flow kinematics and turbulent characteristics of a surge impacting onto a vertical wall. To quantify the flow kinematics, the particle image velocimetry technique was used in the nonaerated region, while the bubble image velocimetry technique was employed to measure the impactinduced turbulent flow with air entrainment. The measured velocity fields of the impactinduced splash confirmed the feasibility of Ko and Yeh's [Coastal Eng. 131, 1–11 (2018)] model employing a solidbody motion assumption of splash that estimates the impact force by bores and surges at the initial impact stage. Velocity fields and streamlines revealed that the main water body overturned backward and formed a large twophase vortex, while a small counter rotating vortex was also formed at the corner of the wallbed junction. The mean velocity magnitude of the small corner vortex is about twothirds that of the main water body. The mean turbulent intensity of these aerated regions is about 3.4 times that of the nonaerated regions. Based on a wavelet transformbased method, the result reveals that the mean turbulence length scale of the aerated region is about twothirds that of the nonaerated region. This study reveals for the first time the quantitative flow field results of the surge impact process, which deepen insight of tsunami risk in coastal engineering, thus improving the accuracy of postdamage prediction in coastal areas.
Physics of Fluids, Volume TSUN2022, Issue 1, March 2023. <br/>Tsunami surges are frequently simulated by dambreak flows over dry beds. The purpose of this study is to quantitatively investigate the flow kinematics and turbulent characteristics of a surge impacting onto a vertical wall. To quantify the flow kinematics, the particle image velocimetry technique was used in the nonaerated region, while the bubble image velocimetry technique was employed to measure the impactinduced turbulent flow with air entrainment. The measured velocity fields of the impactinduced splash confirmed the feasibility of Ko and Yeh's [Coastal Eng. 131, 1–11 (2018)] model employing a solidbody motion assumption of splash that estimates the impact force by bores and surges at the initial impact stage. Velocity fields and streamlines revealed that the main water body overturned backward and formed a large twophase vortex, while a small counter rotating vortex was also formed at the corner of the wallbed junction. The mean velocity magnitude of the small corner vortex is about twothirds that of the main water body. The mean turbulent intensity of these aerated regions is about 3.4 times that of the nonaerated regions. Based on a wavelet transformbased method, the result reveals that the mean turbulence length scale of the aerated region is about twothirds that of the nonaerated region. This study reveals for the first time the quantitative flow field results of the surge impact process, which deepen insight of tsunami risk in coastal engineering, thus improving the accuracy of postdamage prediction in coastal areas.
Experimental study on flow kinematics of dambreak induced surge impacting onto a vertical wall
10.1063/5.0137475@phf.2023.TSUN2022.issue1
Physics of Fluids
20230217T12:09:05Z
© 2023 Author(s).
KuangAn Chang

An unconventional tsunami: 2022 Tonga event
https://aip.scitation.org/doi/10.1063/5.0122830%40phf.2023.TSUN2022.issue1?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/TSUN2022/1">Volume TSUN2022, Issue 1</a>, March 2023. <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 TSUN2022, Issue 1, March 2023. <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@phf.2023.TSUN2022.issue1
Physics of Fluids
20221116T10:41:33Z
© 2022 Author(s).
Peida Han
Xiping Yu

Twolayer twophase material point method simulation of granular landslides and generated tsunami waves
https://aip.scitation.org/doi/10.1063/5.0128867%40phf.2023.TSUN2022.issue1?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/TSUN2022/1">Volume TSUN2022, Issue 1</a>, March 2023. <br/>Numerical modeling of the entire process of tsunamis generation by granular landslides is very difficult and challenging as it involves the soil–water interaction, large deformation of soil, and the fluidization and sedimentation of sand. In this study, a computational model based on the twolayer twophase material point method (MPM) is developed to simulate granularlandslidegenerated tsunamis, wherein the soil–water interaction, large deformation of soil, and fluidization and sedimentation of sand are well modeled. The soil behavior is described using a Mohr–Coulomb model with a nonassociated flow rule, while the water is considered as weakly compressible. Furthermore, three different benchmark problems are simulated. All computed results well agree with the corresponding analytical solution and laboratory test data, verifying the effectiveness of the proposed twolayer twophase MPM for modeling the subaerial and submerged granularlandslidegenerated tsunamis. Additionally, the influence of different soil material parameters on the water wave generated by the subaerial granular landslide is investigated.
Physics of Fluids, Volume TSUN2022, Issue 1, March 2023. <br/>Numerical modeling of the entire process of tsunamis generation by granular landslides is very difficult and challenging as it involves the soil–water interaction, large deformation of soil, and the fluidization and sedimentation of sand. In this study, a computational model based on the twolayer twophase material point method (MPM) is developed to simulate granularlandslidegenerated tsunamis, wherein the soil–water interaction, large deformation of soil, and fluidization and sedimentation of sand are well modeled. The soil behavior is described using a Mohr–Coulomb model with a nonassociated flow rule, while the water is considered as weakly compressible. Furthermore, three different benchmark problems are simulated. All computed results well agree with the corresponding analytical solution and laboratory test data, verifying the effectiveness of the proposed twolayer twophase MPM for modeling the subaerial and submerged granularlandslidegenerated tsunamis. Additionally, the influence of different soil material parameters on the water wave generated by the subaerial granular landslide is investigated.
Twolayer twophase material point method simulation of granular landslides and generated tsunami waves
10.1063/5.0128867@phf.2023.TSUN2022.issue1
Physics of Fluids
20221205T12:16:50Z
© 2022 Author(s).

Referee acknowledgment for 2022
https://aip.scitation.org/doi/10.1063/5.0145926?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>
Referee acknowledgment for 2022
10.1063/5.0145926
Physics of Fluids
20230316T02:06:25Z
© 2023 Author(s).
Alan Jeffrey Giacomin

Recent advances in applying deep reinforcement learning for flow control: Perspectives and future directions
https://aip.scitation.org/doi/10.1063/5.0143913?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Deep reinforcement learning (DRL) has been applied to a variety of problems during the past decade and has provided effective control strategies in highdimensional and nonlinear situations that are challenging to traditional methods. Flourishing applications now spread out into the field of fluid dynamics and specifically active flow control (AFC). In the community of AFC, the encouraging results obtained in twodimensional and chaotic conditions have raised the interest to study increasingly complex flows. In this review, we first provide a general overview of the reinforcementlearning and DRL frameworks, as well as their recent advances. We then focus on the application of DRL to AFC, highlighting the current limitations of the DRL algorithms in this field, and suggesting some of the potential upcoming milestones to reach, as well as open questions that are likely to attract the attention of the fluid mechanics community.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Deep reinforcement learning (DRL) has been applied to a variety of problems during the past decade and has provided effective control strategies in highdimensional and nonlinear situations that are challenging to traditional methods. Flourishing applications now spread out into the field of fluid dynamics and specifically active flow control (AFC). In the community of AFC, the encouraging results obtained in twodimensional and chaotic conditions have raised the interest to study increasingly complex flows. In this review, we first provide a general overview of the reinforcementlearning and DRL frameworks, as well as their recent advances. We then focus on the application of DRL to AFC, highlighting the current limitations of the DRL algorithms in this field, and suggesting some of the potential upcoming milestones to reach, as well as open questions that are likely to attract the attention of the fluid mechanics community.
Recent advances in applying deep reinforcement learning for flow control: Perspectives and future directions
10.1063/5.0143913
Physics of Fluids
20230316T01:57:37Z
© 2023 Author(s).
C. Vignon
J. Rabault
R. Vinuesa

Fundamental review on collision of blast waves
https://aip.scitation.org/doi/10.1063/5.0138156?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The introduction and pinnacle of colliding blast waves research commenced in the 1950s following World War II. Since then, sporadic studies have appeared throughout the literature up until the early 1990s, beyond which a significant contributory gap on the topic ensued. With the interminable proactivity of modern civil and aerospace defense research in the past several decades, investigations on the phenomena of blast wave collisions have fallen behind in comparison. Recent events and applications of offensive and defensive operations have slowly begun to rekindle studies on colliding blast waves in the last few years. However, there remains limitations on the extent of analyses which have yet to be adequately addressed. This review attempts to critically compile and analyze all existing research on blast wave collisions to identify pertinent shortcomings of the present stateoftheart. In addition, related investigations of colliding shock waves and the collision of shock waves and blast waves are also provided to further elaborate on their distinctions to colliding blast waves. Prior to such discussions, the fundamentals of blast wave behaviors in terms of their characteristics, formation, and propagation are presented to pave a background to subsequent advanced topics. Finally, unique classifications of direct and indirect applications of blast wave collisions are presented with modern perspectives. As a result, a classical problem is reawakened toward understanding and addressing highly complex and dynamic shock wave systems in defense applications.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The introduction and pinnacle of colliding blast waves research commenced in the 1950s following World War II. Since then, sporadic studies have appeared throughout the literature up until the early 1990s, beyond which a significant contributory gap on the topic ensued. With the interminable proactivity of modern civil and aerospace defense research in the past several decades, investigations on the phenomena of blast wave collisions have fallen behind in comparison. Recent events and applications of offensive and defensive operations have slowly begun to rekindle studies on colliding blast waves in the last few years. However, there remains limitations on the extent of analyses which have yet to be adequately addressed. This review attempts to critically compile and analyze all existing research on blast wave collisions to identify pertinent shortcomings of the present stateoftheart. In addition, related investigations of colliding shock waves and the collision of shock waves and blast waves are also provided to further elaborate on their distinctions to colliding blast waves. Prior to such discussions, the fundamentals of blast wave behaviors in terms of their characteristics, formation, and propagation are presented to pave a background to subsequent advanced topics. Finally, unique classifications of direct and indirect applications of blast wave collisions are presented with modern perspectives. As a result, a classical problem is reawakened toward understanding and addressing highly complex and dynamic shock wave systems in defense applications.
Fundamental review on collision of blast waves
10.1063/5.0138156
Physics of Fluids
20230317T02:39:41Z
© 2023 Author(s).
Monjee K. Almustafa
Moncef L. Nehdi

100 Years of Giesekus: Beyond science—The person behind the research
https://aip.scitation.org/doi/10.1063/5.0141998?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>On the occasion of the 100th anniversary of Hanswalter Giesekus, his personality and biography shall be given attention within the commemorative publication “100 Years of Giesekus.” Who was the scientist as a human being? What motivation drove his research? What were his interest and meaning in life? The authors, two of the six children of the scientist, show their father to be a multifaceted person, whose ideal was the “universal scholar” at the beginning 20th century. Besides his passion for rheology, he was deeply interested in other areas of physics—especially astronomy—as well as theology, literature, history, music, and art.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>On the occasion of the 100th anniversary of Hanswalter Giesekus, his personality and biography shall be given attention within the commemorative publication “100 Years of Giesekus.” Who was the scientist as a human being? What motivation drove his research? What were his interest and meaning in life? The authors, two of the six children of the scientist, show their father to be a multifaceted person, whose ideal was the “universal scholar” at the beginning 20th century. Besides his passion for rheology, he was deeply interested in other areas of physics—especially astronomy—as well as theology, literature, history, music, and art.
100 Years of Giesekus: Beyond science—The person behind the research
10.1063/5.0141998
Physics of Fluids
20230301T01:51:31Z
© 2023 Author(s).
Andreas Giesekus
Ulrich Giesekus

Analytical model for curvedshock Mach reflection
https://aip.scitation.org/doi/10.1063/5.0139784?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Mach reflection (MR) is an essential component in the development of the shock theory, as the incident shock curvature is found to have a significant effect on the MR patterns. Curvedshock Mach reflection (CMR) is not yet adequately understood due to the rotational complexity behind curved shocks. Here, CMR in steady, planar/axisymmetric flows is analyzed to supplement the wellstudied phenomena caused by obliqueshock Mach reflection (OMR). The solution from the von Neumann's threeshock theory does not fully describe the CMR case. A CMR structure is presented and characterized by an incident shock, reflected shock, Mach stem, and expansion/compression waves over the slipline or occasionally an absence of waves due to pressure equilibrium. On the basis of this CMR structure, an analytical model for predicting the Mach stem in the CMR case is established. The model reduces to the OMR case if the shock curvature is not applicable. Predictions of the Mach stem geometry and shock structure based on the model exhibit better agreement with the numerical results than predictions using previous models. It is found that the circumferential shock curvature plays a key role in the axisymmetric doubly curved CMR case, which results in a different outcome from the planar case.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Mach reflection (MR) is an essential component in the development of the shock theory, as the incident shock curvature is found to have a significant effect on the MR patterns. Curvedshock Mach reflection (CMR) is not yet adequately understood due to the rotational complexity behind curved shocks. Here, CMR in steady, planar/axisymmetric flows is analyzed to supplement the wellstudied phenomena caused by obliqueshock Mach reflection (OMR). The solution from the von Neumann's threeshock theory does not fully describe the CMR case. A CMR structure is presented and characterized by an incident shock, reflected shock, Mach stem, and expansion/compression waves over the slipline or occasionally an absence of waves due to pressure equilibrium. On the basis of this CMR structure, an analytical model for predicting the Mach stem in the CMR case is established. The model reduces to the OMR case if the shock curvature is not applicable. Predictions of the Mach stem geometry and shock structure based on the model exhibit better agreement with the numerical results than predictions using previous models. It is found that the circumferential shock curvature plays a key role in the axisymmetric doubly curved CMR case, which results in a different outcome from the planar case.
Analytical model for curvedshock Mach reflection
10.1063/5.0139784
Physics of Fluids
20230301T03:50:24Z
© 2023 Author(s).

The triple decomposition of the velocity gradient tensor as a standardized real Schur form
https://aip.scitation.org/doi/10.1063/5.0138180?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The triple decomposition of a velocity gradient tensor provides an analysis tool in fluid mechanics by which the flow can be split into a sum of irrotational straining flow, shear flow, and rigid body rotational flow. In 2007, Kolář formulated an optimization problem to compute the triple decomposition [V. Kolář, “Vortex identification: New requirements and limitations,” Int. J. Heat Fluid Flow 28, 638–652 (2007)], and more recently, the triple decomposition has been connected to the Schur form of the associated matrix. We show that the standardized real Schur form, which can be computed by state of the art linear algebra routines, is a solution to the optimization problem posed by Kolář. We also demonstrate why using the standardized variant of the real Schur form makes computation of the triple decomposition more efficient. Furthermore, we illustrate why different structures of the real Schur form correspond to different alignments of the coordinate system with the fluid flow and may, therefore, lead to differences in the resulting triple decomposition. Based on these results, we propose a new, simplified algorithm for computing the triple decomposition, which guarantees consistent results.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The triple decomposition of a velocity gradient tensor provides an analysis tool in fluid mechanics by which the flow can be split into a sum of irrotational straining flow, shear flow, and rigid body rotational flow. In 2007, Kolář formulated an optimization problem to compute the triple decomposition [V. Kolář, “Vortex identification: New requirements and limitations,” Int. J. Heat Fluid Flow 28, 638–652 (2007)], and more recently, the triple decomposition has been connected to the Schur form of the associated matrix. We show that the standardized real Schur form, which can be computed by state of the art linear algebra routines, is a solution to the optimization problem posed by Kolář. We also demonstrate why using the standardized variant of the real Schur form makes computation of the triple decomposition more efficient. Furthermore, we illustrate why different structures of the real Schur form correspond to different alignments of the coordinate system with the fluid flow and may, therefore, lead to differences in the resulting triple decomposition. Based on these results, we propose a new, simplified algorithm for computing the triple decomposition, which guarantees consistent results.
The triple decomposition of the velocity gradient tensor as a standardized real Schur form
10.1063/5.0138180
Physics of Fluids
20230306T10:56:05Z
© 2023 Author(s).
Joel Kronborg
Johan Hoffman

Dual separation control and drag mitigation in high speed flows using viscoelastic materials
https://aip.scitation.org/doi/10.1063/5.0141572?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Boundary layer separation and friction drag form key delimiting phenomena that subvert the aerial platforms from achieving greater efficiency and accessing wider operation envelope. Both these phenomena are significantly aggravated in supersonic platforms due to the interactions between shock waves with the boundary layer that develops over the vehicle surface and within the engines. The present work demonstrates a new paradigm that leverages the native or programmable material properties of the aerostructures to engender simultaneous reduction in the separation scales and mitigation of skin friction drag. As a first step toward realizing this paradigm, the present work demonstrates, for the first time, the simultaneous skin friction drag mitigation in a Mach 2.5 boundary layer and control of shock induced boundary layer separation, both using viscoelastic implants placed under the flow. It is experimentally demonstrated that the appropriately chosen viscoelastic materials can simultaneously reduce the skin friction coefficient at the measurement location by 11% and mitigate the size of a largescale separated flow by up to 28%. The reported performance matches the current generation flow effectors in both separation scale and skin friction mitigation. The present study opens a new application space for soft/programmable materials in high speed aerial vehicles.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Boundary layer separation and friction drag form key delimiting phenomena that subvert the aerial platforms from achieving greater efficiency and accessing wider operation envelope. Both these phenomena are significantly aggravated in supersonic platforms due to the interactions between shock waves with the boundary layer that develops over the vehicle surface and within the engines. The present work demonstrates a new paradigm that leverages the native or programmable material properties of the aerostructures to engender simultaneous reduction in the separation scales and mitigation of skin friction drag. As a first step toward realizing this paradigm, the present work demonstrates, for the first time, the simultaneous skin friction drag mitigation in a Mach 2.5 boundary layer and control of shock induced boundary layer separation, both using viscoelastic implants placed under the flow. It is experimentally demonstrated that the appropriately chosen viscoelastic materials can simultaneously reduce the skin friction coefficient at the measurement location by 11% and mitigate the size of a largescale separated flow by up to 28%. The reported performance matches the current generation flow effectors in both separation scale and skin friction mitigation. The present study opens a new application space for soft/programmable materials in high speed aerial vehicles.
Dual separation control and drag mitigation in high speed flows using viscoelastic materials
10.1063/5.0141572
Physics of Fluids
20230309T12:28:13Z
© 2023 Author(s).
James Walz
Venkat Narayanaswamy

Changing interface conditions in a twofluid rotating flow
https://aip.scitation.org/doi/10.1063/5.0141821?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This experimental study reveals a striking nonlinearphysics phenomenon of fundamental and practical interest—changing conditions at the interface of two swirling immiscible fluids filling a vertical cylindrical container. To this end, we use a new measurement technique significantly advanced compared with prior studies. The rotating bottom disk drives a steady axisymmetric flow of both fluids. The lower fluid makes the centrifugal circulation (CC): It spirals on toroid surfaces going to the periphery near the bottom and going back to the axis near the interface. At a slow rotation (Re = 100), the upper fluid makes the anticentrifugal circulation. As the rotation intensifies (Re = 175), the upperfluid flow reverses into CC near the interfaceaxis intersection. For strong swirl (Re = 500), the CC occurs at the entire interface. In prior studies, the spatial resolution (1 mm) was insufficient to resolve the nearinterface velocity field. Here, we use the advanced (light field) measurement technique, which has significantly better resolution (0.14 mm) and clearly shows that the radial velocity at the interface is negative for small Re and becomes zero for large Re. During these metamorphoses, the topology of the lowerfluid flow remains invariant, the interface has no visible deformation, and the flow is steady and axisymmetric.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This experimental study reveals a striking nonlinearphysics phenomenon of fundamental and practical interest—changing conditions at the interface of two swirling immiscible fluids filling a vertical cylindrical container. To this end, we use a new measurement technique significantly advanced compared with prior studies. The rotating bottom disk drives a steady axisymmetric flow of both fluids. The lower fluid makes the centrifugal circulation (CC): It spirals on toroid surfaces going to the periphery near the bottom and going back to the axis near the interface. At a slow rotation (Re = 100), the upper fluid makes the anticentrifugal circulation. As the rotation intensifies (Re = 175), the upperfluid flow reverses into CC near the interfaceaxis intersection. For strong swirl (Re = 500), the CC occurs at the entire interface. In prior studies, the spatial resolution (1 mm) was insufficient to resolve the nearinterface velocity field. Here, we use the advanced (light field) measurement technique, which has significantly better resolution (0.14 mm) and clearly shows that the radial velocity at the interface is negative for small Re and becomes zero for large Re. During these metamorphoses, the topology of the lowerfluid flow remains invariant, the interface has no visible deformation, and the flow is steady and axisymmetric.
Changing interface conditions in a twofluid rotating flow
10.1063/5.0141821
Physics of Fluids
20230316T01:43:58Z
© 2023 Author(s).
Igor V. Naumov
Sergey G. Skripkin
Alexandr Z. Kvon
Vladimir N. Shtern

Role of ambient pressure on bouncing and coalescence of colliding jets
https://aip.scitation.org/doi/10.1063/5.0146183?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this Letter, the merging vs bouncing response of obliquely oriented colliding jets under elevated and reduced gaseous environment pressures was experimentally examined. Experiments with water and ntetradecane confirmed that the collision outcome transitions from merging to bouncing and then to merging again, when the impact velocity was increased. This behavior which was previously reported for atmospheric pressure has now also been observed at elevated and reduced pressures. New results also show that there exists a critical pressure (0.9 bar for ntetradecane and 5 bar for water) below which increasing pressure promotes bouncing (expands the bouncing regime), while beyond this, merging is promoted (reduces the bouncing regime) instead. This leads to a nonmonotonic influence of pressure on the noncoalescence outcomes of collisional jets, which was not previously reported. The study provides evidence of new behaviors in colliding jets at reduced and elevated pressures, which differs from wellstudied droplet–droplet collisions.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this Letter, the merging vs bouncing response of obliquely oriented colliding jets under elevated and reduced gaseous environment pressures was experimentally examined. Experiments with water and ntetradecane confirmed that the collision outcome transitions from merging to bouncing and then to merging again, when the impact velocity was increased. This behavior which was previously reported for atmospheric pressure has now also been observed at elevated and reduced pressures. New results also show that there exists a critical pressure (0.9 bar for ntetradecane and 5 bar for water) below which increasing pressure promotes bouncing (expands the bouncing regime), while beyond this, merging is promoted (reduces the bouncing regime) instead. This leads to a nonmonotonic influence of pressure on the noncoalescence outcomes of collisional jets, which was not previously reported. The study provides evidence of new behaviors in colliding jets at reduced and elevated pressures, which differs from wellstudied droplet–droplet collisions.
Role of ambient pressure on bouncing and coalescence of colliding jets
10.1063/5.0146183
Physics of Fluids
20230317T03:03:54Z
© 2023 Author(s).
Minglei Li
Abhishek Saha
Chao Sun
Chung K. Law

Acoustic interaction force between two particles immersed in a viscoelastic fluid
https://aip.scitation.org/doi/10.1063/5.0143005?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The interaction acoustic radiation force in a standing plane wave applied to each small solid sphere in a twoparticle system immersed in a viscoelastic fluid is studied in a framework based on perturbation theory. In this work, the first and secondorder perturbation theories are used in the governing equations with considering the upperconvected Maxwell model to obtain mathematical modeling. We use the finite element method to carry out simulations and describe the behavior of the viscoelastic fluid. The mathematical development is validated from three literature case studies: a oneparticle system in a viscous fluid, a twoparticle system in a viscous fluid, and a oneparticle system in a viscoelastic fluid. The novelty of this study is to establish the acoustic interaction force between two spherical particles immersed in a viscoelastic fluid. The results show that the acoustic interaction force between two spheres is greater in a viscous fluid in comparison with the viscoelastic fluid with the same shear viscosity. This behavior is due to the relaxation time effect. It is also indicated that the acoustic interaction force between the particles decreases by the relaxation time and increases by the fluid's viscosity. A mathematical formula is proposed for the acoustic interaction force between particles located close to each other in a viscoelastic fluid.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The interaction acoustic radiation force in a standing plane wave applied to each small solid sphere in a twoparticle system immersed in a viscoelastic fluid is studied in a framework based on perturbation theory. In this work, the first and secondorder perturbation theories are used in the governing equations with considering the upperconvected Maxwell model to obtain mathematical modeling. We use the finite element method to carry out simulations and describe the behavior of the viscoelastic fluid. The mathematical development is validated from three literature case studies: a oneparticle system in a viscous fluid, a twoparticle system in a viscous fluid, and a oneparticle system in a viscoelastic fluid. The novelty of this study is to establish the acoustic interaction force between two spherical particles immersed in a viscoelastic fluid. The results show that the acoustic interaction force between two spheres is greater in a viscous fluid in comparison with the viscoelastic fluid with the same shear viscosity. This behavior is due to the relaxation time effect. It is also indicated that the acoustic interaction force between the particles decreases by the relaxation time and increases by the fluid's viscosity. A mathematical formula is proposed for the acoustic interaction force between particles located close to each other in a viscoelastic fluid.
Acoustic interaction force between two particles immersed in a viscoelastic fluid
10.1063/5.0143005
Physics of Fluids
20230317T03:03:42Z
© 2023 Author(s).
Fatemeh Eslami
Hossein Hamzehpour
Sanaz Derikvandi
S. Amir Bahrani

Coarsegrained model of whole blood hemolysis and morphological analysis of erythrocyte population under nonphysiological shear stress flow environment
https://aip.scitation.org/doi/10.1063/5.0137517?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Erythrocyte dynamics and hemorheology exist inextricably connection. In order to further explore the population dynamics of erythrocytes in nonphysiological shear stress flow and its microscopic hemolysis mechanism, this study improved the coarsegrained erythrocytes damaged model and established the hemoglobin (Hb) diffusion model based on the transport dissipative particle dynamics. The whole blood hemolysis simulation results showed that the red blood cells near the active shear side were more likely to be damaged, and most of the escaping cytoplasm was also concentrated in this side. After the destruction and relaxation of erythrocytes, the cell membrane presents a pathological state of relaxation and swelling. Moreover, we built a deep learning network for recognizing erythrocyte morphology and analyzing the erythrocyte population change rule in nonphysiological shear stress flow. In this study, the clues of the blood shearthinning effect were found from erythrocyte dynamics and coarsegrained simulation. After the shearing starts, the coinstacked erythrocytes are depolymerized. Then, the overturned double concave erythrocytes changed into multilobe erythrocytes. When the flow shear stress gradually increases, most erythrocytes show an ellipsoidal tanktreading movement along the shear direction. Changes in erythrocyte morphology can reduce flow resistance, showing a phenomenon of the whole blood shearthinning effect.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Erythrocyte dynamics and hemorheology exist inextricably connection. In order to further explore the population dynamics of erythrocytes in nonphysiological shear stress flow and its microscopic hemolysis mechanism, this study improved the coarsegrained erythrocytes damaged model and established the hemoglobin (Hb) diffusion model based on the transport dissipative particle dynamics. The whole blood hemolysis simulation results showed that the red blood cells near the active shear side were more likely to be damaged, and most of the escaping cytoplasm was also concentrated in this side. After the destruction and relaxation of erythrocytes, the cell membrane presents a pathological state of relaxation and swelling. Moreover, we built a deep learning network for recognizing erythrocyte morphology and analyzing the erythrocyte population change rule in nonphysiological shear stress flow. In this study, the clues of the blood shearthinning effect were found from erythrocyte dynamics and coarsegrained simulation. After the shearing starts, the coinstacked erythrocytes are depolymerized. Then, the overturned double concave erythrocytes changed into multilobe erythrocytes. When the flow shear stress gradually increases, most erythrocytes show an ellipsoidal tanktreading movement along the shear direction. Changes in erythrocyte morphology can reduce flow resistance, showing a phenomenon of the whole blood shearthinning effect.
Coarsegrained model of whole blood hemolysis and morphological analysis of erythrocyte population under nonphysiological shear stress flow environment
10.1063/5.0137517
Physics of Fluids
20230301T05:45:47Z
© 2023 Author(s).

Bioinspired wake tracking and phase matching of two diagonal flapping swimmers
https://aip.scitation.org/doi/10.1063/5.0136767?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Wake interaction provides hydrodynamic gain and flowaided navigation in fish schools. The lateral spacing [math] and phase angle [math] relative to upstream wake are two important states for downstream swimmers. In this paper, the lateral wake tracking and phase matching of two diagonal flapping swimmers are studied through experiments. Bioinspired differential pressure (DP) sensing on the downstream swimmer is adopted to capture the wake interaction features. Two DP sensing strategies, the symmetrical differential pressure (SDP) and leading edge differential pressure (LDP), are employed to capture the wake interaction features. SDP measures the pressure difference of two symmetrical ports on the two sides of the downstream swimmer, and LDP measures the pressure difference of leading edge port against the two side ports. Onedimensional convolutional neural networks (1D CNN) with a parallel structure are trained to decode wake interaction states ([math] and [math]) based on DP signals. The 1D CNN model is trained and tested offline and is used to estimate the wake interaction states online. Three demonstrations of online lateral wake tracking and phase matching control are carried out. Compared with SDP, LDP predicts [math] and [math] more accurately. It is found that the downstream wakes are more compact after control, which is consistent with high propulsive efficiency mode.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Wake interaction provides hydrodynamic gain and flowaided navigation in fish schools. The lateral spacing [math] and phase angle [math] relative to upstream wake are two important states for downstream swimmers. In this paper, the lateral wake tracking and phase matching of two diagonal flapping swimmers are studied through experiments. Bioinspired differential pressure (DP) sensing on the downstream swimmer is adopted to capture the wake interaction features. Two DP sensing strategies, the symmetrical differential pressure (SDP) and leading edge differential pressure (LDP), are employed to capture the wake interaction features. SDP measures the pressure difference of two symmetrical ports on the two sides of the downstream swimmer, and LDP measures the pressure difference of leading edge port against the two side ports. Onedimensional convolutional neural networks (1D CNN) with a parallel structure are trained to decode wake interaction states ([math] and [math]) based on DP signals. The 1D CNN model is trained and tested offline and is used to estimate the wake interaction states online. Three demonstrations of online lateral wake tracking and phase matching control are carried out. Compared with SDP, LDP predicts [math] and [math] more accurately. It is found that the downstream wakes are more compact after control, which is consistent with high propulsive efficiency mode.
Bioinspired wake tracking and phase matching of two diagonal flapping swimmers
10.1063/5.0136767
Physics of Fluids
20230303T12:58:03Z
© 2023 Author(s).
Wenhua Xu
Guodong Xu
Mingjue Li
Chen Yang

Swimming of the midge larva: Principles and tricks of locomotion at intermediate Reynolds number
https://aip.scitation.org/doi/10.1063/5.0137841?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>At the millimeter scale and in the intermediate Reynolds number (Re) regime, the midge and mosquito larvae can reach swimming speeds of more than one body length per cycle performing a “figure eight” gait, in which their elongated bodies periodically bend nearly into circles and then fully unfold. To elucidate the propulsion mechanism of this cycle of motion, we conducted a threedimensional (3D) numerical study, which investigates the hydrodynamics of undergoing the prescribed kinematics. We found novel propulsion mechanisms, such as modulating the body deformation rate to dynamically increase the maximum net propulsion force, using asymmetric kinematics to generate torque and the appropriate rotation, and controlling the radius of the curled body to manipulate the moment of inertia. The figure eight gait is found to achieve propulsion at a wide range of Re but is most effective at intermediate Re. The results were further validated experimentally, via the development of a soft millimetersized robot that can reach comparable speeds using the figure eight gait.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>At the millimeter scale and in the intermediate Reynolds number (Re) regime, the midge and mosquito larvae can reach swimming speeds of more than one body length per cycle performing a “figure eight” gait, in which their elongated bodies periodically bend nearly into circles and then fully unfold. To elucidate the propulsion mechanism of this cycle of motion, we conducted a threedimensional (3D) numerical study, which investigates the hydrodynamics of undergoing the prescribed kinematics. We found novel propulsion mechanisms, such as modulating the body deformation rate to dynamically increase the maximum net propulsion force, using asymmetric kinematics to generate torque and the appropriate rotation, and controlling the radius of the curled body to manipulate the moment of inertia. The figure eight gait is found to achieve propulsion at a wide range of Re but is most effective at intermediate Re. The results were further validated experimentally, via the development of a soft millimetersized robot that can reach comparable speeds using the figure eight gait.
Swimming of the midge larva: Principles and tricks of locomotion at intermediate Reynolds number
10.1063/5.0137841
Physics of Fluids
20230314T10:16:39Z
© 2023 Author(s).

Vorticity dynamics of fully developed leadingedge vortices on revolving wings undergoing pitchup maneuvers
https://aip.scitation.org/doi/10.1063/5.0143056?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In nature, birds and insects usually execute pitchup maneuvers, which is either an active perching or a passive response against gusts. During such maneuvers, their wings flap with a concomitant noseup rotation around an axis, and thus, both vortex structures and aerodynamic forces of the wings are influenced. This research focuses on the impact of pitchup maneuvers on the evolution and underlying vorticity dynamics of a fully developed leadingedge vortex (LEV), which has received limited interest in previous research. Based on data obtained from numerical simulations, an analysis of vortex dynamics and vorticity transport is conducted at different pitch rates and pitch axis locations. Our findings show that an increase in pitch rate and a shift of pitch axis toward the trailing edge can both terminate the growth of LEV and then initiate its movement toward the trailing edge via strong downward convection. However, the contributions of spanwise convection and vortex stretching (or compression) are distinct in these two scenarios, leading to different lift generations. Other vortextiltingbased mechanisms, e.g., the planetary vorticity tilting and the dualstage radialtangential vortex tilting, are reduced during pitchup maneuvers. Moreover, a rapid pitchup around the leading edge is encouraged to maximize the lift during the maneuver, although this should be accompanied by constraints in flight height and kinetic energy when being applied to guide the perching of bioinspired flapping wing micro air vehicles.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In nature, birds and insects usually execute pitchup maneuvers, which is either an active perching or a passive response against gusts. During such maneuvers, their wings flap with a concomitant noseup rotation around an axis, and thus, both vortex structures and aerodynamic forces of the wings are influenced. This research focuses on the impact of pitchup maneuvers on the evolution and underlying vorticity dynamics of a fully developed leadingedge vortex (LEV), which has received limited interest in previous research. Based on data obtained from numerical simulations, an analysis of vortex dynamics and vorticity transport is conducted at different pitch rates and pitch axis locations. Our findings show that an increase in pitch rate and a shift of pitch axis toward the trailing edge can both terminate the growth of LEV and then initiate its movement toward the trailing edge via strong downward convection. However, the contributions of spanwise convection and vortex stretching (or compression) are distinct in these two scenarios, leading to different lift generations. Other vortextiltingbased mechanisms, e.g., the planetary vorticity tilting and the dualstage radialtangential vortex tilting, are reduced during pitchup maneuvers. Moreover, a rapid pitchup around the leading edge is encouraged to maximize the lift during the maneuver, although this should be accompanied by constraints in flight height and kinetic energy when being applied to guide the perching of bioinspired flapping wing micro air vehicles.
Vorticity dynamics of fully developed leadingedge vortices on revolving wings undergoing pitchup maneuvers
10.1063/5.0143056
Physics of Fluids
20230315T11:50:03Z
© 2023 Author(s).

Fluid–structure interaction and flow sensing of primary cilia in oscillating fluid flows
https://aip.scitation.org/doi/10.1063/5.0140701?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study systematically investigates the interaction between an oscillating flow and primary cilia using numerical simulations. The primary cilia are modeled as elastic filaments with rotatable basal ends to mimic real ciliary deflections. How some governing parameters [i.e., the peak Reynolds number (Repeak), Womersley number (Wo), cilium length (L*), and streamwise spacing interval (Ld*)] regulate fluid–cilia interaction is explored. Our results indicate that within a certain range, both the span of deflection (SD) and the maximal curvature increase with the increase in Repeak, L*, and Ld*, while they decrease as the Wo increases. Compared with other parameters, Ld* affects ciliary deflection less significantly and its impact becomes nearly negligible when the cilia are separated over twice their length. Three typical stretch states are captured. For primary cilia with a short or medium length, an increase in the SD is accompanied by a greater propagation distance of the location of the maximal tensile stress (LMTS). However, this is not the case for long cilia that protrude into 1/3 of the lumen, as the arising third stretch state may greatly suppress the LMTS propagation. Our study further confirms the role of primary cilia in decreasing the wall shear stress (WSS) and altering its oscillating feature. The WSS decrease is more significant for cilia undergoing a larger SD and/or when Ld* is reduced. For a constant Ld*, a larger SD corresponds to a more uneven oscillatory shear index distribution, and the affected (i.e., less oscillatory) region appears to greatly depend on Ld*.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study systematically investigates the interaction between an oscillating flow and primary cilia using numerical simulations. The primary cilia are modeled as elastic filaments with rotatable basal ends to mimic real ciliary deflections. How some governing parameters [i.e., the peak Reynolds number (Repeak), Womersley number (Wo), cilium length (L*), and streamwise spacing interval (Ld*)] regulate fluid–cilia interaction is explored. Our results indicate that within a certain range, both the span of deflection (SD) and the maximal curvature increase with the increase in Repeak, L*, and Ld*, while they decrease as the Wo increases. Compared with other parameters, Ld* affects ciliary deflection less significantly and its impact becomes nearly negligible when the cilia are separated over twice their length. Three typical stretch states are captured. For primary cilia with a short or medium length, an increase in the SD is accompanied by a greater propagation distance of the location of the maximal tensile stress (LMTS). However, this is not the case for long cilia that protrude into 1/3 of the lumen, as the arising third stretch state may greatly suppress the LMTS propagation. Our study further confirms the role of primary cilia in decreasing the wall shear stress (WSS) and altering its oscillating feature. The WSS decrease is more significant for cilia undergoing a larger SD and/or when Ld* is reduced. For a constant Ld*, a larger SD corresponds to a more uneven oscillatory shear index distribution, and the affected (i.e., less oscillatory) region appears to greatly depend on Ld*.
Fluid–structure interaction and flow sensing of primary cilia in oscillating fluid flows
10.1063/5.0140701
Physics of Fluids
20230316T02:04:46Z
© 2023 Author(s).
Jingyu Cui
Yuzhen Jin
Yang Liu
Bingmei M. Fu
Weiwei Yan

Effect of a bend on vortex formation and evolution in a threedimensional stenosed geometry during pulsatile flow
https://aip.scitation.org/doi/10.1063/5.0138825?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Stenosis at arterial bends alters hemodynamics and instigates abnormal disease progression. This configuration is addressed numerically by exploring pulsatile flow (Reynolds number Re = 300–1200; Womersley number Wo = 7.62–15.24) in arteries encountering bend angles of θ = 20°–60°. Individual influences of stenosis and bend on flow dynamics are investigated. Validations against particle image velocimetry experiments for Re = 800 and Wo = 7.62 are carried out in straight and 60° bend stenosed models. For Re = 300–800, the shear layer along the stenosis rolls up into a primary vortex, that is, constrained by the outer wall forming a secondary vortex. At Re = 1200, shear layers undergo instabilities along the poststenotic region and develop new vortices that promote disturbances and induce asymmetries over the crossplane flow structures. These features are not present in a straight stenosed tube, showing that the bend is responsible for flow distortion. During the pulsatile cycle, increasing bend angles intensify the size and strength of vortices, while these are suppressed at higher frequencies. A higher bend of 60° experiences large timeaveraged wall shear stress and oscillatory loads. In time, wall loading spatially circumscribes the poststenotic region followed by wall loading during cycle deceleration. These features are consistent with the skewing of a threedimensional ring structure formed in a stenosed tube that evolves into disintegrated structures in the poststenotic region. Overall, simulations reveal that strongly bent stenosed arteries experience aggravated oscillatory loading. In the biomedical context, such arterial geometries will require special attention.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Stenosis at arterial bends alters hemodynamics and instigates abnormal disease progression. This configuration is addressed numerically by exploring pulsatile flow (Reynolds number Re = 300–1200; Womersley number Wo = 7.62–15.24) in arteries encountering bend angles of θ = 20°–60°. Individual influences of stenosis and bend on flow dynamics are investigated. Validations against particle image velocimetry experiments for Re = 800 and Wo = 7.62 are carried out in straight and 60° bend stenosed models. For Re = 300–800, the shear layer along the stenosis rolls up into a primary vortex, that is, constrained by the outer wall forming a secondary vortex. At Re = 1200, shear layers undergo instabilities along the poststenotic region and develop new vortices that promote disturbances and induce asymmetries over the crossplane flow structures. These features are not present in a straight stenosed tube, showing that the bend is responsible for flow distortion. During the pulsatile cycle, increasing bend angles intensify the size and strength of vortices, while these are suppressed at higher frequencies. A higher bend of 60° experiences large timeaveraged wall shear stress and oscillatory loads. In time, wall loading spatially circumscribes the poststenotic region followed by wall loading during cycle deceleration. These features are consistent with the skewing of a threedimensional ring structure formed in a stenosed tube that evolves into disintegrated structures in the poststenotic region. Overall, simulations reveal that strongly bent stenosed arteries experience aggravated oscillatory loading. In the biomedical context, such arterial geometries will require special attention.
Effect of a bend on vortex formation and evolution in a threedimensional stenosed geometry during pulsatile flow
10.1063/5.0138825
Physics of Fluids
20230316T01:44:06Z
© 2023 Author(s).
Mohammad Owais
Abdullah Y. Usmani
K. Muralidhar

Constitutive modeling of human cornea through fractional calculus approach
https://aip.scitation.org/doi/10.1063/5.0138730?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this work, the fractional calculus approach is considered for modeling the viscoelastic behavior of human cornea. It is observed that the degree of both elasticity and viscosity is easy to describe in terms of the fractional order parameters in such an approach. Modeling of the human cornea when subjected to simple stress up to the level of 250 MPa by fractional order Maxwell model along with the Fractional Kelvin Voigt Viscoelastic Model is reported. For the Maxwell governing fractional equation, two fractional parameters α and β have been considered to model the stress–strain relationship of the human cornea. The analytical solution of the fractional equation has been obtained for different values of α and β using Laplace transform methods. The effect of the fractional parameter values on the stressdeformation nature has been studied. A comparison between experimental values and calculated values for different fractional order of the Maxwell model equation defines the parameters which depict the realtime stress–strain relationship of the human cornea. It has been observed that the fractional model converges to the classical Maxwell model as a special case for α = β = 1.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this work, the fractional calculus approach is considered for modeling the viscoelastic behavior of human cornea. It is observed that the degree of both elasticity and viscosity is easy to describe in terms of the fractional order parameters in such an approach. Modeling of the human cornea when subjected to simple stress up to the level of 250 MPa by fractional order Maxwell model along with the Fractional Kelvin Voigt Viscoelastic Model is reported. For the Maxwell governing fractional equation, two fractional parameters α and β have been considered to model the stress–strain relationship of the human cornea. The analytical solution of the fractional equation has been obtained for different values of α and β using Laplace transform methods. The effect of the fractional parameter values on the stressdeformation nature has been studied. A comparison between experimental values and calculated values for different fractional order of the Maxwell model equation defines the parameters which depict the realtime stress–strain relationship of the human cornea. It has been observed that the fractional model converges to the classical Maxwell model as a special case for α = β = 1.
Constitutive modeling of human cornea through fractional calculus approach
10.1063/5.0138730
Physics of Fluids
20230317T03:04:43Z
© 2023 Author(s).
Dibyendu Mandal
Himadri Chattopadhyay
Kumaresh Halder

Bioinspired in silico microswimmer: Run and tumble kinematics
https://aip.scitation.org/doi/10.1063/5.0142836?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We present an in silico microswimmer motivated by peritrichous bacteria, E. coli, which can run and tumble by spinning their flagellar motors counterclockwise (CCW) or clockwise (CW). Runs are the directed movement driven by a flagellar bundle, and tumbles are reorientations of cells caused by some motors' reversals from CCW to CW. In a viscous fluid without obstacles, our simulations reveal that material properties of the hook and the counterrotation of the cell body are important factors for efficient flagellar bundling and that longer hooks in mutant cell models create an instability and disrupt the bundling process, resulting in a limited range of movement. In the presence of a planar wall, we demonstrate that microswimmers can explore environment near surface by making various types of tumble events as they swim close to the surface. In particular, the variation of tumble duration can lead the microswimmer to run in a wide range of direction. However, we find that cells near surface stay close to the surface even after tumbles, which suggests that the tumble motion may not promote cells' escape from the confinement but promote biofilm formation.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We present an in silico microswimmer motivated by peritrichous bacteria, E. coli, which can run and tumble by spinning their flagellar motors counterclockwise (CCW) or clockwise (CW). Runs are the directed movement driven by a flagellar bundle, and tumbles are reorientations of cells caused by some motors' reversals from CCW to CW. In a viscous fluid without obstacles, our simulations reveal that material properties of the hook and the counterrotation of the cell body are important factors for efficient flagellar bundling and that longer hooks in mutant cell models create an instability and disrupt the bundling process, resulting in a limited range of movement. In the presence of a planar wall, we demonstrate that microswimmers can explore environment near surface by making various types of tumble events as they swim close to the surface. In particular, the variation of tumble duration can lead the microswimmer to run in a wide range of direction. However, we find that cells near surface stay close to the surface even after tumbles, which suggests that the tumble motion may not promote cells' escape from the confinement but promote biofilm formation.
Bioinspired in silico microswimmer: Run and tumble kinematics
10.1063/5.0142836
Physics of Fluids
20230317T03:04:41Z
© 2023 Author(s).

Shape design of an artificial pumplung using highresolution hemodynamic simulation with highperformance computing
https://aip.scitation.org/doi/10.1063/5.0140986?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Accurate and fast prediction of the hemodynamics of the artificial pumplung is critical in the design process. In this study, a comprehensive computational framework, including a sliding mesh method, a coupled free flow and porous media flow model, a hemolysis prediction method, a [math] shear stress transport turbulence model, and solution algorithms, is introduced to accurately predict the velocity field, pressure heads, and hemolysis. The framework is used to do the shape design of an artificial pumplung on a supercomputer. Highresolution hemodynamics simulation results are obtained and analyzed, and the parallel performance of the algorithm is studied. The numerical results indicate that the proposed framework is capable of accurately predicting the velocity field, pressure heads, and hemolysis, and the performance of the designed artificial pumplung meets the biocompatibility requirements. Additionally, the parallel performance results demonstrate the potential of the framework to efficiently perform the design of artificial pumplungs using a large number of processors.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Accurate and fast prediction of the hemodynamics of the artificial pumplung is critical in the design process. In this study, a comprehensive computational framework, including a sliding mesh method, a coupled free flow and porous media flow model, a hemolysis prediction method, a [math] shear stress transport turbulence model, and solution algorithms, is introduced to accurately predict the velocity field, pressure heads, and hemolysis. The framework is used to do the shape design of an artificial pumplung on a supercomputer. Highresolution hemodynamics simulation results are obtained and analyzed, and the parallel performance of the algorithm is studied. The numerical results indicate that the proposed framework is capable of accurately predicting the velocity field, pressure heads, and hemolysis, and the performance of the designed artificial pumplung meets the biocompatibility requirements. Additionally, the parallel performance results demonstrate the potential of the framework to efficiently perform the design of artificial pumplungs using a large number of processors.
Shape design of an artificial pumplung using highresolution hemodynamic simulation with highperformance computing
10.1063/5.0140986
Physics of Fluids
20230317T02:47:24Z
© 2023 Author(s).

Numerical study of the formation and stability of a pair of particles of different sizes in inertial microfluidics
https://aip.scitation.org/doi/10.1063/5.0138640?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The formation of pairs and trains of particles in inertial microfluidics is an important consideration for device design and applications, such as particle focusing and separation. We study the formation and stability of linear and staggered pairs of nearly rigid spherical particles of different sizes in a pressuredriven flow through a straight duct with a rectangular cross section under mild inertia. An inhouse latticeBoltzmannimmersedboundaryfiniteelement code is used for threedimensional simulations. We find that the stability and properties of pairs of heterogeneous particles strongly depend on particle sizes and their size ratio, while the formation of the pairs is also determined by the initial lateral position and the axial order of the particles. Our findings imply that perturbations of particle trajectories caused by other particles, as they are expected to happen even in dilute suspensions, can be important for the formation of stable pairs in inertial microfluidics.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The formation of pairs and trains of particles in inertial microfluidics is an important consideration for device design and applications, such as particle focusing and separation. We study the formation and stability of linear and staggered pairs of nearly rigid spherical particles of different sizes in a pressuredriven flow through a straight duct with a rectangular cross section under mild inertia. An inhouse latticeBoltzmannimmersedboundaryfiniteelement code is used for threedimensional simulations. We find that the stability and properties of pairs of heterogeneous particles strongly depend on particle sizes and their size ratio, while the formation of the pairs is also determined by the initial lateral position and the axial order of the particles. Our findings imply that perturbations of particle trajectories caused by other particles, as they are expected to happen even in dilute suspensions, can be important for the formation of stable pairs in inertial microfluidics.
Numerical study of the formation and stability of a pair of particles of different sizes in inertial microfluidics
10.1063/5.0138640
Physics of Fluids
20230301T02:18:57Z
© 2023 Author(s).
Krishnaveni Thota
Benjamin Owen
Timm Krüger

The rheological performance of shearthickening fluids based on carbon fiber and silica nanocomposite
https://aip.scitation.org/doi/10.1063/5.0138294?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Current available shearthickening fluid (STFs) may suffer from issues such as unsatisfactory energy dissipation performance and unstable dynamic stability for practical engineering applications. This paper investigates the innovated compounded STFs which are fabricated by mixing needlelike carbon fiber powder (CFP) and silicon dioxide (SiO2) into polyethylene glycol (PEG) under proper synthesis conditions. The microstructure and rheological properties of the compounded STFs, namely, CFPSiO2/PEG, are investigated. The interaction between CFP and SiO2 and the shearinduced microstructure are analyzed using scanning electron microscopy. Steadystate rheological tests reveal that compounded STFs with different mass ratios exhibit significant rheological behavior and shearthickening effects. The peak viscosity is demonstrated to be increased from 51.59 (monodispersed STFs) to 574.74 Pa s (compounded STFs), and the critical shear rate decreased from 79.42 to 10.00 s−1 when the mass fraction of CFP is set at 0.2%. The peak viscosity of the compounded STFs is shown to be increased by 313.96% when the plate spacing is increased from 0.25 to 1.00 mm. The dynamic rheological analysis shows that the compounded STFs exhibit excellent energy dissipation capacity at different stages. More importantly, the modulus instability and shearthinning problems of monodispersed STFs could be significantly improved. According to the results, the key performance index of the CFP/SiO2PEG compounded STFs is demonstrated to be improved by ten times or even higher. This work presents a novel type of STFs with high energy dissipation capacity and high dynamic stability for the application of shearthickening fluids composite in engineering practice.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Current available shearthickening fluid (STFs) may suffer from issues such as unsatisfactory energy dissipation performance and unstable dynamic stability for practical engineering applications. This paper investigates the innovated compounded STFs which are fabricated by mixing needlelike carbon fiber powder (CFP) and silicon dioxide (SiO2) into polyethylene glycol (PEG) under proper synthesis conditions. The microstructure and rheological properties of the compounded STFs, namely, CFPSiO2/PEG, are investigated. The interaction between CFP and SiO2 and the shearinduced microstructure are analyzed using scanning electron microscopy. Steadystate rheological tests reveal that compounded STFs with different mass ratios exhibit significant rheological behavior and shearthickening effects. The peak viscosity is demonstrated to be increased from 51.59 (monodispersed STFs) to 574.74 Pa s (compounded STFs), and the critical shear rate decreased from 79.42 to 10.00 s−1 when the mass fraction of CFP is set at 0.2%. The peak viscosity of the compounded STFs is shown to be increased by 313.96% when the plate spacing is increased from 0.25 to 1.00 mm. The dynamic rheological analysis shows that the compounded STFs exhibit excellent energy dissipation capacity at different stages. More importantly, the modulus instability and shearthinning problems of monodispersed STFs could be significantly improved. According to the results, the key performance index of the CFP/SiO2PEG compounded STFs is demonstrated to be improved by ten times or even higher. This work presents a novel type of STFs with high energy dissipation capacity and high dynamic stability for the application of shearthickening fluids composite in engineering practice.
The rheological performance of shearthickening fluids based on carbon fiber and silica nanocomposite
10.1063/5.0138294
Physics of Fluids
20230301T03:50:27Z
© 2023 Author(s).

Reinforcement learning of a multilink swimmer at low Reynolds numbers
https://aip.scitation.org/doi/10.1063/5.0140662?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The use of machine learning techniques in the development of microscopic swimmers has drawn considerable attention in recent years. In particular, reinforcement learning has been shown useful in enabling swimmers to learn effective propulsion strategies through its interactions with the surroundings. In this work, we apply a reinforcement learning approach to identify swimming gaits of a multilink model swimmer. The swimmer consists of multiple rigid links connected serially with hinges, which can rotate freely to change the relative angles between neighboring links. Purcell [“Life at low Reynolds number,” Am. J. Phys. 45, 3 (1977)] demonstrated how the particular case of a threelink swimmer (now known as Purcell's swimmer) can perform a prescribed sequence of hinge rotation to generate selfpropulsion in the absence of inertia. Here, without relying on any prior knowledge of lowReynoldsnumber locomotion, we first demonstrate the use of reinforcement learning in identifying the classical swimming gaits of Purcell's swimmer for case of three links. We next examine the new swimming gaits acquired by the learning process as the number of links increases. We also consider the scenarios when only a single hinge is allowed to rotate at a time and when simultaneous rotation of multiple hinges is allowed. We contrast the difference in the locomotory gaits learned by the swimmers in these scenarios and discuss their propulsion performance. Taken together, our results demonstrate how a simple reinforcement learning technique can be applied to identify both classical and new swimming gaits at low Reynolds numbers.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The use of machine learning techniques in the development of microscopic swimmers has drawn considerable attention in recent years. In particular, reinforcement learning has been shown useful in enabling swimmers to learn effective propulsion strategies through its interactions with the surroundings. In this work, we apply a reinforcement learning approach to identify swimming gaits of a multilink model swimmer. The swimmer consists of multiple rigid links connected serially with hinges, which can rotate freely to change the relative angles between neighboring links. Purcell [“Life at low Reynolds number,” Am. J. Phys. 45, 3 (1977)] demonstrated how the particular case of a threelink swimmer (now known as Purcell's swimmer) can perform a prescribed sequence of hinge rotation to generate selfpropulsion in the absence of inertia. Here, without relying on any prior knowledge of lowReynoldsnumber locomotion, we first demonstrate the use of reinforcement learning in identifying the classical swimming gaits of Purcell's swimmer for case of three links. We next examine the new swimming gaits acquired by the learning process as the number of links increases. We also consider the scenarios when only a single hinge is allowed to rotate at a time and when simultaneous rotation of multiple hinges is allowed. We contrast the difference in the locomotory gaits learned by the swimmers in these scenarios and discuss their propulsion performance. Taken together, our results demonstrate how a simple reinforcement learning technique can be applied to identify both classical and new swimming gaits at low Reynolds numbers.
Reinforcement learning of a multilink swimmer at low Reynolds numbers
10.1063/5.0140662
Physics of Fluids
20230301T02:12:05Z
© 2023 Author(s).
Ke Qin
Zonghao Zou
Lailai Zhu
On Shun Pak

Negative thermophoresis of nanoparticles in liquids
https://aip.scitation.org/doi/10.1063/5.0133385?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The thermophoresis of suspended particles in a fluid is usually from high to low temperature. In the present paper, the negative thermophoresis (from low to high temperature) of nanoparticles in liquids is investigated by molecular dynamics simulations. It is found that the solid–liquid intermolecular coupling strength has a significant effect on the direction and magnitude of the thermophoretic force. Positive thermophoresis can be observed for strong couplings, while negative thermophoresis emerges for weak couplings. The negative thermophoresis is induced by the density gradient which pushes the particle from high to low density. Based on the analysis of the potential mean force of the solid–liquid interfacial layer, it is revealed that the switch between positive and negative thermophoresis is associated with the sign change of the averaged potential mean force for the interfacial layer. Therefore, the sign of the averaged potential mean force can be used as a criterion to predict the occurrence of negative thermophoresis. The results of this work provide insights for the microscopic manipulation of nanoparticles.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The thermophoresis of suspended particles in a fluid is usually from high to low temperature. In the present paper, the negative thermophoresis (from low to high temperature) of nanoparticles in liquids is investigated by molecular dynamics simulations. It is found that the solid–liquid intermolecular coupling strength has a significant effect on the direction and magnitude of the thermophoretic force. Positive thermophoresis can be observed for strong couplings, while negative thermophoresis emerges for weak couplings. The negative thermophoresis is induced by the density gradient which pushes the particle from high to low density. Based on the analysis of the potential mean force of the solid–liquid interfacial layer, it is revealed that the switch between positive and negative thermophoresis is associated with the sign change of the averaged potential mean force for the interfacial layer. Therefore, the sign of the averaged potential mean force can be used as a criterion to predict the occurrence of negative thermophoresis. The results of this work provide insights for the microscopic manipulation of nanoparticles.
Negative thermophoresis of nanoparticles in liquids
10.1063/5.0133385
Physics of Fluids
20230302T01:42:41Z
© 2023 Author(s).

The periodic secondary flow of OldroydB fluids driven by direct electric field in a rectangular curved channel
https://aip.scitation.org/doi/10.1063/5.0138394?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The electroosmotic flow of OldroydB fluids in a 90° curved tube with a rectangular section under a direct electric field is numerically studied. By introducing elastic forces into the force balance of viscous, electric, and centrifugal forces, another secondary flow pattern is found in addition to the stable state for Newtonian fluids, i.e., the periodic oscillation state. In this oscillating state, the position of the maximum velocity periodically moves from the center to the position near the wall. Meanwhile, a symmetric vortex can be periodically observed in the streamline figures. The secondary flow oscillates when the Deborah number De or the dimensionless wall potential [math] is sufficiently large, and the oscillating frequency increases with a larger Deborah number De or a larger dimensionless wall potential [math]. A phase diagram of the secondary flow as it depends on the Deborah number De and the dimensionless wall potential [math] is presented. There is a critical Deborah number [math] for a given wall potential [math], and the secondary flow become periodically oscillating at [math]. The critical Deborah number [math] decreases as the value of the dimensionless wall potential [math] increases. Moreover, the critical Deborah number should be larger than 0.2 even though the wall potential [math] further increases, i.e., [math]. At [math], the elastic forces are small, and the secondary flow is stable rather than oscillating similar to the phenomena of Newtonian fluids.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The electroosmotic flow of OldroydB fluids in a 90° curved tube with a rectangular section under a direct electric field is numerically studied. By introducing elastic forces into the force balance of viscous, electric, and centrifugal forces, another secondary flow pattern is found in addition to the stable state for Newtonian fluids, i.e., the periodic oscillation state. In this oscillating state, the position of the maximum velocity periodically moves from the center to the position near the wall. Meanwhile, a symmetric vortex can be periodically observed in the streamline figures. The secondary flow oscillates when the Deborah number De or the dimensionless wall potential [math] is sufficiently large, and the oscillating frequency increases with a larger Deborah number De or a larger dimensionless wall potential [math]. A phase diagram of the secondary flow as it depends on the Deborah number De and the dimensionless wall potential [math] is presented. There is a critical Deborah number [math] for a given wall potential [math], and the secondary flow become periodically oscillating at [math]. The critical Deborah number [math] decreases as the value of the dimensionless wall potential [math] increases. Moreover, the critical Deborah number should be larger than 0.2 even though the wall potential [math] further increases, i.e., [math]. At [math], the elastic forces are small, and the secondary flow is stable rather than oscillating similar to the phenomena of Newtonian fluids.
The periodic secondary flow of OldroydB fluids driven by direct electric field in a rectangular curved channel
10.1063/5.0138394
Physics of Fluids
20230302T01:30:02Z
© 2023 Author(s).

Diffusiophoresis of hydrophobic spherical particles in a solution of general electrolyte
https://aip.scitation.org/doi/10.1063/5.0141490?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The present article deals with the diffusiophoresis of hydrophobic rigid colloids bearing arbitrary ζpotential. We derived the generic expression for the diffusiophoretic velocity of such a colloid exposed in an externally applied concentration gradient of the general electrolyte solution. The derived expression takes into account the relaxation effect and is applicable for all values of surface ζpotential and hydrodynamic slip length at large κa ([math]), where [math] is the thickness of the electric double layer and a is the particle radius. We further derived several closedform expressions for particle velocity derived under various electrostatic and hydrodynamic conditions when the particle is exposed in an applied concentration gradient of binary symmetric (e.g., [math]), asymmetric (1:2, 2:1, 3:1, 1:3), and a mixed electrolyte (mixture of 1:1 and 2:1 electrolytes). The results for diffusiophoretic velocity are further illustrated graphically to indicate the mutual interaction of chemiphoresis, induced electrophoresis due to unequal mobilities of cations and anions of the electrolyte, and the mechanism by which the sufficiently charged particle migrates opposite to the direction of the applied concentration gradient. The impact of hydrophobicity is further discussed.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The present article deals with the diffusiophoresis of hydrophobic rigid colloids bearing arbitrary ζpotential. We derived the generic expression for the diffusiophoretic velocity of such a colloid exposed in an externally applied concentration gradient of the general electrolyte solution. The derived expression takes into account the relaxation effect and is applicable for all values of surface ζpotential and hydrodynamic slip length at large κa ([math]), where [math] is the thickness of the electric double layer and a is the particle radius. We further derived several closedform expressions for particle velocity derived under various electrostatic and hydrodynamic conditions when the particle is exposed in an applied concentration gradient of binary symmetric (e.g., [math]), asymmetric (1:2, 2:1, 3:1, 1:3), and a mixed electrolyte (mixture of 1:1 and 2:1 electrolytes). The results for diffusiophoretic velocity are further illustrated graphically to indicate the mutual interaction of chemiphoresis, induced electrophoresis due to unequal mobilities of cations and anions of the electrolyte, and the mechanism by which the sufficiently charged particle migrates opposite to the direction of the applied concentration gradient. The impact of hydrophobicity is further discussed.
Diffusiophoresis of hydrophobic spherical particles in a solution of general electrolyte
10.1063/5.0141490
Physics of Fluids
20230306T10:56:03Z
© 2023 Author(s).
Susmita Samanta
Paramita Mahapatra
H. Ohshima
Partha P. Gopmandal

Dropletinduced optical effects in an optomicrofluidic crossconfiguration system
https://aip.scitation.org/doi/10.1063/5.0138475?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A comprehensive description of all the optical phenomena occurring when light interacts with a moving dispersed phase in a constrained environment such as a real microfluidic channel is needed to perform a quantitative analysis as well as predictive one. This requires identifying fingerprints in the detected optical signal that are doubtlessly correlated with the shape and content type of the dispersed phase from those connected to uncertainties of the optical detection systems and/or instabilities in the microfluidics apparatus leading to dispersed phase size distribution. This article aims to model all the dropletinduced optical effects in an optomicrofluidic crossconfiguration system and quantify how diffraction, transmission, absorbance, and reflection contribute to the overall response in the detected intensity after lightmatter interaction. The model has been tested in the case of water droplets dispersed in hexadecane continuous phase as generated in an optomicrofluidic platform where optical waveguides are fully integrated with the microfluidic channels, so that light illuminates the flowing droplets from the channel wall and collected on the opposite side. A critical discussion of the impact of geometry and constrains is proposed as well as the impact of each contribute in terms of fingerprints in the detected signal. The good agreement obtained demonstrates the potentialities of both the derived model and the crossconfiguration, getting information on droplet characteristics from the intensity arising from its light interaction.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A comprehensive description of all the optical phenomena occurring when light interacts with a moving dispersed phase in a constrained environment such as a real microfluidic channel is needed to perform a quantitative analysis as well as predictive one. This requires identifying fingerprints in the detected optical signal that are doubtlessly correlated with the shape and content type of the dispersed phase from those connected to uncertainties of the optical detection systems and/or instabilities in the microfluidics apparatus leading to dispersed phase size distribution. This article aims to model all the dropletinduced optical effects in an optomicrofluidic crossconfiguration system and quantify how diffraction, transmission, absorbance, and reflection contribute to the overall response in the detected intensity after lightmatter interaction. The model has been tested in the case of water droplets dispersed in hexadecane continuous phase as generated in an optomicrofluidic platform where optical waveguides are fully integrated with the microfluidic channels, so that light illuminates the flowing droplets from the channel wall and collected on the opposite side. A critical discussion of the impact of geometry and constrains is proposed as well as the impact of each contribute in terms of fingerprints in the detected signal. The good agreement obtained demonstrates the potentialities of both the derived model and the crossconfiguration, getting information on droplet characteristics from the intensity arising from its light interaction.
Dropletinduced optical effects in an optomicrofluidic crossconfiguration system
10.1063/5.0138475
Physics of Fluids
20230310T12:19:47Z
© 2023 Author(s).
Leonardo Zanini
Cinzia Sada

Diffusion effects on mixed convective peristaltic flow of a biviscous Bingham nanofluid through a porous medium with convective boundary conditions
https://aip.scitation.org/doi/10.1063/5.0142003?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The study of heat transfer and peristaltic pumping of magnetohydrodynamic biofluids has many physiological applications, such as heart–lung machines during surgeries, dialysis, vitamin injections, and cancer treatment. Also, it has many industrial applications, such as pharmaceutical fluid production, filtration, and dispensing cosmetic/glue emulsions with no contamination. Furthermore, the biviscous Bingham nanofluid model is the best for several bio/industrial fluids. Therefore, the impact of Hall current, thermal radiation, and crossdiffusion on the mixed convection peristaltic pumping of a biviscous Bingham nanofluid in a porous medium is considered. Also, we focus on the flexibility of the walls along with the convective boundary conditions. We adopted the lubrication strategy to reduce the system’s complexity. The system of nondimensional partial differential equations along with the pertinent boundary conditions is solved by using a regular perturbation method (RPM) for several sets of values of the dimensionless parameters. The expressions for the temperature, concentration, velocity, and heat transfer coefficient are obtained analytically. The impact of the relevant parameters on the velocity, temperature, coefficient of heat transfer, concentration, skin friction coefficient, Nusselt number, and trapping is discussed in depth with the help of graphical illustrations and tables. The results indicate that the velocity distribution is reduced with growing Darcy parameter and concentration Grashof number. Intensifying the magnetic parameter results in shrinking the trapped bolus. Decay in the heat transfer coefficient is observed for rising values of the radiation parameter. The current findings are compared with the existing studies in the literature and are found to agree very well for special cases. Moreover, the closed form solution (RPM) is compared with the numerical solution (BVC5C, MATLAB) for validation.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The study of heat transfer and peristaltic pumping of magnetohydrodynamic biofluids has many physiological applications, such as heart–lung machines during surgeries, dialysis, vitamin injections, and cancer treatment. Also, it has many industrial applications, such as pharmaceutical fluid production, filtration, and dispensing cosmetic/glue emulsions with no contamination. Furthermore, the biviscous Bingham nanofluid model is the best for several bio/industrial fluids. Therefore, the impact of Hall current, thermal radiation, and crossdiffusion on the mixed convection peristaltic pumping of a biviscous Bingham nanofluid in a porous medium is considered. Also, we focus on the flexibility of the walls along with the convective boundary conditions. We adopted the lubrication strategy to reduce the system’s complexity. The system of nondimensional partial differential equations along with the pertinent boundary conditions is solved by using a regular perturbation method (RPM) for several sets of values of the dimensionless parameters. The expressions for the temperature, concentration, velocity, and heat transfer coefficient are obtained analytically. The impact of the relevant parameters on the velocity, temperature, coefficient of heat transfer, concentration, skin friction coefficient, Nusselt number, and trapping is discussed in depth with the help of graphical illustrations and tables. The results indicate that the velocity distribution is reduced with growing Darcy parameter and concentration Grashof number. Intensifying the magnetic parameter results in shrinking the trapped bolus. Decay in the heat transfer coefficient is observed for rising values of the radiation parameter. The current findings are compared with the existing studies in the literature and are found to agree very well for special cases. Moreover, the closed form solution (RPM) is compared with the numerical solution (BVC5C, MATLAB) for validation.
Diffusion effects on mixed convective peristaltic flow of a biviscous Bingham nanofluid through a porous medium with convective boundary conditions
10.1063/5.0142003
Physics of Fluids
20230310T12:20:00Z
© 2023 Author(s).
M. Ajithkumar
P. Lakshminarayana
K. Vajravelu

Flowinduced transition of compound droplet to composite microfiber in a channel with sudden contraction
https://aip.scitation.org/doi/10.1063/5.0137904?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The deformation behavior and hydrodynamic stability of a threedimensional Newtonian singlecore compound droplet during flow in a channel with sudden contraction were studied by numerical modeling. This research was motivated by the quest for conditions of the steady transition of a compound droplet into a composite microfiber, whose core is stretched as much as the shell. With this aim, the dynamics and morphology evolution of the compound droplet were analyzed in detail as functions of capillary number, coretoshell relative viscosities, interfacial tensions, and the relative initial core radius. It was found that the effective elongation of the core occurs either with a significant increase in the shell viscosity relative to the ambient fluid or with a decrease in the core viscosity with respect to the shell. In this case, as the composite droplet advances into the narrowing zone of the canal, it continues to stretch, becoming a bulletshaped composite microfiber. A new mechanism of disintegration of the compound droplet was revealed, which is caused by the core destabilizing effect and manifests itself either with an increase in the relative core/shell interfacial tension or the relative core viscosity.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The deformation behavior and hydrodynamic stability of a threedimensional Newtonian singlecore compound droplet during flow in a channel with sudden contraction were studied by numerical modeling. This research was motivated by the quest for conditions of the steady transition of a compound droplet into a composite microfiber, whose core is stretched as much as the shell. With this aim, the dynamics and morphology evolution of the compound droplet were analyzed in detail as functions of capillary number, coretoshell relative viscosities, interfacial tensions, and the relative initial core radius. It was found that the effective elongation of the core occurs either with a significant increase in the shell viscosity relative to the ambient fluid or with a decrease in the core viscosity with respect to the shell. In this case, as the composite droplet advances into the narrowing zone of the canal, it continues to stretch, becoming a bulletshaped composite microfiber. A new mechanism of disintegration of the compound droplet was revealed, which is caused by the core destabilizing effect and manifests itself either with an increase in the relative core/shell interfacial tension or the relative core viscosity.
Flowinduced transition of compound droplet to composite microfiber in a channel with sudden contraction
10.1063/5.0137904
Physics of Fluids
20230314T11:25:50Z
© 2023 Author(s).
S. A. Vagner
S. A. Patlazhan

Slip and jump coefficients for general gas–surface interactions according to the moment method
https://aip.scitation.org/doi/10.1063/5.0142861?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We develop a moment method based on the Hermite series of the arbitrary order to calculate viscousslip, thermalslip, and temperaturejump coefficients for general gassurface scattering kernels. Under some usual assumptions of scattering kernels, the solvability is obtained by showing the positive definiteness of the symmetric coefficient matrix in the boundary conditions. For gas flows with the Cercignani–Lampis gas–surface interaction and inversepowerlaw intermolecular potentials, the model can capture the slip and jump coefficients accurately with elegant analytic expressions. On the one hand, the proposed method can apply to the cases of arbitrary order moments with increasing accuracy. On the other hand, the explicit formulas for loworder situations are simpler and more accurate than some existing results in references. Therefore, one may apply these formulas in slip and jump conditions to improve the accuracy of macroscopic fluid dynamic models for gas flows.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We develop a moment method based on the Hermite series of the arbitrary order to calculate viscousslip, thermalslip, and temperaturejump coefficients for general gassurface scattering kernels. Under some usual assumptions of scattering kernels, the solvability is obtained by showing the positive definiteness of the symmetric coefficient matrix in the boundary conditions. For gas flows with the Cercignani–Lampis gas–surface interaction and inversepowerlaw intermolecular potentials, the model can capture the slip and jump coefficients accurately with elegant analytic expressions. On the one hand, the proposed method can apply to the cases of arbitrary order moments with increasing accuracy. On the other hand, the explicit formulas for loworder situations are simpler and more accurate than some existing results in references. Therefore, one may apply these formulas in slip and jump conditions to improve the accuracy of macroscopic fluid dynamic models for gas flows.
Slip and jump coefficients for general gas–surface interactions according to the moment method
10.1063/5.0142861
Physics of Fluids
20230315T11:50:06Z
© 2023 Author(s).

Interaction between the oil droplet in water and wetted wall: Force model and motion law
https://aip.scitation.org/doi/10.1063/5.0141934?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>To investigate the force model and motion law of oil droplets in water near the wetted wall, oil droplets with R1 = 0.29–0.62 mm and oil films with R2 = 1–6 mm are solved numerically. In addition to buoyancy, flow resistance, and added mass force, the filminduced force triggered by the wetted wall constraint is also introduced into the force model. The drainage process is described using the Stokes–Reynolds equation, and the Young–Laplace equation is used to calculate the pressure within the water film. The results show that the force model can be coupled with the Stokes–Reynolds–Young–Laplace model equation to better describe the drainage dynamics near the wetted wall. The pressure distribution law is closely related to the shape of the water film, especially when the oil–water interface is in the shape of a dimple, which can lead to the formation of negative pressure zones within the water film. The maximum pressure first grows in an exponential, then logarithmic pattern and eventually approaches the equivalent Laplace pressure. Around the critical size, the direction of the filminduced force changes and the form of action switches between driving and drag forces. The filminduced force's dominant effect is strongest when the curvature radius of the oil film is comparable to the droplet size.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>To investigate the force model and motion law of oil droplets in water near the wetted wall, oil droplets with R1 = 0.29–0.62 mm and oil films with R2 = 1–6 mm are solved numerically. In addition to buoyancy, flow resistance, and added mass force, the filminduced force triggered by the wetted wall constraint is also introduced into the force model. The drainage process is described using the Stokes–Reynolds equation, and the Young–Laplace equation is used to calculate the pressure within the water film. The results show that the force model can be coupled with the Stokes–Reynolds–Young–Laplace model equation to better describe the drainage dynamics near the wetted wall. The pressure distribution law is closely related to the shape of the water film, especially when the oil–water interface is in the shape of a dimple, which can lead to the formation of negative pressure zones within the water film. The maximum pressure first grows in an exponential, then logarithmic pattern and eventually approaches the equivalent Laplace pressure. Around the critical size, the direction of the filminduced force changes and the form of action switches between driving and drag forces. The filminduced force's dominant effect is strongest when the curvature radius of the oil film is comparable to the droplet size.
Interaction between the oil droplet in water and wetted wall: Force model and motion law
10.1063/5.0141934
Physics of Fluids
20230316T02:04:51Z
© 2023 Author(s).
Feng Rong
Limin He
Yuling Lǚ
Xiaolei Lu

Retraction and bouncing dynamics of nanodroplets upon impact on superhydrophobic surfaces
https://aip.scitation.org/doi/10.1063/5.0140920?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This work investigates the retraction and bouncing dynamics of an impacting lowviscosity nanodroplet on superhydrophobic surfaces via molecular dynamics simulations, aiming to reveal the scaling laws of retraction and bouncing velocities and to establish the relationship between them. The retraction velocity, Vre, is found to scale as Vre ∼ Dmax/τc,n, where Dmax is the maximum spreading diameter, τc,n = (D0/V0)We1/2Oh1/3 is the inertialcapillaryviscous time, and We and Oh are the Weber number and Ohnesorge number, respectively. The bouncing stems from the collision of the retracting rim at the center of the nanodroplet, leading to the bouncing velocity scaling as the retraction velocity. Combining the relationship of Vre ∼ Dmax/τc,n with the scaling law of Dmax ∼ We1/2Oh1/3D0 yields both the retraction and bouncing velocities scaling as the impact velocity, indicating that both the retraction and bouncing velocities of lowviscosity nanodroplets on a superhydrophobic surface depend merely on the impact velocity. An energy analysis shows that the proportion of the surface energy at the maximum spreading state (Es,max) to the initial kinetic energy (Ek,ini) follows Es,max/Ek,ini ∼ Oh2/3, whereas the proportion of the bouncing kinetic energy (Ek,b) to the surface energy at the maximum spreading state follows Ek,b/Es,max ∼ Oh−2/3, leading to constant Ek,b/Ek,ini and also constant restitution coefficient for lowviscosity nanodroplets impacting superhydrophobic surfaces.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This work investigates the retraction and bouncing dynamics of an impacting lowviscosity nanodroplet on superhydrophobic surfaces via molecular dynamics simulations, aiming to reveal the scaling laws of retraction and bouncing velocities and to establish the relationship between them. The retraction velocity, Vre, is found to scale as Vre ∼ Dmax/τc,n, where Dmax is the maximum spreading diameter, τc,n = (D0/V0)We1/2Oh1/3 is the inertialcapillaryviscous time, and We and Oh are the Weber number and Ohnesorge number, respectively. The bouncing stems from the collision of the retracting rim at the center of the nanodroplet, leading to the bouncing velocity scaling as the retraction velocity. Combining the relationship of Vre ∼ Dmax/τc,n with the scaling law of Dmax ∼ We1/2Oh1/3D0 yields both the retraction and bouncing velocities scaling as the impact velocity, indicating that both the retraction and bouncing velocities of lowviscosity nanodroplets on a superhydrophobic surface depend merely on the impact velocity. An energy analysis shows that the proportion of the surface energy at the maximum spreading state (Es,max) to the initial kinetic energy (Ek,ini) follows Es,max/Ek,ini ∼ Oh2/3, whereas the proportion of the bouncing kinetic energy (Ek,b) to the surface energy at the maximum spreading state follows Ek,b/Es,max ∼ Oh−2/3, leading to constant Ek,b/Ek,ini and also constant restitution coefficient for lowviscosity nanodroplets impacting superhydrophobic surfaces.
Retraction and bouncing dynamics of nanodroplets upon impact on superhydrophobic surfaces
10.1063/5.0140920
Physics of Fluids
20230316T02:59:16Z
© 2023 Author(s).

Electroosmotic mixing of nonNewtonian fluid in an optimized geometry connected with a modulated microchamber
https://aip.scitation.org/doi/10.1063/5.0144762?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The main objective of this work is to enhance the micromixing of different species transported through the electrokinetic mechanism applicable in labonachip devices used in BioMEMS. In this process, it is essential to predict the efficiency and precision of the micromixture for the quick and correct mixing. In this paper, a numerical study is conducted to investigate the mixing quantification of the electroosmotic micromixer with a nozzle–diffuser shaped channel connected to reservoirs located at both ends of the channel with a microchamber located in the middle of the channel modulated with an inner rectangular obstacle. Since enhancing mixing quality is the paramount factor, this study examines how the design of the mixing chamber (circular and triangular), the size of the inner obstacle, the conical angle of the nozzle–diffuser channel, and the electric double layer height influence the flow inside the electroosmotic micromixer. Numerical simulations have been performed by using the Poisson–Nernst–Planck based Cauchy momentum equations for a nonNewtonian powerlaw fluid. This study focuses on both the mixing enhancement and the performance evaluation factor by lowering the pressure drop with variation of geometric modulation. The reservoir end wall effects are considered for the flow rate and mixing of the powerlaw fluids with variation of different flow parameters. After obtaining the optimal values of the effective parameters used in the micromixers for the experiments, regardless of the geometry of the obstacles, the present model is formulated and validated, and the results are presented. According to the findings, it is observed that the height and width of the inner obstacle, Debye–Hückel parameter, and the slope of the channel have a significant role in the overall mixing quality. The mixing efficiency is improved up to 90% for Newtonian fluid and 96% for shear thickening fluid by using obstacle fitted in the microchamber of the system. In addition, the results demonstrate that shear thickening fluids have better mixing performance than shear thinning fluids, which can be helpful in the fabrication of advanced micromixers.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The main objective of this work is to enhance the micromixing of different species transported through the electrokinetic mechanism applicable in labonachip devices used in BioMEMS. In this process, it is essential to predict the efficiency and precision of the micromixture for the quick and correct mixing. In this paper, a numerical study is conducted to investigate the mixing quantification of the electroosmotic micromixer with a nozzle–diffuser shaped channel connected to reservoirs located at both ends of the channel with a microchamber located in the middle of the channel modulated with an inner rectangular obstacle. Since enhancing mixing quality is the paramount factor, this study examines how the design of the mixing chamber (circular and triangular), the size of the inner obstacle, the conical angle of the nozzle–diffuser channel, and the electric double layer height influence the flow inside the electroosmotic micromixer. Numerical simulations have been performed by using the Poisson–Nernst–Planck based Cauchy momentum equations for a nonNewtonian powerlaw fluid. This study focuses on both the mixing enhancement and the performance evaluation factor by lowering the pressure drop with variation of geometric modulation. The reservoir end wall effects are considered for the flow rate and mixing of the powerlaw fluids with variation of different flow parameters. After obtaining the optimal values of the effective parameters used in the micromixers for the experiments, regardless of the geometry of the obstacles, the present model is formulated and validated, and the results are presented. According to the findings, it is observed that the height and width of the inner obstacle, Debye–Hückel parameter, and the slope of the channel have a significant role in the overall mixing quality. The mixing efficiency is improved up to 90% for Newtonian fluid and 96% for shear thickening fluid by using obstacle fitted in the microchamber of the system. In addition, the results demonstrate that shear thickening fluids have better mixing performance than shear thinning fluids, which can be helpful in the fabrication of advanced micromixers.
Electroosmotic mixing of nonNewtonian fluid in an optimized geometry connected with a modulated microchamber
10.1063/5.0144762
Physics of Fluids
20230317T02:47:22Z
© 2023 Author(s).
M. Majhi
A. K. Nayak
B. Weigand

Finite droplets vs long droplets: Discrepancy in release conditions in a microscopic constricted channel
https://aip.scitation.org/doi/10.1063/5.0139025?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Conditions of release of trapped droplets in constricted channels are of great significance in various domains, including microfluidic development and enhanced oil recovery. In our previous studies, a detailed and quantitative analysis of the threshold pressure needed to release a droplet from a constricted channel has been performed. However, droplets may exist in real applications as long droplets, which may exhibit different behavior than finite droplets. Therefore, in this study, direct numerical simulations, combining the fluid flow equations and the phasefield method, have been conducted on threedimensional constrained channels to investigate discrepancies in release conditions of finite droplets and long droplets. The results have shown that for a finite droplet, the maximum pressure increases with the increase in the contact angle, whereas for a long droplet, the maximum pressure is almost the same both in the waterwet and neutralwet conditions. Effects of droplet size on the release pressure have also been studied. For the finite droplet and at the waterwet condition (θ = 45°), the minimum release pressure increases linearly with the droplet length, while for the long droplet at similar conditions, the minimum release pressure does not change much as the length of the droplet increases. Furthermore, the release pressure decreases with the increased tapering angle.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Conditions of release of trapped droplets in constricted channels are of great significance in various domains, including microfluidic development and enhanced oil recovery. In our previous studies, a detailed and quantitative analysis of the threshold pressure needed to release a droplet from a constricted channel has been performed. However, droplets may exist in real applications as long droplets, which may exhibit different behavior than finite droplets. Therefore, in this study, direct numerical simulations, combining the fluid flow equations and the phasefield method, have been conducted on threedimensional constrained channels to investigate discrepancies in release conditions of finite droplets and long droplets. The results have shown that for a finite droplet, the maximum pressure increases with the increase in the contact angle, whereas for a long droplet, the maximum pressure is almost the same both in the waterwet and neutralwet conditions. Effects of droplet size on the release pressure have also been studied. For the finite droplet and at the waterwet condition (θ = 45°), the minimum release pressure increases linearly with the droplet length, while for the long droplet at similar conditions, the minimum release pressure does not change much as the length of the droplet increases. Furthermore, the release pressure decreases with the increased tapering angle.
Finite droplets vs long droplets: Discrepancy in release conditions in a microscopic constricted channel
10.1063/5.0139025
Physics of Fluids
20230302T01:42:39Z
© 2023 Author(s).
Gloire Imani
Munezero Ntibahanana

Dynamics of cocurrent gas–liquid film flow through a slippery channel
https://aip.scitation.org/doi/10.1063/5.0139030?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We consider a thin liquid film in a wide inclined channel being driven by gravity and cocurrent turbulent gas flow. The bottom plate with which the liquid is in contact with is taken to be slippery, and we impose the classic Navier slip condition at this substrate. Such a setting finds application in technological processes as well as nature (e.g., distillation, absorption, and cooling devices). The gas–liquid problem can be decoupled by making certain reasonable assumptions. Under these assumptions, we solve the gas problem to obtain the tangential and normal stresses acting at the wavy gas–liquid interface for arbitrary waviness. In modeling the liquid layer dynamics, we make use of the stresses computed in the gas problem as inputs to the interface boundary conditions. We develop the longwave model and the weightedintegral boundary layer (WIBL) model to describe the thin film dynamics. We perform a linear stability of these reduced order models to scrutinize the effect of wall slip, liquid flow rate, and the gas shear on the stability of the flat film solution. It is found that the wall slip promotes the instability of the flat interface. Furthermore, we compute solitary wave solutions of the WIBL model by implementing Keller's pseudoarc length algorithm on a periodic domain. We observe that the wave speed as well as the wave amplitude are attenuated on incrementing the liquid slip at the substrate. We corroborate these findings with the timedependent computations of the nonlinear WIBL model.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We consider a thin liquid film in a wide inclined channel being driven by gravity and cocurrent turbulent gas flow. The bottom plate with which the liquid is in contact with is taken to be slippery, and we impose the classic Navier slip condition at this substrate. Such a setting finds application in technological processes as well as nature (e.g., distillation, absorption, and cooling devices). The gas–liquid problem can be decoupled by making certain reasonable assumptions. Under these assumptions, we solve the gas problem to obtain the tangential and normal stresses acting at the wavy gas–liquid interface for arbitrary waviness. In modeling the liquid layer dynamics, we make use of the stresses computed in the gas problem as inputs to the interface boundary conditions. We develop the longwave model and the weightedintegral boundary layer (WIBL) model to describe the thin film dynamics. We perform a linear stability of these reduced order models to scrutinize the effect of wall slip, liquid flow rate, and the gas shear on the stability of the flat film solution. It is found that the wall slip promotes the instability of the flat interface. Furthermore, we compute solitary wave solutions of the WIBL model by implementing Keller's pseudoarc length algorithm on a periodic domain. We observe that the wave speed as well as the wave amplitude are attenuated on incrementing the liquid slip at the substrate. We corroborate these findings with the timedependent computations of the nonlinear WIBL model.
Dynamics of cocurrent gas–liquid film flow through a slippery channel
10.1063/5.0139030
Physics of Fluids
20230303T12:57:56Z
© 2023 Author(s).
Rajagopal Vellingiri

Evaporation of twin drops: Effect of acoustics and spacing
https://aip.scitation.org/doi/10.1063/5.0137944?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The study investigates how an acoustic field influences evaporation and internal circulation of twin drops when their inbetween horizontal spacing varies. The acoustic source is a simple sine wave (i) with and (ii) without white noise at various frequencies. The circulation and outer flow are visualized. Maximum evaporation rate and circulation are found for the lowest frequency and highest spacing. The rate rises with the spacing for a given frequency up to a critical distance. The evaporation becomes almost identical beyond the critical spacing. A correlation among the spacing, evaporation rate, and outer flow velocity is demonstrated. The rate becomes lowest for a given frequency at the least spacing since the vapors accumulated in the surrounding are not swept out by the acousticinduced flow. The visualization shows a horizontal outer flow, which becomes vertical with the rise in spacing because the acoustic wave can sweep the vapor out. The horizontal flow for the least spacing transforms itself to vertical when the wave amplitude is raised. The evaporation thus rises because the wave now sweeps the vapors out. We show that the perception that any acoustic wave enhances the evaporation of multiple nearby drops is incorrect. The evaporation and circulation decline faster with the rise in frequency since the surrounding flow becomes weak. Thus, we show how the spacing influences the evaporation when acoustic is incident and how the evaporation can be raised by sweeping the accumulated vapor out using higher amplitude acoustics for the closer drops.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The study investigates how an acoustic field influences evaporation and internal circulation of twin drops when their inbetween horizontal spacing varies. The acoustic source is a simple sine wave (i) with and (ii) without white noise at various frequencies. The circulation and outer flow are visualized. Maximum evaporation rate and circulation are found for the lowest frequency and highest spacing. The rate rises with the spacing for a given frequency up to a critical distance. The evaporation becomes almost identical beyond the critical spacing. A correlation among the spacing, evaporation rate, and outer flow velocity is demonstrated. The rate becomes lowest for a given frequency at the least spacing since the vapors accumulated in the surrounding are not swept out by the acousticinduced flow. The visualization shows a horizontal outer flow, which becomes vertical with the rise in spacing because the acoustic wave can sweep the vapor out. The horizontal flow for the least spacing transforms itself to vertical when the wave amplitude is raised. The evaporation thus rises because the wave now sweeps the vapors out. We show that the perception that any acoustic wave enhances the evaporation of multiple nearby drops is incorrect. The evaporation and circulation decline faster with the rise in frequency since the surrounding flow becomes weak. Thus, we show how the spacing influences the evaporation when acoustic is incident and how the evaporation can be raised by sweeping the accumulated vapor out using higher amplitude acoustics for the closer drops.
Evaporation of twin drops: Effect of acoustics and spacing
10.1063/5.0137944
Physics of Fluids
20230307T11:18:57Z
© 2023 Author(s).
Aadil Kureshee
S. Narayanan
Deepak Kumar Mandal

Thermocapillary migration of a deformed droplet in the combined vertical temperature gradient and thermal radiation
https://aip.scitation.org/doi/10.1063/5.0142144?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Thermocapillary migration of a deformed droplet in the combined vertical temperature gradient and thermal radiations with uniform and nonuniform fluxes is first analyzed. The creeping flow solutions show that the deformed droplet has a slender or a cardioid shape, which depends on the form of the radiation flux. The deviation from a sphere depends not only on the viscosity and the conductivity ratios of twophase fluids but also on capillary and thermal radiation numbers. Moreover, in the roles of interfacial rheology on thermocapillary migration of a deformed droplet, only the surface dilatational viscosity and the surface internal energy can reduce the steady migration velocity, but the surface shear viscosity has not any effects on the steady migration velocity. The surface shear and dilatational viscosities affect the deformation of the droplet by increasing the viscosity ratio of twophase fluids. The surface internal energy directly reduces the deformation of the droplet. However, the deformed droplet still keeps its original shape without the influence of interfacial rheology. Furthermore, it is found that, based on the net force balance condition of the droplet, the normal stress balance at the interface can be used to determine the steady migration velocity, which is not affected by the surface deformation in the creeping flow. From the expressions of the normal/the tangential stress balance, it can be proved that the surface shear viscosity does not affect the steady migration velocity. The results could not only provide a valuable understanding of thermocapillary migration of a deformed droplet with/without the interfacial rheology in a vertical temperature gradient controlled by thermal radiation but also inspire its potential practical applications in microgravity and microfluidic fields.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Thermocapillary migration of a deformed droplet in the combined vertical temperature gradient and thermal radiations with uniform and nonuniform fluxes is first analyzed. The creeping flow solutions show that the deformed droplet has a slender or a cardioid shape, which depends on the form of the radiation flux. The deviation from a sphere depends not only on the viscosity and the conductivity ratios of twophase fluids but also on capillary and thermal radiation numbers. Moreover, in the roles of interfacial rheology on thermocapillary migration of a deformed droplet, only the surface dilatational viscosity and the surface internal energy can reduce the steady migration velocity, but the surface shear viscosity has not any effects on the steady migration velocity. The surface shear and dilatational viscosities affect the deformation of the droplet by increasing the viscosity ratio of twophase fluids. The surface internal energy directly reduces the deformation of the droplet. However, the deformed droplet still keeps its original shape without the influence of interfacial rheology. Furthermore, it is found that, based on the net force balance condition of the droplet, the normal stress balance at the interface can be used to determine the steady migration velocity, which is not affected by the surface deformation in the creeping flow. From the expressions of the normal/the tangential stress balance, it can be proved that the surface shear viscosity does not affect the steady migration velocity. The results could not only provide a valuable understanding of thermocapillary migration of a deformed droplet with/without the interfacial rheology in a vertical temperature gradient controlled by thermal radiation but also inspire its potential practical applications in microgravity and microfluidic fields.
Thermocapillary migration of a deformed droplet in the combined vertical temperature gradient and thermal radiation
10.1063/5.0142144
Physics of Fluids
20230307T11:19:02Z
© 2023 Author(s).

The dynamics of directional transport of a droplet in programmable electrowetting channel
https://aip.scitation.org/doi/10.1063/5.0139965?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Directional fluid transport by electrowetting is an effective method for fluid management both on Earth and in the space environment. Exact control of the process is always hard because the fundamental dynamics of fluid flow and interface are not well understood. In this study, we examine the process of a sensible droplet transported directionally in an electrowetting channel. The electrodes of the channel are programmed to actuate the droplet at the most effective manner. We build a numerical model based on the phase field method, and a dynamic contact angle model is incorporated in the model. Based on simulated results, the basic process of droplet deformation and motion is explained. Three different stages are observed when the droplet starts to move in the electrowetting channel. The droplet can be transported at a high velocity of 17 mm/s at a voltage of V = 80 V. A wide range of influence factors, including voltage, droplet size, friction factor, pinning force, channel height, gravity level, and tilted angle of the channel, are considered. The contact line friction increases almost linearly with the contact line friction coefficient and the pinning force, both retarding the motion of the droplet at parabolic relations. With an increase in the gravity level, the transport velocity of large droplet decreases. However, the droplet smaller than the capillary length shows quite good antigravity capability, which can be transported smoothly even when the channel is tilted by 90° in a normal gravity.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Directional fluid transport by electrowetting is an effective method for fluid management both on Earth and in the space environment. Exact control of the process is always hard because the fundamental dynamics of fluid flow and interface are not well understood. In this study, we examine the process of a sensible droplet transported directionally in an electrowetting channel. The electrodes of the channel are programmed to actuate the droplet at the most effective manner. We build a numerical model based on the phase field method, and a dynamic contact angle model is incorporated in the model. Based on simulated results, the basic process of droplet deformation and motion is explained. Three different stages are observed when the droplet starts to move in the electrowetting channel. The droplet can be transported at a high velocity of 17 mm/s at a voltage of V = 80 V. A wide range of influence factors, including voltage, droplet size, friction factor, pinning force, channel height, gravity level, and tilted angle of the channel, are considered. The contact line friction increases almost linearly with the contact line friction coefficient and the pinning force, both retarding the motion of the droplet at parabolic relations. With an increase in the gravity level, the transport velocity of large droplet decreases. However, the droplet smaller than the capillary length shows quite good antigravity capability, which can be transported smoothly even when the channel is tilted by 90° in a normal gravity.
The dynamics of directional transport of a droplet in programmable electrowetting channel
10.1063/5.0139965
Physics of Fluids
20230308T12:22:34Z
© 2023 Author(s).

Numerical simulations of miscible displacement in an inclined channel by lattice Boltzmann method
https://aip.scitation.org/doi/10.1063/5.0135734?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The interfacial instability between miscible fluids in a channel is determined by many factors, such as viscosity contrast and the inclination angle. Considering the effect of the gravity field, we investigate the displacement phenomenon between two miscible fluids with different viscosities in an inclined channel. The results show that when the concentration Rayleigh number [math], the inclination angle θ ranges from [math] to [math], and the natural logarithm of the viscosity ratio R > 0; there are three fluid–fluid interfacial instability regions, namely, viscous fingering, “Kelvin–Helmholtz” (K–H) instability, and “Rayleigh–Taylor” (R–T) instability. A scaling analysis is developed to describe the time evolution of the displacement as described by the displacement efficiency at a fixed viscous ratio. Our analysis indicates that in the viscous fingering region, the time evolution of the displacement efficiency gradually increases with t scaling due to fingering formations; in the K–H and R–T regions, the displacement efficiency rapidly increases with [math]. When considering the effect of the viscosity ratio in the K–H instability region, the displacement efficiency scales as [math]. In addition, when the inclination angle is negative or R < 0, the instability phenomenon is not obvious, and the displacement efficiency decreases as the inclination angle or R decreases.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The interfacial instability between miscible fluids in a channel is determined by many factors, such as viscosity contrast and the inclination angle. Considering the effect of the gravity field, we investigate the displacement phenomenon between two miscible fluids with different viscosities in an inclined channel. The results show that when the concentration Rayleigh number [math], the inclination angle θ ranges from [math] to [math], and the natural logarithm of the viscosity ratio R > 0; there are three fluid–fluid interfacial instability regions, namely, viscous fingering, “Kelvin–Helmholtz” (K–H) instability, and “Rayleigh–Taylor” (R–T) instability. A scaling analysis is developed to describe the time evolution of the displacement as described by the displacement efficiency at a fixed viscous ratio. Our analysis indicates that in the viscous fingering region, the time evolution of the displacement efficiency gradually increases with t scaling due to fingering formations; in the K–H and R–T regions, the displacement efficiency rapidly increases with [math]. When considering the effect of the viscosity ratio in the K–H instability region, the displacement efficiency scales as [math]. In addition, when the inclination angle is negative or R < 0, the instability phenomenon is not obvious, and the displacement efficiency decreases as the inclination angle or R decreases.
Numerical simulations of miscible displacement in an inclined channel by lattice Boltzmann method
10.1063/5.0135734
Physics of Fluids
20230310T12:33:07Z
© 2023 Author(s).

Marangoni plumes in miscible spreading
https://aip.scitation.org/doi/10.1063/5.0137335?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We present a study of novel, surface tension driven plumes that form at the periphery of fast expanding, circular ethanol–water films that emanate from millimeter sized ethanol–water drops floating at the surface of a deep water layer. Visualizing these plumes that are azimuthally uniformly spaced, using floating particles, we measure their lengths (lp), radial velocities (Up), and mean azimuthal spacings (λp). We show through a model that a balance between the surface tension force across lp and the viscous resistance in an underlying boundary layer results in [math], where [math] is a Marangoni length scale and δbl is the boundary layer thickness. The model also predicts that [math], where [math] is a velocity scale balancing inertia and surface tension and [math] is the velocity scale of momentum diffusion. These predictions are shown to be in agreement with our experimentally observed variations of lp and Up. The observed variation of λp, which we show not to match the predictions of any of the available instability theories, is shown to scale as [math], where Ohd is the drop Ohnesorge number, rf is the film radius, and ξ and χ are the viscosity and the density ratios.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We present a study of novel, surface tension driven plumes that form at the periphery of fast expanding, circular ethanol–water films that emanate from millimeter sized ethanol–water drops floating at the surface of a deep water layer. Visualizing these plumes that are azimuthally uniformly spaced, using floating particles, we measure their lengths (lp), radial velocities (Up), and mean azimuthal spacings (λp). We show through a model that a balance between the surface tension force across lp and the viscous resistance in an underlying boundary layer results in [math], where [math] is a Marangoni length scale and δbl is the boundary layer thickness. The model also predicts that [math], where [math] is a velocity scale balancing inertia and surface tension and [math] is the velocity scale of momentum diffusion. These predictions are shown to be in agreement with our experimentally observed variations of lp and Up. The observed variation of λp, which we show not to match the predictions of any of the available instability theories, is shown to scale as [math], where Ohd is the drop Ohnesorge number, rf is the film radius, and ξ and χ are the viscosity and the density ratios.
Marangoni plumes in miscible spreading
10.1063/5.0137335
Physics of Fluids
20230310T12:20:09Z
© 2023 Author(s).
Anurag Pant
Baburaj A. Puthenveettil
Sreeram K. Kalpathy

Control of boundary slip by interfacial nanobubbles: A perspective from molecular dynamics simulations
https://aip.scitation.org/doi/10.1063/5.0141614?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Enhancing boundary slip using interfacial nanobubbles (INBs) has gained significant interest in nanofluidic transport. In this study, we conducted a comprehensive investigation on the influence of INBs on boundary conditions for both smooth and rough substrates using molecular dynamics simulations. We analyzed the impact of INB protrusion angle, coverage percentage, quantity, and fluidity on the slip length. Our results showed that INBs always increase the slip length on a smooth substrate, with a linear increase in slip length observed with increasing surface coverage. On a rough substrate, we found that the protrusion angle, quantity, and fluidity of INBs play a crucial role in determining the slip length. Smaller protrusion angles and fewer quantities of INBs were found to be more favorable for enhancing the slip length when the INB coverage is fixed, while the correlation between boundary slip and INB quantity depended on the wetting state of the substrate when the size of the INBs was fixed with a low protrusion angle. Additionally, we revealed that the fluidity of gas molecules inside the INBs dominated the enhancement of slip length by INBs. Overall, our findings are expected to provide valuable insight into drag reduction based on INBs.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Enhancing boundary slip using interfacial nanobubbles (INBs) has gained significant interest in nanofluidic transport. In this study, we conducted a comprehensive investigation on the influence of INBs on boundary conditions for both smooth and rough substrates using molecular dynamics simulations. We analyzed the impact of INB protrusion angle, coverage percentage, quantity, and fluidity on the slip length. Our results showed that INBs always increase the slip length on a smooth substrate, with a linear increase in slip length observed with increasing surface coverage. On a rough substrate, we found that the protrusion angle, quantity, and fluidity of INBs play a crucial role in determining the slip length. Smaller protrusion angles and fewer quantities of INBs were found to be more favorable for enhancing the slip length when the INB coverage is fixed, while the correlation between boundary slip and INB quantity depended on the wetting state of the substrate when the size of the INBs was fixed with a low protrusion angle. Additionally, we revealed that the fluidity of gas molecules inside the INBs dominated the enhancement of slip length by INBs. Overall, our findings are expected to provide valuable insight into drag reduction based on INBs.
Control of boundary slip by interfacial nanobubbles: A perspective from molecular dynamics simulations
10.1063/5.0141614
Physics of Fluids
20230314T12:08:33Z
© 2023 Author(s).

A semiempirical force balancebased model to capture sessile droplet spread on smooth surfaces: A moving front kinetic Monte Carlo study
https://aip.scitation.org/doi/10.1063/5.0139638?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study reports the development of a semiempirical force balancebased moving front kinetic Monte Carlo (FBMFkMC) model to describe droplet spreading on a smooth surface. The proposed model depicts the statebystate evolution of a sessile droplet in a stochastic manner that captures the molecularlevel events taking place in an accurate yet efficient manner. In the developed model, the movement of the droplet triple contact line is depicted using rate expressions that detail the probability that the contact line will locally advance over a set distance at each time point. These rate expressions are derived based on the force balance acting upon the droplet interface, which is captured using analytical inertial and capillary expressions from the literature. This work furthermore derives a new semiempirical expression to depict the viscous damping force acting on the droplet. The derived viscous force term depends on a fitted parameter [math], whose value was observed to vary solely depending on the droplet liquid as captured predominantly by the droplet Ohnesorge number. The proposed FBMFkMC approach is subsequently validated using data obtained both from conducted experiments and from the literature to support the robustness of the framework. The predictive capabilities of the developed model are further inspected to provide insights on the sessile droplet system behavior.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study reports the development of a semiempirical force balancebased moving front kinetic Monte Carlo (FBMFkMC) model to describe droplet spreading on a smooth surface. The proposed model depicts the statebystate evolution of a sessile droplet in a stochastic manner that captures the molecularlevel events taking place in an accurate yet efficient manner. In the developed model, the movement of the droplet triple contact line is depicted using rate expressions that detail the probability that the contact line will locally advance over a set distance at each time point. These rate expressions are derived based on the force balance acting upon the droplet interface, which is captured using analytical inertial and capillary expressions from the literature. This work furthermore derives a new semiempirical expression to depict the viscous damping force acting on the droplet. The derived viscous force term depends on a fitted parameter [math], whose value was observed to vary solely depending on the droplet liquid as captured predominantly by the droplet Ohnesorge number. The proposed FBMFkMC approach is subsequently validated using data obtained both from conducted experiments and from the literature to support the robustness of the framework. The predictive capabilities of the developed model are further inspected to provide insights on the sessile droplet system behavior.
A semiempirical force balancebased model to capture sessile droplet spread on smooth surfaces: A moving front kinetic Monte Carlo study
10.1063/5.0139638
Physics of Fluids
20230314T10:16:47Z
© 2023 Author(s).
Donovan Chaffart
Luis A. RicardezSandoval

Pulsed coaxial dropondemand electrohydrodynamic printing
https://aip.scitation.org/doi/10.1063/5.0141214?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This research demonstrates pulsed electrohydrodynamic dropondemand (DoD) printing as a novel technique for synthesizing core–shell microparticles in a controlled manner. In this regard, a multiphase and multiphysics model is presented for coaxial electrohydrodynamic printing. The governing partial differential equations of the model are discretized using the finite element method, and a suitable numerical scheme is adopted to solve the system of discretized equations. The experimental results in the literature are used to validate the numerical model. Utilizing the validated model, the effects of continuousdirect current (DC) voltage and pulsedDC voltage are examined on the behavior of a compound meniscus (composed of ethylene glycol core and olive oil shell) and the droplet formation process. According to the results obtained, the onset voltage of the compound meniscus is 3330 V, which agrees with the scale analysis. Furthermore, increasing continuousDC voltage results in longer breakup length, shorter breakup time, faster droplet velocity, and shorter jetting start time. In addition, increasing pulsedDC voltage duration leads to an increase in the breakup length and droplet velocity. Most importantly, it is possible to control the inertia of the coaxial spindle by controlling the pulsedDC voltage magnitude and duration to ensure that a core–shell droplet separates from the meniscus in every pulse with the shortest breakup length and the minimum satellite droplets possible. It is generally recommended to keep the pulse duration and amplitude low enough to prevent the long breakup length and irregularities in the printed pattern; however, they must be sufficiently large to sustain the microdripping mode.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This research demonstrates pulsed electrohydrodynamic dropondemand (DoD) printing as a novel technique for synthesizing core–shell microparticles in a controlled manner. In this regard, a multiphase and multiphysics model is presented for coaxial electrohydrodynamic printing. The governing partial differential equations of the model are discretized using the finite element method, and a suitable numerical scheme is adopted to solve the system of discretized equations. The experimental results in the literature are used to validate the numerical model. Utilizing the validated model, the effects of continuousdirect current (DC) voltage and pulsedDC voltage are examined on the behavior of a compound meniscus (composed of ethylene glycol core and olive oil shell) and the droplet formation process. According to the results obtained, the onset voltage of the compound meniscus is 3330 V, which agrees with the scale analysis. Furthermore, increasing continuousDC voltage results in longer breakup length, shorter breakup time, faster droplet velocity, and shorter jetting start time. In addition, increasing pulsedDC voltage duration leads to an increase in the breakup length and droplet velocity. Most importantly, it is possible to control the inertia of the coaxial spindle by controlling the pulsedDC voltage magnitude and duration to ensure that a core–shell droplet separates from the meniscus in every pulse with the shortest breakup length and the minimum satellite droplets possible. It is generally recommended to keep the pulse duration and amplitude low enough to prevent the long breakup length and irregularities in the printed pattern; however, they must be sufficiently large to sustain the microdripping mode.
Pulsed coaxial dropondemand electrohydrodynamic printing
10.1063/5.0141214
Physics of Fluids
20230315T11:59:12Z
© 2023 Author(s).
Mostafa Jamshidian
Kaivan Mohammadi
Ali Moosavi
Siamak Kazemzadeh Hannani

Dynamics of growth and detachment of single hydrogen bubble on horizontal and vertical microelectrode surfaces considering liquid microlayer structure in water electrolysis
https://aip.scitation.org/doi/10.1063/5.0141648?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The dynamics of the growth and detachment of a single hydrogen bubble on both the horizontal and vertical microelectrode surfaces in water electrolysis were synthetically investigated by combining the numerical simulation, force balance analysis, and available experimental data. Approximately, multiple steady simulation cases with different bubble diameters for different growth instances were conducted to state the actual unsteady bubble growth and detachment behavior. The numerical simulations of the temperature distribution considering the heat transfer caused by the liquid microlayer and induced Marangoni convection effects were performed. Then, a force balance model for predicting the bubble detachment diameter was developed by fully utilizing the simulated multiphysical field parameters and the experimental results of some key bubble geometric parameters. The presented numerical model and the force balance model were validated by comparing them with previous experimental data on the potential and the bubble detachment diameter, respectively. The simulation results indicate a significantly larger potential value occurs within the microlayer, and hence, the Joule heat of the electrolyte is mainly generated in the microlayer and then transferred to the bulk flow region. Obviously, the temperature gradient distribution is formed at the bubble interface, causing unstable Marangoni convection structure. The distribution patterns and evolutions of the electrolyte temperature, Marangoni convection velocity, and microlayer thickness for the horizontal and vertical microelectrode systems are significantly different. The present force balance model presents higher prediction accuracy for the bubble detachment diameters. Moreover, the indepth force analysis results reveal that some dominant forces influence the bubble growth and detachment.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The dynamics of the growth and detachment of a single hydrogen bubble on both the horizontal and vertical microelectrode surfaces in water electrolysis were synthetically investigated by combining the numerical simulation, force balance analysis, and available experimental data. Approximately, multiple steady simulation cases with different bubble diameters for different growth instances were conducted to state the actual unsteady bubble growth and detachment behavior. The numerical simulations of the temperature distribution considering the heat transfer caused by the liquid microlayer and induced Marangoni convection effects were performed. Then, a force balance model for predicting the bubble detachment diameter was developed by fully utilizing the simulated multiphysical field parameters and the experimental results of some key bubble geometric parameters. The presented numerical model and the force balance model were validated by comparing them with previous experimental data on the potential and the bubble detachment diameter, respectively. The simulation results indicate a significantly larger potential value occurs within the microlayer, and hence, the Joule heat of the electrolyte is mainly generated in the microlayer and then transferred to the bulk flow region. Obviously, the temperature gradient distribution is formed at the bubble interface, causing unstable Marangoni convection structure. The distribution patterns and evolutions of the electrolyte temperature, Marangoni convection velocity, and microlayer thickness for the horizontal and vertical microelectrode systems are significantly different. The present force balance model presents higher prediction accuracy for the bubble detachment diameters. Moreover, the indepth force analysis results reveal that some dominant forces influence the bubble growth and detachment.
Dynamics of growth and detachment of single hydrogen bubble on horizontal and vertical microelectrode surfaces considering liquid microlayer structure in water electrolysis
10.1063/5.0141648
Physics of Fluids
20230315T11:50:00Z
© 2023 Author(s).

The contact time of reboundingcoalescing droplets on rectangularridged superhydrophobic surfaces
https://aip.scitation.org/doi/10.1063/5.0138372?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>On a rectangularridged superhydrophobic surface, the contact time of the reboundingcoalescing droplet is for the first time investigated via lattice Boltzmann method simulations, where the reboundingcoalescing droplet is caused by an impinging droplet coalescing with an adhesive droplet. The simulation results show that at constant initial radii of impinging droplets, R0, and various initial radii of adhesive droplets, R1, the contact time of rebounding–coalescing droplets depends not only on the impact condition but also on the surface condition. Under various impact conditions, that is, with increased Weber numbers of We = 1–30, the contact time is gradually reduced, and then nearly constant, and eventually constant after slightly reduced at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is gradually reduced, then increased, and eventually constant. It indicates that the contact time of reboundingcoalescing droplets is affected by the initial radii of adhesive droplets. Under different surface conditions, that is, with increased spacing distances between adhesive droplets and ridges of L = 3–17, the contact time is reduced at the low Weber number of We = 3, constant at the moderate Weber number of We = 12, and increased at the high Weber number of We = 28 at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is reduced at both low and moderate Weber numbers of We = 3 and 12, and constant at the high Weber number of We = 28. It indicates that under different surface conditions, the contact time of reboundingcoalescing droplets is also affected by the initial radii of adhesive droplets.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>On a rectangularridged superhydrophobic surface, the contact time of the reboundingcoalescing droplet is for the first time investigated via lattice Boltzmann method simulations, where the reboundingcoalescing droplet is caused by an impinging droplet coalescing with an adhesive droplet. The simulation results show that at constant initial radii of impinging droplets, R0, and various initial radii of adhesive droplets, R1, the contact time of rebounding–coalescing droplets depends not only on the impact condition but also on the surface condition. Under various impact conditions, that is, with increased Weber numbers of We = 1–30, the contact time is gradually reduced, and then nearly constant, and eventually constant after slightly reduced at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is gradually reduced, then increased, and eventually constant. It indicates that the contact time of reboundingcoalescing droplets is affected by the initial radii of adhesive droplets. Under different surface conditions, that is, with increased spacing distances between adhesive droplets and ridges of L = 3–17, the contact time is reduced at the low Weber number of We = 3, constant at the moderate Weber number of We = 12, and increased at the high Weber number of We = 28 at R0 = 35 and R1 = 25. However, at R0 = 35 and R1 = 10, it is reduced at both low and moderate Weber numbers of We = 3 and 12, and constant at the high Weber number of We = 28. It indicates that under different surface conditions, the contact time of reboundingcoalescing droplets is also affected by the initial radii of adhesive droplets.
The contact time of reboundingcoalescing droplets on rectangularridged superhydrophobic surfaces
10.1063/5.0138372
Physics of Fluids
20230316T02:06:28Z
© 2023 Author(s).

Modeling the evaporation of sessile drops deformed by gravity on hydrophilic and hydrophobic substrates
https://aip.scitation.org/doi/10.1063/5.0143575?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Evaporation of sessile drops deformed by gravity is quantified by an analytical–numerical approach. The shape of the drops is defined by minimizing the interfacial and potential drop energies, following a variational integral approach, for a wide range of drop sizes (from 2.7 [math] to 1.4 ml for water drops) and contact angles for both hydrophilic and hydrophobic substrates. The extension of an analytical model for drop evaporation, which accounts for the effect of the Stefan flow and the temperature dependence of thermophysical properties, to the present conditions reduces the problem to the solution of a Laplace equation, which is then numerically calculated using COMSOL Multiphysics®. The vapor fluxes and evaporation rates are then quantified, and the systematic approach to the problem allows the derivation of two correlations, for hydrophilic and hydrophobic substrates, respectively, that can be used to correct the evaporation rate calculated for a drop of the same volume and contact angle in the absence of gravity effects.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Evaporation of sessile drops deformed by gravity is quantified by an analytical–numerical approach. The shape of the drops is defined by minimizing the interfacial and potential drop energies, following a variational integral approach, for a wide range of drop sizes (from 2.7 [math] to 1.4 ml for water drops) and contact angles for both hydrophilic and hydrophobic substrates. The extension of an analytical model for drop evaporation, which accounts for the effect of the Stefan flow and the temperature dependence of thermophysical properties, to the present conditions reduces the problem to the solution of a Laplace equation, which is then numerically calculated using COMSOL Multiphysics®. The vapor fluxes and evaporation rates are then quantified, and the systematic approach to the problem allows the derivation of two correlations, for hydrophilic and hydrophobic substrates, respectively, that can be used to correct the evaporation rate calculated for a drop of the same volume and contact angle in the absence of gravity effects.
Modeling the evaporation of sessile drops deformed by gravity on hydrophilic and hydrophobic substrates
10.1063/5.0143575
Physics of Fluids
20230316T01:44:05Z
© 2023 Author(s).
S. Tonini
G. E. Cossali

Dynamic analysis of symmetric oscillation and turning characteristics of a flexible fin underwater robot propelled by double fins
https://aip.scitation.org/doi/10.1063/5.0136565?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this study, the dynamics of the symmetric oscillation and turning characteristics of a flexible fin underwater robot propelled by two fins were studied. First, a threedimensional model of a robot was established using threedimensional software. Then, a fluid simulation experiment was conducted and a dynamic model of a flexible fin was established. The deformation of the flexible fin during symmetric undulations was studied. A motion equation for the wave track of the outer edge of the fin surface was also established. This motion equation was simulated and verified. Finally, an experimental prototype was fabricated to verify the simulation results. The results show that if the robot fish oscillates symmetrically using bilateral flexible pectoral fins, it can remain suspended, float vertically, or dive in the water. Its average turning speed can reach 0.8 rad/s, its straight running speed is 0.5 m/s, and its vertical ascending and descending speed is 0.1 m/s. Because a turn made by the robot fish is only driven by its pectoral fins, its turning radius is 0. The results show that the flexible fin underwater robot provides more abundant turning methods, better maneuverability, and higher turning efficiency. This research into the motion of the robot body for different wave parameters when the two fins move together provides a theoretical basis for the cooperative motion of two fins.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this study, the dynamics of the symmetric oscillation and turning characteristics of a flexible fin underwater robot propelled by two fins were studied. First, a threedimensional model of a robot was established using threedimensional software. Then, a fluid simulation experiment was conducted and a dynamic model of a flexible fin was established. The deformation of the flexible fin during symmetric undulations was studied. A motion equation for the wave track of the outer edge of the fin surface was also established. This motion equation was simulated and verified. Finally, an experimental prototype was fabricated to verify the simulation results. The results show that if the robot fish oscillates symmetrically using bilateral flexible pectoral fins, it can remain suspended, float vertically, or dive in the water. Its average turning speed can reach 0.8 rad/s, its straight running speed is 0.5 m/s, and its vertical ascending and descending speed is 0.1 m/s. Because a turn made by the robot fish is only driven by its pectoral fins, its turning radius is 0. The results show that the flexible fin underwater robot provides more abundant turning methods, better maneuverability, and higher turning efficiency. This research into the motion of the robot body for different wave parameters when the two fins move together provides a theoretical basis for the cooperative motion of two fins.
Dynamic analysis of symmetric oscillation and turning characteristics of a flexible fin underwater robot propelled by double fins
10.1063/5.0136565
Physics of Fluids
20230316T01:57:39Z
© 2023 Author(s).

Numerical study of drop impact on slippery lubricated surfaces
https://aip.scitation.org/doi/10.1063/5.0137313?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We numerically study drop impact on slippery lubricated surfaces at varied impact speeds to comprehend the cloaking of the water drop by the lubricant. We employ a multimaterial and multiphase interface reconstruction method to capture the interaction between the drop and the lubricants of varying interfacial tensions. We demonstrate that cloaking occurs when lubricant water interfacial tensions are low and impact speeds are low. Our research demonstrates that the thickness of the encapsulating lubricant layer varies over time. At moderate impact speeds of 0.25 and 0.5 m/s, the drop displaces a large amount of lubricant, generating a lubricant–water jet, as we also demonstrate. At high impact speeds of 5 and 30 m/s, a secondary impingement forms, which displaces a significant amount of lubricant to reveal the underneath substrate that was not visible at lower impact speeds. Finally, we investigate the drop impact on lubricant infused microwells with varying spacing. We find that small spacing between the microwell walls can limit lubricant drainage and displacement. The substrates with microwells exhibit far less splashing than those without. Furthermore, we demonstrate that microwells are better at preserving lubricants than substrates without microwells.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We numerically study drop impact on slippery lubricated surfaces at varied impact speeds to comprehend the cloaking of the water drop by the lubricant. We employ a multimaterial and multiphase interface reconstruction method to capture the interaction between the drop and the lubricants of varying interfacial tensions. We demonstrate that cloaking occurs when lubricant water interfacial tensions are low and impact speeds are low. Our research demonstrates that the thickness of the encapsulating lubricant layer varies over time. At moderate impact speeds of 0.25 and 0.5 m/s, the drop displaces a large amount of lubricant, generating a lubricant–water jet, as we also demonstrate. At high impact speeds of 5 and 30 m/s, a secondary impingement forms, which displaces a significant amount of lubricant to reveal the underneath substrate that was not visible at lower impact speeds. Finally, we investigate the drop impact on lubricant infused microwells with varying spacing. We find that small spacing between the microwell walls can limit lubricant drainage and displacement. The substrates with microwells exhibit far less splashing than those without. Furthermore, we demonstrate that microwells are better at preserving lubricants than substrates without microwells.
Numerical study of drop impact on slippery lubricated surfaces
10.1063/5.0137313
Physics of Fluids
20230317T02:39:47Z
© 2023 Author(s).
Ahmed Islam
Yongsheng Lian

Stability of oblique liquid curtains with surface tension
https://aip.scitation.org/doi/10.1063/5.0143532?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Oblique (nonvertical) liquid curtains are examined under the assumption that the Froude number is large. As shown previously [E. S. Benilov, “Oblique liquid curtains with a large Froude number,” J. Fluid Mech. 861, 328 (2019)], their structure depends on the Weber number: if We < 1 (strong surface tension), the Navier–Stokes equations admit asymptotic solutions describing curtains bending upward, i.e., against gravity. In the present paper, it is shown that such curtains are unstable with respect to small perturbations of the flow parameters at the outlet: they give rise to a disturbance traveling downstream and becoming singular near the curtain's terminal point (where the liquid runs out of the initial supply of kinetic energy). It is argued that, since the instability is spatially localized, the curtain can be stabilized by a properly positioned collection nozzle. All curtains with We > 1 bend downward and are shown to be stable.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Oblique (nonvertical) liquid curtains are examined under the assumption that the Froude number is large. As shown previously [E. S. Benilov, “Oblique liquid curtains with a large Froude number,” J. Fluid Mech. 861, 328 (2019)], their structure depends on the Weber number: if We < 1 (strong surface tension), the Navier–Stokes equations admit asymptotic solutions describing curtains bending upward, i.e., against gravity. In the present paper, it is shown that such curtains are unstable with respect to small perturbations of the flow parameters at the outlet: they give rise to a disturbance traveling downstream and becoming singular near the curtain's terminal point (where the liquid runs out of the initial supply of kinetic energy). It is argued that, since the instability is spatially localized, the curtain can be stabilized by a properly positioned collection nozzle. All curtains with We > 1 bend downward and are shown to be stable.
Stability of oblique liquid curtains with surface tension
10.1063/5.0143532
Physics of Fluids
20230317T03:03:51Z
© 2023 Author(s).
E. S. Benilov

Parallel and perpendicular flows of a couple stress fluid past a solid cylinder in cell model: Slip condition
https://aip.scitation.org/doi/10.1063/5.0135866?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The axisymmetric steady flow of a couple stress fluid between two concentric cylinders with a slip effect is investigated with the help of the cell model technique. Here, the inner cylinder is rigid, and the outer cylinder is fictitious. The tangential slip, vanishing of normal velocity, and zero couple stress conditions are applied on the inner cylindrical surface. In addition, zero shear stress (Happel's model), continuity of normal velocity component, and zero couple stress conditions are used on the outer cylindrical surface. We consider two flow problems: the first is the parallel flow, and the second is the perpendicular flow to the cylinder in the cell model. Also, we have discussed the random case. For all the cases, the Kozeny constant is calculated. We described some special cases and compared them with wellknown results. The effects of slip and couple stress parameters on the Kozeny constant with fixed value of couple stress viscosity parameter are presented graphically. The influence of the couple stress viscosity parameter on the Kozeny constant with fixed values of couple stress, and slip parameters for parallel flow are expressed graphically. The numerical values for the Kozeny constant for different values of fractional void volume are tabulated. We also obtained the results of the consistent couple stress theory as a special case.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The axisymmetric steady flow of a couple stress fluid between two concentric cylinders with a slip effect is investigated with the help of the cell model technique. Here, the inner cylinder is rigid, and the outer cylinder is fictitious. The tangential slip, vanishing of normal velocity, and zero couple stress conditions are applied on the inner cylindrical surface. In addition, zero shear stress (Happel's model), continuity of normal velocity component, and zero couple stress conditions are used on the outer cylindrical surface. We consider two flow problems: the first is the parallel flow, and the second is the perpendicular flow to the cylinder in the cell model. Also, we have discussed the random case. For all the cases, the Kozeny constant is calculated. We described some special cases and compared them with wellknown results. The effects of slip and couple stress parameters on the Kozeny constant with fixed value of couple stress viscosity parameter are presented graphically. The influence of the couple stress viscosity parameter on the Kozeny constant with fixed values of couple stress, and slip parameters for parallel flow are expressed graphically. The numerical values for the Kozeny constant for different values of fractional void volume are tabulated. We also obtained the results of the consistent couple stress theory as a special case.
Parallel and perpendicular flows of a couple stress fluid past a solid cylinder in cell model: Slip condition
10.1063/5.0135866
Physics of Fluids
20230301T01:51:28Z
© 2023 Author(s).
Priya Sarkar
Krishna Prasad Madasu

Underwater oscillations of rigid plates with Hshaped cross sections: An experimental study to explore their flow physics
https://aip.scitation.org/doi/10.1063/5.0141889?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this work, we present a comprehensive experimental study on the problem of harmonic oscillations of rigid plates with Hshaped cross sections submerged in a quiescent, Newtonian, incompressible, viscous fluid environment. Motivated by recent results on the minimization of hydrodynamic damping for transversely oscillating flat plates, we conduct a detailed qualitative and quantitative experimental investigation of the flow physics created by the presence of the flanges, that is, the vertical segments in the plate cross section. Specifically, the main goal is to elucidate the effect of flange size on various aspects of fluid–structure interaction, by primarily investigating the dynamics of vortex shedding and convection. We perform particle image velocimetry experiments over a broad range of oscillation amplitudes, frequencies, and flange sizetowidth ratios by leveraging the identification of pathlines, vortex shedding and dynamics, distinctive hydrodynamic regimes, and steady streaming. The fundamental contributions of this work include novel hydrodynamic regime phase diagrams demonstrating the effect of flange ratio on regime transitions, and in the investigation of their relation to qualitatively distinct patterns of vortex–vortex and vortex–structure interactions. Finally, we discuss steady streaming, identifying primary, and secondary structures as a function of the governing parameters.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this work, we present a comprehensive experimental study on the problem of harmonic oscillations of rigid plates with Hshaped cross sections submerged in a quiescent, Newtonian, incompressible, viscous fluid environment. Motivated by recent results on the minimization of hydrodynamic damping for transversely oscillating flat plates, we conduct a detailed qualitative and quantitative experimental investigation of the flow physics created by the presence of the flanges, that is, the vertical segments in the plate cross section. Specifically, the main goal is to elucidate the effect of flange size on various aspects of fluid–structure interaction, by primarily investigating the dynamics of vortex shedding and convection. We perform particle image velocimetry experiments over a broad range of oscillation amplitudes, frequencies, and flange sizetowidth ratios by leveraging the identification of pathlines, vortex shedding and dynamics, distinctive hydrodynamic regimes, and steady streaming. The fundamental contributions of this work include novel hydrodynamic regime phase diagrams demonstrating the effect of flange ratio on regime transitions, and in the investigation of their relation to qualitatively distinct patterns of vortex–vortex and vortex–structure interactions. Finally, we discuss steady streaming, identifying primary, and secondary structures as a function of the governing parameters.
Underwater oscillations of rigid plates with Hshaped cross sections: An experimental study to explore their flow physics
10.1063/5.0141889
Physics of Fluids
20230301T01:51:25Z
© 2023 Author(s).
Burak Gulsacan
Matteo Aureli

Multiscale analysis of solute dispersion in nonNewtonian flows in a tube with wall absorption
https://aip.scitation.org/doi/10.1063/5.0130789?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study presents the twodimensional concentration distribution of a solute cloud for nonNewtonian fluid in a tube flow with wall absorption. The nonNewtonian fluid models, such as the Carreau–Yasuda and Carreau fluid models, are helpful in investigating solute dispersion in the bloodstream and have also been effective in understanding hemodynamics. The multiscale method of homogenization is used here to analyze the dispersion of solute through a straight tube for Carreau–Yasuda and Carreau fluids, which represents the shearthinning nature. Most of the previous studies are mainly focused on determining the dispersion coefficient and mean concentration distribution for nonNewtonian fluids. Apart from those in our study, we also derived analytical expressions for the twodimensional concentration distribution for Carreau–Yasuda and Carreau fluids. As the exact peak position of the twodimensional concentration is a concern in reallife applications rather than that of mean concentration, the effects of wall absorption parameter ([math]), the Weissenberg number (We), Yasuda parameter (a), and powerlaw index (n) on solute concentration distribution are discussed. Comparison between the present results and previous results of solute dispersion for nonNewtonian as well as Newtonian fluids are also enclosed in this study. Results reveal that the mean concentration decreases with increasing values of We because of an increase in the dispersion coefficient. Carreau–Yasuda and Carreau fluids act like Newtonian fluid for very small values of We. At the initial stage, the solute concentration exhibits transverse nonuniformity and then becomes uniform over a larger timescale. The effects of nonNewtonian parameters such as We, a, and n on transverse variation are also studied. It is noted that parameters n, We, and a have no significant impacts on the nonuniformity of the transverse concentration variation on both sides of the tube centroid, but that is not the case for the wall absorption parameter. It is observed that wall absorption results in significant transverse concentration nonuniformity across the tube cross section even after large times.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study presents the twodimensional concentration distribution of a solute cloud for nonNewtonian fluid in a tube flow with wall absorption. The nonNewtonian fluid models, such as the Carreau–Yasuda and Carreau fluid models, are helpful in investigating solute dispersion in the bloodstream and have also been effective in understanding hemodynamics. The multiscale method of homogenization is used here to analyze the dispersion of solute through a straight tube for Carreau–Yasuda and Carreau fluids, which represents the shearthinning nature. Most of the previous studies are mainly focused on determining the dispersion coefficient and mean concentration distribution for nonNewtonian fluids. Apart from those in our study, we also derived analytical expressions for the twodimensional concentration distribution for Carreau–Yasuda and Carreau fluids. As the exact peak position of the twodimensional concentration is a concern in reallife applications rather than that of mean concentration, the effects of wall absorption parameter ([math]), the Weissenberg number (We), Yasuda parameter (a), and powerlaw index (n) on solute concentration distribution are discussed. Comparison between the present results and previous results of solute dispersion for nonNewtonian as well as Newtonian fluids are also enclosed in this study. Results reveal that the mean concentration decreases with increasing values of We because of an increase in the dispersion coefficient. Carreau–Yasuda and Carreau fluids act like Newtonian fluid for very small values of We. At the initial stage, the solute concentration exhibits transverse nonuniformity and then becomes uniform over a larger timescale. The effects of nonNewtonian parameters such as We, a, and n on transverse variation are also studied. It is noted that parameters n, We, and a have no significant impacts on the nonuniformity of the transverse concentration variation on both sides of the tube centroid, but that is not the case for the wall absorption parameter. It is observed that wall absorption results in significant transverse concentration nonuniformity across the tube cross section even after large times.
Multiscale analysis of solute dispersion in nonNewtonian flows in a tube with wall absorption
10.1063/5.0130789
Physics of Fluids
20230301T02:18:58Z
© 2023 Author(s).
Aruna A
Swarup Barik

Development of a generalized Richards equation for predicting spontaneous imbibition of highly shearthinning liquids in gas recovery applications
https://aip.scitation.org/doi/10.1063/5.0141564?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A new generalized Richards equation (GRE) valid for highly shearthinning liquids obeying the powerlaw model is developed using the concept of the effective viscosity. The mathematical model developed this way is validated against experimental data reported recently for onedimensional spontaneous imbibition of two pusher liquids by a tight sandstone. The GRE model was then used for evaluating the applicability of shearthinning liquids for enhanced gas recovery. For a homogenous tight sandstone, it is shown that shearthinning can dramatically shorten the time needed for the gas recovery to reach equilibrium. Based on the obtained numerical results, the mass of the gas recovered using spontaneous imbibition is increased if use is made of highly shearthinning liquids. At prolonged times, however, it is predicted that gas recovery might slightly drop below its Newtonian counterpart even for highly shearthinning fluids. The effect was attributed to the fact that, in spontaneous imbibition, the viscosity of powerlaw fluids increases with time and can eventually become larger than its Newtonian counterpart. For a twolayered nonhomogeneous system, numerical results suggest that depending on the microstructure of the two layers, the liquid mass uptake can be smaller than that of the homogenous case. It is predicted that if the liquid is sufficiently shearthinning, gas recovery can reach levels much above the homogeneous case.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A new generalized Richards equation (GRE) valid for highly shearthinning liquids obeying the powerlaw model is developed using the concept of the effective viscosity. The mathematical model developed this way is validated against experimental data reported recently for onedimensional spontaneous imbibition of two pusher liquids by a tight sandstone. The GRE model was then used for evaluating the applicability of shearthinning liquids for enhanced gas recovery. For a homogenous tight sandstone, it is shown that shearthinning can dramatically shorten the time needed for the gas recovery to reach equilibrium. Based on the obtained numerical results, the mass of the gas recovered using spontaneous imbibition is increased if use is made of highly shearthinning liquids. At prolonged times, however, it is predicted that gas recovery might slightly drop below its Newtonian counterpart even for highly shearthinning fluids. The effect was attributed to the fact that, in spontaneous imbibition, the viscosity of powerlaw fluids increases with time and can eventually become larger than its Newtonian counterpart. For a twolayered nonhomogeneous system, numerical results suggest that depending on the microstructure of the two layers, the liquid mass uptake can be smaller than that of the homogenous case. It is predicted that if the liquid is sufficiently shearthinning, gas recovery can reach levels much above the homogeneous case.
Development of a generalized Richards equation for predicting spontaneous imbibition of highly shearthinning liquids in gas recovery applications
10.1063/5.0141564
Physics of Fluids
20230302T02:38:39Z
© 2023 Author(s).
H. Asadi
M. PourjafarChelikdani
S. M. Taghavi
K. Sadeghy

Binary coalescence of nonNewtonian droplets under an electric field: A numerical study
https://aip.scitation.org/doi/10.1063/5.0136588?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We numerically investigate the effect of electrohydrodynamics on a nonNewtonian droplet pair suspended in a Newtonian medium. The leaky dielectric model is implemented to study the response of emulsion drops in an externally applied electric field. Subsequently, the nonNewtonian drop behavior is incorporated using the power law model, whereby three different fluid behaviors are considered for the drops: Newtonian, Shear thinning, and Shear thickening. We validated our numerical model with the available literature data, and the results are in good agreement. The droplets' deformation and net motion are investigated for a range of electrical permittivity ratios of the droplet medium with respect to the surrounding fluid. In this study, four distinct regimes are identified based on the net drop pair motion and the circulation pattern that develops due to the electric stresses inside and around the drops. Furthermore, it is observed that the droplet deformation and their net motion are fastest for the pseudoplastic drops and slowest for dilatant drops. We devised a simple ratiobased model to understand this behavior. The inferences drawn from this study will help contribute to a better understanding of the behavior of nonlinear fluids under an electric field.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We numerically investigate the effect of electrohydrodynamics on a nonNewtonian droplet pair suspended in a Newtonian medium. The leaky dielectric model is implemented to study the response of emulsion drops in an externally applied electric field. Subsequently, the nonNewtonian drop behavior is incorporated using the power law model, whereby three different fluid behaviors are considered for the drops: Newtonian, Shear thinning, and Shear thickening. We validated our numerical model with the available literature data, and the results are in good agreement. The droplets' deformation and net motion are investigated for a range of electrical permittivity ratios of the droplet medium with respect to the surrounding fluid. In this study, four distinct regimes are identified based on the net drop pair motion and the circulation pattern that develops due to the electric stresses inside and around the drops. Furthermore, it is observed that the droplet deformation and their net motion are fastest for the pseudoplastic drops and slowest for dilatant drops. We devised a simple ratiobased model to understand this behavior. The inferences drawn from this study will help contribute to a better understanding of the behavior of nonlinear fluids under an electric field.
Binary coalescence of nonNewtonian droplets under an electric field: A numerical study
10.1063/5.0136588
Physics of Fluids
20230303T12:58:00Z
© 2023 Author(s).
Joy Mandal
Deep Chatterjee
Sandip Sarkar

A nuclear magnetic resonance proxy model for predicting movable fluid of rocks based on adaptive ensemble learning
https://aip.scitation.org/doi/10.1063/5.0140372?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The movable fluid percentage and movable fluid porosity of rocks are important parameters for evaluating the development potential of petroleum reservoirs, which are usually determined by expensive and timeconsuming lowfield nuclear magnetic resonance (NMR) experiments combined with centrifugation. In this study, an NMR proxy model based on adaptive ensemble learning was proposed to predict the rock movable fluid indexes efficiently and economically. We established adaptive ensemble learning via an opposite political optimizer (AELOPO), which adaptively combines 33 base learners through political optimization to increase the prediction accuracy of the NMR proxy model. To improve the generalization ability of the AELOPO, oppositionbased learning was introduced to improve the global search speed and stability of the political optimizer. Accessible petrophysical parameters, such as rock density, porosity, permeability, average throat radius, and maximum throat radius, were used as a training set, a validation set, and a test set. The prediction results show that our new strategy outperforms the other 33 base learners, with R2 (coefficient of determination) values of 84.64% in movable fluid percentage and 74.09% in movable fluid porosity.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The movable fluid percentage and movable fluid porosity of rocks are important parameters for evaluating the development potential of petroleum reservoirs, which are usually determined by expensive and timeconsuming lowfield nuclear magnetic resonance (NMR) experiments combined with centrifugation. In this study, an NMR proxy model based on adaptive ensemble learning was proposed to predict the rock movable fluid indexes efficiently and economically. We established adaptive ensemble learning via an opposite political optimizer (AELOPO), which adaptively combines 33 base learners through political optimization to increase the prediction accuracy of the NMR proxy model. To improve the generalization ability of the AELOPO, oppositionbased learning was introduced to improve the global search speed and stability of the political optimizer. Accessible petrophysical parameters, such as rock density, porosity, permeability, average throat radius, and maximum throat radius, were used as a training set, a validation set, and a test set. The prediction results show that our new strategy outperforms the other 33 base learners, with R2 (coefficient of determination) values of 84.64% in movable fluid percentage and 74.09% in movable fluid porosity.
A nuclear magnetic resonance proxy model for predicting movable fluid of rocks based on adaptive ensemble learning
10.1063/5.0140372
Physics of Fluids
20230307T11:18:49Z
© 2023 Author(s).

Microflow investigation on laying process in Al2O3 stereolithography forming
https://aip.scitation.org/doi/10.1063/5.0141852?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>When printing Al2O3 parts by stereolithography technology, the laying process is an extremely important part. In the current work, the referred flow analysis was numerically investigated. The rheological behavior was measured to determine the rheological type of the slurry. According to the fitting analysis, a Sisko model was available to describe the nonNewtonian behavior. Then, the modified multiple relaxation time lattice Boltzmann method was proposed and validated to effectively improve the stability of the simulation. Based on the proposed method, the situations without and with printed solids in the previous layer were investigated by a series of simulations. The laying velocity and layer thickness were considered as two important factors on the laying process. When the situation without printed solids in the previous layer is analyzed, the streamlines and flow velocities curves were almost horizontal. With different laying velocities, the flow velocities show obvious differences at the same thickness. With different layer thicknesses, the difference is mainly embodied in the vertical velocity component. When the printed solid is considered, the solid seriously affected the smooth flow. The vortices appeared near the printed solid, which also caused the disturbance in both horizontal and vertical velocity components. The mentioned interfering factors indicated different actions on the flow. The research will contribute to understanding the flow of the laying process. It can help to select suitable laying velocity and layer thickness to avoid severe flow velocity fluctuation and redundant vertical velocity components.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>When printing Al2O3 parts by stereolithography technology, the laying process is an extremely important part. In the current work, the referred flow analysis was numerically investigated. The rheological behavior was measured to determine the rheological type of the slurry. According to the fitting analysis, a Sisko model was available to describe the nonNewtonian behavior. Then, the modified multiple relaxation time lattice Boltzmann method was proposed and validated to effectively improve the stability of the simulation. Based on the proposed method, the situations without and with printed solids in the previous layer were investigated by a series of simulations. The laying velocity and layer thickness were considered as two important factors on the laying process. When the situation without printed solids in the previous layer is analyzed, the streamlines and flow velocities curves were almost horizontal. With different laying velocities, the flow velocities show obvious differences at the same thickness. With different layer thicknesses, the difference is mainly embodied in the vertical velocity component. When the printed solid is considered, the solid seriously affected the smooth flow. The vortices appeared near the printed solid, which also caused the disturbance in both horizontal and vertical velocity components. The mentioned interfering factors indicated different actions on the flow. The research will contribute to understanding the flow of the laying process. It can help to select suitable laying velocity and layer thickness to avoid severe flow velocity fluctuation and redundant vertical velocity components.
Microflow investigation on laying process in Al2O3 stereolithography forming
10.1063/5.0141852
Physics of Fluids
20230309T12:28:18Z
© 2023 Author(s).
Weiwei Wu
Xu Deng
Shuang Ding
Yanjun Zhang
Bing Tang
Binquan Shi

Numerical study of viscoelastic flow around an oscillating circular cylinder
https://aip.scitation.org/doi/10.1063/5.0141254?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The viscoelasticityinduced fluid–structure interaction studies have a significant influence on practical applications. To clarify the lockin phenomenon and the wake topology of the vibrating cylinder placed in the viscoelastic flow, the OldroydB fluid flows around an oscillating circular cylinder have been numerically investigated at Re = 10 and Re = 60, respectively. The governing equations are solved by the coupling of the squarerootconformation representation approach and the discontinuous Galerkin method in framework of the highorder dual splitting scheme. In addition, the arbitrary Lagrangian–Eulerian formulation is implemented in the coupling procedure in order to account for the interaction between the fluid and the oscillating body in the flow field. With this, complex boundary movements can be tackled simply and efficiently. In numerical simulation, the force coefficients and the wake structures of vortex and stress are discussed in some detail. At Re = 10, when the frequency of cylinder is small, it is obvious that the vortex shedding takes place in the wake. As the frequency increases, almost no obvious vortex shedding is observed. Also, the wake still oscillates at the same frequency of the cylinder for all cases, even for high Wi numbers. However, different wake modes of vortex and stress are found for various frequencies at Re = 60 and Wi = 0.1. In the lockin region, the 2S mode of wake type are observed. Beyond the lockin region, the wake type is no longer 2S, but the formation of vortex shedding and stress distribution in the far wake recovers to its natural mode. These numerical results open up a new field of study for viscoelastic fluids.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The viscoelasticityinduced fluid–structure interaction studies have a significant influence on practical applications. To clarify the lockin phenomenon and the wake topology of the vibrating cylinder placed in the viscoelastic flow, the OldroydB fluid flows around an oscillating circular cylinder have been numerically investigated at Re = 10 and Re = 60, respectively. The governing equations are solved by the coupling of the squarerootconformation representation approach and the discontinuous Galerkin method in framework of the highorder dual splitting scheme. In addition, the arbitrary Lagrangian–Eulerian formulation is implemented in the coupling procedure in order to account for the interaction between the fluid and the oscillating body in the flow field. With this, complex boundary movements can be tackled simply and efficiently. In numerical simulation, the force coefficients and the wake structures of vortex and stress are discussed in some detail. At Re = 10, when the frequency of cylinder is small, it is obvious that the vortex shedding takes place in the wake. As the frequency increases, almost no obvious vortex shedding is observed. Also, the wake still oscillates at the same frequency of the cylinder for all cases, even for high Wi numbers. However, different wake modes of vortex and stress are found for various frequencies at Re = 60 and Wi = 0.1. In the lockin region, the 2S mode of wake type are observed. Beyond the lockin region, the wake type is no longer 2S, but the formation of vortex shedding and stress distribution in the far wake recovers to its natural mode. These numerical results open up a new field of study for viscoelastic fluids.
Numerical study of viscoelastic flow around an oscillating circular cylinder
10.1063/5.0141254
Physics of Fluids
20230310T12:19:58Z
© 2023 Author(s).

Hirota bilinear method and multisoliton interaction of electrostatic waves driven by cubic nonlinearity in pairion–electron plasmas
https://aip.scitation.org/doi/10.1063/5.0142447?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Multisoliton interaction of nonlinear ion sound waves in a pairion–electron (PIE) plasma having nonMaxwellian electrons including Kappa, Cairns, and generalized two spectral index distribution functions is studied. To this end, a modified Korteweg–de Vries (mKdV) equation is obtained to investigate the ionacoustic waves in a PIE plasma at a critical plasma composition. The effects of temperature and density ratios and the nonMaxwellian electron velocity distributions on the overtaking interaction of solitons are explored in detail. The results reveal that both hump (positive peak) and dip (negative peak) solitons can propagate for the physical model under consideration. Two and threesoliton interactions are presented, and the novel features of interacting compressive and rarefactive solitons are highlighted. The present investigation may be useful in laboratory plasmas where PIE plasmas have been reported.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Multisoliton interaction of nonlinear ion sound waves in a pairion–electron (PIE) plasma having nonMaxwellian electrons including Kappa, Cairns, and generalized two spectral index distribution functions is studied. To this end, a modified Korteweg–de Vries (mKdV) equation is obtained to investigate the ionacoustic waves in a PIE plasma at a critical plasma composition. The effects of temperature and density ratios and the nonMaxwellian electron velocity distributions on the overtaking interaction of solitons are explored in detail. The results reveal that both hump (positive peak) and dip (negative peak) solitons can propagate for the physical model under consideration. Two and threesoliton interactions are presented, and the novel features of interacting compressive and rarefactive solitons are highlighted. The present investigation may be useful in laboratory plasmas where PIE plasmas have been reported.
Hirota bilinear method and multisoliton interaction of electrostatic waves driven by cubic nonlinearity in pairion–electron plasmas
10.1063/5.0142447
Physics of Fluids
20230313T11:26:08Z
© 2023 Author(s).
Nazia Batool
W. Masood
M. Siddiq
Albandari W. Alrowaily
Sherif M. E. Ismaeel
S. A. ElTantawy

Modified pressure and vorticity variables using Helmholtz decomposition for solution of the incompressible flow equations
https://aip.scitation.org/doi/10.1063/5.0139754?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Incompressible Navier–Stokes equations are reformulated using the Helmholtz decomposition of a velocity vector into rotational and potential components. By substituting the decomposed velocity in the time derivative term in a momentum equation, the potential component representing a gradient of a pressurelike term is combined with the gradient of the pressure modifying the physical pressure field. The rotational component representing a curl of a vortexlike vector is combined with the vorticity vector, making a nonphysical vorticity vector that modifies the fluid viscosity. Thus, the unsteady Navier–Stokes equation is transformed into an explicitly steadystate form in terms of nonprimitive variables. The stream function vector is governed by a parabolic equation in time, while the vorticity vector is governed by the Poisson function with a source term function of the convection stretching and time dependency of the physical flow vorticity. Therefore, the resulting system of equations is numerically independent of the cell Reynolds number stability condition that hunted the convection–diffusion Navier–Stokes equation. Numerical results are obtained for the twodimensional driven cavity problem for Reynolds number of 1000 with 21 [math] 21, 41 [math] 41, 81 [math] 81, and 161 [math] 161 grid points. The computational grids correspond to cell Reynolds numbers 25, 12.5, 6.25, and 3.125, respectively. The computed results are smooth in all cases and validate the method.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Incompressible Navier–Stokes equations are reformulated using the Helmholtz decomposition of a velocity vector into rotational and potential components. By substituting the decomposed velocity in the time derivative term in a momentum equation, the potential component representing a gradient of a pressurelike term is combined with the gradient of the pressure modifying the physical pressure field. The rotational component representing a curl of a vortexlike vector is combined with the vorticity vector, making a nonphysical vorticity vector that modifies the fluid viscosity. Thus, the unsteady Navier–Stokes equation is transformed into an explicitly steadystate form in terms of nonprimitive variables. The stream function vector is governed by a parabolic equation in time, while the vorticity vector is governed by the Poisson function with a source term function of the convection stretching and time dependency of the physical flow vorticity. Therefore, the resulting system of equations is numerically independent of the cell Reynolds number stability condition that hunted the convection–diffusion Navier–Stokes equation. Numerical results are obtained for the twodimensional driven cavity problem for Reynolds number of 1000 with 21 [math] 21, 41 [math] 41, 81 [math] 81, and 161 [math] 161 grid points. The computational grids correspond to cell Reynolds numbers 25, 12.5, 6.25, and 3.125, respectively. The computed results are smooth in all cases and validate the method.
Modified pressure and vorticity variables using Helmholtz decomposition for solution of the incompressible flow equations
10.1063/5.0139754
Physics of Fluids
20230313T11:26:13Z
© 2023 Author(s).
Kaurab Gautam
Shaaban Abdallah

Viscous heating and instability of the adiabatic buoyant flows in a horizontal channel
https://aip.scitation.org/doi/10.1063/5.0144878?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The stability of buoyant flows occurring in the mixed convection regime for a viscous fluid in a horizontal planeparallel channel with adiabatic walls is investigated. The basic flow features a parallel velocity field under stationary state conditions. There exists a duality of flows, for every prescribed value of the mass flow rate across the channel crosssection, caused by the combined actions of viscous dissipation and the buoyancy force. As pointed out in a previous study, only the primary branch of the dual solutions is compatible with the Oberbeck–Boussinesq approximation. Thus, the stability analysis will be focused on the stability of such flows. The onset of the thermal instability with smallamplitude perturbations of the basic flow is investigated by assuming a very large Prandtl number, which is equivalent to a creeping flow regime. The neutral stability curves and the critical parametric conditions for the onset of instability are determined numerically.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The stability of buoyant flows occurring in the mixed convection regime for a viscous fluid in a horizontal planeparallel channel with adiabatic walls is investigated. The basic flow features a parallel velocity field under stationary state conditions. There exists a duality of flows, for every prescribed value of the mass flow rate across the channel crosssection, caused by the combined actions of viscous dissipation and the buoyancy force. As pointed out in a previous study, only the primary branch of the dual solutions is compatible with the Oberbeck–Boussinesq approximation. Thus, the stability analysis will be focused on the stability of such flows. The onset of the thermal instability with smallamplitude perturbations of the basic flow is investigated by assuming a very large Prandtl number, which is equivalent to a creeping flow regime. The neutral stability curves and the critical parametric conditions for the onset of instability are determined numerically.
Viscous heating and instability of the adiabatic buoyant flows in a horizontal channel
10.1063/5.0144878
Physics of Fluids
20230314T10:16:34Z
© 2023 Author(s).
A. Barletta
M. Celli
D. A. S. Rees

Stabilization of the flat Poiseuilletype flow for viscoelastic polymeric liquid
https://aip.scitation.org/doi/10.1063/5.0140477?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This paper presents a numerical study for the problem of the onedimensional flow of viscoelastic liquid polymers between two parallel plates. The equations of a rheologically modified Vinogradov–Pokrovskii (mVP) model is used for the formulation of the problem. It is shown that the problem could have multiple steadystate solutions. The evaluation of nonsteady solutions was performed to see if the timedependent solutions got eventually attracted by the steady ones. Also for the case of multiple steady solutions, it was checked which one attracts the nonsteady solution if any. The evaluation of timedependent solutions was used to estimate the stability of equilibrium states. It is revealed that stable steadystate regimes of the problem exist under certain conditions, and also there could be no more than one stable regime for any given set of parameters. The calculations were performed to estimate the values of Reynolds and Weissenberg numbers corresponding to either stable or unstable steady regimes. The result indicates that instability of the steady flow could possibly occur for arbitrary low Reynolds numbers under certain balance of viscous and elastic forces.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This paper presents a numerical study for the problem of the onedimensional flow of viscoelastic liquid polymers between two parallel plates. The equations of a rheologically modified Vinogradov–Pokrovskii (mVP) model is used for the formulation of the problem. It is shown that the problem could have multiple steadystate solutions. The evaluation of nonsteady solutions was performed to see if the timedependent solutions got eventually attracted by the steady ones. Also for the case of multiple steady solutions, it was checked which one attracts the nonsteady solution if any. The evaluation of timedependent solutions was used to estimate the stability of equilibrium states. It is revealed that stable steadystate regimes of the problem exist under certain conditions, and also there could be no more than one stable regime for any given set of parameters. The calculations were performed to estimate the values of Reynolds and Weissenberg numbers corresponding to either stable or unstable steady regimes. The result indicates that instability of the steady flow could possibly occur for arbitrary low Reynolds numbers under certain balance of viscous and elastic forces.
Stabilization of the flat Poiseuilletype flow for viscoelastic polymeric liquid
10.1063/5.0140477
Physics of Fluids
20230316T01:43:56Z
© 2023 Author(s).
Roman Semenko

Combined internal and external natural convection of Bingham plastics in a cavity using a lattice Boltzmann method
https://aip.scitation.org/doi/10.1063/5.0142490?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Natural convection of Bingham plastics in a cavity with differentially heated walls and an internal heat source is investigated numerically. The governing dimensional and nondimensional macroscopic equations are presented, and the constitutive equation is written based on an exact Bingham model. The implemented lattice Boltzmann method is explained and showed how to derive the presented governing equations. The code is validated and verified against previous studies and exhibited a good agreement. The results are demonstrated and discussed for various nondimensional parameters of Rayleigh (R = 102–104), Rayleigh–Roberts (RR = 102–106), Prandtl (Pr = 0.1–100), Bingham (Bn), and Yield (Y) numbers. The effects of the parameters are depicted on isotherms, yielded/unyielded zones, streamlines, and the lines of temperatures and velocities in the middle of the cavity. The maximum (or critical) Yield number ([math]) is found in the studied parameters and reported. The Yield number is independent of the Rayleigh and Prandtl numbers in a fixed ratio of R and RR (Δ = RR/R), like the external and internal convection. However, the alteration of Δ changes the unique value of the Yield number. We considered the three ratios of Δ = 1, 10, and 100 and the single maximum Yield number of the ratios for zero inclined angles ([math]) were observed at [math] and 0.38, respectively. The increase in the inclined angle counterclockwise expands the unyielded zones and declines the maximum Yield number.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Natural convection of Bingham plastics in a cavity with differentially heated walls and an internal heat source is investigated numerically. The governing dimensional and nondimensional macroscopic equations are presented, and the constitutive equation is written based on an exact Bingham model. The implemented lattice Boltzmann method is explained and showed how to derive the presented governing equations. The code is validated and verified against previous studies and exhibited a good agreement. The results are demonstrated and discussed for various nondimensional parameters of Rayleigh (R = 102–104), Rayleigh–Roberts (RR = 102–106), Prandtl (Pr = 0.1–100), Bingham (Bn), and Yield (Y) numbers. The effects of the parameters are depicted on isotherms, yielded/unyielded zones, streamlines, and the lines of temperatures and velocities in the middle of the cavity. The maximum (or critical) Yield number ([math]) is found in the studied parameters and reported. The Yield number is independent of the Rayleigh and Prandtl numbers in a fixed ratio of R and RR (Δ = RR/R), like the external and internal convection. However, the alteration of Δ changes the unique value of the Yield number. We considered the three ratios of Δ = 1, 10, and 100 and the single maximum Yield number of the ratios for zero inclined angles ([math]) were observed at [math] and 0.38, respectively. The increase in the inclined angle counterclockwise expands the unyielded zones and declines the maximum Yield number.
Combined internal and external natural convection of Bingham plastics in a cavity using a lattice Boltzmann method
10.1063/5.0142490
Physics of Fluids
20230316T02:04:39Z
© 2023 Author(s).
Gholamreza Kefayati

Effect of cavity aspect ratio on mixed convective heat transfer phenomenon inside a liddriven cavity due to elastic turbulence
https://aip.scitation.org/doi/10.1063/5.0143472?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study performs extensive numerical simulations to investigate how the aspect ratio (AR) of a liddriven cavity influences the onset of elastic instability and elastic turbulence and the subsequent mixed convective heat transfer rate inside it. To this end, we utilize the finite volume method based open source code OpenFOAM along with Rheotool to solve the mass, momentum, energy, and viscoelastic constitutive equations. We find that the dependency of the cavity AR on the heat transfer rate is highly complicated depending upon the values of the Richardson (Ri) and Prandtl numbers (Pr). At low values of Ri, the heat transfer rate continuously decreases with AR irrespective of the value of the Prandtl number and the fluid type, i.e., Newtonian or viscoelastic. The same trend is also observed at high values of Ri and low values of Pr. At these combinations of Ri and Pr, the heat transfer rate is always higher in viscoelastic fluids than in Newtonian fluids due to the presence of elastic turbulence in the former fluids. However, a different trend is observed at high values of both Ri and Pr. At this combination of Ri and Pr, the heat transfer rate increases with AR in Newtonian fluids, whereas it decreases in viscoelastic fluids. Therefore, at high values of AR, Ri, and Pr, the heat transfer rate is higher in Newtonian fluids than that in viscoelastic fluids despite the presence of elastic turbulence in the latter fluids. This is in contrast to the assumption that the elastic turbulence phenomenon always increases the rate of transport processes. A possible explanation for this behavior is provided in this study. Along with the heat transfer aspects, we also provide a detailed discussion on how the cavity aspect ratio influences the corresponding flow dynamics inside the cavity. In particular, we find that the onset of the elastic instability (and the subsequent elastic turbulence) phenomenon is delayed to higher values of the Weissenberg number as the cavity aspect ratio increases. This is in line with prior experimental studies reported in the literature.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study performs extensive numerical simulations to investigate how the aspect ratio (AR) of a liddriven cavity influences the onset of elastic instability and elastic turbulence and the subsequent mixed convective heat transfer rate inside it. To this end, we utilize the finite volume method based open source code OpenFOAM along with Rheotool to solve the mass, momentum, energy, and viscoelastic constitutive equations. We find that the dependency of the cavity AR on the heat transfer rate is highly complicated depending upon the values of the Richardson (Ri) and Prandtl numbers (Pr). At low values of Ri, the heat transfer rate continuously decreases with AR irrespective of the value of the Prandtl number and the fluid type, i.e., Newtonian or viscoelastic. The same trend is also observed at high values of Ri and low values of Pr. At these combinations of Ri and Pr, the heat transfer rate is always higher in viscoelastic fluids than in Newtonian fluids due to the presence of elastic turbulence in the former fluids. However, a different trend is observed at high values of both Ri and Pr. At this combination of Ri and Pr, the heat transfer rate increases with AR in Newtonian fluids, whereas it decreases in viscoelastic fluids. Therefore, at high values of AR, Ri, and Pr, the heat transfer rate is higher in Newtonian fluids than that in viscoelastic fluids despite the presence of elastic turbulence in the latter fluids. This is in contrast to the assumption that the elastic turbulence phenomenon always increases the rate of transport processes. A possible explanation for this behavior is provided in this study. Along with the heat transfer aspects, we also provide a detailed discussion on how the cavity aspect ratio influences the corresponding flow dynamics inside the cavity. In particular, we find that the onset of the elastic instability (and the subsequent elastic turbulence) phenomenon is delayed to higher values of the Weissenberg number as the cavity aspect ratio increases. This is in line with prior experimental studies reported in the literature.
Effect of cavity aspect ratio on mixed convective heat transfer phenomenon inside a liddriven cavity due to elastic turbulence
10.1063/5.0143472
Physics of Fluids
20230317T02:43:25Z
© 2023 Author(s).
S. Gupta
C. Sasmal

“Absolute zero” temperature in a vertically vibrated granular system
https://aip.scitation.org/doi/10.1063/5.0140106?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In thermodynamics, absolute zero is the coldest temperature and will never be reached because the thermal motion of microscopic particles never ceases. However, this situation could be changed for a collection of macroscopic particles. In the present paper, an experimental study was performed to explore the possible lowest temperature in a vertically vibrated granular system. It was found that the granular “absolute zero” temperature appears when the vibrating intensity is adjusted to about 4.6 times the gravitational acceleration. At this temperature, the macroscopic particles are arranged closely and behave like a rigid body without relative motions during the vibration. Near the absolute zero, inelastic collisions and energy transfer are responsible for the variation of granular temperature with time and vibrating parameters. Interestingly, the temperature variation reveals that the vibrated macroscopic particles are neither a crystal nor an amorphous system. This study introduces the granular entropy, including entropy generation and entropy flow, to describe the order of the vibrated particles. The entropy change could be illustrated by the temperature profiles of the granular system and its outside. It was also found that, unlike microscopic particles, which could maintain their entropy at a constant temperature, a granular system necessarily behaves completely disordered unless the system achieves the absolute zero granular temperature.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In thermodynamics, absolute zero is the coldest temperature and will never be reached because the thermal motion of microscopic particles never ceases. However, this situation could be changed for a collection of macroscopic particles. In the present paper, an experimental study was performed to explore the possible lowest temperature in a vertically vibrated granular system. It was found that the granular “absolute zero” temperature appears when the vibrating intensity is adjusted to about 4.6 times the gravitational acceleration. At this temperature, the macroscopic particles are arranged closely and behave like a rigid body without relative motions during the vibration. Near the absolute zero, inelastic collisions and energy transfer are responsible for the variation of granular temperature with time and vibrating parameters. Interestingly, the temperature variation reveals that the vibrated macroscopic particles are neither a crystal nor an amorphous system. This study introduces the granular entropy, including entropy generation and entropy flow, to describe the order of the vibrated particles. The entropy change could be illustrated by the temperature profiles of the granular system and its outside. It was also found that, unlike microscopic particles, which could maintain their entropy at a constant temperature, a granular system necessarily behaves completely disordered unless the system achieves the absolute zero granular temperature.
“Absolute zero” temperature in a vertically vibrated granular system
10.1063/5.0140106
Physics of Fluids
20230301T01:51:33Z
© 2023 Author(s).

Effect of turbulence modulation caused by thread structure on coaxial airblast atomization
https://aip.scitation.org/doi/10.1063/5.0134754?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study aims at investigating the influence of turbulence modulation caused by a thread structure on coaxial airblast atomization by means of highspeed flow visualizations and droplet particle size techniques. The medium in the central channel of an atomizer is water while the annular channel is airflow. The results show that the thread structure added to the inner surface of an annular channel plays an important role in atomization effect. The generated liquid ligaments on the jet present more structures, which grow shorter and breakup faster than that without thread. To compare the difference in jet breakup length and droplet diameter caused by the thread structure, we establish a new breakup length model and then use the ratio of structure factors to describe the change in the droplet diameter. The results in this experiment confirm the improvement of atomization performance brought by optimization design of the thread structure.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study aims at investigating the influence of turbulence modulation caused by a thread structure on coaxial airblast atomization by means of highspeed flow visualizations and droplet particle size techniques. The medium in the central channel of an atomizer is water while the annular channel is airflow. The results show that the thread structure added to the inner surface of an annular channel plays an important role in atomization effect. The generated liquid ligaments on the jet present more structures, which grow shorter and breakup faster than that without thread. To compare the difference in jet breakup length and droplet diameter caused by the thread structure, we establish a new breakup length model and then use the ratio of structure factors to describe the change in the droplet diameter. The results in this experiment confirm the improvement of atomization performance brought by optimization design of the thread structure.
Effect of turbulence modulation caused by thread structure on coaxial airblast atomization
10.1063/5.0134754
Physics of Fluids
20230301T02:38:05Z
© 2023 Author(s).

Dissimilar cavitation dynamics and damage patterns produced by parallel fiber alignment to the stone surface in holmium:yttrium aluminum garnet laser lithotripsy
https://aip.scitation.org/doi/10.1063/5.0139741?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Recent studies indicate that cavitation may play a vital role in laser lithotripsy. However, the underlying bubble dynamics and associated damage mechanisms are largely unknown. In this study, we use ultrahighspeed shadowgraph imaging, hydrophone measurements, threedimensional passive cavitation mapping (3DPCM), and phantom test to investigate the transient dynamics of vapor bubbles induced by a holmium:yttrium aluminum garnet laser and their correlation with solid damage. We vary the standoff distance (SD) between the fiber tip and solid boundary under parallel fiber alignment and observe several distinctive features in bubble dynamics. First, long pulsed laser irradiation and solid boundary interaction create an elongated “pearshaped” bubble that collapses asymmetrically and forms multiple jets in sequence. Second, unlike nanosecond laserinduced cavitation bubbles, jet impact on solid boundary generates negligible pressure transients and causes no direct damage. A noncircular toroidal bubble forms, particularly following the primary and secondary bubble collapses at SD = 1.0 and 3.0 mm, respectively. We observe three intensified bubble collapses with strong shock wave emissions: the intensified bubble collapse by shock wave, the ensuing reflected shock wave from the solid boundary, and selfintensified collapse of an inverted “triangleshaped” or “horseshoeshaped” bubble. Third, highspeed shadowgraph imaging and 3DPCM confirm that the shock origins from the distinctive bubble collapse form either two discrete spots or a “smilingface” shape. The spatial collapse pattern is consistent with the similar BegoStone surface damage, suggesting that the shockwave emissions during the intensified asymmetric collapse of the pearshaped bubble are decisive for the solid damage.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Recent studies indicate that cavitation may play a vital role in laser lithotripsy. However, the underlying bubble dynamics and associated damage mechanisms are largely unknown. In this study, we use ultrahighspeed shadowgraph imaging, hydrophone measurements, threedimensional passive cavitation mapping (3DPCM), and phantom test to investigate the transient dynamics of vapor bubbles induced by a holmium:yttrium aluminum garnet laser and their correlation with solid damage. We vary the standoff distance (SD) between the fiber tip and solid boundary under parallel fiber alignment and observe several distinctive features in bubble dynamics. First, long pulsed laser irradiation and solid boundary interaction create an elongated “pearshaped” bubble that collapses asymmetrically and forms multiple jets in sequence. Second, unlike nanosecond laserinduced cavitation bubbles, jet impact on solid boundary generates negligible pressure transients and causes no direct damage. A noncircular toroidal bubble forms, particularly following the primary and secondary bubble collapses at SD = 1.0 and 3.0 mm, respectively. We observe three intensified bubble collapses with strong shock wave emissions: the intensified bubble collapse by shock wave, the ensuing reflected shock wave from the solid boundary, and selfintensified collapse of an inverted “triangleshaped” or “horseshoeshaped” bubble. Third, highspeed shadowgraph imaging and 3DPCM confirm that the shock origins from the distinctive bubble collapse form either two discrete spots or a “smilingface” shape. The spatial collapse pattern is consistent with the similar BegoStone surface damage, suggesting that the shockwave emissions during the intensified asymmetric collapse of the pearshaped bubble are decisive for the solid damage.
Dissimilar cavitation dynamics and damage patterns produced by parallel fiber alignment to the stone surface in holmium:yttrium aluminum garnet laser lithotripsy
10.1063/5.0139741
Physics of Fluids
20230302T01:30:04Z
© 2023 Author(s).
Arpit Mishra
Georgy Sankin

Porous media flooding mechanism of nanoparticleenhanced emulsification system
https://aip.scitation.org/doi/10.1063/5.0141815?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study carried out interfacial tension (IFT) testing, sand surface element analysis and scanning electron microscope imaging, rock–oil–emulsification system interaction testing, and microstructure, droplet size distribution, and stability of oil in water (O/W) emulsion to clarify the porous media flooding mechanism of a hydrophilic nanoSiO2 enhanced emulsification system. The results show that by adding a small amount of nanoSiO2 (0.01 wt. %) into an anionic surfactant fatty alcohol polyoxyethylene ether sodium hydroxypropyl sulfonate (AEOSHS) solution (0.5 wt. %), the IFT of oil–water was effectively reduced, the adsorption loss of AEOSHS on the formation sand surface was reduced by more than 70%, and the droplet size of the formed O/W emulsion was reduced by 50%. This greatly improves the effective concentration of AEOSHS and emulsifies the heavy oil ability in the formation away from the injection well. Moreover, the spreading ability of oil on the core surface is greatly reduced, and the width of the diffusion zone is narrowed. Meanwhile, a very clear dividing line of oil can be seen, which shows that the wettability of the core has changed to water wet. The stability of the formed O/W emulsion was further enhanced, and the coalescence and migration process of the droplet is extremely slow. The oil recovery of the AEOSHS + nanoSiO2 system can effectively increase 21.95% of the original oil in place. Both the sandpacked tube experiment and the microscopic visual oil flooding experiment show that the system can not only expand the swept volume but also improve the oil displacement efficiency, which means that the combined system can significantly improve the oil displacement effect.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study carried out interfacial tension (IFT) testing, sand surface element analysis and scanning electron microscope imaging, rock–oil–emulsification system interaction testing, and microstructure, droplet size distribution, and stability of oil in water (O/W) emulsion to clarify the porous media flooding mechanism of a hydrophilic nanoSiO2 enhanced emulsification system. The results show that by adding a small amount of nanoSiO2 (0.01 wt. %) into an anionic surfactant fatty alcohol polyoxyethylene ether sodium hydroxypropyl sulfonate (AEOSHS) solution (0.5 wt. %), the IFT of oil–water was effectively reduced, the adsorption loss of AEOSHS on the formation sand surface was reduced by more than 70%, and the droplet size of the formed O/W emulsion was reduced by 50%. This greatly improves the effective concentration of AEOSHS and emulsifies the heavy oil ability in the formation away from the injection well. Moreover, the spreading ability of oil on the core surface is greatly reduced, and the width of the diffusion zone is narrowed. Meanwhile, a very clear dividing line of oil can be seen, which shows that the wettability of the core has changed to water wet. The stability of the formed O/W emulsion was further enhanced, and the coalescence and migration process of the droplet is extremely slow. The oil recovery of the AEOSHS + nanoSiO2 system can effectively increase 21.95% of the original oil in place. Both the sandpacked tube experiment and the microscopic visual oil flooding experiment show that the system can not only expand the swept volume but also improve the oil displacement efficiency, which means that the combined system can significantly improve the oil displacement effect.
Porous media flooding mechanism of nanoparticleenhanced emulsification system
10.1063/5.0141815
Physics of Fluids
20230303T12:57:46Z
© 2023 Author(s).

Criterion for bubble encapsulation on drop impact onto a liquid film
https://aip.scitation.org/doi/10.1063/5.0138901?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The phenomenon of bubble encapsulation results from droplet impact on a liquid film for specific impact conditions, but there is no established criterion for predicting its onset. Phenomenon visualization from two perspectives, the common lateral perspective and a bottom perspective, provided insights into the dynamics and formation mechanisms. Namely, the bottom shadowgraphs show capillary wavy patterns and perturbations imposed on the steady liquid film, which suggests a greater role of the liquid film in the onset of bubble encapsulation. Also, some considerations about the cavity development underneath the bubble limited by the solid wall allow concluding that the cavity shape is independent of the bubble encapsulation phenomenon. Additionally, using the bottom shadowgraphs, the crown closure time shows a systematic decrease in the dimensionless film thickness of [math], which will be subject of future work. Finally, while most drop impact correlations focus on using the droplets' characteristics and thermophysical properties, the experimental results point in a different direction. Considering correlations relating the Ohnesorge and Reynolds numbers, the new criterion for the onset of bubble encapsulation uses drop characteristics and properties in the Reynolds number, while the liquid film thickness and thermophysical properties are used in the Ohnesorge number because most of the crown material comes from the liquid film. Therefore, the criterion based on 100% occurrence of bubble encapsulation is not a threshold, but a range: [math], with [math]. Other authors observed this phenomenon and despite being outside the validation range of this correlation, the values are close to their boundaries.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The phenomenon of bubble encapsulation results from droplet impact on a liquid film for specific impact conditions, but there is no established criterion for predicting its onset. Phenomenon visualization from two perspectives, the common lateral perspective and a bottom perspective, provided insights into the dynamics and formation mechanisms. Namely, the bottom shadowgraphs show capillary wavy patterns and perturbations imposed on the steady liquid film, which suggests a greater role of the liquid film in the onset of bubble encapsulation. Also, some considerations about the cavity development underneath the bubble limited by the solid wall allow concluding that the cavity shape is independent of the bubble encapsulation phenomenon. Additionally, using the bottom shadowgraphs, the crown closure time shows a systematic decrease in the dimensionless film thickness of [math], which will be subject of future work. Finally, while most drop impact correlations focus on using the droplets' characteristics and thermophysical properties, the experimental results point in a different direction. Considering correlations relating the Ohnesorge and Reynolds numbers, the new criterion for the onset of bubble encapsulation uses drop characteristics and properties in the Reynolds number, while the liquid film thickness and thermophysical properties are used in the Ohnesorge number because most of the crown material comes from the liquid film. Therefore, the criterion based on 100% occurrence of bubble encapsulation is not a threshold, but a range: [math], with [math]. Other authors observed this phenomenon and despite being outside the validation range of this correlation, the values are close to their boundaries.
Criterion for bubble encapsulation on drop impact onto a liquid film
10.1063/5.0138901
Physics of Fluids
20230306T10:56:16Z
© 2023 Author(s).
D. Ribeiro
A. R. R. Silva
M. R. O. Panão

Theoretical adjustment of metalorganic chemical vapor deposition process parameters for highquality gallium nitride epitaxial films
https://aip.scitation.org/doi/10.1063/5.0141060?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Semiconductor thin films for electronic devices are usually produced through processes such as chemical vapor deposition, which requires careful control of the gas flow, heat distribution, and concentration distribution over the substrate to ensure a uniform deposition rate and thickness. Herein, a systematic method is proposed for the theoretical adjustment of metalorganic chemical vapor deposition (MOCVD) process parameters. To this end, a GaNMOCVD reactor with a vertical injection structure was simulated based on computational fluid dynamics to analyze the stable flow under a fixed flow rate. The orthogonal experimental design was used to analyze the influence of process conditions on film quality. A neural network and genetic algorithm were used to optimize the inlet flow under the stable flow state to render the coefficient of variation <3%. Under these conditions, the flow field in the reactor was stabilized to ensure a uniform thickness for the deposited film. This study provides not only an effective solution for highquality epitaxial growth but also a theoretical basis for subsequent experiments and equipment improvement.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Semiconductor thin films for electronic devices are usually produced through processes such as chemical vapor deposition, which requires careful control of the gas flow, heat distribution, and concentration distribution over the substrate to ensure a uniform deposition rate and thickness. Herein, a systematic method is proposed for the theoretical adjustment of metalorganic chemical vapor deposition (MOCVD) process parameters. To this end, a GaNMOCVD reactor with a vertical injection structure was simulated based on computational fluid dynamics to analyze the stable flow under a fixed flow rate. The orthogonal experimental design was used to analyze the influence of process conditions on film quality. A neural network and genetic algorithm were used to optimize the inlet flow under the stable flow state to render the coefficient of variation <3%. Under these conditions, the flow field in the reactor was stabilized to ensure a uniform thickness for the deposited film. This study provides not only an effective solution for highquality epitaxial growth but also a theoretical basis for subsequent experiments and equipment improvement.
Theoretical adjustment of metalorganic chemical vapor deposition process parameters for highquality gallium nitride epitaxial films
10.1063/5.0141060
Physics of Fluids
20230307T11:19:01Z
© 2023 Author(s).

Steady threedimensional patterns in gravitydriven film flow down an inclined sinusoidal bottom contour
https://aip.scitation.org/doi/10.1063/5.0140841?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We experimentally studied gravitydriven film flow in an inclined corrugated channel. Beyond a critical Reynolds number, threedimensional patterns appear. We identified two different types of patterns: a synchronous and a checkerboard one. While the synchronous pattern appears at all inclination angles studied, we observed the checkerboard one only at higher inclination angles and Reynolds numbers. The patterns suppress traveling waves and stabilize the steady flow. We characterize the patterns and their generation and provide a flowregime map.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We experimentally studied gravitydriven film flow in an inclined corrugated channel. Beyond a critical Reynolds number, threedimensional patterns appear. We identified two different types of patterns: a synchronous and a checkerboard one. While the synchronous pattern appears at all inclination angles studied, we observed the checkerboard one only at higher inclination angles and Reynolds numbers. The patterns suppress traveling waves and stabilize the steady flow. We characterize the patterns and their generation and provide a flowregime map.
Steady threedimensional patterns in gravitydriven film flow down an inclined sinusoidal bottom contour
10.1063/5.0140841
Physics of Fluids
20230307T11:19:13Z
© 2023 Author(s).
B. AlShamaa
T. Kahraman
A. Wierschem

Effect of combining multijet component with axial swirl blade on evaporation in a spouted bed
https://aip.scitation.org/doi/10.1063/5.0138735?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>To improve the fluidization behavior and the heat and mass transfer process in a spouted bed, a multijet–axialswirlblade spouted bed (MJASB SB) was developed. The water evaporation process of the MJASB SB was simulated and compared with those of the conventional spouted bed (CSB) and an integral multijet spoutfluidized bed (IMJSFB). The simulation results showed that the MJASB SB combined the staged spouting action of multijet with the swirling action of the axial swirl blade, which promoted particle turbulence in the annulus region and ensured effective particle mixing. The swirl number of the MJASB SB ranged from 0.0816 to 2.7239 with enhanced vortex intensity, thus promoting momentum and heat transfer of gas and particles in the spouted bed. The MJASB SB had a higher slip velocity than the other two bed types, which indicates that the combined internal structure could improve the fluidization state of the bed and intensify the movement and mixing of phases in the spouted bed. The threephase temperature, water evaporation rate, and gas humidity of the MJASB SB were higher than those of the CSB and IMJSFB, and water evaporation occurred in an enlarged region in the MJASB SB. The mass transfer intensification factors I of the MJASB SB (2.62) and IMJSFB (1.92) were 91% and 161% higher than that of the CSB (1), respectively, indicating that the combined internal structure of the MJASB SB significantly contributed to the water evaporation process.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>To improve the fluidization behavior and the heat and mass transfer process in a spouted bed, a multijet–axialswirlblade spouted bed (MJASB SB) was developed. The water evaporation process of the MJASB SB was simulated and compared with those of the conventional spouted bed (CSB) and an integral multijet spoutfluidized bed (IMJSFB). The simulation results showed that the MJASB SB combined the staged spouting action of multijet with the swirling action of the axial swirl blade, which promoted particle turbulence in the annulus region and ensured effective particle mixing. The swirl number of the MJASB SB ranged from 0.0816 to 2.7239 with enhanced vortex intensity, thus promoting momentum and heat transfer of gas and particles in the spouted bed. The MJASB SB had a higher slip velocity than the other two bed types, which indicates that the combined internal structure could improve the fluidization state of the bed and intensify the movement and mixing of phases in the spouted bed. The threephase temperature, water evaporation rate, and gas humidity of the MJASB SB were higher than those of the CSB and IMJSFB, and water evaporation occurred in an enlarged region in the MJASB SB. The mass transfer intensification factors I of the MJASB SB (2.62) and IMJSFB (1.92) were 91% and 161% higher than that of the CSB (1), respectively, indicating that the combined internal structure of the MJASB SB significantly contributed to the water evaporation process.
Effect of combining multijet component with axial swirl blade on evaporation in a spouted bed
10.1063/5.0138735
Physics of Fluids
20230308T12:22:37Z
© 2023 Author(s).

Proper orthogonal decomposition analysis of the cavitating flow around a hydrofoil with an insight on the kinetic characteristics
https://aip.scitation.org/doi/10.1063/5.0138773?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this research, the cavitating flow around a NACA0015 (National Advisory Committee for Aeronautics) hydrofoil obtained by the largeeddy simulation method is analyzed using the proper orthogonal decomposition (POD) theory. Various fundamental mechanisms have been investigated thoroughly, including the reentrant jet behavior, pressure gradient mechanism, vortex dynamics, and dynamic properties of the hydrofoil. The influence of the vortex dynamics, pressure mechanism, and temporal/spatial evolution is revealed. The POD decomposition indicates that the first four dominant POD modes occupy 97.4% of the entire energy. Based on the vortex force field extracted from the first four single POD modes, it is found that the liftanddrag characteristics in the cavitating flow are determined by the specific spatial distribution of mode vortex structures. In addition, the coupling of velocity pulsations and pressure fluctuations is carried out to obtain the POD modal pressure gradient field, which reveals that the pressure gradient has a close connection with the cavity evolution. Furthermore, the vortex force and pressure gradient are reconstructed using the first four modes, 17 modes, and 160 modes, which indicates that the loworder POD modes without the impact of smallscale structures and noise can clearly capture the fundamental aspects of the flow field.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this research, the cavitating flow around a NACA0015 (National Advisory Committee for Aeronautics) hydrofoil obtained by the largeeddy simulation method is analyzed using the proper orthogonal decomposition (POD) theory. Various fundamental mechanisms have been investigated thoroughly, including the reentrant jet behavior, pressure gradient mechanism, vortex dynamics, and dynamic properties of the hydrofoil. The influence of the vortex dynamics, pressure mechanism, and temporal/spatial evolution is revealed. The POD decomposition indicates that the first four dominant POD modes occupy 97.4% of the entire energy. Based on the vortex force field extracted from the first four single POD modes, it is found that the liftanddrag characteristics in the cavitating flow are determined by the specific spatial distribution of mode vortex structures. In addition, the coupling of velocity pulsations and pressure fluctuations is carried out to obtain the POD modal pressure gradient field, which reveals that the pressure gradient has a close connection with the cavity evolution. Furthermore, the vortex force and pressure gradient are reconstructed using the first four modes, 17 modes, and 160 modes, which indicates that the loworder POD modes without the impact of smallscale structures and noise can clearly capture the fundamental aspects of the flow field.
Proper orthogonal decomposition analysis of the cavitating flow around a hydrofoil with an insight on the kinetic characteristics
10.1063/5.0138773
Physics of Fluids
20230309T12:28:10Z
© 2023 Author(s).

Investigation of cavitating vortex rope instabilities and its suppression inside a Francis turbine model with Thoma number variation
https://aip.scitation.org/doi/10.1063/5.0140973?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Cavitating vortex rope at part load (PL) condition at lower values of the Thoma number ([math]) induces severe pressure fluctuation and efficiency reduction in a Francis turbine, which ultimately hinders continuous energy production. Installation of fins at draft tube (DT) can mitigate these instabilities and can safeguard the turbine operation with lower maintenance costs. The effect of fins on hydraulic performance and internal flow physics at PL condition with the variation of [math] is examined in the present numerical investigation. For the two extreme opposite values of [math], the flow characteristics are predicted accurately for the turbine with and without fins by conducting transient simulations using ANSYSCFX. The numerical findings on the structured and unstructured grid points are validated with the experimental results. The turbine's performance remains constant for higher values of Thoma numbers, and as the value decreases, the performance declines. The cavitation vortex rope formation inside the DT with fins is mitigated significantly at the minimum [math], while at the maximum value, the vortex rope with bubble generation is restricted. Compared to the without fin case, the swirl intensity is minimized remarkably (68%) with the presence of fins at the lowest [math]. The maximum cavitation rate is manifested by the DT without fins, which is about 60% higher than the DT with fins. At minimum [math], extreme pressure pulsations are induced inside the DT without fins, which are reduced by 43% in the finned draft tube. Therefore, stable energy production is maximized with the installation of fins at both Thoma numbers.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Cavitating vortex rope at part load (PL) condition at lower values of the Thoma number ([math]) induces severe pressure fluctuation and efficiency reduction in a Francis turbine, which ultimately hinders continuous energy production. Installation of fins at draft tube (DT) can mitigate these instabilities and can safeguard the turbine operation with lower maintenance costs. The effect of fins on hydraulic performance and internal flow physics at PL condition with the variation of [math] is examined in the present numerical investigation. For the two extreme opposite values of [math], the flow characteristics are predicted accurately for the turbine with and without fins by conducting transient simulations using ANSYSCFX. The numerical findings on the structured and unstructured grid points are validated with the experimental results. The turbine's performance remains constant for higher values of Thoma numbers, and as the value decreases, the performance declines. The cavitation vortex rope formation inside the DT with fins is mitigated significantly at the minimum [math], while at the maximum value, the vortex rope with bubble generation is restricted. Compared to the without fin case, the swirl intensity is minimized remarkably (68%) with the presence of fins at the lowest [math]. The maximum cavitation rate is manifested by the DT without fins, which is about 60% higher than the DT with fins. At minimum [math], extreme pressure pulsations are induced inside the DT without fins, which are reduced by 43% in the finned draft tube. Therefore, stable energy production is maximized with the installation of fins at both Thoma numbers.
Investigation of cavitating vortex rope instabilities and its suppression inside a Francis turbine model with Thoma number variation
10.1063/5.0140973
Physics of Fluids
20230310T12:19:51Z
© 2023 Author(s).
Mohammad Abu Shahzer
Mohamed Murshid Shamsuddeen

Pair trajectories of uncharged conducting spheres in an electric field
https://aip.scitation.org/doi/10.1063/5.0142014?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this paper, we study the role of electrostatic forces on pair trajectories of two uncharged conducting spheres subject to an external electric field. We consider the hydrodynamic interactions between the spheres as they move relative to one another. Previous studies have shown that electricfieldinduced forces on a twosphere system are always attractive, except for the configuration when the line joining the centers is perpendicular to the external electric field. In the current study, we derive the asymptotic form of the interparticle force induced by the electric field in the lubrication limit for arbitrary size ratios. The attractive electric force diverges as the separation approaches zero. Thus, our calculation shows that the electricfieldinduced forces can overcome the continuum lubrication resistance and allow finite time contact between the surfaces of two spherical conductors. We calculate the asymptotic variation of interparticle separation using the nearfield asymptotic expressions for the electricfieldinduced forces, exploring the role of hydrodynamic interactions in interparticle motion parallel and perpendicular to the electric field.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this paper, we study the role of electrostatic forces on pair trajectories of two uncharged conducting spheres subject to an external electric field. We consider the hydrodynamic interactions between the spheres as they move relative to one another. Previous studies have shown that electricfieldinduced forces on a twosphere system are always attractive, except for the configuration when the line joining the centers is perpendicular to the external electric field. In the current study, we derive the asymptotic form of the interparticle force induced by the electric field in the lubrication limit for arbitrary size ratios. The attractive electric force diverges as the separation approaches zero. Thus, our calculation shows that the electricfieldinduced forces can overcome the continuum lubrication resistance and allow finite time contact between the surfaces of two spherical conductors. We calculate the asymptotic variation of interparticle separation using the nearfield asymptotic expressions for the electricfieldinduced forces, exploring the role of hydrodynamic interactions in interparticle motion parallel and perpendicular to the electric field.
Pair trajectories of uncharged conducting spheres in an electric field
10.1063/5.0142014
Physics of Fluids
20230313T11:26:01Z
© 2023 Author(s).
Natarajan Thiruvenkadam
Pijush Patra
Vishwanath Kadaba Puttanna
Anubhab Roy

Selforganizing singleline particle trains with differently shaped particles in a channel flow
https://aip.scitation.org/doi/10.1063/5.0139574?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The inertial migration of differently shaped rectangular particles and elliptical particles in a channel flow and the selforganization of singleline particle trains are studied using the lattice Boltzmann method. The effects of particle shape, particle aspect ratio (α), Reynolds number (Re), blockage ratio (k), and particle concentration (Φ) on selforganizing singleline particle trains are explored. The results show that a singleline particle train is dynamically formed, with circular particle trains having a more pronounced dynamic process than rectangular and elliptical particle train. The inclination of height (IH) for the particles in the train is the main reason for the dynamic formation of a singleline particle train. Due to the changes of orientation angle under different flow conditions, the rectangular particle trains always have a larger IH and smaller interparticle spacing than the elliptical particle trains when the train is just formed. The effect of α on the spacing of elliptical particle trains is more sensitive than other shapes. Rectangular particles and elliptical particles with large Φ and Re and small k are prone to selforganize the singleline particle trains with stable spacing for a long travel distance. With increasing Φ, Re, and k, IH increases and the interparticle spacing decreases.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The inertial migration of differently shaped rectangular particles and elliptical particles in a channel flow and the selforganization of singleline particle trains are studied using the lattice Boltzmann method. The effects of particle shape, particle aspect ratio (α), Reynolds number (Re), blockage ratio (k), and particle concentration (Φ) on selforganizing singleline particle trains are explored. The results show that a singleline particle train is dynamically formed, with circular particle trains having a more pronounced dynamic process than rectangular and elliptical particle train. The inclination of height (IH) for the particles in the train is the main reason for the dynamic formation of a singleline particle train. Due to the changes of orientation angle under different flow conditions, the rectangular particle trains always have a larger IH and smaller interparticle spacing than the elliptical particle trains when the train is just formed. The effect of α on the spacing of elliptical particle trains is more sensitive than other shapes. Rectangular particles and elliptical particles with large Φ and Re and small k are prone to selforganize the singleline particle trains with stable spacing for a long travel distance. With increasing Φ, Re, and k, IH increases and the interparticle spacing decreases.
Selforganizing singleline particle trains with differently shaped particles in a channel flow
10.1063/5.0139574
Physics of Fluids
20230315T11:50:08Z
© 2023 Author(s).

Reverse to forward density segregation depending on gas inflow velocity in vibrated fluidized beds
https://aip.scitation.org/doi/10.1063/5.0138556?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Particle density segregations in vibrated fluidized beds depending on gas inflow velocity under the same vertical vibration condition are studied. Coarsegraining discrete element method and computational fluid dynamics numerical simulations are employed to capture the behaviors of reverse segregation in which heavy particles are located above light particles at zero gas inflow velocity or at velocities considerably lower than the minimum fluidization velocity of light particles. Furthermore, upon increasing the gas inflow velocity slightly, the forward segregation occurs, such that heavy particles are located below light particles. The mechanisms are also elucidated using the simulation results. Because of the relative motions between the particles and bed caused by vertical vibration, negative gauge pressure is observed to be dependent on the vibration phase. In the reverse segregation case, the accumulative effect of the downward gas pressure gradient force induced by vibration overcomes the upward force of the forced air flow. The wall friction transports both the heavy and light particles in the vicinity of the sidewall to the bed bottom, where the local void fraction is comparatively high and reverse segregation mainly occurs. Reverse segregation results from the combined effects of the downward gas pressure gradient force, particle transport, and local formation of the high void region. The increase in gas inflow velocity enhances the upward pressure gradient force, resulting in forward segregation.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Particle density segregations in vibrated fluidized beds depending on gas inflow velocity under the same vertical vibration condition are studied. Coarsegraining discrete element method and computational fluid dynamics numerical simulations are employed to capture the behaviors of reverse segregation in which heavy particles are located above light particles at zero gas inflow velocity or at velocities considerably lower than the minimum fluidization velocity of light particles. Furthermore, upon increasing the gas inflow velocity slightly, the forward segregation occurs, such that heavy particles are located below light particles. The mechanisms are also elucidated using the simulation results. Because of the relative motions between the particles and bed caused by vertical vibration, negative gauge pressure is observed to be dependent on the vibration phase. In the reverse segregation case, the accumulative effect of the downward gas pressure gradient force induced by vibration overcomes the upward force of the forced air flow. The wall friction transports both the heavy and light particles in the vicinity of the sidewall to the bed bottom, where the local void fraction is comparatively high and reverse segregation mainly occurs. Reverse segregation results from the combined effects of the downward gas pressure gradient force, particle transport, and local formation of the high void region. The increase in gas inflow velocity enhances the upward pressure gradient force, resulting in forward segregation.
Reverse to forward density segregation depending on gas inflow velocity in vibrated fluidized beds
10.1063/5.0138556
Physics of Fluids
20230316T01:57:55Z
© 2023 Author(s).

Underwater gas bubbles produced by droplet impact: Mechanism to trigger volumetric oscillations
https://aip.scitation.org/doi/10.1063/5.0140484?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Oscillating gas bubbles formed when droplets collide with a water surface are studied experimentally. Over a short time interval, before and after the bubble detachment, the bubble surface curvature changes drastically, causing a pulse of Laplace pressure. The leading edge of the pulse occurs before the bubble detachment, and the rear (negative) edge falls on the stage of an already closed bubble, which, like a resonator, is excited into volume oscillations on the natural frequency while simultaneously emitting an acoustic wavepacket. The amplitude and steepness of the pulse are inversely related to the size of the bubble, thereby ensuring that the dynamic parameters of the triggering pulse correspond to the natural frequency of the bubble. The release of pressure during the negative trailing edge of the pulse initiates the beginning of volumetric oscillations from the expansion phase and the acoustic packet from the positive halfwave.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Oscillating gas bubbles formed when droplets collide with a water surface are studied experimentally. Over a short time interval, before and after the bubble detachment, the bubble surface curvature changes drastically, causing a pulse of Laplace pressure. The leading edge of the pulse occurs before the bubble detachment, and the rear (negative) edge falls on the stage of an already closed bubble, which, like a resonator, is excited into volume oscillations on the natural frequency while simultaneously emitting an acoustic wavepacket. The amplitude and steepness of the pulse are inversely related to the size of the bubble, thereby ensuring that the dynamic parameters of the triggering pulse correspond to the natural frequency of the bubble. The release of pressure during the negative trailing edge of the pulse initiates the beginning of volumetric oscillations from the expansion phase and the acoustic packet from the positive halfwave.
Underwater gas bubbles produced by droplet impact: Mechanism to trigger volumetric oscillations
10.1063/5.0140484
Physics of Fluids
20230316T01:44:01Z
© 2023 Author(s).
V. E. Prokhorov

Acoustically driven translation of a single bubble in pulsed traveling ultrasonic waves
https://aip.scitation.org/doi/10.1063/5.0138484?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The acoustic radiation force has been proven as an effective mechanism for displacing particles and bubbles, but it has been mainly applied in a standing wave mode in microfluidics. Alternatively, the use of pulsed traveling acoustic waves could enable new options, but its transient dynamic, which entails the additional complexities of pulse timing, reflections, and the type of waveform, has not yet been fully investigated. To better understand these transient effects, a transient numerical solution and an experimental testbed were developed to gain insights into the displacement of microbubbles when exposed to on and offperiods of pulsed traveling waves. In this study, a practical sinusoid tone burst excitation at a driving frequency of 0.5 MHz is investigated. Our numerical and experimental results were found to be in good agreement, with only a 13% deviation in the acoustically driven velocity. With greater detail from the numerical solution at a sampling rate of 1 GHz, the fundamental mechanism for the bubble translation was revealed. It was found that the added mass force, gained through the onperiod of the pulse, continued to drive the bubble throughout the offperiod, enabling a large total displacement, even in the case of low dutycycle (2%) pulsing. In addition, the results showed greater translational velocity is possible with a lower number of cycles for the same input acoustic energy (constant duty cycle and acoustic pressure amplitude). Overall, this study proposes a new, practical, and scalable approach for the acoustic manipulation of microbubbles for scientific, biomedical, and industrial applications.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The acoustic radiation force has been proven as an effective mechanism for displacing particles and bubbles, but it has been mainly applied in a standing wave mode in microfluidics. Alternatively, the use of pulsed traveling acoustic waves could enable new options, but its transient dynamic, which entails the additional complexities of pulse timing, reflections, and the type of waveform, has not yet been fully investigated. To better understand these transient effects, a transient numerical solution and an experimental testbed were developed to gain insights into the displacement of microbubbles when exposed to on and offperiods of pulsed traveling waves. In this study, a practical sinusoid tone burst excitation at a driving frequency of 0.5 MHz is investigated. Our numerical and experimental results were found to be in good agreement, with only a 13% deviation in the acoustically driven velocity. With greater detail from the numerical solution at a sampling rate of 1 GHz, the fundamental mechanism for the bubble translation was revealed. It was found that the added mass force, gained through the onperiod of the pulse, continued to drive the bubble throughout the offperiod, enabling a large total displacement, even in the case of low dutycycle (2%) pulsing. In addition, the results showed greater translational velocity is possible with a lower number of cycles for the same input acoustic energy (constant duty cycle and acoustic pressure amplitude). Overall, this study proposes a new, practical, and scalable approach for the acoustic manipulation of microbubbles for scientific, biomedical, and industrial applications.
Acoustically driven translation of a single bubble in pulsed traveling ultrasonic waves
10.1063/5.0138484
Physics of Fluids
20230316T02:04:40Z
© 2023 Author(s).
Philippe Blanloeuil
Darson D. Li
Robert A. Taylor
Tracie J. Barber

Volume flow rate calculation model of nonfull pipe multiphase flow based on ultrasonic sensors
https://aip.scitation.org/doi/10.1063/5.0139031?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In the oil and gas industries, it is crucial to employ appropriate drilling fluids in order to maintain equilibrium of formation pressure throughout the various stages of drilling operations. During the recycling process, the drilling fluid may precipitate gas and as a result exhibit nonfull pipe flow upon return to the surface. Accurate measurement of the volume flow rate of the drilling fluid is imperative in obtaining valuable information from the bottom of the well. Commonly, onsite drilling operations use a multiphase target flowmeter in conjunction with an empirical model to rectify calculation results. However, the returned multiphase flow that is not fully in the pipe and its liquid component exhibits corrosive properties, making it a challenge for traditional invasive measurement methods to achieve adequate accuracy over an extended period. Therefore, the theoretical potential of utilizing noncontact ultrasonic sensors for measuring the multiphase volume flow rate of the nonfull pipe flow is significant. In this research, an apparent flow velocity calculation model was established by integrating the ultrasonic Doppler shift model and pipeline fluid mechanics utilizing a fourchannel ultrasonic array. Subsequently, the invariant scattering convolution—long shortterm memory) network was trained on the datafused ultrasonic signal to identify the liquid level. The velocityarea method was also employed to establish a new multiphase volume flow calculation model. To evaluate the validity of the proposed model, comparison experiments of liquid singlephase flow and liquid–solid twophase flow were conducted. The experimental results show that, compared with the comparative flow measurement system, the accuracy of the ultrasonic flow measurement system is reduced by 0.965%, the nonlinear error by 2.293%, the average relative error by 2.570%, the standard deviation by 1.395, and the root mean square error by 14.394.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In the oil and gas industries, it is crucial to employ appropriate drilling fluids in order to maintain equilibrium of formation pressure throughout the various stages of drilling operations. During the recycling process, the drilling fluid may precipitate gas and as a result exhibit nonfull pipe flow upon return to the surface. Accurate measurement of the volume flow rate of the drilling fluid is imperative in obtaining valuable information from the bottom of the well. Commonly, onsite drilling operations use a multiphase target flowmeter in conjunction with an empirical model to rectify calculation results. However, the returned multiphase flow that is not fully in the pipe and its liquid component exhibits corrosive properties, making it a challenge for traditional invasive measurement methods to achieve adequate accuracy over an extended period. Therefore, the theoretical potential of utilizing noncontact ultrasonic sensors for measuring the multiphase volume flow rate of the nonfull pipe flow is significant. In this research, an apparent flow velocity calculation model was established by integrating the ultrasonic Doppler shift model and pipeline fluid mechanics utilizing a fourchannel ultrasonic array. Subsequently, the invariant scattering convolution—long shortterm memory) network was trained on the datafused ultrasonic signal to identify the liquid level. The velocityarea method was also employed to establish a new multiphase volume flow calculation model. To evaluate the validity of the proposed model, comparison experiments of liquid singlephase flow and liquid–solid twophase flow were conducted. The experimental results show that, compared with the comparative flow measurement system, the accuracy of the ultrasonic flow measurement system is reduced by 0.965%, the nonlinear error by 2.293%, the average relative error by 2.570%, the standard deviation by 1.395, and the root mean square error by 14.394.
Volume flow rate calculation model of nonfull pipe multiphase flow based on ultrasonic sensors
10.1063/5.0139031
Physics of Fluids
20230316T02:04:43Z
© 2023 Author(s).

Twoparticle method for liquid–solid twophase mixed flow
https://aip.scitation.org/doi/10.1063/5.0140599?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Liquid–solid twophase flows are a very important class of multiphase flow problems widely existing in industry and nature. This paper establishes a twophase model for liquid–solid twophase flows considering multiphase states of granular media. The volume fraction is defined by the solid phase, determining the material properties of the two phases, and momentum is exchanged between the phases by drag and pressure gradient forces. On this basis, a twoparticle method for simulating the liquid–solid twophase flow is proposed by coupling smoothed particle hydrodynamics with smoothed discrete particle hydrodynamics. The coupling framework for the twoparticle method is constructed, and the coupling between the algorithms is realized through interphase momentum exchange, volume fraction constraint, and field variable sharing. The liquid phase density changes are divided into two types. One is caused by weak compressibility, and the other is caused by changes in the solid phase volume fraction. The former is used to calculate the liquidphase flow field, and the latter is used to calculate the twophase coupling to solve the problem of sudden bulk density changes in the liquid phase caused by changes in particle volume fractions. The twoparticle method maintains the dual advantages of the particle method for free interface tracking and material point tracking for particles. The new method is validated using a series of fundamental test cases, and comparison with experimental results shows that the new method is suitable for resolving liquid–solid twophase flow problems and has significant practical value for future simulations of mudflow motions, coastal breakwaters, and landslide surges.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Liquid–solid twophase flows are a very important class of multiphase flow problems widely existing in industry and nature. This paper establishes a twophase model for liquid–solid twophase flows considering multiphase states of granular media. The volume fraction is defined by the solid phase, determining the material properties of the two phases, and momentum is exchanged between the phases by drag and pressure gradient forces. On this basis, a twoparticle method for simulating the liquid–solid twophase flow is proposed by coupling smoothed particle hydrodynamics with smoothed discrete particle hydrodynamics. The coupling framework for the twoparticle method is constructed, and the coupling between the algorithms is realized through interphase momentum exchange, volume fraction constraint, and field variable sharing. The liquid phase density changes are divided into two types. One is caused by weak compressibility, and the other is caused by changes in the solid phase volume fraction. The former is used to calculate the liquidphase flow field, and the latter is used to calculate the twophase coupling to solve the problem of sudden bulk density changes in the liquid phase caused by changes in particle volume fractions. The twoparticle method maintains the dual advantages of the particle method for free interface tracking and material point tracking for particles. The new method is validated using a series of fundamental test cases, and comparison with experimental results shows that the new method is suitable for resolving liquid–solid twophase flow problems and has significant practical value for future simulations of mudflow motions, coastal breakwaters, and landslide surges.
Twoparticle method for liquid–solid twophase mixed flow
10.1063/5.0140599
Physics of Fluids
20230316T01:57:35Z
© 2023 Author(s).

Datadriven methods for lowdimensional representation and state identification for the spatiotemporal structure of cavitation flow fields
https://aip.scitation.org/doi/10.1063/5.0145453?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Computational Fluid Dynamics (CFD) generates highdimensional spatiotemporal data. The datadriven method approach to extracting physical information from CFD has attracted widespread concern in fluid mechanics. While good results have been obtained for some benchmark problems, the performance on complex flow field problems has not been extensively studied. In this paper, we use a dimensionality reduction approach to preserve the main features of the flow field. Based on this, we perform unsupervised identification of flow field states using a clustering approach that applies datadriven analysis to the spatiotemporal structure of complex threedimensional unsteady cavitation flows. The result shows that the datadriven method can effectively represent the changes in the spatial structure of the unsteady flow field over time and to visualize changes in the quasiperiodic state of the flow. Furthermore, we demonstrate that the combination of principal component analysis and Toeplitz inverse covariancebased clustering can identify different states of the cavitated flow field with high accuracy. This suggests that the method has great potential for application in complex flow phenomena.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Computational Fluid Dynamics (CFD) generates highdimensional spatiotemporal data. The datadriven method approach to extracting physical information from CFD has attracted widespread concern in fluid mechanics. While good results have been obtained for some benchmark problems, the performance on complex flow field problems has not been extensively studied. In this paper, we use a dimensionality reduction approach to preserve the main features of the flow field. Based on this, we perform unsupervised identification of flow field states using a clustering approach that applies datadriven analysis to the spatiotemporal structure of complex threedimensional unsteady cavitation flows. The result shows that the datadriven method can effectively represent the changes in the spatial structure of the unsteady flow field over time and to visualize changes in the quasiperiodic state of the flow. Furthermore, we demonstrate that the combination of principal component analysis and Toeplitz inverse covariancebased clustering can identify different states of the cavitated flow field with high accuracy. This suggests that the method has great potential for application in complex flow phenomena.
Datadriven methods for lowdimensional representation and state identification for the spatiotemporal structure of cavitation flow fields
10.1063/5.0145453
Physics of Fluids
20230317T03:04:45Z
© 2023 Author(s).

Heat and mass transport from neutrally suspended oblate spheroid in simple shear flow
https://aip.scitation.org/doi/10.1063/5.0140778?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Through highfidelity numerical simulation based on the lattice Boltzmann method, we have conducted an indepth study on the heat and mass transport from an oblate spheroid neutrally suspended in a simple shear flow. In the simulation, the temperature and mass concentration are modeled as a passive scalar released at the surface of the spheroid. The fluid dynamics induced by the interaction between the carrier fluid and the suspended spheroid, as well as the resultant scalar transport process, have been extensively investigated. A coupled transport mechanism comprising several components of the flow around the oblate spheroid has been identified. The effects of the Reynolds number and the aspect ratio of the spheroid on the flow characteristics and scalar transport rate are examined. The variation of the nondimensional scalar transport rate suggests that the effect of spheroid shape on scalar transfer rate can be decoupled from the effects of Peclet and Reynolds numbers, which facilitates the development of a correlation of scalar transfer rate for oblate spheroids based on the welldeveloped correlations for a sphere.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Through highfidelity numerical simulation based on the lattice Boltzmann method, we have conducted an indepth study on the heat and mass transport from an oblate spheroid neutrally suspended in a simple shear flow. In the simulation, the temperature and mass concentration are modeled as a passive scalar released at the surface of the spheroid. The fluid dynamics induced by the interaction between the carrier fluid and the suspended spheroid, as well as the resultant scalar transport process, have been extensively investigated. A coupled transport mechanism comprising several components of the flow around the oblate spheroid has been identified. The effects of the Reynolds number and the aspect ratio of the spheroid on the flow characteristics and scalar transport rate are examined. The variation of the nondimensional scalar transport rate suggests that the effect of spheroid shape on scalar transfer rate can be decoupled from the effects of Peclet and Reynolds numbers, which facilitates the development of a correlation of scalar transfer rate for oblate spheroids based on the welldeveloped correlations for a sphere.
Heat and mass transport from neutrally suspended oblate spheroid in simple shear flow
10.1063/5.0140778
Physics of Fluids
20230317T02:43:21Z
© 2023 Author(s).
Yanxing Wang
Hui Wan
Ruben Gonzalez Pizarro
Seokbin Lim
Fangjun Shu

Collision regimes and dynamic behaviors of a viscous droplet impacting on a spherical particle at high temperatures
https://aip.scitation.org/doi/10.1063/5.0138103?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Understanding droplet–particle collision behaviors is essential for the pyrohydrolysis process of pickling liquor, where the pickling liquor is sprayed and dried into particles. This study systematically investigated the collision characteristics between a viscous droplet and a heated stainless steel particle whose temperature ranged from 100 to 700 °C. The results indicate that the vapor thrust promotes droplet rebound in smaller spreading diameters but induces disintegration in larger spreading diameters under the film boiling regime. The collision regime map is summarized, and the transition thresholds of collision patterns are significantly increased with increasing liquid viscosity. The spreading factor and contact line velocity are strongly affected by the particle temperature at high liquid viscosities. In addition, the hydrophobic nature of particle surface in film boiling regime is favorable for droplet spreading. The dynamic contact angle significantly depends on the particle temperature and droplet properties. The dimensionless contact time shows a power law dependency on the Weber number in the rebound pattern, but it is almost a constant in the disintegration pattern.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Understanding droplet–particle collision behaviors is essential for the pyrohydrolysis process of pickling liquor, where the pickling liquor is sprayed and dried into particles. This study systematically investigated the collision characteristics between a viscous droplet and a heated stainless steel particle whose temperature ranged from 100 to 700 °C. The results indicate that the vapor thrust promotes droplet rebound in smaller spreading diameters but induces disintegration in larger spreading diameters under the film boiling regime. The collision regime map is summarized, and the transition thresholds of collision patterns are significantly increased with increasing liquid viscosity. The spreading factor and contact line velocity are strongly affected by the particle temperature at high liquid viscosities. In addition, the hydrophobic nature of particle surface in film boiling regime is favorable for droplet spreading. The dynamic contact angle significantly depends on the particle temperature and droplet properties. The dimensionless contact time shows a power law dependency on the Weber number in the rebound pattern, but it is almost a constant in the disintegration pattern.
Collision regimes and dynamic behaviors of a viscous droplet impacting on a spherical particle at high temperatures
10.1063/5.0138103
Physics of Fluids
20230306T10:56:19Z
© 2023 Author(s).

Drag reduction by flapping a pair of flexible filaments behind a cylinder
https://aip.scitation.org/doi/10.1063/5.0139372?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The hydrodynamic mechanism of drag reduction by flapping a pair of flexible filaments behind a cylinder was explored using the penalty immersed boundary method. The effects of the phase difference between two filaments, the attachment height, and the flapping amplitude on drag reduction were examined. The flapping filaments weaken the vortex shedding via the destructive interaction between the vortices with the opposite signal. The clapping (outofphase) flexible filaments experience a lower friction drag and reduce a form drag of the cylinder, showing a better drag reduction than the snaking (inphase) flexible filaments and the clapping rigid filaments. A minimum drag is obtained at an appropriate attachment height and flapping amplitude that avoid collision of the filaments and weaken the shearlayer–filaments interaction. The effectiveness ratio of the clapping filaments is higher than that of the snaking filaments. Energy saving can be achieved by avoiding the shear layer–filament interaction at a low flapping amplitude, whereas the filaments can further reduce the drag with greater energy consumption at an appropriate flapping amplitude. In addition, the total drag decreases with increasing Reynolds number, accompanied by a transition of the wake pattern from the 2S mode to the P + S mode.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The hydrodynamic mechanism of drag reduction by flapping a pair of flexible filaments behind a cylinder was explored using the penalty immersed boundary method. The effects of the phase difference between two filaments, the attachment height, and the flapping amplitude on drag reduction were examined. The flapping filaments weaken the vortex shedding via the destructive interaction between the vortices with the opposite signal. The clapping (outofphase) flexible filaments experience a lower friction drag and reduce a form drag of the cylinder, showing a better drag reduction than the snaking (inphase) flexible filaments and the clapping rigid filaments. A minimum drag is obtained at an appropriate attachment height and flapping amplitude that avoid collision of the filaments and weaken the shearlayer–filaments interaction. The effectiveness ratio of the clapping filaments is higher than that of the snaking filaments. Energy saving can be achieved by avoiding the shear layer–filament interaction at a low flapping amplitude, whereas the filaments can further reduce the drag with greater energy consumption at an appropriate flapping amplitude. In addition, the total drag decreases with increasing Reynolds number, accompanied by a transition of the wake pattern from the 2S mode to the P + S mode.
Drag reduction by flapping a pair of flexible filaments behind a cylinder
10.1063/5.0139372
Physics of Fluids
20230307T11:19:17Z
© 2023 Author(s).

Flow control of an elastically mounted square cylinder by using an attached flexible plate
https://aip.scitation.org/doi/10.1063/5.0139662?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This work numerically investigates the flow control of an elastically mounted square cylinder by using an attached plate placing downstream. The flow control effect by using a long solid/flexible plate for a circular cylinder has been widely discussed. However, the effect of a short plate is not clear. In addition, few knowledge is known for VIV (vortexinduced vibration) suppression effect for a square cylinder by using a solid/flexible plate. The present work systematically studies the influence of plate length and flexibility on VIV suppression for an elastically mounted square cylinder at Reynolds number 150. First, the effect of a solid plate with nondimensional length varying in a wide range [math] is analyzed. Significant VIV suppression can be achieved by using an attached solid plate, even with a plate at a short length. The influence of flexibility is more complex. A short flexible plate is less effective than a solid plate with the same length. On the other hand, a long flexible plate with medium flexibility can further enhance VIV suppression. A maximum 96% reduction in the vibration amplitude can be achieved by using a long flexible plate with optimal flexibility. Additionally, two VIV suppression mechanisms for an elastically mounted square cylinder with an attached plate are concluded, and the influence of flexibility for both short and long plates is also analyzed.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This work numerically investigates the flow control of an elastically mounted square cylinder by using an attached plate placing downstream. The flow control effect by using a long solid/flexible plate for a circular cylinder has been widely discussed. However, the effect of a short plate is not clear. In addition, few knowledge is known for VIV (vortexinduced vibration) suppression effect for a square cylinder by using a solid/flexible plate. The present work systematically studies the influence of plate length and flexibility on VIV suppression for an elastically mounted square cylinder at Reynolds number 150. First, the effect of a solid plate with nondimensional length varying in a wide range [math] is analyzed. Significant VIV suppression can be achieved by using an attached solid plate, even with a plate at a short length. The influence of flexibility is more complex. A short flexible plate is less effective than a solid plate with the same length. On the other hand, a long flexible plate with medium flexibility can further enhance VIV suppression. A maximum 96% reduction in the vibration amplitude can be achieved by using a long flexible plate with optimal flexibility. Additionally, two VIV suppression mechanisms for an elastically mounted square cylinder with an attached plate are concluded, and the influence of flexibility for both short and long plates is also analyzed.
Flow control of an elastically mounted square cylinder by using an attached flexible plate
10.1063/5.0139662
Physics of Fluids
20230308T12:22:25Z
© 2023 Author(s).

Effect of radial velocity profiles on axial dispersion in packed beds: Transient formulation
https://aip.scitation.org/doi/10.1063/5.0139689?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>For a singlephase flow in a packed bed, systematic radial velocity profiles promote the axial dispersion of a solute identified as transcolumn dispersion and quantified by the corresponding dispersion coefficient. In a recent contribution, we evaluated the magnitude of such a dispersive effect for a long enough bed, i.e., the asymptotic behavior. However, in many practical cases, this last condition is not accomplished, and the dispersion coefficient will be lower than the asymptotic value. The development of the transcolumn dispersion is addressed based on a twodimensional twozone model and the application of the Taylor–Aris method of moments. The results show satisfactory agreement compared with available literature data. The effect of the vesseltoparticle diameter ratio on the development of the transcolumn dispersion coefficient is also explored. As the initial growth rate of the dispersion coefficient is lower, the higher the diameter ratio, and the opposite trend holds for the asymptotic value, the net effect of the diameter ratio weakens up to distances of some tens of particle diameter. This result can be identified as one of the reasons for the controversy that still prevails in assessing the contribution of the transcolumn dispersion to the total axial dispersion. Further aspects discussed concern suitable approximations to evaluate the development of the transcolumn dispersion coefficient and the comparison between the results from the Taylor–Aris method of moments and from the residence time distribution approach.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>For a singlephase flow in a packed bed, systematic radial velocity profiles promote the axial dispersion of a solute identified as transcolumn dispersion and quantified by the corresponding dispersion coefficient. In a recent contribution, we evaluated the magnitude of such a dispersive effect for a long enough bed, i.e., the asymptotic behavior. However, in many practical cases, this last condition is not accomplished, and the dispersion coefficient will be lower than the asymptotic value. The development of the transcolumn dispersion is addressed based on a twodimensional twozone model and the application of the Taylor–Aris method of moments. The results show satisfactory agreement compared with available literature data. The effect of the vesseltoparticle diameter ratio on the development of the transcolumn dispersion coefficient is also explored. As the initial growth rate of the dispersion coefficient is lower, the higher the diameter ratio, and the opposite trend holds for the asymptotic value, the net effect of the diameter ratio weakens up to distances of some tens of particle diameter. This result can be identified as one of the reasons for the controversy that still prevails in assessing the contribution of the transcolumn dispersion to the total axial dispersion. Further aspects discussed concern suitable approximations to evaluate the development of the transcolumn dispersion coefficient and the comparison between the results from the Taylor–Aris method of moments and from the residence time distribution approach.
Effect of radial velocity profiles on axial dispersion in packed beds: Transient formulation
10.1063/5.0139689
Physics of Fluids
20230308T12:22:43Z
© 2023 Author(s).
Carlos D. Luzi
Osvaldo M. Martinez
Guillermo F. Barreto

Understanding multiregime detonation development for hydrogen and syngas fuels
https://aip.scitation.org/doi/10.1063/5.0139872?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Autoignition and detonation development are foundational events in the combustion community and are fundamentally relevant to engine knocking and detonation propulsion. Autoignitioninduced reaction front propagation modes have been extensively investigated, addressing the role of thermal and concentration inhomogeneities. In this work, we have further investigated the nonmonotonic response of detonation development to temperature gradients for lowcarbon fuels (hydrogen and syngas) and have found additional detonation regimes, which can depict the panorama of reaction front propagation modes. Results show that separate detonation regimes can be observed when hotspot sizes are below some critical thresholds, with the first corresponding to the known “Bradley detonation peninsula” and the second newly identified featuring broader detonation regions. Despite this, distinct combustion characteristics are observed in the demarcation of detonation regimes between hydrogen and syngas fuels. Specifically, the upper branch of the first detonation regimes for hydrogen is sensitive to temperature gradients at various hotspot sizes, while it exhibits similar behaviors in the lower branch of the second one for syngas, which results in narrower detonation regions. Meanwhile, hydrogen possesses a larger critical hotspot size compared to syngas, and the underlying mechanism is ascribed to the chemical reactivity when hotspot autoignition and the difference of energy density between hotspot interior and exterior. Finally, various detonation regimes are summarized in dimensionless detonation diagrams, in which hydrogen and syngas show similar distributions of detonation peninsula. Despite this, those distinctions in the detonation characteristics between hydrogen and syngas can still be manifested quantitatively. The current work can provide useful insights into knocking inhabitation and detonation promotion.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Autoignition and detonation development are foundational events in the combustion community and are fundamentally relevant to engine knocking and detonation propulsion. Autoignitioninduced reaction front propagation modes have been extensively investigated, addressing the role of thermal and concentration inhomogeneities. In this work, we have further investigated the nonmonotonic response of detonation development to temperature gradients for lowcarbon fuels (hydrogen and syngas) and have found additional detonation regimes, which can depict the panorama of reaction front propagation modes. Results show that separate detonation regimes can be observed when hotspot sizes are below some critical thresholds, with the first corresponding to the known “Bradley detonation peninsula” and the second newly identified featuring broader detonation regions. Despite this, distinct combustion characteristics are observed in the demarcation of detonation regimes between hydrogen and syngas fuels. Specifically, the upper branch of the first detonation regimes for hydrogen is sensitive to temperature gradients at various hotspot sizes, while it exhibits similar behaviors in the lower branch of the second one for syngas, which results in narrower detonation regions. Meanwhile, hydrogen possesses a larger critical hotspot size compared to syngas, and the underlying mechanism is ascribed to the chemical reactivity when hotspot autoignition and the difference of energy density between hotspot interior and exterior. Finally, various detonation regimes are summarized in dimensionless detonation diagrams, in which hydrogen and syngas show similar distributions of detonation peninsula. Despite this, those distinctions in the detonation characteristics between hydrogen and syngas can still be manifested quantitatively. The current work can provide useful insights into knocking inhabitation and detonation promotion.
Understanding multiregime detonation development for hydrogen and syngas fuels
10.1063/5.0139872
Physics of Fluids
20230309T12:28:16Z
© 2023 Author(s).

Equivalent permeability model of dualporosity and bidispersed porous media based on the intermingled fractal units
https://aip.scitation.org/doi/10.1063/5.0140041?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Dualporosity and bidispersed porous media (DBPM) widely exist in geotechnical engineering, material engineering, soil science, and groundwater exploitation. Therefore, it is significant to quantify the relationship between permeability and matrix–fracture structure parameters for mastering fluid's seepage and transport characteristics. Hence, this paper derives an analytical solution of equivalent permeability for DBPM based on the intermingled fractal units (IFU). The developed model considers the capillary pressure of fractures and capillaries and the tortuosity of fractures and capillaries. Specifically, the number of porous matrix fractal units in IFU is quantified, and then, the dimensionless permeability is calculated, defined as the ratio of the permeability of [math] matrix fractal units to a single fracture fractal unit. The results reveal that equivalent permeability is mainly contributed by fracture permeability. Next, the second dimensionless permeability is defined to compare further and quantify the permeable ability of fracture and porous matrix. The results highlight that the permeability difference between a single fracture fractal unit and a single porous matrix fractal unit is approximately 7–11 orders of magnitude. Overall, through this paper, the preferential flow mechanism of DBPM can be better described and understood by introducing the above two dimensionless permeabilities and analyzing the influence of structural parameters on them.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Dualporosity and bidispersed porous media (DBPM) widely exist in geotechnical engineering, material engineering, soil science, and groundwater exploitation. Therefore, it is significant to quantify the relationship between permeability and matrix–fracture structure parameters for mastering fluid's seepage and transport characteristics. Hence, this paper derives an analytical solution of equivalent permeability for DBPM based on the intermingled fractal units (IFU). The developed model considers the capillary pressure of fractures and capillaries and the tortuosity of fractures and capillaries. Specifically, the number of porous matrix fractal units in IFU is quantified, and then, the dimensionless permeability is calculated, defined as the ratio of the permeability of [math] matrix fractal units to a single fracture fractal unit. The results reveal that equivalent permeability is mainly contributed by fracture permeability. Next, the second dimensionless permeability is defined to compare further and quantify the permeable ability of fracture and porous matrix. The results highlight that the permeability difference between a single fracture fractal unit and a single porous matrix fractal unit is approximately 7–11 orders of magnitude. Overall, through this paper, the preferential flow mechanism of DBPM can be better described and understood by introducing the above two dimensionless permeabilities and analyzing the influence of structural parameters on them.
Equivalent permeability model of dualporosity and bidispersed porous media based on the intermingled fractal units
10.1063/5.0140041
Physics of Fluids
20230310T12:19:55Z
© 2023 Author(s).

Effect of Stefan flow on the flow field and heat transfer near wall of supercritical carbon dioxide flowing over a stationary spherical particle
https://aip.scitation.org/doi/10.1063/5.0141213?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In industrial applications, the phenomenon of scCO2 (supercritical carbon dioxide) flowing over particles is quite common. Considering that the scCO2 is chemically inactive but has high solubility, the pure Stefan flow will present without the related diffusion of a chemical reaction component and reaction heat, during the process of a spherical particle in the solid phase dissolved in a system of scCO2. To this, particle resolvedirect numerical simulation without considering the role of gravity and buoyancy is employed in this paper to investigate the hightemperature scCO2 flowing over a lowtemperature stationary sphere with the uniformly, normally, and outward distributed Stefan flow on its surface, with the above cases conducted in the process of small variations on physical properties of scCO2. We present a series of variables in the flow field and temperature field near the sphere surface to study the effects of Stefan flow on them compared with cases without Stefan flow. Related distribution details of the velocity boundary layer and the temperature boundary layer near the sphere surface under conditions with or without the Stefan flow are also presented and analyzed. Different from other similar studies, our study also pays more attention to variables of the local fluid field as well as temperature field near the surface of the spherical particle. The results show that the presence of Stefan flow will reduce flow resistance of the freestream but inhibits heat transfer performance. Simpler correlations in form compared with previous wellestablished correlations are presented and are used to describe the operating conditions proposed herein.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In industrial applications, the phenomenon of scCO2 (supercritical carbon dioxide) flowing over particles is quite common. Considering that the scCO2 is chemically inactive but has high solubility, the pure Stefan flow will present without the related diffusion of a chemical reaction component and reaction heat, during the process of a spherical particle in the solid phase dissolved in a system of scCO2. To this, particle resolvedirect numerical simulation without considering the role of gravity and buoyancy is employed in this paper to investigate the hightemperature scCO2 flowing over a lowtemperature stationary sphere with the uniformly, normally, and outward distributed Stefan flow on its surface, with the above cases conducted in the process of small variations on physical properties of scCO2. We present a series of variables in the flow field and temperature field near the sphere surface to study the effects of Stefan flow on them compared with cases without Stefan flow. Related distribution details of the velocity boundary layer and the temperature boundary layer near the sphere surface under conditions with or without the Stefan flow are also presented and analyzed. Different from other similar studies, our study also pays more attention to variables of the local fluid field as well as temperature field near the surface of the spherical particle. The results show that the presence of Stefan flow will reduce flow resistance of the freestream but inhibits heat transfer performance. Simpler correlations in form compared with previous wellestablished correlations are presented and are used to describe the operating conditions proposed herein.
Effect of Stefan flow on the flow field and heat transfer near wall of supercritical carbon dioxide flowing over a stationary spherical particle
10.1063/5.0141213
Physics of Fluids
20230314T10:16:42Z
© 2023 Author(s).

Lifting Stokes' paradox by accelerating flow past a circular cylinder and extension of the analysis to the sphere
https://aip.scitation.org/doi/10.1063/5.0141560?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>It is known that Stokes' paradox exhibits in various flow conditions, most notably, in flow past a twodimensional (2D) circular cylinder. In this study, we provide an example through detailed analytical solution that Stokes' paradox can be lifted by accelerating flow past a stationary cylinder. The analysis is also extended to the case of the accelerating flow past a stationary sphere although in this case, there is no Stokes' paradox. The effects of the acceleration parameter on the flow streamlines, the pressure, and the vorticity distributions, as well as on the drag coefficient, are investigated. The drag comprises the potential component and vorticity component, which are further due to form drag and frictional drag receiving a separate investigation. However, the drag decomposition is also examined the viewpoint of the force decomposition: the total drag = the potential component + surface vorticity component + volume vorticity component.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>It is known that Stokes' paradox exhibits in various flow conditions, most notably, in flow past a twodimensional (2D) circular cylinder. In this study, we provide an example through detailed analytical solution that Stokes' paradox can be lifted by accelerating flow past a stationary cylinder. The analysis is also extended to the case of the accelerating flow past a stationary sphere although in this case, there is no Stokes' paradox. The effects of the acceleration parameter on the flow streamlines, the pressure, and the vorticity distributions, as well as on the drag coefficient, are investigated. The drag comprises the potential component and vorticity component, which are further due to form drag and frictional drag receiving a separate investigation. However, the drag decomposition is also examined the viewpoint of the force decomposition: the total drag = the potential component + surface vorticity component + volume vorticity component.
Lifting Stokes' paradox by accelerating flow past a circular cylinder and extension of the analysis to the sphere
10.1063/5.0141560
Physics of Fluids
20230317T02:47:20Z
© 2023 Author(s).

Hydrodynamic instability of oddviscosityinduced shearimposed falling film
https://aip.scitation.org/doi/10.1063/5.0137425?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this paper, an analysis of linear and weakly nonlinear stability for an oddviscosityinduced shearimposed falling film over an inclined plane is performed. Using the Chebyshev spectral collocation approach, the linear effect for disturbance of arbitrary wavenumbers is numerically examined by solving the Orr–Sommerfeld eigenvalue problem within the framework of normal mode analysis. The study reveals that instability rises with increasing external shear in the streamwise direction. However, as external shear rises in the reverse flow direction, wave energy is dissipated, and the surface wave stabilizes. Furthermore, the longwave expansion method is applied to calculate the nonlinear surface deformation expression, and it is found that the odd viscosity has the ability to stabilize the fluid flow instability caused by a positive shear force. The investigation of weakly nonlinear stability is also performed using the multiple scale method, which led to the Ginzburg–Landau equation of the nonlinear surface deformation equation. The corresponding results confirm the significant effect of both imposed shear and odd viscosity coefficient on the existent subcritical unstable and supercritical stable zones along with unconditional and explosive zones near the threshold of the film flow instability. The bandwidth of the subcritical stable zone mitigates for the higher viscosity ratio while it enhances the flowdirected potent imposed shear. Additionally, the amplitude and phase speed of nonlinear waves in the supercritical stable regime rise with increasing induced shear in the fluid flow direction and gradually decrease with increasing the value of the odd viscosity coefficient.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this paper, an analysis of linear and weakly nonlinear stability for an oddviscosityinduced shearimposed falling film over an inclined plane is performed. Using the Chebyshev spectral collocation approach, the linear effect for disturbance of arbitrary wavenumbers is numerically examined by solving the Orr–Sommerfeld eigenvalue problem within the framework of normal mode analysis. The study reveals that instability rises with increasing external shear in the streamwise direction. However, as external shear rises in the reverse flow direction, wave energy is dissipated, and the surface wave stabilizes. Furthermore, the longwave expansion method is applied to calculate the nonlinear surface deformation expression, and it is found that the odd viscosity has the ability to stabilize the fluid flow instability caused by a positive shear force. The investigation of weakly nonlinear stability is also performed using the multiple scale method, which led to the Ginzburg–Landau equation of the nonlinear surface deformation equation. The corresponding results confirm the significant effect of both imposed shear and odd viscosity coefficient on the existent subcritical unstable and supercritical stable zones along with unconditional and explosive zones near the threshold of the film flow instability. The bandwidth of the subcritical stable zone mitigates for the higher viscosity ratio while it enhances the flowdirected potent imposed shear. Additionally, the amplitude and phase speed of nonlinear waves in the supercritical stable regime rise with increasing induced shear in the fluid flow direction and gradually decrease with increasing the value of the odd viscosity coefficient.
Hydrodynamic instability of oddviscosityinduced shearimposed falling film
10.1063/5.0137425
Physics of Fluids
20230301T04:26:06Z
© 2023 Author(s).
Dipankar Paul
Md. Mouzakkir Hossain
Harekrushna Behera

Resonatorlike behavior of a wallbounded precessing vortex core in a diffuser with wall asymmetries
https://aip.scitation.org/doi/10.1063/5.0140025?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This paper reports a detailed investigation of the interaction between a wallbounded precessing vortex core (PVC) occurring in swirling flows after vortex breakdown and a wall asymmetry. Experiments are carried out in an axisymmetric diffuser downstream of an axial swirl generator inducing a swirling flow with a swirl number of S = 1.1. Wall pressure measurements and twocomponent particle image velocimetry (PIV) are conducted for Reynolds numbers (Re) ranging from 20 000 to 76 000 in the initial axisymmetric configuration and several asymmetric configurations, with an additional cylindrical protrusion placed on the diffuser wall at different streamwise and circumferential positions. It is first confirmed that synchronous pressure fluctuations at the PVC frequency are only produced in asymmetric configurations. Furthermore, the analysis of the pressure data in several asymmetric configurations revealed for the first time a resonatorlike behavior of a wallbounded PVC. While a change of the protrusion circumferential position in a given cross section of the diffuser only affects the phase of the synchronous pressure fluctuations, the amplitude of the latter features successive minima (pressure node) and maxima (pressure antinode) as the protrusion is moved along the diffuser in the streamwise direction. In addition, as the protrusion is moved closer to a pressure node, the phase of the synchronous pressure fluctuations exhibits a sudden variation of [math]. Similar results are observed for all tested values of Reynolds number, whereas the PVC frequency linearly increases with Re. A reconstruction of the PVC helical structure based on PIV measurements showed that these consecutive pressure nodes are spaced by a distance equal to approximately one third of the PVC helical pitch. Finally, it also revealed that two different states are observed, depending on the position of the protrusion along the diffuser: the synchronous pressure component reaches its maximum value as the PVC center is approaching either its closest or farthest angular position with respect to the protrusion. The transition from one state to another one depends on the streamwise position of the protrusion with respect to the pressure nodes. These unprecedented experimental observations pave the way to novel theoretical developments for a better understanding and modeling of synchronous pressure fluctuations induced by wallbounded PVC in asymmetric geometries.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This paper reports a detailed investigation of the interaction between a wallbounded precessing vortex core (PVC) occurring in swirling flows after vortex breakdown and a wall asymmetry. Experiments are carried out in an axisymmetric diffuser downstream of an axial swirl generator inducing a swirling flow with a swirl number of S = 1.1. Wall pressure measurements and twocomponent particle image velocimetry (PIV) are conducted for Reynolds numbers (Re) ranging from 20 000 to 76 000 in the initial axisymmetric configuration and several asymmetric configurations, with an additional cylindrical protrusion placed on the diffuser wall at different streamwise and circumferential positions. It is first confirmed that synchronous pressure fluctuations at the PVC frequency are only produced in asymmetric configurations. Furthermore, the analysis of the pressure data in several asymmetric configurations revealed for the first time a resonatorlike behavior of a wallbounded PVC. While a change of the protrusion circumferential position in a given cross section of the diffuser only affects the phase of the synchronous pressure fluctuations, the amplitude of the latter features successive minima (pressure node) and maxima (pressure antinode) as the protrusion is moved along the diffuser in the streamwise direction. In addition, as the protrusion is moved closer to a pressure node, the phase of the synchronous pressure fluctuations exhibits a sudden variation of [math]. Similar results are observed for all tested values of Reynolds number, whereas the PVC frequency linearly increases with Re. A reconstruction of the PVC helical structure based on PIV measurements showed that these consecutive pressure nodes are spaced by a distance equal to approximately one third of the PVC helical pitch. Finally, it also revealed that two different states are observed, depending on the position of the protrusion along the diffuser: the synchronous pressure component reaches its maximum value as the PVC center is approaching either its closest or farthest angular position with respect to the protrusion. The transition from one state to another one depends on the streamwise position of the protrusion with respect to the pressure nodes. These unprecedented experimental observations pave the way to novel theoretical developments for a better understanding and modeling of synchronous pressure fluctuations induced by wallbounded PVC in asymmetric geometries.
Resonatorlike behavior of a wallbounded precessing vortex core in a diffuser with wall asymmetries
10.1063/5.0140025
Physics of Fluids
20230301T02:18:56Z
© 2023 Author(s).
Arthur Favrel

Predicting the effect of inertia, rotation, and magnetic field on the onset of convection in a bidispersive porous medium using machine learning techniques
https://aip.scitation.org/doi/10.1063/5.0138421?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Effects of the magnetic field and inertia on the onset of thermal convection in a horizontal bidispersive porous layer, rotating about a vertical axis, are analyzed. The Darcy equation with same temperature in the micro and macrophases is used to characterize the fluid motion. The Vadasz number is taken into account in a generalized Darcy equation for the macrophase. The eigenvalue problem obtained from the linear stability analysis is solved analytically for free–free boundaries. Moving one step further from the traditional linear stability analysis, machine learning tools are introduced in this paper to include the effect of multiple parameters on the marginal state of the system. Machine learning techniques have been implemented to identify the mode of instability with respect to different parameters. In particular, classification algorithms, namely, Artificial Neural Networks (ANN) and Support vector machine, are used to examine the onset of oscillatory convection and stationary convection. The required data for training of the algorithms are generated from the results of linear stability analysis. It is found that ANN with the sufficient number of hidden layers along with good choice of training dataset can predict the mode of instability even on the small variation in a given parameter. The combined effect of rotation, magnetic field, and inertia is to reduce the oscillatory mode of instability; hence, the system exhibits the steady mode of instability for a significant region in the three dimensional space comprising the Taylor number, the Hartman number, and the Vadasz number.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Effects of the magnetic field and inertia on the onset of thermal convection in a horizontal bidispersive porous layer, rotating about a vertical axis, are analyzed. The Darcy equation with same temperature in the micro and macrophases is used to characterize the fluid motion. The Vadasz number is taken into account in a generalized Darcy equation for the macrophase. The eigenvalue problem obtained from the linear stability analysis is solved analytically for free–free boundaries. Moving one step further from the traditional linear stability analysis, machine learning tools are introduced in this paper to include the effect of multiple parameters on the marginal state of the system. Machine learning techniques have been implemented to identify the mode of instability with respect to different parameters. In particular, classification algorithms, namely, Artificial Neural Networks (ANN) and Support vector machine, are used to examine the onset of oscillatory convection and stationary convection. The required data for training of the algorithms are generated from the results of linear stability analysis. It is found that ANN with the sufficient number of hidden layers along with good choice of training dataset can predict the mode of instability even on the small variation in a given parameter. The combined effect of rotation, magnetic field, and inertia is to reduce the oscillatory mode of instability; hence, the system exhibits the steady mode of instability for a significant region in the three dimensional space comprising the Taylor number, the Hartman number, and the Vadasz number.
Predicting the effect of inertia, rotation, and magnetic field on the onset of convection in a bidispersive porous medium using machine learning techniques
10.1063/5.0138421
Physics of Fluids
20230301T01:51:37Z
© 2023 Author(s).
Mahesh Singh
Ravi Ragoju
G. Shiva Kumar Reddy
Chinnamuthu Subramani

On the interaction of Tollmien–Schlichting waves with a wallembedded Helmholtz resonator
https://aip.scitation.org/doi/10.1063/5.0141685?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The influence of a wallembedded Helmholtz resonator on the development and stability of Tollmien–Schlichting (TS) waves is investigated numerically and experimentally for a range of frequencies extending from below to above resonance. Interactions are found to be limited in the nearwall region and toward the trailing edge of the resonator orifice while at the same time being linear nature. The dynamic response of the flowexcited resonator is shown to have a fixed phase relation with respect to the TSwaves, indicating that only amplification of the latter can be achieved. The same resonant behavior is maintained regardless of whether the resonator is flowexcited or acoustically excited. Thus, it is suggested that pressure perturbations propagate perpendicularly and acoustically within the resonator throat and cavity. The amplification observed in the vicinity of the resonator displays features typical of TSwave scattering; however, it is confirmed that this is not solely the result of mean flow distortion due to the geometry and recirculation region. Instead, the results indicate that the phenomenology is a consequence of the combination of scattering, localized nonmodal growth, and wallforcing in the wallnormal direction due to resonance.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The influence of a wallembedded Helmholtz resonator on the development and stability of Tollmien–Schlichting (TS) waves is investigated numerically and experimentally for a range of frequencies extending from below to above resonance. Interactions are found to be limited in the nearwall region and toward the trailing edge of the resonator orifice while at the same time being linear nature. The dynamic response of the flowexcited resonator is shown to have a fixed phase relation with respect to the TSwaves, indicating that only amplification of the latter can be achieved. The same resonant behavior is maintained regardless of whether the resonator is flowexcited or acoustically excited. Thus, it is suggested that pressure perturbations propagate perpendicularly and acoustically within the resonator throat and cavity. The amplification observed in the vicinity of the resonator displays features typical of TSwave scattering; however, it is confirmed that this is not solely the result of mean flow distortion due to the geometry and recirculation region. Instead, the results indicate that the phenomenology is a consequence of the combination of scattering, localized nonmodal growth, and wallforcing in the wallnormal direction due to resonance.
On the interaction of Tollmien–Schlichting waves with a wallembedded Helmholtz resonator
10.1063/5.0141685
Physics of Fluids
20230306T10:55:58Z
© 2023 Author(s).
T. Michelis
C. de Koning
M. Kotsonis

Electrohydrodynamic viscous fingering of leaky dielectric fluids in a channel
https://aip.scitation.org/doi/10.1063/5.0140068?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Viscous fingering is a commonly observed interfacial instability during fluid displacement, where a fingerlike shape is formed at the fluid interface when a more viscous fluid is displaced by a less viscous fluid. In this study, a hybrid numerical model based on the lattice Boltzmann method and finite difference method is developed for investigating the control of viscous fingering of leaky dielectric fluids confined in a channel using electrohydrodynamics. Extensive simulations are carried out for studying the effects of the strength and direction of the electric field as well as the fluid properties, including the permittivity ratio and conductivity ratio, on viscous fingering. It is shown that a horizontal electric field, i.e., when the direction of the electrical field is perpendicular to the direction of fluid motion, can either promote or suppress the viscous fingering, depending on the permittivity ratio and conductivity ratio. For a vertical electric field, the extent of promotion of viscous fingering first decreases and then increases with the increase in conductivity ratio at a constant permittivity ratio. Also, various interfacial morphologies, such as broad fingers and thin jets, are observed under different fluid properties. A phase diagram for both the horizontal and vertical electric field is established based on the simulations with different permittivity and conductivity ratios to characterize the interfacial morphologies. This study offers insight into the electrohydrodynamic effects on the viscous fingering of leaky dielectric fluids, which could facilitate the control of multiphase flow in various applications, such as enhanced oil recovery and coupled chromatographic systems for separation.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Viscous fingering is a commonly observed interfacial instability during fluid displacement, where a fingerlike shape is formed at the fluid interface when a more viscous fluid is displaced by a less viscous fluid. In this study, a hybrid numerical model based on the lattice Boltzmann method and finite difference method is developed for investigating the control of viscous fingering of leaky dielectric fluids confined in a channel using electrohydrodynamics. Extensive simulations are carried out for studying the effects of the strength and direction of the electric field as well as the fluid properties, including the permittivity ratio and conductivity ratio, on viscous fingering. It is shown that a horizontal electric field, i.e., when the direction of the electrical field is perpendicular to the direction of fluid motion, can either promote or suppress the viscous fingering, depending on the permittivity ratio and conductivity ratio. For a vertical electric field, the extent of promotion of viscous fingering first decreases and then increases with the increase in conductivity ratio at a constant permittivity ratio. Also, various interfacial morphologies, such as broad fingers and thin jets, are observed under different fluid properties. A phase diagram for both the horizontal and vertical electric field is established based on the simulations with different permittivity and conductivity ratios to characterize the interfacial morphologies. This study offers insight into the electrohydrodynamic effects on the viscous fingering of leaky dielectric fluids, which could facilitate the control of multiphase flow in various applications, such as enhanced oil recovery and coupled chromatographic systems for separation.
Electrohydrodynamic viscous fingering of leaky dielectric fluids in a channel
10.1063/5.0140068
Physics of Fluids
20230307T11:19:05Z
© 2023 Author(s).
Jiachen Zhao
Zhongzheng Wang
Yuantong Gu
Emilie Sauret

Fluid dynamic mathematical aspects of supernova remnants
https://aip.scitation.org/doi/10.1063/5.0123930?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Supernovae—explosions of stars—are a central problem in astrophysics since they contain information on the entire process of stellar evolution and nucleosynthesis. Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities, developing during the supernova blast, lead to intense interfacial RT/RM mixing of the star's materials and couple astrophysical to atomic scales. This work analyzes some fluid dynamic mathematical aspects of the titanic task of supernova's blast. We handle mathematical challenges of RT/RM dynamics in supernova relevant conditions by directly linking the conservation laws governing RT/RM dynamics to symmetrybased momentum model, by exactly deriving the model parameters in the scaledependent and scaleinvariant regimes, and by exploring the special selfsimilar class for RT/RM interfacial mixing with variable accelerations. We reveal that RT/RM dynamics is strongly influenced by deterministic (the initial and the flow) conditions in the scaledependent linear and nonlinear regimes and in the selfsimilar mixing regime. The theory outcomes are consistent with the observations of supernova remnants, explain the results of the scaled laboratory experiments in high energy density plasmas, and yield the design of future experiments for the accurate quantification of RT/RM dynamics in supernova relevant conditions. We find that from fluid dynamic mathematical perspectives, supernovae can be regarded as an astrophysical initial value problem. Along with the guidance of what explodes at microscopic scales, supernova remnants encapsulate information on the explosion hydrodynamics and the associated deterministic conditions at macroscopic scales. We urge such effects be considered in interpretations of the observational data.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Supernovae—explosions of stars—are a central problem in astrophysics since they contain information on the entire process of stellar evolution and nucleosynthesis. Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities, developing during the supernova blast, lead to intense interfacial RT/RM mixing of the star's materials and couple astrophysical to atomic scales. This work analyzes some fluid dynamic mathematical aspects of the titanic task of supernova's blast. We handle mathematical challenges of RT/RM dynamics in supernova relevant conditions by directly linking the conservation laws governing RT/RM dynamics to symmetrybased momentum model, by exactly deriving the model parameters in the scaledependent and scaleinvariant regimes, and by exploring the special selfsimilar class for RT/RM interfacial mixing with variable accelerations. We reveal that RT/RM dynamics is strongly influenced by deterministic (the initial and the flow) conditions in the scaledependent linear and nonlinear regimes and in the selfsimilar mixing regime. The theory outcomes are consistent with the observations of supernova remnants, explain the results of the scaled laboratory experiments in high energy density plasmas, and yield the design of future experiments for the accurate quantification of RT/RM dynamics in supernova relevant conditions. We find that from fluid dynamic mathematical perspectives, supernovae can be regarded as an astrophysical initial value problem. Along with the guidance of what explodes at microscopic scales, supernova remnants encapsulate information on the explosion hydrodynamics and the associated deterministic conditions at macroscopic scales. We urge such effects be considered in interpretations of the observational data.
Fluid dynamic mathematical aspects of supernova remnants
10.1063/5.0123930
Physics of Fluids
20230308T06:25:51Z
© 2023 Author(s).
Snezhana I. Abarzhi
Desmond L. Hill
Kurt C. Williams
Jiahe T. Li
Bruce A. Remington
David Martinez
W. David Arnett

Roles of hydrogen bonding interactions and hydrophobic effects on enhanced water structure strength in aqueous alcohol solutions
https://aip.scitation.org/doi/10.1063/5.0142699?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The structure and dynamics of water in aqueous alcohol solutions were explored using twodimensional Raman correlation spectroscopy (2D RamanCOS) combined with the density functional theory (DFT). The spectral changes in the H–O–H bending and O:H stretching modes demonstrated that ethanol and npropanol induced an enhancement of the water structure compared to methanol. The extent of this effect was related to the length of the alkyl chain. Comparative studies with aqueous ethylene glycol solution revealed that an enhanced water structure stemmed mainly from hydrophobic effects rather than hydrogen bonding (Hbonding) interactions. Alcoholinduced waterspecific structural transitions were further analyzed using 2D RamanCOS, which showed that the free OH and strong Hbond structure of water respond preferentially to changes in alcohol content, inducing a transition in the weak Hbond structure of water. In addition, the 2D RamanCOS results indicated that the CH3 stretching mode of alcohol responds preferentially to variations in water content compared to other C–H vibrational modes. Finally, the details of the alcoholinduced water structural transitions were calculated using DFT. The 2D RamanCOS combined with DFT calculations provided insight into alcoholinduced water structural transitions and can be easily extended to other studies of waterorganic chemistry.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The structure and dynamics of water in aqueous alcohol solutions were explored using twodimensional Raman correlation spectroscopy (2D RamanCOS) combined with the density functional theory (DFT). The spectral changes in the H–O–H bending and O:H stretching modes demonstrated that ethanol and npropanol induced an enhancement of the water structure compared to methanol. The extent of this effect was related to the length of the alkyl chain. Comparative studies with aqueous ethylene glycol solution revealed that an enhanced water structure stemmed mainly from hydrophobic effects rather than hydrogen bonding (Hbonding) interactions. Alcoholinduced waterspecific structural transitions were further analyzed using 2D RamanCOS, which showed that the free OH and strong Hbond structure of water respond preferentially to changes in alcohol content, inducing a transition in the weak Hbond structure of water. In addition, the 2D RamanCOS results indicated that the CH3 stretching mode of alcohol responds preferentially to variations in water content compared to other C–H vibrational modes. Finally, the details of the alcoholinduced water structural transitions were calculated using DFT. The 2D RamanCOS combined with DFT calculations provided insight into alcoholinduced water structural transitions and can be easily extended to other studies of waterorganic chemistry.
Roles of hydrogen bonding interactions and hydrophobic effects on enhanced water structure strength in aqueous alcohol solutions
10.1063/5.0142699
Physics of Fluids
20230308T12:22:29Z
© 2023 Author(s).

On the origin of spanwise vortex deformations during the secondary instability stage in compressible mixing layers
https://aip.scitation.org/doi/10.1063/5.0140632?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The threedimensionality of turbulence initiates with spanwise vortex deformations associated with the amplification of threedimensional disturbance modes. However, the origin of spanwise vortex deformations is still not well understood. In this paper, compressible mixing layers are performed via direct numerical simulation (DNS). Two typical types of secondary instabilities producing spanwise vortex deformations are of consideration: fundamental instability and subharmonic instability. Based on the fast Fourier transform and DNS data, a lowrank velocity model v0 is obtained to demonstrate that spanwise vortex deformations are originated from a linear superposition of fundamental norm mode, a pair of fundamental or subharmonic oblique modes, and the mean mode. Through observing flow structures of the above norm and oblique modes, a striking feature is found that the velocity model v0 containing deformed spanwise vortices can be decomposed into three new velocity models v1, v2, and v3 containing relatively simplified counterparts (spanwise or oblique vortices). Then, the instability mechanism of the latter vortices is explored by analyzing the position relationship between the function of the generalized inflection points and cores of relatively simplified vortices. We find that an inviscid inflectional instability mechanism is responsible for the formation of spanwise and oblique vortices. Based on the above findings, a view is first proposed that spanwise vortex deformations with aligned and staggered patterns are a joint result of the parametric resonant mechanism and the inviscid inflectional instability mechanism.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The threedimensionality of turbulence initiates with spanwise vortex deformations associated with the amplification of threedimensional disturbance modes. However, the origin of spanwise vortex deformations is still not well understood. In this paper, compressible mixing layers are performed via direct numerical simulation (DNS). Two typical types of secondary instabilities producing spanwise vortex deformations are of consideration: fundamental instability and subharmonic instability. Based on the fast Fourier transform and DNS data, a lowrank velocity model v0 is obtained to demonstrate that spanwise vortex deformations are originated from a linear superposition of fundamental norm mode, a pair of fundamental or subharmonic oblique modes, and the mean mode. Through observing flow structures of the above norm and oblique modes, a striking feature is found that the velocity model v0 containing deformed spanwise vortices can be decomposed into three new velocity models v1, v2, and v3 containing relatively simplified counterparts (spanwise or oblique vortices). Then, the instability mechanism of the latter vortices is explored by analyzing the position relationship between the function of the generalized inflection points and cores of relatively simplified vortices. We find that an inviscid inflectional instability mechanism is responsible for the formation of spanwise and oblique vortices. Based on the above findings, a view is first proposed that spanwise vortex deformations with aligned and staggered patterns are a joint result of the parametric resonant mechanism and the inviscid inflectional instability mechanism.
On the origin of spanwise vortex deformations during the secondary instability stage in compressible mixing layers
10.1063/5.0140632
Physics of Fluids
20230310T12:19:49Z
© 2023 Author(s).

Receptivity and its influence on transition prediction of a hypersonic boundary layer over a small bluntness cone
https://aip.scitation.org/doi/10.1063/5.0141000?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Hypersonic boundarylayer receptivity is investigated using direct numerical simulation for Mach 6 flow over a 0.79 mm nose radius, 5° halfangle cone to slow acoustic waves. Two ways of exciting the second mode are discovered: For a lowfrequency forcing (region I where f < 370 kHz), the first mode is initially excited and subsequently evolves into the second mode, whereas for a highfrequency forcing (region II where [math] 370 kHz), the second mode is excited through the disturbances in the entropy layer. The receptivity mechanisms corresponding to the first and second modes are consistent with those of a previously investigated large bluntness cone. The receptivity coefficient shows a dramatical decrease with the frequency for disturbances in region I while a slight increase for disturbances in region II. The results show that the correlation between the receptivity coefficient of the first mode and the frequency proposed for large bluntness cones also applies to the current case. Furthermore, with receptivity and the measured freestream noise spectra taken into account, it is found that at the measured transition location, the disturbance predicted by the traditional eN method is no longer the most amplified disturbance, since the disturbances with lower frequencies can acquire larger amplitudes due to their large receptivity coefficients despite their smaller growth rates.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Hypersonic boundarylayer receptivity is investigated using direct numerical simulation for Mach 6 flow over a 0.79 mm nose radius, 5° halfangle cone to slow acoustic waves. Two ways of exciting the second mode are discovered: For a lowfrequency forcing (region I where f < 370 kHz), the first mode is initially excited and subsequently evolves into the second mode, whereas for a highfrequency forcing (region II where [math] 370 kHz), the second mode is excited through the disturbances in the entropy layer. The receptivity mechanisms corresponding to the first and second modes are consistent with those of a previously investigated large bluntness cone. The receptivity coefficient shows a dramatical decrease with the frequency for disturbances in region I while a slight increase for disturbances in region II. The results show that the correlation between the receptivity coefficient of the first mode and the frequency proposed for large bluntness cones also applies to the current case. Furthermore, with receptivity and the measured freestream noise spectra taken into account, it is found that at the measured transition location, the disturbance predicted by the traditional eN method is no longer the most amplified disturbance, since the disturbances with lower frequencies can acquire larger amplitudes due to their large receptivity coefficients despite their smaller growth rates.
Receptivity and its influence on transition prediction of a hypersonic boundary layer over a small bluntness cone
10.1063/5.0141000
Physics of Fluids
20230315T11:49:50Z
© 2023 Author(s).

Numerical reproduction of the spiral wave visualized experimentally in a widegap spherical Couette flow
https://aip.scitation.org/doi/10.1063/5.0144365?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Spherical Couette flow experiments were conducted according to the work of Egbers and Rath [Acta Mech. 111, 125–140 (1995)]. While the value of the critical Reynolds number obtained by the previous experiments was in good agreement with the numerical prediction, it has remained a question why a spiral wave bifurcating over the critical Reynolds number can be visualized even by a classical flow visualization technique like the mixing of a small amount of aluminum flakes to the working fluid. In the present study, through visualization using aluminum flakes drifting on a horizontal plane illuminated by a laser sheet, the flow was identified as a spiral wave with azimuthal wavenumber m = 3, using the experimentally obtained and numerically deduced comparison between phase velocities. By solving the equation of motion for the infinitesimal planar particles advecting in the flow field of the spiral wave, a visual distribution of reflected light was virtually reproduced, which is in good agreement with the experimentally obtained picture.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Spherical Couette flow experiments were conducted according to the work of Egbers and Rath [Acta Mech. 111, 125–140 (1995)]. While the value of the critical Reynolds number obtained by the previous experiments was in good agreement with the numerical prediction, it has remained a question why a spiral wave bifurcating over the critical Reynolds number can be visualized even by a classical flow visualization technique like the mixing of a small amount of aluminum flakes to the working fluid. In the present study, through visualization using aluminum flakes drifting on a horizontal plane illuminated by a laser sheet, the flow was identified as a spiral wave with azimuthal wavenumber m = 3, using the experimentally obtained and numerically deduced comparison between phase velocities. By solving the equation of motion for the infinitesimal planar particles advecting in the flow field of the spiral wave, a visual distribution of reflected light was virtually reproduced, which is in good agreement with the experimentally obtained picture.
Numerical reproduction of the spiral wave visualized experimentally in a widegap spherical Couette flow
10.1063/5.0144365
Physics of Fluids
20230316T01:44:04Z
© 2023 Author(s).

Effects of entropy layer on the boundary layer over hypersonic blunt cones considering chemical reactions
https://aip.scitation.org/doi/10.1063/5.0139146?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this study, the effects of entropy layer on the boundary layer over hypersonic blunt cones for a thermochemical equilibrium gas are investigated. The flow and stability characteristics of the entropy and boundary layers are presented considering the chemical reactions. It is found that the entropy layer has little influence on the inner layer inside the boundary layer. The inner layer thickness increases when chemical equilibrium is considered, which stems from the enhanced viscosity protection near the wall surface. At the leading edge of the blunt cone, due to the effects of the equilibrium gas, the temperature in the boundary and entropy layers decreases and the boundarylayeredge parameters significantly change. The entropy layer gradually vanishes along the downstream direction, and the effects of chemical reactions are concentrated in the boundary layer. The entropy swallowing point of the equilibrium gas does not significantly differ from that of the perfect gas. For all the analyzed cases, chemical reactions stabilize the entropy layer instability modes. The instability region and the frequency range of the equilibrium gas decrease, and the growth rate of the most unstable perturbation of the equilibrium gas is considerably smaller than that of the perfect gas. Moreover, the results show that the equilibrium gas has a minor influence on the process of the blunt cone boundary layer solution tending to the sharp cone solution.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this study, the effects of entropy layer on the boundary layer over hypersonic blunt cones for a thermochemical equilibrium gas are investigated. The flow and stability characteristics of the entropy and boundary layers are presented considering the chemical reactions. It is found that the entropy layer has little influence on the inner layer inside the boundary layer. The inner layer thickness increases when chemical equilibrium is considered, which stems from the enhanced viscosity protection near the wall surface. At the leading edge of the blunt cone, due to the effects of the equilibrium gas, the temperature in the boundary and entropy layers decreases and the boundarylayeredge parameters significantly change. The entropy layer gradually vanishes along the downstream direction, and the effects of chemical reactions are concentrated in the boundary layer. The entropy swallowing point of the equilibrium gas does not significantly differ from that of the perfect gas. For all the analyzed cases, chemical reactions stabilize the entropy layer instability modes. The instability region and the frequency range of the equilibrium gas decrease, and the growth rate of the most unstable perturbation of the equilibrium gas is considerably smaller than that of the perfect gas. Moreover, the results show that the equilibrium gas has a minor influence on the process of the blunt cone boundary layer solution tending to the sharp cone solution.
Effects of entropy layer on the boundary layer over hypersonic blunt cones considering chemical reactions
10.1063/5.0139146
Physics of Fluids
20230317T02:40:20Z
© 2023 Author(s).

Active transition control by synthetic jets in a hypersonic boundary layer
https://aip.scitation.org/doi/10.1063/5.0141091?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We investigate by direct numerical simulation the active control of laminarturbulent transition in a hypersonic flatplate boundary layer at a freestream Mach number of 5.86. The control mechanism is a synthetic jet. Based upon the linear stability theory of Mack, in hypersonic flow the important path to transition involves a highfrequency, secondmode fundamental resonance. Through systematic investigation, we reveal that the forcing the boundary layer with a synthetic jet at appropriate combinations of amplitude and frequency suppresses the second mode and delays transition. To gain physical insights into the major control mechanism, we employ the momentum potential theory (MPT) to analyze the flows with and without control. Essentially, the underlying control mechanism relies on an intriguing effect of the synthetic jet via generating the outward radiated wave structures, which are identified to split the upstream acoustic and vortical components. The splitting treatment presents the secondmode energy to drop sharply after the flow passes through the synthetic jet slot. The MPT sourceterm analysis reveals that the significantly suppressed nearwall source terms are responsible for suppressing the second mode downstream. Compared with the vortical and thermal source terms, the acoustic source term is found to be suppressed most. The kinetic budget analysis further reveals that the splitting treatment is related to the nonparallel effect and the nonlinear interaction.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We investigate by direct numerical simulation the active control of laminarturbulent transition in a hypersonic flatplate boundary layer at a freestream Mach number of 5.86. The control mechanism is a synthetic jet. Based upon the linear stability theory of Mack, in hypersonic flow the important path to transition involves a highfrequency, secondmode fundamental resonance. Through systematic investigation, we reveal that the forcing the boundary layer with a synthetic jet at appropriate combinations of amplitude and frequency suppresses the second mode and delays transition. To gain physical insights into the major control mechanism, we employ the momentum potential theory (MPT) to analyze the flows with and without control. Essentially, the underlying control mechanism relies on an intriguing effect of the synthetic jet via generating the outward radiated wave structures, which are identified to split the upstream acoustic and vortical components. The splitting treatment presents the secondmode energy to drop sharply after the flow passes through the synthetic jet slot. The MPT sourceterm analysis reveals that the significantly suppressed nearwall source terms are responsible for suppressing the second mode downstream. Compared with the vortical and thermal source terms, the acoustic source term is found to be suppressed most. The kinetic budget analysis further reveals that the splitting treatment is related to the nonparallel effect and the nonlinear interaction.
Active transition control by synthetic jets in a hypersonic boundary layer
10.1063/5.0141091
Physics of Fluids
20230317T02:47:40Z
© 2023 Author(s).
GuoHui Zhuang
ZhenHua Wan
ChuangChao Ye
ZhenBing Luo
NanSheng Liu
DeJun Sun
XiYun Lu

Enhanced effective diffusion in subwavelength, axonscale microchannels using surface acoustic waves
https://aip.scitation.org/doi/10.1063/5.0134605?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Excitation using surface acoustic waves (SAW) has demonstrated efficacy in improving microscale particle/chemical transport due to its ability to generate microscale wavelengths. However, the effects of acoustic stimulation on transport processes along the length of subwavelength microchannels and their underlying mechanisms, essential for longrange transport, have not been examined in detail. In this work, we investigate diffusion along the length of subwavelength microchannels using experimental and simulation approaches, demonstrating enhanced transport under SAW excitation. The microchannelbased enhanced diffusion mechanisms are further studied by investigating the acoustic pressure and streaming fields, finding that the degree of enhancement is a function of applied power, microchannel dimensions, and viscosity. This microchannelbased diffusion enhancement approach is applicable to microfluidic and biomedical microscale transport enhancement, with the findings here being relevant to acousticbased micromixing and neurodegenerative therapies.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Excitation using surface acoustic waves (SAW) has demonstrated efficacy in improving microscale particle/chemical transport due to its ability to generate microscale wavelengths. However, the effects of acoustic stimulation on transport processes along the length of subwavelength microchannels and their underlying mechanisms, essential for longrange transport, have not been examined in detail. In this work, we investigate diffusion along the length of subwavelength microchannels using experimental and simulation approaches, demonstrating enhanced transport under SAW excitation. The microchannelbased enhanced diffusion mechanisms are further studied by investigating the acoustic pressure and streaming fields, finding that the degree of enhancement is a function of applied power, microchannel dimensions, and viscosity. This microchannelbased diffusion enhancement approach is applicable to microfluidic and biomedical microscale transport enhancement, with the findings here being relevant to acousticbased micromixing and neurodegenerative therapies.
Enhanced effective diffusion in subwavelength, axonscale microchannels using surface acoustic waves
10.1063/5.0134605
Physics of Fluids
20230301T01:53:37Z
© 2023 Author(s).
Danli Peng
Wei Tong
David J. Collins
Michael R. Ibbotson
Steven Prawer
Melanie E. M. Stamp

Large eddy simulation of flow around two sidebyside circular cylinders at Reynolds number 3900
https://aip.scitation.org/doi/10.1063/5.0131708?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This paper investigates the flow dynamics around two circular cylinders in a sidebyside arrangement with different spacing ratios (T/D, T is the centertocenter cylinder spacing and D is the diameter) under a subcritical Reynolds number condition (Re = 3900). A threedimensional (3D) numerical model was developed with Large Eddy Simulation (LES) technique. The model was well validated against published data of flow around a single cylinder at Re = 3900. Numerical simulations were conducted for flow around two sidebyside circular cylinders with T/D = 1.2, 1.5, 1.75, 2, 2.5, 3, 3.5, and 4. Based on the LES results, three wake regimes were identified: single bluff body regime (T/D = 1.2), biased flow regime (T/D = 1.5–2), and parallel vortex streets regime (T/D = 2.5–4). In the single bluff body regime with T/D = 1.2, the stable deflection of gap flow is also observed which indicates that there may exist a transition state from the single bluff body regime to the biased flow regime. In biased flow regime, the pairing and merging process of the outer vortices with the inner vortices are analyzed. The occurrence of the flipflopping phenomenon is found to be related to the merging tendency between gapside vortices in narrow wake region and freeflowside vortices in wide wake region, and the relative phase of gap side vortices in transient state. In the parallel vortex streets regime, the phase relation of the vortex shedding process was analyzed. The time proportions of the inphase mode and antiphase mode are found to be varied with spacing ratio. As the spacing ratio increases, the wakes behind the cylinders lose their dependency on the antiphase mode. The results of the present study were compared with the existing results at other Reynolds numbers. It is found that vortex shedding manner during the flipover transitions is closely related to the spacing ratios and is independent of the Reynolds number.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This paper investigates the flow dynamics around two circular cylinders in a sidebyside arrangement with different spacing ratios (T/D, T is the centertocenter cylinder spacing and D is the diameter) under a subcritical Reynolds number condition (Re = 3900). A threedimensional (3D) numerical model was developed with Large Eddy Simulation (LES) technique. The model was well validated against published data of flow around a single cylinder at Re = 3900. Numerical simulations were conducted for flow around two sidebyside circular cylinders with T/D = 1.2, 1.5, 1.75, 2, 2.5, 3, 3.5, and 4. Based on the LES results, three wake regimes were identified: single bluff body regime (T/D = 1.2), biased flow regime (T/D = 1.5–2), and parallel vortex streets regime (T/D = 2.5–4). In the single bluff body regime with T/D = 1.2, the stable deflection of gap flow is also observed which indicates that there may exist a transition state from the single bluff body regime to the biased flow regime. In biased flow regime, the pairing and merging process of the outer vortices with the inner vortices are analyzed. The occurrence of the flipflopping phenomenon is found to be related to the merging tendency between gapside vortices in narrow wake region and freeflowside vortices in wide wake region, and the relative phase of gap side vortices in transient state. In the parallel vortex streets regime, the phase relation of the vortex shedding process was analyzed. The time proportions of the inphase mode and antiphase mode are found to be varied with spacing ratio. As the spacing ratio increases, the wakes behind the cylinders lose their dependency on the antiphase mode. The results of the present study were compared with the existing results at other Reynolds numbers. It is found that vortex shedding manner during the flipover transitions is closely related to the spacing ratios and is independent of the Reynolds number.
Large eddy simulation of flow around two sidebyside circular cylinders at Reynolds number 3900
10.1063/5.0131708
Physics of Fluids
20230301T01:51:29Z
© 2023 Author(s).

Experimental investigation on cylinder noise and its reductions by identifying aerodynamic sound sources in flow fields
https://aip.scitation.org/doi/10.1063/5.0138080?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Through anechoic wind tunnel tests, this study comprehensively investigates the noise and drag reductions on a circular cylinder with dimples. Dimples built on a surface pattern fabric cover the cylinder surface as one of the passive flow control methods. The force, noise, and flow field measurements are performed at diameterbased Reynolds numbers ranging from [math] to [math], covering the subcritical, critical, and supercritical regimes. The force and noise measurement results show that dimple fabric simultaneously reduces noise and drag in the critical regime. The changes in flow structures were characterized by the Timeresolved Particle Image Velocimetry (TRPIV) measurements. Based on the vortex sound theory, the flow analysis shows that the dominant sound sources are found to be concentrated near the cylinder surface, which is caused by the unsteady vortex motions near the separation locations during the process of vortex shedding. The crosscorrelation between the synchronized TRPIV and microphone measurements further supports the conclusions. Moreover, the cylinder noise reductions controlled by the dimples are directly associated with the reduced sound sources in the critical and supercritical regimes, corresponding to the reduced strength of the vortex shedding.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Through anechoic wind tunnel tests, this study comprehensively investigates the noise and drag reductions on a circular cylinder with dimples. Dimples built on a surface pattern fabric cover the cylinder surface as one of the passive flow control methods. The force, noise, and flow field measurements are performed at diameterbased Reynolds numbers ranging from [math] to [math], covering the subcritical, critical, and supercritical regimes. The force and noise measurement results show that dimple fabric simultaneously reduces noise and drag in the critical regime. The changes in flow structures were characterized by the Timeresolved Particle Image Velocimetry (TRPIV) measurements. Based on the vortex sound theory, the flow analysis shows that the dominant sound sources are found to be concentrated near the cylinder surface, which is caused by the unsteady vortex motions near the separation locations during the process of vortex shedding. The crosscorrelation between the synchronized TRPIV and microphone measurements further supports the conclusions. Moreover, the cylinder noise reductions controlled by the dimples are directly associated with the reduced sound sources in the critical and supercritical regimes, corresponding to the reduced strength of the vortex shedding.
Experimental investigation on cylinder noise and its reductions by identifying aerodynamic sound sources in flow fields
10.1063/5.0138080
Physics of Fluids
20230301T02:12:04Z
© 2023 Author(s).

Effects of vortex generator on the hydrodynamic characteristics of hydrofoil and horizontal axis tidal turbine
https://aip.scitation.org/doi/10.1063/5.0137951?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Tidal turbine blades are prone to flow separation in the boundary layer under high speed or high angle of attack, which will reduce energy efficiency and even the stall damage of the blades. This paper proposes introducing the flow control theory of vortex generators (VGs) to tidal turbines and studying the influence of VGs on the hydrodynamic characteristics of the tidal turbine blades. First, a numerical study is performed to investigate the effects of VGS on the hydrodynamic performance of the National Advisory Committee for Aeronautics (NACA) 4418 hydrofoil. The impact of different parameters, such as VG arrangement, spacing, height, and length, on the hydrodynamic performance of hydrofoil is studied by the computational fluid dynamics method. The results show that VGs can effectively suppress the flow separation and improve the maximum lift coefficient of the hydrofoil. The influence of VGs on flow separation characteristics of horizontal axis tidal turbines is studied by the CFD method. The results show that the flow separation of turbine blades mainly occurs at the root part of the suction surface, and the flow separation region expands radially as the flow velocity increases. VGs can effectively reduce the flow separation area on the suction side of turbine blades by suppressing the flow separation effect. Compared with the turbine blades without VGs, the power coefficient of turbine blades with VGs is increased by up to 5%. The flume experiment verifies the accuracy of the simulation results.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Tidal turbine blades are prone to flow separation in the boundary layer under high speed or high angle of attack, which will reduce energy efficiency and even the stall damage of the blades. This paper proposes introducing the flow control theory of vortex generators (VGs) to tidal turbines and studying the influence of VGs on the hydrodynamic characteristics of the tidal turbine blades. First, a numerical study is performed to investigate the effects of VGS on the hydrodynamic performance of the National Advisory Committee for Aeronautics (NACA) 4418 hydrofoil. The impact of different parameters, such as VG arrangement, spacing, height, and length, on the hydrodynamic performance of hydrofoil is studied by the computational fluid dynamics method. The results show that VGs can effectively suppress the flow separation and improve the maximum lift coefficient of the hydrofoil. The influence of VGs on flow separation characteristics of horizontal axis tidal turbines is studied by the CFD method. The results show that the flow separation of turbine blades mainly occurs at the root part of the suction surface, and the flow separation region expands radially as the flow velocity increases. VGs can effectively reduce the flow separation area on the suction side of turbine blades by suppressing the flow separation effect. Compared with the turbine blades without VGs, the power coefficient of turbine blades with VGs is increased by up to 5%. The flume experiment verifies the accuracy of the simulation results.
Effects of vortex generator on the hydrodynamic characteristics of hydrofoil and horizontal axis tidal turbine
10.1063/5.0137951
Physics of Fluids
20230301T02:12:08Z
© 2023 Author(s).

Hydrodynamics of a rotor–stator spinning disk reactor: Investigations by largeeddy simulation
https://aip.scitation.org/doi/10.1063/5.0137405?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this work, computational fluid dynamics are used to study the hydrodynamics in a complete rotor–stator spinning disk reactor with throughflow. Largeeddy simulations of OpenFOAM 9 were used to capture the turbulent structures of the flow in combination with the walladapting local eddy viscosity subgridscale model. The method was validated based on residence time distributions (RTDs) for a range of rotational Reynolds numbers (Re = [math] = 3.2–52 × 104) and a dimensionless flow rate (Cw = Q [math]) of 150 and G = 0.0303 (G = h [math]). The experimental RTD was obtained from tracer experiments with UV/VIS flow cells. From the RTD, the plug flow (PFR) volume fraction, the Péclet number, and the radial position (rtrans) where the flow changes from PFR into ideally mixed were determined by using an engineering model based on axial dispersion. For the turbulent cases, good agreement based on the RTD curve, PFR volume, the Péclet number, and rtrans was found. Furthermore, the boundary layer thickness on the rotor and stator and the entrainment coefficient were in good agreement with the literature. Finally, the turbulent intensity was analyzed illustrating a high intensity at the rim of the rotor and was 10% larger in centripetal flow compared to centrifugal flow.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this work, computational fluid dynamics are used to study the hydrodynamics in a complete rotor–stator spinning disk reactor with throughflow. Largeeddy simulations of OpenFOAM 9 were used to capture the turbulent structures of the flow in combination with the walladapting local eddy viscosity subgridscale model. The method was validated based on residence time distributions (RTDs) for a range of rotational Reynolds numbers (Re = [math] = 3.2–52 × 104) and a dimensionless flow rate (Cw = Q [math]) of 150 and G = 0.0303 (G = h [math]). The experimental RTD was obtained from tracer experiments with UV/VIS flow cells. From the RTD, the plug flow (PFR) volume fraction, the Péclet number, and the radial position (rtrans) where the flow changes from PFR into ideally mixed were determined by using an engineering model based on axial dispersion. For the turbulent cases, good agreement based on the RTD curve, PFR volume, the Péclet number, and rtrans was found. Furthermore, the boundary layer thickness on the rotor and stator and the entrainment coefficient were in good agreement with the literature. Finally, the turbulent intensity was analyzed illustrating a high intensity at the rim of the rotor and was 10% larger in centripetal flow compared to centrifugal flow.
Hydrodynamics of a rotor–stator spinning disk reactor: Investigations by largeeddy simulation
10.1063/5.0137405
Physics of Fluids
20230303T12:57:48Z
© 2023 Author(s).
C. J. W. Hop
R. Jansen
M. Besten
A. Chaudhuri
M. W. Baltussen
J. van der Schaaf

Thermal largeeddy simulation methods to model highly anisothermal and turbulent flows
https://aip.scitation.org/doi/10.1063/5.0139433?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Thermal largeeddy simulations (TLES) of highly anisothermal and turbulent channel flows are assessed using direct numerical simulations (DNS). The investigated conditions are representative of solar receivers used in concentrated solar power towers. Four thermal operating conditions are considered. They aim to study several locations in the solar receiver. They are distinguished by different temperature profiles and thus different wall heat fluxes. The mean friction Reynolds number is close to 800 for all the simulations. The Navier–Stokes equations are solved under the lowMachnumber approximation. The nonlinear terms corresponding to the velocity–velocity and the velocity–temperature correlations are modeled. Functional, structural, and mixed models are investigated. An extension of the anisotropic minimum dissipation (AMD) model to compressible case and twolayer mixed models are proposed and assessed. Fourthorder and secondorder centered schemes are tested for the discretization of the momentum convection term. First, a global assessment of 16TLES approaches on mean quantities and correlations for three different meshes is performed in reference conditions. Then, three of the TLES are selected for more detailed analyses. The mesh effect and the influence of the thermal conditions on the model accuracy are investigated. These detailed studies consist of the comparison of the relative error of the TLES on mean quantities and correlations and the visualization of the normalized profiles as functions of the wallnormal distance. The results highlight the good agreement of twolayer mixed models consisting of the combination of the Bardina and the AMD models with the DNS for the three tested meshes.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Thermal largeeddy simulations (TLES) of highly anisothermal and turbulent channel flows are assessed using direct numerical simulations (DNS). The investigated conditions are representative of solar receivers used in concentrated solar power towers. Four thermal operating conditions are considered. They aim to study several locations in the solar receiver. They are distinguished by different temperature profiles and thus different wall heat fluxes. The mean friction Reynolds number is close to 800 for all the simulations. The Navier–Stokes equations are solved under the lowMachnumber approximation. The nonlinear terms corresponding to the velocity–velocity and the velocity–temperature correlations are modeled. Functional, structural, and mixed models are investigated. An extension of the anisotropic minimum dissipation (AMD) model to compressible case and twolayer mixed models are proposed and assessed. Fourthorder and secondorder centered schemes are tested for the discretization of the momentum convection term. First, a global assessment of 16TLES approaches on mean quantities and correlations for three different meshes is performed in reference conditions. Then, three of the TLES are selected for more detailed analyses. The mesh effect and the influence of the thermal conditions on the model accuracy are investigated. These detailed studies consist of the comparison of the relative error of the TLES on mean quantities and correlations and the visualization of the normalized profiles as functions of the wallnormal distance. The results highlight the good agreement of twolayer mixed models consisting of the combination of the Bardina and the AMD models with the DNS for the three tested meshes.
Thermal largeeddy simulation methods to model highly anisothermal and turbulent flows
10.1063/5.0139433
Physics of Fluids
20230303T12:58:09Z
© 2023 Author(s).
M. David
A. Toutant
F. Bataille

Decomposition of the skinfriction coefficient of compressible boundary layers
https://aip.scitation.org/doi/10.1063/5.0142129?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We derive an integral formula for the skinfriction coefficient of compressible boundary layers by extending the formula of Elnahhas and Johnson [“On the enhancement of boundary layer skin friction by turbulence: An angular momentum approach,” J. Fluid Mech. 940, A36 (2022)] for incompressible boundary layers. The skinfriction coefficient is decomposed into the sum of the contributions of the laminar coefficient, the change of the dynamic viscosity with the temperature, the Favre–Reynolds stresses, and the mean flow. This decomposition is applied to numerical data for laminar and turbulent boundary layers, and the role of each term on the wallshear stress is quantified. We also show that the threefold integration identity of Gomez et al. [“Contribution of Reynolds stress distribution to the skin friction in compressible turbulent channel flows,” Phys. Rev. E 79(3), 035301 (2009)] and the twofold integration identities of Wenzel et al. [“About the influences of compressibility, heat transfer and pressure gradients in compressible turbulent boundary layers,” J. Fluid Mech. 930, A1 (2022)] and Xu et al. [“Skinfriction and heattransfer decompositions in hypersonic transitional and turbulent boundary layers,” J. Fluid Mech. 941, A4 (2022)] for turbulent boundary layers all simplify to the compressible von Kármán momentum integral equation when the upper limit of integration is asymptotically large. The dependence of these identities on the upper integration bound is studied. By using asymptotic methods, we prove that the multipleintegration identity of Wenzel et al. [“About the influences of compressibility, heat transfer and pressure gradients in compressible turbulent boundary layers,” J. Fluid Mech. 930, A1 (2022)] degenerates to the definition of the skinfriction coefficient when the number of integrations is asymptotically large.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We derive an integral formula for the skinfriction coefficient of compressible boundary layers by extending the formula of Elnahhas and Johnson [“On the enhancement of boundary layer skin friction by turbulence: An angular momentum approach,” J. Fluid Mech. 940, A36 (2022)] for incompressible boundary layers. The skinfriction coefficient is decomposed into the sum of the contributions of the laminar coefficient, the change of the dynamic viscosity with the temperature, the Favre–Reynolds stresses, and the mean flow. This decomposition is applied to numerical data for laminar and turbulent boundary layers, and the role of each term on the wallshear stress is quantified. We also show that the threefold integration identity of Gomez et al. [“Contribution of Reynolds stress distribution to the skin friction in compressible turbulent channel flows,” Phys. Rev. E 79(3), 035301 (2009)] and the twofold integration identities of Wenzel et al. [“About the influences of compressibility, heat transfer and pressure gradients in compressible turbulent boundary layers,” J. Fluid Mech. 930, A1 (2022)] and Xu et al. [“Skinfriction and heattransfer decompositions in hypersonic transitional and turbulent boundary layers,” J. Fluid Mech. 941, A4 (2022)] for turbulent boundary layers all simplify to the compressible von Kármán momentum integral equation when the upper limit of integration is asymptotically large. The dependence of these identities on the upper integration bound is studied. By using asymptotic methods, we prove that the multipleintegration identity of Wenzel et al. [“About the influences of compressibility, heat transfer and pressure gradients in compressible turbulent boundary layers,” J. Fluid Mech. 930, A1 (2022)] degenerates to the definition of the skinfriction coefficient when the number of integrations is asymptotically large.
Decomposition of the skinfriction coefficient of compressible boundary layers
10.1063/5.0142129
Physics of Fluids
20230303T12:57:52Z
© 2023 Author(s).
Pierre Ricco
Lian Duan

Research and numerical analysis of floating offshore wind turbine based on a novel extended tuned mass damper
https://aip.scitation.org/doi/10.1063/5.0130881?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Offshore wind turbines will be developed from shallow water to deep water to meet the rapid growth of wind power generation. Floating offshore wind turbine (FOWT) faces complex load challenges, which endanger its safety and service life. Hence, it is urgent to develop a novel damping device to improve the stability of FOWT. In this paper, a novel extended tuned mass damper (ETMD) is proposed. On this basis, a linear quadratic regulator is added to realize the design and simulation of the extended active tuned mass damper (EATMD) control system to reduce the surge response of FOWT. Numerical analysis shows that under the control of ETMD, the surge response of the tower is reduced by 73%, and the frequency modulation width is increased by 55%. Here, under the control of EATMD, the displacement, velocity, and acceleration of the surge response of the tower are decreased by 47.0%, 30.7%, and 24.2%, respectively.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Offshore wind turbines will be developed from shallow water to deep water to meet the rapid growth of wind power generation. Floating offshore wind turbine (FOWT) faces complex load challenges, which endanger its safety and service life. Hence, it is urgent to develop a novel damping device to improve the stability of FOWT. In this paper, a novel extended tuned mass damper (ETMD) is proposed. On this basis, a linear quadratic regulator is added to realize the design and simulation of the extended active tuned mass damper (EATMD) control system to reduce the surge response of FOWT. Numerical analysis shows that under the control of ETMD, the surge response of the tower is reduced by 73%, and the frequency modulation width is increased by 55%. Here, under the control of EATMD, the displacement, velocity, and acceleration of the surge response of the tower are decreased by 47.0%, 30.7%, and 24.2%, respectively.
Research and numerical analysis of floating offshore wind turbine based on a novel extended tuned mass damper
10.1063/5.0130881
Physics of Fluids
20230307T11:19:20Z
© 2023 Author(s).

Investigation of audible sound waves on the heat transfer characteristics of airtoair heat exchange systems
https://aip.scitation.org/doi/10.1063/5.0139945?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>To reveal the mechanism of the effect of audible sound waves on the heat transfer process, the flow and heat transfer characteristics of an airtoair heat exchanger were analyzed by incidence of sound waves with different intensities on its cold, hot, and both sides. The results showed that the sound waves incident on the cold side enhanced the heat exchange between the unstable cold flow and hot surface, which decreased the surface temperature of the latter with an increasing sound pressure level (SPL). In contrast, the sound waves incident on the hot side increased the surface temperature, thereby enhancing the heat transfer performance. When the SPL increased to 140 dB, the average surface heat flux increased by 8.22% and 15.19% under the sound waves incident on the cold and hot sides, respectively, whereas the sound energy efficiency was relatively higher with the sound waves incident on the cold side. Additionally, under the synergetic effect of the incidence of sound waves on both sides on the flow characteristics, the average surface heat flux increased by 25.56%. It was higher than the summation of the corresponding fluxes under the incidence of sound waves on single side, while the sound energy efficiency decreased under high SPL. The results indicated that sound waves incident on both sides can effectively enhance the heat transfer performance. This research is significant for the application of sound waves on the heat transfer process of airtoair heat exchange systems.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>To reveal the mechanism of the effect of audible sound waves on the heat transfer process, the flow and heat transfer characteristics of an airtoair heat exchanger were analyzed by incidence of sound waves with different intensities on its cold, hot, and both sides. The results showed that the sound waves incident on the cold side enhanced the heat exchange between the unstable cold flow and hot surface, which decreased the surface temperature of the latter with an increasing sound pressure level (SPL). In contrast, the sound waves incident on the hot side increased the surface temperature, thereby enhancing the heat transfer performance. When the SPL increased to 140 dB, the average surface heat flux increased by 8.22% and 15.19% under the sound waves incident on the cold and hot sides, respectively, whereas the sound energy efficiency was relatively higher with the sound waves incident on the cold side. Additionally, under the synergetic effect of the incidence of sound waves on both sides on the flow characteristics, the average surface heat flux increased by 25.56%. It was higher than the summation of the corresponding fluxes under the incidence of sound waves on single side, while the sound energy efficiency decreased under high SPL. The results indicated that sound waves incident on both sides can effectively enhance the heat transfer performance. This research is significant for the application of sound waves on the heat transfer process of airtoair heat exchange systems.
Investigation of audible sound waves on the heat transfer characteristics of airtoair heat exchange systems
10.1063/5.0139945
Physics of Fluids
20230307T11:18:53Z
© 2023 Author(s).
Chang Guo
Wei Wei
Cong Wang
Zhigang Liu
Lin Guo
Ming Gao

Turbulent flow on a rotating disk
https://aip.scitation.org/doi/10.1063/5.0137638?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The dissipation theorem is applied to describe fully developed turbulent flow on a rotating disk. A complicated coordinate transformation is proposed to solve the problem. The similarity exponent m becomes a parameter of the problem and can vary gradually with the Reynolds number. This solution may span the entire range from laminar to fully developed turbulent flow, although not accounting for all features observed in the transition region. The profiles of the various quantities are required to be well behaved, and this requirement is used to determine m at different Reynolds numbers. The results are in harmony with the literature and show how different geometries can be treated in a unified manner, with differences arising naturally from those geometric differences but with utilization of similar forms for the decay of the dissipation.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The dissipation theorem is applied to describe fully developed turbulent flow on a rotating disk. A complicated coordinate transformation is proposed to solve the problem. The similarity exponent m becomes a parameter of the problem and can vary gradually with the Reynolds number. This solution may span the entire range from laminar to fully developed turbulent flow, although not accounting for all features observed in the transition region. The profiles of the various quantities are required to be well behaved, and this requirement is used to determine m at different Reynolds numbers. The results are in harmony with the literature and show how different geometries can be treated in a unified manner, with differences arising naturally from those geometric differences but with utilization of similar forms for the decay of the dissipation.
Turbulent flow on a rotating disk
10.1063/5.0137638
Physics of Fluids
20230307T11:19:25Z
© 2023 Author(s).
John Newman

On the cylinder noise and drag reductions in different Reynolds number ranges using surface pattern fabrics
https://aip.scitation.org/doi/10.1063/5.0138074?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study experimentally investigates the potential of using surface pattern fabrics for the cylinder noise and drag control in different Reynolds number ranges. The aerodynamic and aeroacoustic effects were evaluated through the noise and force measurements in an anechoic wind tunnel. It was observed that the noise and drag reductions take place simultaneously but in different Reynolds number ranges, corresponding to the cylinder flow in different flow regimes, e.g., subcritical, critical, and supercritical flow regimes. Microphone arc array measurements reveal that the suppression of the Aeolian tone in the critical regime is the major cause of noise reductions, and the noise directivity gradually loses dipole features in the critical and supercritical flow regimes, which is probably related to the reduced lift fluctuation coefficient and the spanwise segment of the sound sources. Further hotwire wake survey revealed significant changes in flow dynamics, which explain the variations of noise and drag in different flow regimes. We have shown for the first time that fabric with different surface patterns can effectively reduce cylinder drag and noise in different Reynolds number ranges. Since the Reynolds number is a key factor that determines the flow state in practical engineering applications, e.g., cycling aerodynamics, this study suggests that optimal drag and noise reductions can be realized by employing the combinations of different surface pattern fabrics to account for the Reynolds number effects.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study experimentally investigates the potential of using surface pattern fabrics for the cylinder noise and drag control in different Reynolds number ranges. The aerodynamic and aeroacoustic effects were evaluated through the noise and force measurements in an anechoic wind tunnel. It was observed that the noise and drag reductions take place simultaneously but in different Reynolds number ranges, corresponding to the cylinder flow in different flow regimes, e.g., subcritical, critical, and supercritical flow regimes. Microphone arc array measurements reveal that the suppression of the Aeolian tone in the critical regime is the major cause of noise reductions, and the noise directivity gradually loses dipole features in the critical and supercritical flow regimes, which is probably related to the reduced lift fluctuation coefficient and the spanwise segment of the sound sources. Further hotwire wake survey revealed significant changes in flow dynamics, which explain the variations of noise and drag in different flow regimes. We have shown for the first time that fabric with different surface patterns can effectively reduce cylinder drag and noise in different Reynolds number ranges. Since the Reynolds number is a key factor that determines the flow state in practical engineering applications, e.g., cycling aerodynamics, this study suggests that optimal drag and noise reductions can be realized by employing the combinations of different surface pattern fabrics to account for the Reynolds number effects.
On the cylinder noise and drag reductions in different Reynolds number ranges using surface pattern fabrics
10.1063/5.0138074
Physics of Fluids
20230307T11:18:51Z
© 2023 Author(s).

Effects of combustion on the nearwall turbulence and performance for supersonic hydrogen film cooling using large eddy simulation
https://aip.scitation.org/doi/10.1063/5.0139355?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Supersonic film cooling using fuel on board is an effective way to simultaneously shield the huge heat and momentum flux transported from the mainstream to the wall in a scramjet engine. The selfignition and combustion of the injected fuel film will significantly change the turbulent transport behavior in the boundary layer. To reveal the effects of the boundary layer combustion on the nearwall turbulence and wall fluxes, large eddy simulations (LES) of the Burrows–Kurkov supersonic combustion experiment using hydrogen as a film are performed based on the inhouse solver scramjetFoam. The solver successfully captures the additional skin friction reduction phenomenon induced by the boundary layer combustion compared to other numerical works using LES in the public literature. The results reveal that further increased anisotropy of turbulence combined with the lowdensity region contributes to a remarkable suppression of turbulent transport processes in the wallnormal direction. The selfignition point of the hydrogen film is found to oscillate back and forth in a span of 80 mm, which significantly enhances turbulence in the boundary layer. However, the increased turbulent fluctuating velocity is mainly concentrated in the streamwise direction, while the other two components are suppressed instead. The findings are also essential for improving engineering computations based on the Reynolds averaged simulation method.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Supersonic film cooling using fuel on board is an effective way to simultaneously shield the huge heat and momentum flux transported from the mainstream to the wall in a scramjet engine. The selfignition and combustion of the injected fuel film will significantly change the turbulent transport behavior in the boundary layer. To reveal the effects of the boundary layer combustion on the nearwall turbulence and wall fluxes, large eddy simulations (LES) of the Burrows–Kurkov supersonic combustion experiment using hydrogen as a film are performed based on the inhouse solver scramjetFoam. The solver successfully captures the additional skin friction reduction phenomenon induced by the boundary layer combustion compared to other numerical works using LES in the public literature. The results reveal that further increased anisotropy of turbulence combined with the lowdensity region contributes to a remarkable suppression of turbulent transport processes in the wallnormal direction. The selfignition point of the hydrogen film is found to oscillate back and forth in a span of 80 mm, which significantly enhances turbulence in the boundary layer. However, the increased turbulent fluctuating velocity is mainly concentrated in the streamwise direction, while the other two components are suppressed instead. The findings are also essential for improving engineering computations based on the Reynolds averaged simulation method.
Effects of combustion on the nearwall turbulence and performance for supersonic hydrogen film cooling using large eddy simulation
10.1063/5.0139355
Physics of Fluids
20230307T11:19:11Z
© 2023 Author(s).

Physically based formula for the maximum scour depth induced by a propeller jet
https://aip.scitation.org/doi/10.1063/5.0140666?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In the last two decades, many studies have been dedicated to the employment of predictive formulas able to assess the maximum scour depth produced by propeller jets. Owing to the complexity of the phenomenon, most of the literature formulas were built on empirical arguments, making them susceptible to scale issues and not fully coherent with the physics underpinning the scouring problem. Recent studies exploited the phenomenological theory of turbulence and the paradigms of sediment incipientmotion theory to derive a predictive formula of the maximum equilibrium scour depth in different cases: scour produced by jet, scour at bridge piers, and scour downstream of hydrokinetic turbines. In the present study, using the same considerations of the aforementioned works, we propose a new model that allows the derivation of a predictive physically based formula that includes all the relevant parameters controlling the scouring process induced by the propeller rotation. The theory is validated at the laboratory scale with experimental published data: a good agreement between theoretical predictions and literature data is shown. The proposed design formula clearly indicates that, when tested against experimental data, it leads to lower scattering levels than those obtained with empirical literature formulas.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In the last two decades, many studies have been dedicated to the employment of predictive formulas able to assess the maximum scour depth produced by propeller jets. Owing to the complexity of the phenomenon, most of the literature formulas were built on empirical arguments, making them susceptible to scale issues and not fully coherent with the physics underpinning the scouring problem. Recent studies exploited the phenomenological theory of turbulence and the paradigms of sediment incipientmotion theory to derive a predictive formula of the maximum equilibrium scour depth in different cases: scour produced by jet, scour at bridge piers, and scour downstream of hydrokinetic turbines. In the present study, using the same considerations of the aforementioned works, we propose a new model that allows the derivation of a predictive physically based formula that includes all the relevant parameters controlling the scouring process induced by the propeller rotation. The theory is validated at the laboratory scale with experimental published data: a good agreement between theoretical predictions and literature data is shown. The proposed design formula clearly indicates that, when tested against experimental data, it leads to lower scattering levels than those obtained with empirical literature formulas.
Physically based formula for the maximum scour depth induced by a propeller jet
10.1063/5.0140666
Physics of Fluids
20230307T11:19:19Z
© 2023 Author(s).
Francesco Coscarella
Giuseppe Curulli
Nadia Penna
Roberto Gaudio

Comparison of flow characteristics behind squareback bluffbodies with and without wheels
https://aip.scitation.org/doi/10.1063/5.0138305?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The wake dynamics of two referenced variations of the squareback Windsor model with and without wheels is numerically studied by performing improved delayed detached eddy simulation. Numerical assessments are validated against publicly available experimental data. The focus of this study is on the wake states influenced by the wheels and the thick oncoming floor boundary layer. Results show that the addition of the wheels significantly changes the aerodynamic forces, the underbody flow, and the wake topology. The wake bistability is also enhanced with wheels in place due to the increased curvature of lateral shear layers in the near wake. However, the bistable behavior is largely suppressed when immersed in a thick boundary layer. These alterations depend on the degree of interaction between the wake recirculation and the bottom flow, and such degree is strongly affected by the underbody flow momentum. The evolution of loworder flow organizations and complementary spectral analysis highlight the differences in the coherent dynamics of the wake. The finding of this present work suggests that the wake bistability behind the squareback body can exist not only for a simplified geometry but also for a more realistic car with wheels in realworld upstream conditions.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The wake dynamics of two referenced variations of the squareback Windsor model with and without wheels is numerically studied by performing improved delayed detached eddy simulation. Numerical assessments are validated against publicly available experimental data. The focus of this study is on the wake states influenced by the wheels and the thick oncoming floor boundary layer. Results show that the addition of the wheels significantly changes the aerodynamic forces, the underbody flow, and the wake topology. The wake bistability is also enhanced with wheels in place due to the increased curvature of lateral shear layers in the near wake. However, the bistable behavior is largely suppressed when immersed in a thick boundary layer. These alterations depend on the degree of interaction between the wake recirculation and the bottom flow, and such degree is strongly affected by the underbody flow momentum. The evolution of loworder flow organizations and complementary spectral analysis highlight the differences in the coherent dynamics of the wake. The finding of this present work suggests that the wake bistability behind the squareback body can exist not only for a simplified geometry but also for a more realistic car with wheels in realworld upstream conditions.
Comparison of flow characteristics behind squareback bluffbodies with and without wheels
10.1063/5.0138305
Physics of Fluids
20230308T12:22:47Z
© 2023 Author(s).
Siniša Krajnović

Onedimensional turbulence modeling of compressible flows. I. Conservative Eulerian formulation and application to supersonic channel flow
https://aip.scitation.org/doi/10.1063/5.0125514?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Accurate but economical modeling of supersonic turbulent boundary layers is a standing challenge due to the intricate entanglement of temperature, density, and velocity fluctuations on top of the meanfield variation. Application of the van Driest transformation may describe well the mean state but cannot provide detailed flow information. This lackin modeling coarse and finescale variability is addressed by the present study using a stochastic onedimensional turbulence (ODT) model. ODT is a simulation methodology that represents the evolution of turbulent flow in a lowdimensional stochastic way. In this study, ODT is extended to fully compressible flows. An Eulerian framework and a conservative form of the governing equations serve as the basis of the compressible ODT model. Computational methods for statistical properties based on ODT realizations are also extended to compressible flows, and a comprehensive way of turbulent kinetic energy budget calculation based on compressible ODT is put forward for the first time. Two canonical direct numerical simulation cases of supersonic isothermalwall channel flow at Mach numbers 1.5 and 3.0 with bulk Reynolds numbers 3000 and 4880, respectively, are used to validate the extended model. A rigorous numerical validation is presented, including the firstorder mean statistics, the secondorder root mean square statistics, and higherorder turbulent fluctuation statistics. In ODT results, both mean and root mean square profiles are accurately captured in the nearwall region. Nearwall temperature spectra reveal that temperature fluctuations are amplified at all turbulent scales as the effects of compressibility increase. This phenomenon is caused by intensified viscous heating at a higher Mach number, which is indicated by the steeper profiles of viscous turbulent kinetic energy budget terms in the very nearwall region. The low computational cost and predictive capabilities of ODT suggest that it is a promising approach for detailed modeling of highly turbulent compressible boundary layers. Furthermore, it is found that the ODT model requires a Machnumberdependent increase in a viscous penalty parameter Z in wallbounded turbulent flows to enable accurate capture of the buffer layer.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Accurate but economical modeling of supersonic turbulent boundary layers is a standing challenge due to the intricate entanglement of temperature, density, and velocity fluctuations on top of the meanfield variation. Application of the van Driest transformation may describe well the mean state but cannot provide detailed flow information. This lackin modeling coarse and finescale variability is addressed by the present study using a stochastic onedimensional turbulence (ODT) model. ODT is a simulation methodology that represents the evolution of turbulent flow in a lowdimensional stochastic way. In this study, ODT is extended to fully compressible flows. An Eulerian framework and a conservative form of the governing equations serve as the basis of the compressible ODT model. Computational methods for statistical properties based on ODT realizations are also extended to compressible flows, and a comprehensive way of turbulent kinetic energy budget calculation based on compressible ODT is put forward for the first time. Two canonical direct numerical simulation cases of supersonic isothermalwall channel flow at Mach numbers 1.5 and 3.0 with bulk Reynolds numbers 3000 and 4880, respectively, are used to validate the extended model. A rigorous numerical validation is presented, including the firstorder mean statistics, the secondorder root mean square statistics, and higherorder turbulent fluctuation statistics. In ODT results, both mean and root mean square profiles are accurately captured in the nearwall region. Nearwall temperature spectra reveal that temperature fluctuations are amplified at all turbulent scales as the effects of compressibility increase. This phenomenon is caused by intensified viscous heating at a higher Mach number, which is indicated by the steeper profiles of viscous turbulent kinetic energy budget terms in the very nearwall region. The low computational cost and predictive capabilities of ODT suggest that it is a promising approach for detailed modeling of highly turbulent compressible boundary layers. Furthermore, it is found that the ODT model requires a Machnumberdependent increase in a viscous penalty parameter Z in wallbounded turbulent flows to enable accurate capture of the buffer layer.
Onedimensional turbulence modeling of compressible flows. I. Conservative Eulerian formulation and application to supersonic channel flow
10.1063/5.0125514
Physics of Fluids
20230308T12:52:49Z
© 2023 Author(s).
Heiko Schmidt
Marten Klein

Onedimensional turbulence modeling of compressible flows: II. Full compressible modification and application to shock–turbulence interaction
https://aip.scitation.org/doi/10.1063/5.0137435?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Onedimensional turbulence (ODT) is a simulation methodology that represents the essential physics of threedimensional turbulence through stochastic resolution of the full range of length and time scales on a onedimensional domain. In the present study, full compressible modifications are incorporated into ODT methodology, based on an Eulerian framework and a conservative form of the governing equations. In the deterministic part of this approach, a shock capturing scheme is introduced for the first time. In the stochastic part, onedimensional eddy events are modeled and sampled according to standard methods for compressible flow simulation. Time advancement adjustments are made to balance comparable time steps between the deterministic and stochastic parts in compressible flows. Canonical shock–turbulence interaction cases involving Richtmyer–Meshkov instability at Mach numbers 1.24, 1.5, and 1.98 are simulated to validate the extended model. The ODT results are compared with available reference data from large eddy simulations and laboratory experiments. The introduction of a shock capturing scheme significantly improves the performance of the ODT method, and the results for turbulent kinetic energy are qualitatively improved compared with those of a previous compressible Lagrangian ODT method [Jozefik et al., “Simulation of shock–turbulence interaction in nonreactive flow and in turbulent deflagration and detonation regimes using onedimensional turbulence,” Combust. Flame 164, 53 (2016)]. For the time evolution of profiles of the turbulent mixing zone width, ensembleaveraged density, and specific heat ratio, the new model also yields good to reasonable results. Furthermore, it is found that the viscous penalty parameter Z of the ODT model is insensitive to compressibility effects in turbulent flows without wall effects. A small value of Z is appropriate for turbulent flows with weak wall effects, and the parameter Z serves to suppress extremely small eddy events that would be dissipated instantly by viscosity.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Onedimensional turbulence (ODT) is a simulation methodology that represents the essential physics of threedimensional turbulence through stochastic resolution of the full range of length and time scales on a onedimensional domain. In the present study, full compressible modifications are incorporated into ODT methodology, based on an Eulerian framework and a conservative form of the governing equations. In the deterministic part of this approach, a shock capturing scheme is introduced for the first time. In the stochastic part, onedimensional eddy events are modeled and sampled according to standard methods for compressible flow simulation. Time advancement adjustments are made to balance comparable time steps between the deterministic and stochastic parts in compressible flows. Canonical shock–turbulence interaction cases involving Richtmyer–Meshkov instability at Mach numbers 1.24, 1.5, and 1.98 are simulated to validate the extended model. The ODT results are compared with available reference data from large eddy simulations and laboratory experiments. The introduction of a shock capturing scheme significantly improves the performance of the ODT method, and the results for turbulent kinetic energy are qualitatively improved compared with those of a previous compressible Lagrangian ODT method [Jozefik et al., “Simulation of shock–turbulence interaction in nonreactive flow and in turbulent deflagration and detonation regimes using onedimensional turbulence,” Combust. Flame 164, 53 (2016)]. For the time evolution of profiles of the turbulent mixing zone width, ensembleaveraged density, and specific heat ratio, the new model also yields good to reasonable results. Furthermore, it is found that the viscous penalty parameter Z of the ODT model is insensitive to compressibility effects in turbulent flows without wall effects. A small value of Z is appropriate for turbulent flows with weak wall effects, and the parameter Z serves to suppress extremely small eddy events that would be dissipated instantly by viscosity.
Onedimensional turbulence modeling of compressible flows: II. Full compressible modification and application to shock–turbulence interaction
10.1063/5.0137435
Physics of Fluids
20230308T12:52:52Z
© 2023 Author(s).
Heiko Schmidt
Marten Klein

The aerodynamic effects of forelimb pose on the gliding flight of Draco lizards
https://aip.scitation.org/doi/10.1063/5.0137154?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Gliding arboreal lizards in the genus Draco possess a pair of patagia, which are thin wing membranes supported by highly elongated thoracic ribs and can be actively folded and unfolded. The uniqueness of Draco gliding flight is that the forelimbs of Draco can move freely independent of the patagia, which are the main lifting surfaces. During the main glide phase, the entire forelimbs are straightened, abducted from the body, and held very close to the patagial leading edges. The reasons for adopting this abducted pose have not been investigated before, especially from the perspective of fluid physics. In this study, wind tunnel experiments and computational simulations are conducted to compare the aerodynamic performances of the abducted pose with two other poses, which have the forelimbs held away from the patagial leading edges. The results show that the abducted pose leads to the highest maximum lift coefficient. This aerodynamic advantage is caused by the larger leadingedge radius due to the abducted forelimbs and small gaps between the abducted forelimbs and the patagial leading edges. Furthermore, it is found that the low aspect ratio of the patagium (0.985) allows the wingtip vortex to energize the flow over the top patagial surface at high angles of attack, which leads to a gentle stall characteristic. The current results also show the existence of distinct leadingedge vortices up to moderate angles of attack. Overall, this work deepens our understanding of the gliding flight aerodynamics of Draco lizards and is useful for future artificial flying machine applications.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Gliding arboreal lizards in the genus Draco possess a pair of patagia, which are thin wing membranes supported by highly elongated thoracic ribs and can be actively folded and unfolded. The uniqueness of Draco gliding flight is that the forelimbs of Draco can move freely independent of the patagia, which are the main lifting surfaces. During the main glide phase, the entire forelimbs are straightened, abducted from the body, and held very close to the patagial leading edges. The reasons for adopting this abducted pose have not been investigated before, especially from the perspective of fluid physics. In this study, wind tunnel experiments and computational simulations are conducted to compare the aerodynamic performances of the abducted pose with two other poses, which have the forelimbs held away from the patagial leading edges. The results show that the abducted pose leads to the highest maximum lift coefficient. This aerodynamic advantage is caused by the larger leadingedge radius due to the abducted forelimbs and small gaps between the abducted forelimbs and the patagial leading edges. Furthermore, it is found that the low aspect ratio of the patagium (0.985) allows the wingtip vortex to energize the flow over the top patagial surface at high angles of attack, which leads to a gentle stall characteristic. The current results also show the existence of distinct leadingedge vortices up to moderate angles of attack. Overall, this work deepens our understanding of the gliding flight aerodynamics of Draco lizards and is useful for future artificial flying machine applications.
The aerodynamic effects of forelimb pose on the gliding flight of Draco lizards
10.1063/5.0137154
Physics of Fluids
20230310T12:20:04Z
© 2023 Author(s).

Dynamics of multiscale vortical structures behind a barchan dune
https://aip.scitation.org/doi/10.1063/5.0131631?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this study, multiscale vortical structures and vortex dynamics around a fixedbed barchan dune have been experimentally investigated based on the particle image velocimetry technique, wavelet transform, and the finitetime Lyapunov exponent method. It was found that the dynamic characteristics of a dune wake are predominated by large and intermediatescale coherent structures. Quadrant analysis of the Reynoldsstress distribution for the corresponding wavelet components revealed that ejection and sweep events are the main contributors to the whole field, while outward and inward interaction events just dominate the region near the dune crest. In addition, the process of ejection and sweeping motions associated with the turbulent bursting sequence can also be captured by applying proper orthogonal decomposition analysis of the decomposed velocity field for the different wavelet components. Finally, a continuous development process of the different wavelet scale structures in the shear layer was visualized in the Lagrangian framework. The smallscale waves grow exponentially and gradually develop into largerscale vortices when convected downstream until the reattachment point, and largerscale vortices break into the smaller ones.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this study, multiscale vortical structures and vortex dynamics around a fixedbed barchan dune have been experimentally investigated based on the particle image velocimetry technique, wavelet transform, and the finitetime Lyapunov exponent method. It was found that the dynamic characteristics of a dune wake are predominated by large and intermediatescale coherent structures. Quadrant analysis of the Reynoldsstress distribution for the corresponding wavelet components revealed that ejection and sweep events are the main contributors to the whole field, while outward and inward interaction events just dominate the region near the dune crest. In addition, the process of ejection and sweeping motions associated with the turbulent bursting sequence can also be captured by applying proper orthogonal decomposition analysis of the decomposed velocity field for the different wavelet components. Finally, a continuous development process of the different wavelet scale structures in the shear layer was visualized in the Lagrangian framework. The smallscale waves grow exponentially and gradually develop into largerscale vortices when convected downstream until the reattachment point, and largerscale vortices break into the smaller ones.
Dynamics of multiscale vortical structures behind a barchan dune
10.1063/5.0131631
Physics of Fluids
20230310T12:20:12Z
© 2023 Author(s).

Conditional statistics of Reynolds stress in open channel flows with modeled canopies of homogeneous and heterogeneous density
https://aip.scitation.org/doi/10.1063/5.0141128?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The flow structures under the effects of heterogeneous canopies have been shown to be significantly different from those under the effects of homogeneous canopies. The purpose of this study is to investigate how the changes in density and density uniformity of the canopy affect the turbulent characteristics of the flow in a partially vegetated channel. A comparative experiment is conducted, including two cases of homogeneous canopy with different densities and one case of heterogeneous canopy consisting of alternating sparse and dense vegetation patches. While the lateral profiles of Reynolds stress, magnitudes of quadrant motions, and highorder moments of velocity fluctuations present a high similarity within the shear layer, variations in both the density and density uniformity of the canopy markedly affect the turbulence at the interface between the canopy and the main channel. The results show that canopy density heterogeneity enhances the momentum exchange at the interface and promotes the penetration of stressdriven flow into the sparse vegetation patch while inhibiting its penetration into the dense vegetation patch. An analogy can be drawn between the canopy flow with sufficient density and the turbulent roughwall boundary layers based on the turbulent statistics within the shear layer. Furthermore, the effect of increased canopy density on the flow corresponds well to the effect of decreased wall roughness. By using the cumulant expansion method, the assumption of structural similarity present in wallbounded flows is found to be applicable to the canopy flows considered in this study.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The flow structures under the effects of heterogeneous canopies have been shown to be significantly different from those under the effects of homogeneous canopies. The purpose of this study is to investigate how the changes in density and density uniformity of the canopy affect the turbulent characteristics of the flow in a partially vegetated channel. A comparative experiment is conducted, including two cases of homogeneous canopy with different densities and one case of heterogeneous canopy consisting of alternating sparse and dense vegetation patches. While the lateral profiles of Reynolds stress, magnitudes of quadrant motions, and highorder moments of velocity fluctuations present a high similarity within the shear layer, variations in both the density and density uniformity of the canopy markedly affect the turbulence at the interface between the canopy and the main channel. The results show that canopy density heterogeneity enhances the momentum exchange at the interface and promotes the penetration of stressdriven flow into the sparse vegetation patch while inhibiting its penetration into the dense vegetation patch. An analogy can be drawn between the canopy flow with sufficient density and the turbulent roughwall boundary layers based on the turbulent statistics within the shear layer. Furthermore, the effect of increased canopy density on the flow corresponds well to the effect of decreased wall roughness. By using the cumulant expansion method, the assumption of structural similarity present in wallbounded flows is found to be applicable to the canopy flows considered in this study.
Conditional statistics of Reynolds stress in open channel flows with modeled canopies of homogeneous and heterogeneous density
10.1063/5.0141128
Physics of Fluids
20230310T12:19:53Z
© 2023 Author(s).

Comparative study of two combined blowing and suction flow control methods on pitching airfoils
https://aip.scitation.org/doi/10.1063/5.0138962?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A comparative study of two combined blowing/suction flow control methods was conducted on the pitching airfoil using the unsteady Reynoldsaveraged Navier–Stokes (URANS) method. One used leadingedge blowing and trailingedge suction, which is referred to as a coflow jet (CFJ), and a conformal slot CFJ (CCFJ) was adopted. Another used leadingedge suction combined with trailingedge blowing, which was called reversed CFJ (RCFJ). The S809 airfoil was used as the baseline as its stall characteristic is suitable for separation flow or stall control research. Aerodynamic coefficients of these two combined blowing/suction methods were compared under nostall, mildstall, and deepstall cases. The net gain of output power was also discussed if CFJ methods were used for wind energy applications. The influence mechanism of these two methods on the flow around the airfoil was revealed. The results showed that the CCFJ is suitable for the nostall and mildstall cases, while the RCFJ is suitable for the deepstall case. Leadingedge suction is more stable than leadingedge blowing when suppressing the dynamic stall. The leadingedge jet flow will cause dynamic stalls when it is detached from the airfoil surface, while the detached jet flow can block the development of the separation when it is placed on the trailing edge.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A comparative study of two combined blowing/suction flow control methods was conducted on the pitching airfoil using the unsteady Reynoldsaveraged Navier–Stokes (URANS) method. One used leadingedge blowing and trailingedge suction, which is referred to as a coflow jet (CFJ), and a conformal slot CFJ (CCFJ) was adopted. Another used leadingedge suction combined with trailingedge blowing, which was called reversed CFJ (RCFJ). The S809 airfoil was used as the baseline as its stall characteristic is suitable for separation flow or stall control research. Aerodynamic coefficients of these two combined blowing/suction methods were compared under nostall, mildstall, and deepstall cases. The net gain of output power was also discussed if CFJ methods were used for wind energy applications. The influence mechanism of these two methods on the flow around the airfoil was revealed. The results showed that the CCFJ is suitable for the nostall and mildstall cases, while the RCFJ is suitable for the deepstall case. Leadingedge suction is more stable than leadingedge blowing when suppressing the dynamic stall. The leadingedge jet flow will cause dynamic stalls when it is detached from the airfoil surface, while the detached jet flow can block the development of the separation when it is placed on the trailing edge.
Comparative study of two combined blowing and suction flow control methods on pitching airfoils
10.1063/5.0138962
Physics of Fluids
20230313T11:26:19Z
© 2023 Author(s).

Measurement of hypersonic turbulent boundary layer on a flat plate using cylindrical focused laser differential interferometer
https://aip.scitation.org/doi/10.1063/5.0141681?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A modified CylindricalFocused Laser Differential Interferometer (CFLDI) system was used to measure the density fluctuations generated by a Mach 6 turbulent boundary layer on a flat plate. The amplitude, spectral statistics, and correlation scale of the density fluctuations were analyzed at different wallnormal heights throughout the boundary layer. Direct numerical simulation was performed under the similar condition to verify the experimental results. The results show that the CFLDI system herewith can accurately represent the statistical characteristics of density fluctuations with a bandwidth of 3–200 kHz in the hypersonic turbulent boundary layer. As the distance from the wall increases, the amplitude of density fluctuations keeps increasing until y/δ ≈ 0.8 and levels off after y/δ ≈ 1.5. As the measurement position moves into the freestream, the characteristic frequency of the density fluctuation decreases rapidly, while its integral timescale gradually increases. Similar phenomena were also observed for pressure fluctuations.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A modified CylindricalFocused Laser Differential Interferometer (CFLDI) system was used to measure the density fluctuations generated by a Mach 6 turbulent boundary layer on a flat plate. The amplitude, spectral statistics, and correlation scale of the density fluctuations were analyzed at different wallnormal heights throughout the boundary layer. Direct numerical simulation was performed under the similar condition to verify the experimental results. The results show that the CFLDI system herewith can accurately represent the statistical characteristics of density fluctuations with a bandwidth of 3–200 kHz in the hypersonic turbulent boundary layer. As the distance from the wall increases, the amplitude of density fluctuations keeps increasing until y/δ ≈ 0.8 and levels off after y/δ ≈ 1.5. As the measurement position moves into the freestream, the characteristic frequency of the density fluctuation decreases rapidly, while its integral timescale gradually increases. Similar phenomena were also observed for pressure fluctuations.
Measurement of hypersonic turbulent boundary layer on a flat plate using cylindrical focused laser differential interferometer
10.1063/5.0141681
Physics of Fluids
20230314T10:16:31Z
© 2023 Author(s).

Quantification and investigation of pressure fluctuation intensity in a multistage electric submersible pump
https://aip.scitation.org/doi/10.1063/5.0136664?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Pressure fluctuation is an important factor affecting the stability of rotating machinery. Electric submersible pumps (ESPs) are generally arranged in a multistage series structure, and its internal unsteady flow is extremely easy to propagate and develop in the lengthy flow passage, which brings about differences in the characteristics of pressure fluctuations in each stage. In contrast to the conventional method of processing pressure fluctuation signals, we propose a parameter called “energy flow density (EFD)” of pressure pulsation by analogy with the definition of wave intensity, in order to directly quantify the intensity of pressure fluctuations. Here, we study these pressure fluctuation characteristics using a typical threestage ESP as the research object. We apply theoretical analysis, numerical simulation, and test verification. First, in comparisons between numerical predictions of pressure fluctuation and test results, the period, amplitude, and phase of pulsation curves are highly consistent, verifying the accuracy of the numerical method employed in this paper. Next, the mechanism underlying the pressure fluctuations and the characteristics of interstage interference are investigated through flow field analysis. Subsequently, the results of the evaluation of the pressure fluctuations based on EFD processing are compared with those obtained in the conventional way. The results are consistent in terms of characterizing the multistage ESP pressure fluctuation characteristics, but the conventional method does not reflect subtle differences due to interstage propagation and coupling. However, the EFD method combines the amplitudes of all signals and provides the intensity of pressure fluctuations directly, which reflects interstage differences. Our results provide a theoretical basis for improving the operational stability of ESPs connected in a multistage series and have practical engineering significance.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Pressure fluctuation is an important factor affecting the stability of rotating machinery. Electric submersible pumps (ESPs) are generally arranged in a multistage series structure, and its internal unsteady flow is extremely easy to propagate and develop in the lengthy flow passage, which brings about differences in the characteristics of pressure fluctuations in each stage. In contrast to the conventional method of processing pressure fluctuation signals, we propose a parameter called “energy flow density (EFD)” of pressure pulsation by analogy with the definition of wave intensity, in order to directly quantify the intensity of pressure fluctuations. Here, we study these pressure fluctuation characteristics using a typical threestage ESP as the research object. We apply theoretical analysis, numerical simulation, and test verification. First, in comparisons between numerical predictions of pressure fluctuation and test results, the period, amplitude, and phase of pulsation curves are highly consistent, verifying the accuracy of the numerical method employed in this paper. Next, the mechanism underlying the pressure fluctuations and the characteristics of interstage interference are investigated through flow field analysis. Subsequently, the results of the evaluation of the pressure fluctuations based on EFD processing are compared with those obtained in the conventional way. The results are consistent in terms of characterizing the multistage ESP pressure fluctuation characteristics, but the conventional method does not reflect subtle differences due to interstage propagation and coupling. However, the EFD method combines the amplitudes of all signals and provides the intensity of pressure fluctuations directly, which reflects interstage differences. Our results provide a theoretical basis for improving the operational stability of ESPs connected in a multistage series and have practical engineering significance.
Quantification and investigation of pressure fluctuation intensity in a multistage electric submersible pump
10.1063/5.0136664
Physics of Fluids
20230315T11:50:16Z
© 2023 Author(s).

A thorough experimental investigation on airfoil turbulence interaction noise
https://aip.scitation.org/doi/10.1063/5.0142704?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This paper on airfoil turbulence interaction noise reveals the nature of the relation between the distortion type of turbulent structures and radiated farfield noise. The turbulence interaction phenomenon is explored through comprehensive simultaneous hotwire, surface pressure, and farfield noise measurements. Two grid turbulence cases are utilized to examine the effect of the coherent structure's length scale compared to the airfoil's leadingedge radius. The results show that the turbulent structures with a size comparable to the leadingedge radius disperse into smaller threedimensional structures, losing their spatial coherence in the vicinity of the stagnation point. In contrast, the structures with larger integral length scales distort into highly coherent twodimensional structures, yielding an increase in the surface pressure fluctuation energy spectra and the chordwise extent of the affected area by the interaction phenomenon, which is found to be responsible for the increased levels of farfield noise. The turbulence characteristics of the flow far upstream of the stagnation point determine the unsteady loading behavior at the stagnation point yet have little influence on the unsteady loading of the full airfoil chord. The stagnation point velocity fluctuations manifest a strong link to the remainder of the airfoil chord, as well as the nearfield hydrodynamic to farfield acoustic signal coherence, while demonstrating no communication with the surface pressure fluctuations at the stagnation point.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This paper on airfoil turbulence interaction noise reveals the nature of the relation between the distortion type of turbulent structures and radiated farfield noise. The turbulence interaction phenomenon is explored through comprehensive simultaneous hotwire, surface pressure, and farfield noise measurements. Two grid turbulence cases are utilized to examine the effect of the coherent structure's length scale compared to the airfoil's leadingedge radius. The results show that the turbulent structures with a size comparable to the leadingedge radius disperse into smaller threedimensional structures, losing their spatial coherence in the vicinity of the stagnation point. In contrast, the structures with larger integral length scales distort into highly coherent twodimensional structures, yielding an increase in the surface pressure fluctuation energy spectra and the chordwise extent of the affected area by the interaction phenomenon, which is found to be responsible for the increased levels of farfield noise. The turbulence characteristics of the flow far upstream of the stagnation point determine the unsteady loading behavior at the stagnation point yet have little influence on the unsteady loading of the full airfoil chord. The stagnation point velocity fluctuations manifest a strong link to the remainder of the airfoil chord, as well as the nearfield hydrodynamic to farfield acoustic signal coherence, while demonstrating no communication with the surface pressure fluctuations at the stagnation point.
A thorough experimental investigation on airfoil turbulence interaction noise
10.1063/5.0142704
Physics of Fluids
20230316T02:04:49Z
© 2023 Author(s).
L. Bowen
A. Celik
M. Azarpeyvand

Influence of dualaxial swirler configuration on hydrodynamic stability in combustor
https://aip.scitation.org/doi/10.1063/5.0139259?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>To improve hydrodynamic stability in a combustor, an unsteady flow analysis method is needed. Hence, the proper orthogonal decomposition (POD) method based on a large eddy simulation (LES) unsteady flow field and corresponding experimental verification were utilized to analyze and assess the influence of the precession vortex core (PVC) motion law on the pulsation downstream of different swirler configurations. The pulsation outcomes of the unsteady simulation match the experimental data quite well, with case 1 having the highest pulsation quantity. The POD analysis reveals that the majority of pulsation energy is concentrated in the first two modes. The results of the motion state, regularity of the time coefficient, and frequency characteristics also demonstrate that the combustor's PVC features are compatible with modes 1 and 2. There is an optimum value of swirl number, 0.72, for which the flow field's stability has the lowest degree of disturbance. Moreover, the airfoil vane's stability is beyond that of the straight vane. The mean flow field and the coherent flow field in the pulsation flow field of case 3 are the most beneficial factors concerning combustion stability, and the unstable aspects of the transition flow field and the turbulent flow field are the least in shape and energy ratio. The data procured from the experiment certify that there is no characteristic frequency of pulsation within 5000 Hz that exists in case 3.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>To improve hydrodynamic stability in a combustor, an unsteady flow analysis method is needed. Hence, the proper orthogonal decomposition (POD) method based on a large eddy simulation (LES) unsteady flow field and corresponding experimental verification were utilized to analyze and assess the influence of the precession vortex core (PVC) motion law on the pulsation downstream of different swirler configurations. The pulsation outcomes of the unsteady simulation match the experimental data quite well, with case 1 having the highest pulsation quantity. The POD analysis reveals that the majority of pulsation energy is concentrated in the first two modes. The results of the motion state, regularity of the time coefficient, and frequency characteristics also demonstrate that the combustor's PVC features are compatible with modes 1 and 2. There is an optimum value of swirl number, 0.72, for which the flow field's stability has the lowest degree of disturbance. Moreover, the airfoil vane's stability is beyond that of the straight vane. The mean flow field and the coherent flow field in the pulsation flow field of case 3 are the most beneficial factors concerning combustion stability, and the unstable aspects of the transition flow field and the turbulent flow field are the least in shape and energy ratio. The data procured from the experiment certify that there is no characteristic frequency of pulsation within 5000 Hz that exists in case 3.
Influence of dualaxial swirler configuration on hydrodynamic stability in combustor
10.1063/5.0139259
Physics of Fluids
20230317T02:43:30Z
© 2023 Author(s).

Timeaveraged flow field behind a transversely spinning sphere: An experimental study
https://aip.scitation.org/doi/10.1063/5.0141058?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The aerodynamic forces on a sphere with a rough surface were measured in a water tunnel at a Reynolds number of 7930 and for a range of spinning ratios (α) from 0 to 6.0. The timeaveraged flow fields were also measured using particle image velocimetry. The effect of the spinning ratio α on the flow was found to show distinct trends in different regimes, including [math]; [math]; [math]; [math]; and [math]. The study identified two critical spinning ratios, where the flow underwent significant changes. The first change occurred in regime II, where the boundary layer over one side of the sphere transitioned from laminar to turbulent, leading to a significant modification in the lift force on the sphere. The second significant change took place across regimes II and III, where the boundary flow in the vicinity of the entire sphere became turbulent. Beyond this range, with [math], the high spinning rate disturbed the incoming flow, resulting in lessefficient downwash production. The lift increased with α at a slower rate compared to other regimes, and the lessefficient downwash production caused a decrease in drag as more momentum was directed downstream in the horizontal direction.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The aerodynamic forces on a sphere with a rough surface were measured in a water tunnel at a Reynolds number of 7930 and for a range of spinning ratios (α) from 0 to 6.0. The timeaveraged flow fields were also measured using particle image velocimetry. The effect of the spinning ratio α on the flow was found to show distinct trends in different regimes, including [math]; [math]; [math]; [math]; and [math]. The study identified two critical spinning ratios, where the flow underwent significant changes. The first change occurred in regime II, where the boundary layer over one side of the sphere transitioned from laminar to turbulent, leading to a significant modification in the lift force on the sphere. The second significant change took place across regimes II and III, where the boundary flow in the vicinity of the entire sphere became turbulent. Beyond this range, with [math], the high spinning rate disturbed the incoming flow, resulting in lessefficient downwash production. The lift increased with α at a slower rate compared to other regimes, and the lessefficient downwash production caused a decrease in drag as more momentum was directed downstream in the horizontal direction.
Timeaveraged flow field behind a transversely spinning sphere: An experimental study
10.1063/5.0141058
Physics of Fluids
20230317T02:43:28Z
© 2023 Author(s).

An entropy viscosity method for large eddy simulation of turbulent thermal flow in a rotor–stator cavity
https://aip.scitation.org/doi/10.1063/5.0140005?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Turbulent flow and heat transfer in a rotor–stator cavity have fundamental importance in both academia of turbulence research and the industry of rotating turbomachinery. The main characteristic of the flow is that there is the centrifugal Ekman layer on the rotor and the centripetal Bödewadt layer on the stator, which are separated by a central rotating core. In this paper, an entropy viscosity subgrid model based on the large eddy simulation (LES) method is proposed to solve the complex flow with heat transfer in a rotating frame at high Reynolds numbers. The method is fully validated by the simulation of turbulent thermal flow in a closed stator–rotor cavity up to [math]. By performing 12 simulations, the sensitivity of the simulation results to mesh resolution and the free parameters of entropy viscosity are systematically studied, and the proper range for the parameters is determined. In particular, it is found that the prediction on the mean flow and fluctuation from the simple turbulent diffusivity model, which scales linearly with the eddy viscosity, is as accurate as that from the alternative model that is a more computationally complex model. Despite the comparable accuracy, the entropy viscositybased LES uses the mesh resolution twoorder lower than that of direct numerical simulation; therefore, it is feasible to apply the LES to the flow at the practical Reynolds number in an aircraft engine, that is, [math].
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Turbulent flow and heat transfer in a rotor–stator cavity have fundamental importance in both academia of turbulence research and the industry of rotating turbomachinery. The main characteristic of the flow is that there is the centrifugal Ekman layer on the rotor and the centripetal Bödewadt layer on the stator, which are separated by a central rotating core. In this paper, an entropy viscosity subgrid model based on the large eddy simulation (LES) method is proposed to solve the complex flow with heat transfer in a rotating frame at high Reynolds numbers. The method is fully validated by the simulation of turbulent thermal flow in a closed stator–rotor cavity up to [math]. By performing 12 simulations, the sensitivity of the simulation results to mesh resolution and the free parameters of entropy viscosity are systematically studied, and the proper range for the parameters is determined. In particular, it is found that the prediction on the mean flow and fluctuation from the simple turbulent diffusivity model, which scales linearly with the eddy viscosity, is as accurate as that from the alternative model that is a more computationally complex model. Despite the comparable accuracy, the entropy viscositybased LES uses the mesh resolution twoorder lower than that of direct numerical simulation; therefore, it is feasible to apply the LES to the flow at the practical Reynolds number in an aircraft engine, that is, [math].
An entropy viscosity method for large eddy simulation of turbulent thermal flow in a rotor–stator cavity
10.1063/5.0140005
Physics of Fluids
20230317T02:39:44Z
© 2023 Author(s).

An integral method to determine mean skin friction in turbulent boundary layers
https://aip.scitation.org/doi/10.1063/5.0142609?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study is concerned with accurately determining the mean skin friction in a zeropressuregradient turbulent boundary layer. By assuming a linear relation for the weighted total shear stress in the nearwall region, an integral method to evaluate the skin friction is proposed. The method requires the wallnormal profiles of the mean streamwise velocity and Reynolds shear stress within the range of [math] at only one streamwise location, where [math] is the boundary layer thickness. A number of direct numerical simulation and experimental data available in the literature are employed to validate the accuracy of the method over a wide range of Reynolds numbers. The skin friction coefficient obtained using the proposed method is found to be within [math] in agreement with the published values in both the smooth and roughwall turbulent boundary layers. A comparison of the present approach with several existing methods is presented, showing that the proposed skin friction relation is robust and accurate.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study is concerned with accurately determining the mean skin friction in a zeropressuregradient turbulent boundary layer. By assuming a linear relation for the weighted total shear stress in the nearwall region, an integral method to evaluate the skin friction is proposed. The method requires the wallnormal profiles of the mean streamwise velocity and Reynolds shear stress within the range of [math] at only one streamwise location, where [math] is the boundary layer thickness. A number of direct numerical simulation and experimental data available in the literature are employed to validate the accuracy of the method over a wide range of Reynolds numbers. The skin friction coefficient obtained using the proposed method is found to be within [math] in agreement with the published values in both the smooth and roughwall turbulent boundary layers. A comparison of the present approach with several existing methods is presented, showing that the proposed skin friction relation is robust and accurate.
An integral method to determine mean skin friction in turbulent boundary layers
10.1063/5.0142609
Physics of Fluids
20230317T02:43:22Z
© 2023 Author(s).

Numerical investigation on aerodynamic noise of flow past a cylinder with different spanwise lengths
https://aip.scitation.org/doi/10.1063/5.0139731?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A numerical investigation is conducted on aerodynamic noise of flow past a circular cylinder with different spanwise lengths (0.5πD, πD, 2πD, and 4πD) at Re = 10 000, where D is the diameter of the cylinder. The nearfield pressure and velocity fields are predicted through Large Eddy Simulation, and then, the acoustic analogy is used to obtain the farfield noise. The results show good agreements for both the near and far field with the data from inhouse experiments and the literature. Though the spanwise length has limited influence on the power spectral density of the nearfield velocity and pressure fluctuations at different spanwise locations, substantial differences are observed for the spanwise pressure coherence and nearwake structures. The 0.5πD case shows primarily twodimensional flow features immediately behind the cylinder compared to the other three cases, resulting in the overprediction of the spanwise pressure coherence, which has strong implications for the farfield noise prediction. With the spanwise length correction, the differences in overall noise magnitudes of the different cases diminish. Nevertheless, the 2πD and 4πD cases better capture the first and second harmonics of the vortex shedding and its associated directivities than the other two cases, showing the importance of sufficient spanwise lengths in predicting noise from flow past a cylinder.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A numerical investigation is conducted on aerodynamic noise of flow past a circular cylinder with different spanwise lengths (0.5πD, πD, 2πD, and 4πD) at Re = 10 000, where D is the diameter of the cylinder. The nearfield pressure and velocity fields are predicted through Large Eddy Simulation, and then, the acoustic analogy is used to obtain the farfield noise. The results show good agreements for both the near and far field with the data from inhouse experiments and the literature. Though the spanwise length has limited influence on the power spectral density of the nearfield velocity and pressure fluctuations at different spanwise locations, substantial differences are observed for the spanwise pressure coherence and nearwake structures. The 0.5πD case shows primarily twodimensional flow features immediately behind the cylinder compared to the other three cases, resulting in the overprediction of the spanwise pressure coherence, which has strong implications for the farfield noise prediction. With the spanwise length correction, the differences in overall noise magnitudes of the different cases diminish. Nevertheless, the 2πD and 4πD cases better capture the first and second harmonics of the vortex shedding and its associated directivities than the other two cases, showing the importance of sufficient spanwise lengths in predicting noise from flow past a cylinder.
Numerical investigation on aerodynamic noise of flow past a cylinder with different spanwise lengths
10.1063/5.0139731
Physics of Fluids
20230317T02:39:52Z
© 2023 Author(s).
Guanjiang Chen
Bin Zang
Mahdi Azarpeyvand

A quasionedimensional model for the stagnation streamline in hypersonic magnetohydrodynamic flows
https://aip.scitation.org/doi/10.1063/5.0138366?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The flow near the stagnation streamline of a blunt body is often attracted and analyzed by using the approximation of local similarity, which reduces the equations of motion to a system of ordinary differential equations. To efficiently calculate the stagnationstreamline parameters in hypersonic magnetohydrodynamic (MHD) flows, an improved quasionedimensional model for MHD flows is developed in the present paper. The Lorentz force is first incorporated into the original dimensionally reduced Navier–Stokes equations to compensate for its effect. Detailed comparisons about the shock standoff distance and the stagnation point heat flux are conducted with the twodimensional Navier–Stokes calculations for flows around the orbital reentry experiment model, including gas flows in thermochemical nonequilibrium under different magnetic field strengths. Results show that the shock curvature should be considered in the quasionedimensional model to prevent accuracy reduction due to the deviation from the local similarity assumption, particularly for hypersonic MHD flows, where the shock standoff distance will increase with larger magnetic strength. Then, the shock curvature parameter is introduced to compensate for the shock curvature effect. A good agreement between the improved quasionedimensional and the twodimensional fullfield simulations is achieved, indicating that the proposed model enables an efficient and reliable evaluation of stagnationstreamline quantities under hypersonic MHD flows.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The flow near the stagnation streamline of a blunt body is often attracted and analyzed by using the approximation of local similarity, which reduces the equations of motion to a system of ordinary differential equations. To efficiently calculate the stagnationstreamline parameters in hypersonic magnetohydrodynamic (MHD) flows, an improved quasionedimensional model for MHD flows is developed in the present paper. The Lorentz force is first incorporated into the original dimensionally reduced Navier–Stokes equations to compensate for its effect. Detailed comparisons about the shock standoff distance and the stagnation point heat flux are conducted with the twodimensional Navier–Stokes calculations for flows around the orbital reentry experiment model, including gas flows in thermochemical nonequilibrium under different magnetic field strengths. Results show that the shock curvature should be considered in the quasionedimensional model to prevent accuracy reduction due to the deviation from the local similarity assumption, particularly for hypersonic MHD flows, where the shock standoff distance will increase with larger magnetic strength. Then, the shock curvature parameter is introduced to compensate for the shock curvature effect. A good agreement between the improved quasionedimensional and the twodimensional fullfield simulations is achieved, indicating that the proposed model enables an efficient and reliable evaluation of stagnationstreamline quantities under hypersonic MHD flows.
A quasionedimensional model for the stagnation streamline in hypersonic magnetohydrodynamic flows
10.1063/5.0138366
Physics of Fluids
20230301T03:50:30Z
© 2023 Author(s).

Simulation and validation of the effective power absorbed by a nonequilibrium plasma flow inside the mediumpower inductively coupled plasma wind tunnel
https://aip.scitation.org/doi/10.1063/5.0141093?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A nonequilibrium magnetohydrodynamic model coupled with a power absorption model was established to calculate the effective power absorbed by the plasma flow inside a 110 kW mediumpower inductively coupled plasma wind tunnel. This magnetohydrodynamic model takes into account the coupling of Navier–Stokes equations, electromagnetic field equations, five species and eight chemical reactions of nitrogen, and a fourtemperature model. Moreover, the power absorption model not only considers the power loss from the power supply system but also the coupling efficiency between plasma and the inductive coils. First, the anode loss of an electronic tube and its oscillator circuit efficiency is calculated, respectively, to obtain the total power loss from a radio frequency power supply system. Second, a transformer circuit model of the inductively coupled plasma (ICP) is established to calculate the coupling efficiency between the coil and plasma. Third, the effective power absorbed by the plasma flow and the pathways of the power losses of a mediumpower ICP wind tunnel are obtained and discussed. Finally, the flowfield properties of the plasma flow, which are simulated by solving the Navier–Stokes equations coupled with the power absorption model, are obtained and analyzed. Furthermore, the simulated results are compared with corresponding experimental data, and they agree well with each other. It is found that the power loss of the electron tube oscillator accounts for 40%. It is the most dominant part of the total power loss. The effective power absorbed by a plasma flow is about 33.6% for the 110kW inductively coupled plasma wind tunnel.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A nonequilibrium magnetohydrodynamic model coupled with a power absorption model was established to calculate the effective power absorbed by the plasma flow inside a 110 kW mediumpower inductively coupled plasma wind tunnel. This magnetohydrodynamic model takes into account the coupling of Navier–Stokes equations, electromagnetic field equations, five species and eight chemical reactions of nitrogen, and a fourtemperature model. Moreover, the power absorption model not only considers the power loss from the power supply system but also the coupling efficiency between plasma and the inductive coils. First, the anode loss of an electronic tube and its oscillator circuit efficiency is calculated, respectively, to obtain the total power loss from a radio frequency power supply system. Second, a transformer circuit model of the inductively coupled plasma (ICP) is established to calculate the coupling efficiency between the coil and plasma. Third, the effective power absorbed by the plasma flow and the pathways of the power losses of a mediumpower ICP wind tunnel are obtained and discussed. Finally, the flowfield properties of the plasma flow, which are simulated by solving the Navier–Stokes equations coupled with the power absorption model, are obtained and analyzed. Furthermore, the simulated results are compared with corresponding experimental data, and they agree well with each other. It is found that the power loss of the electron tube oscillator accounts for 40%. It is the most dominant part of the total power loss. The effective power absorbed by a plasma flow is about 33.6% for the 110kW inductively coupled plasma wind tunnel.
Simulation and validation of the effective power absorbed by a nonequilibrium plasma flow inside the mediumpower inductively coupled plasma wind tunnel
10.1063/5.0141093
Physics of Fluids
20230301T03:50:32Z
© 2023 Author(s).

Theoretical analysis on macromesoscopic gas flow performances in gas dynamic bearing with three pads
https://aip.scitation.org/doi/10.1063/5.0135537?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The Reynolds equation based on the continuum medium assumption fails to meet the accuracy requirements of numerical simulation for mesoscale gas flow. In this research, the gas flow performances and bearing performances of gas dynamic bearing with three pads (GDBTPs) are theoretically analyzed from macroscopic to mesoscopic perspectives. A modified lattice Boltzmann equation is exploited considering the wall effect ψ(y/λ) with gas density ratio ρ/ρref, and the dimensionless gas flow velocity is analyzed for smooth, square cavity, halfsine asperity, triangular asperity, and a combination of surface morphologies. A modified Reynolds equation considering the gas compressibility and gas rarefaction effect is developed to study the static bearing performances of GDBTPs. Results show that the relative roughness Δh and asperities geometries are key factors to affect the mesoscale gas flow characteristics. The loadcarrying capacity of GDBTPs increases with the growth of lengthtodiameter ratio L/D, rotational speed ω, and eccentricity ratio ɛ and decreases with the increase of gas film thickness hg.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The Reynolds equation based on the continuum medium assumption fails to meet the accuracy requirements of numerical simulation for mesoscale gas flow. In this research, the gas flow performances and bearing performances of gas dynamic bearing with three pads (GDBTPs) are theoretically analyzed from macroscopic to mesoscopic perspectives. A modified lattice Boltzmann equation is exploited considering the wall effect ψ(y/λ) with gas density ratio ρ/ρref, and the dimensionless gas flow velocity is analyzed for smooth, square cavity, halfsine asperity, triangular asperity, and a combination of surface morphologies. A modified Reynolds equation considering the gas compressibility and gas rarefaction effect is developed to study the static bearing performances of GDBTPs. Results show that the relative roughness Δh and asperities geometries are key factors to affect the mesoscale gas flow characteristics. The loadcarrying capacity of GDBTPs increases with the growth of lengthtodiameter ratio L/D, rotational speed ω, and eccentricity ratio ɛ and decreases with the increase of gas film thickness hg.
Theoretical analysis on macromesoscopic gas flow performances in gas dynamic bearing with three pads
10.1063/5.0135537
Physics of Fluids
20230302T01:30:04Z
© 2023 Author(s).

The Riemann problem for a traffic flow model
https://aip.scitation.org/doi/10.1063/5.0141732?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A traffic flow model describing the formation and dynamics of traffic jams was introduced by Berthelin et al. [“A model for the formation and evolution of traffic jams,” Arch. Ration. Mech. Anal. 187, 185–220 (2008)], which consists of a pressureless gas dynamics system under a maximal constraint on the density and can be derived from the Aw–Rascle model under the constraint condition [math] by letting the traffic pressure vanish. In this paper, we give up this constraint condition and consider the following form: [math]in which [math]. The Riemann problem for the above traffic flow model is constructively solved. The delta shock wave arises in the Riemann solutions, although the system is strictly hyperbolic, its first eigenvalue is genuinely nonlinear, and the second eigenvalue is linearly degenerate. Furthermore, we clarify the generalized Rankine–Hugoniot relations and δentropy condition. The position, strength, and propagation speed of the delta shock wave are obtained from the generalized Rankine–Hugoniot conditions. The delta shock may be useful for the description of the serious traffic jam. More importantly, it is proved that the limits of the Riemann solutions of the above traffic flow model are exactly those of the pressureless gas dynamics system with the same Riemann initial data as the traffic pressure vanishes.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A traffic flow model describing the formation and dynamics of traffic jams was introduced by Berthelin et al. [“A model for the formation and evolution of traffic jams,” Arch. Ration. Mech. Anal. 187, 185–220 (2008)], which consists of a pressureless gas dynamics system under a maximal constraint on the density and can be derived from the Aw–Rascle model under the constraint condition [math] by letting the traffic pressure vanish. In this paper, we give up this constraint condition and consider the following form: [math]in which [math]. The Riemann problem for the above traffic flow model is constructively solved. The delta shock wave arises in the Riemann solutions, although the system is strictly hyperbolic, its first eigenvalue is genuinely nonlinear, and the second eigenvalue is linearly degenerate. Furthermore, we clarify the generalized Rankine–Hugoniot relations and δentropy condition. The position, strength, and propagation speed of the delta shock wave are obtained from the generalized Rankine–Hugoniot conditions. The delta shock may be useful for the description of the serious traffic jam. More importantly, it is proved that the limits of the Riemann solutions of the above traffic flow model are exactly those of the pressureless gas dynamics system with the same Riemann initial data as the traffic pressure vanishes.
The Riemann problem for a traffic flow model
10.1063/5.0141732
Physics of Fluids
20230306T10:56:06Z
© 2023 Author(s).
Zhiqiang Shao

Numerical analysis for film cooling characteristics of trenched hole under the effects of ribdisturbed feed flow and curved surface
https://aip.scitation.org/doi/10.1063/5.0142276?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Film holes are extensively utilized to protect the blade external surface by ejecting coolant, forming a protective film, and separating the hot gas and blade surface. Trenched holes, caused by the turbine blade coated with thermal barrier, greatly feature better film cooling performance than traditional cylinder holes. In this study, the effects of a ribdisturbed feed flow on the film cooling performance of trenched holes are studied through the numerical method. Two typical rib attacking angles, 45° and 135°, are compared with the blowing ratio increasing from 0.5 to 2.0. The effects of the curved surface (convex and concave) are also included. Numerical results prove that film effectiveness with the coolant fed by the ribdisturbed internal flow is sensitive to the blowing ratio. The ribturbulated cooling flow entering the film hole is featured with different swirling states; therefore, the interaction between the mainstream and the cooling air of varied swirling state leads to different film coverage and effectiveness. Overall, 135° ribs induce better adiabatic film cooling effectiveness than 45° ribs, with a maximum improvement 34.9% at M = 0.5. Film effectiveness on the convex surface is better than that on flat and concave surfaces. Areaaveraged η on convex and concave surfaces is, respectively, 4.7% higher and 6.2% lower than that on the flat surface. Normal pressure gradient established on the convex surface contributes to reducing the turbulence intensity and improving the film lateral coverage.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Film holes are extensively utilized to protect the blade external surface by ejecting coolant, forming a protective film, and separating the hot gas and blade surface. Trenched holes, caused by the turbine blade coated with thermal barrier, greatly feature better film cooling performance than traditional cylinder holes. In this study, the effects of a ribdisturbed feed flow on the film cooling performance of trenched holes are studied through the numerical method. Two typical rib attacking angles, 45° and 135°, are compared with the blowing ratio increasing from 0.5 to 2.0. The effects of the curved surface (convex and concave) are also included. Numerical results prove that film effectiveness with the coolant fed by the ribdisturbed internal flow is sensitive to the blowing ratio. The ribturbulated cooling flow entering the film hole is featured with different swirling states; therefore, the interaction between the mainstream and the cooling air of varied swirling state leads to different film coverage and effectiveness. Overall, 135° ribs induce better adiabatic film cooling effectiveness than 45° ribs, with a maximum improvement 34.9% at M = 0.5. Film effectiveness on the convex surface is better than that on flat and concave surfaces. Areaaveraged η on convex and concave surfaces is, respectively, 4.7% higher and 6.2% lower than that on the flat surface. Normal pressure gradient established on the convex surface contributes to reducing the turbulence intensity and improving the film lateral coverage.
Numerical analysis for film cooling characteristics of trenched hole under the effects of ribdisturbed feed flow and curved surface
10.1063/5.0142276
Physics of Fluids
20230306T10:55:57Z
© 2023 Author(s).

Influence of thermochemical nonequilibrium on expansion tube air test conditions: A numerical study
https://aip.scitation.org/doi/10.1063/5.0141281?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Using a Lagrangian solver, thermochemical nonequilibrium simulations are performed for the entire range of practical operating conditions of expansion tubes to isolate the influence of nonequilibrium and identify key features in largescale facilities. Particular attention is given not only to the influence of the nonequilibrium unsteady expansion but also to the influences of the nonequilibrium region behind the primary shock and nonideal secondary diaphragm rupture. The nonequilibrium unsteady expansion is found to be the most influential process in the test flow—it can significantly influence the flow properties and cause significant temporal variations in the properties during the test time. The nonequilibrium unsteady expansion is also found to accelerate the secondary shock and contact surface. The nonideal secondary diaphragm rupture is found to increase the amount of nonequilibrium in the test flow due to the generation of a reflected shock. The nonequilibrium region behind the primary shock may be considered negligible in most conditions. Regarding the creation of thermochemical equilibrium test conditions, important factors for achieving this include having a high acceleration tube fill pressure, largescale facility, and high total enthalpy. The combined effects of viscosity and nonequilibrium are postulated, and the results are supported by experimental works that report consistent findings. To provide an idea of the sensitivity of the numerical configuration, simulations of fixedvolume reactors at various deexcitation conditions are performed using different nonequilibrium models.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Using a Lagrangian solver, thermochemical nonequilibrium simulations are performed for the entire range of practical operating conditions of expansion tubes to isolate the influence of nonequilibrium and identify key features in largescale facilities. Particular attention is given not only to the influence of the nonequilibrium unsteady expansion but also to the influences of the nonequilibrium region behind the primary shock and nonideal secondary diaphragm rupture. The nonequilibrium unsteady expansion is found to be the most influential process in the test flow—it can significantly influence the flow properties and cause significant temporal variations in the properties during the test time. The nonequilibrium unsteady expansion is also found to accelerate the secondary shock and contact surface. The nonideal secondary diaphragm rupture is found to increase the amount of nonequilibrium in the test flow due to the generation of a reflected shock. The nonequilibrium region behind the primary shock may be considered negligible in most conditions. Regarding the creation of thermochemical equilibrium test conditions, important factors for achieving this include having a high acceleration tube fill pressure, largescale facility, and high total enthalpy. The combined effects of viscosity and nonequilibrium are postulated, and the results are supported by experimental works that report consistent findings. To provide an idea of the sensitivity of the numerical configuration, simulations of fixedvolume reactors at various deexcitation conditions are performed using different nonequilibrium models.
Influence of thermochemical nonequilibrium on expansion tube air test conditions: A numerical study
10.1063/5.0141281
Physics of Fluids
20230310T12:19:45Z
© 2023 Author(s).

Numerical study on the scattering of acoustic waves by a compact vortex
https://aip.scitation.org/doi/10.1063/5.0140006?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A new family of compact vortex models is developed and taken as base vortical flows to numerically study the acoustic scattering by solving the twodimensional Euler equations in the time domain with highorder accurate finitedifference methods and nonreflecting boundary conditions. The computations of scattered fields with very small amplitude are found to be in excellent agreement with a benchmark provided by previous studies. Simulations for the scattering from a Taylor vortex reveal that the amplitude of the scattered fields is strongly influenced by two dimensionless quantities, the vortex strength Mv based on the maximal velocity of the vortex, and the acoustic lengthscale ratio [math] defined as the acoustic wavelength relative to the vortex core size. To have a deep understanding of the roles played by these two quantities, another significant quantity used for describing quantitatively the total amount of scattering, namely, scattered sound power, is introduced. Thereupon, on the basis of a global analysis of scale effects of these two dimensionless quantities on the scattered sound power, the scattering defined in a physical coordinate system with Mv and [math] is divided into three domains, longwave domain, resonance domain, and geometricalacoustics domain. For each domain, we examine the influence of Mv and [math] in detail and derive the explicit scaling laws involved in the strength of the scattered field and these two dimensionless quantities separately. Furthermore, the computations for the scattering from a highorder compact vortex are conducted at a wide range of Mv and [math] and compared with the results from the Taylor vortex in each domain to gain some insights into the acoustic scattering by a compact vortex.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A new family of compact vortex models is developed and taken as base vortical flows to numerically study the acoustic scattering by solving the twodimensional Euler equations in the time domain with highorder accurate finitedifference methods and nonreflecting boundary conditions. The computations of scattered fields with very small amplitude are found to be in excellent agreement with a benchmark provided by previous studies. Simulations for the scattering from a Taylor vortex reveal that the amplitude of the scattered fields is strongly influenced by two dimensionless quantities, the vortex strength Mv based on the maximal velocity of the vortex, and the acoustic lengthscale ratio [math] defined as the acoustic wavelength relative to the vortex core size. To have a deep understanding of the roles played by these two quantities, another significant quantity used for describing quantitatively the total amount of scattering, namely, scattered sound power, is introduced. Thereupon, on the basis of a global analysis of scale effects of these two dimensionless quantities on the scattered sound power, the scattering defined in a physical coordinate system with Mv and [math] is divided into three domains, longwave domain, resonance domain, and geometricalacoustics domain. For each domain, we examine the influence of Mv and [math] in detail and derive the explicit scaling laws involved in the strength of the scattered field and these two dimensionless quantities separately. Furthermore, the computations for the scattering from a highorder compact vortex are conducted at a wide range of Mv and [math] and compared with the results from the Taylor vortex in each domain to gain some insights into the acoustic scattering by a compact vortex.
Numerical study on the scattering of acoustic waves by a compact vortex
10.1063/5.0140006
Physics of Fluids
20230310T12:20:07Z
© 2023 Author(s).

A highorder nonlinear limiter for discontinuous Galerkin method on parallel adaptive Cartesian grids
https://aip.scitation.org/doi/10.1063/5.0138993?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The discontinuous Galerkin (DG) method has been widely adopted due to its excellent properties. However, the problem of designing a class of highorder limiter that takes into account accuracy, compactness, efficiency, and robustness has long been an open question in simulating compressible flow with strong discontinuities. In this paper, a highorder multiresolution weighted essentially nonoscillatory (MRWENO) limiter is designed for the DG method on a parallel adaptive Cartesian grid, based directly on the weak solution to a polynomial obtained by the DG method. It can gradually be reduced to firstorder accuracy in the vicinity of discontinuities while maintaining the excellent features of the DG method. Thus, it essentially has nonoscillatory characteristics in nonsmooth regions with respect to the adaptive Cartesian grids. An improved shock detection technique is adopted as an indicator to identify troubled cells, which forms a highorder limiting procedure. A highorder MRWENO limiter is used for both two and threedimensional cases to reconstruct different degrees of freedom on adaptive Cartesian grids. If the mesh is refined or coarsened, the details of the implementation algorithm are presented to determine how the hanging nodes are modulated and how the numerical solutions are redefined on such adaptive Cartesian grids. The parallelization of this method can be achieved by linking to the octreebased adaptive mesh refinement library called p4est. Finally, the low dissipation, shock capture ability, and load balancing of the highorder DG method with an MRWENO limiter may enhance the resolutions of nearby strong discontinuities in adaptive Cartesian grids.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The discontinuous Galerkin (DG) method has been widely adopted due to its excellent properties. However, the problem of designing a class of highorder limiter that takes into account accuracy, compactness, efficiency, and robustness has long been an open question in simulating compressible flow with strong discontinuities. In this paper, a highorder multiresolution weighted essentially nonoscillatory (MRWENO) limiter is designed for the DG method on a parallel adaptive Cartesian grid, based directly on the weak solution to a polynomial obtained by the DG method. It can gradually be reduced to firstorder accuracy in the vicinity of discontinuities while maintaining the excellent features of the DG method. Thus, it essentially has nonoscillatory characteristics in nonsmooth regions with respect to the adaptive Cartesian grids. An improved shock detection technique is adopted as an indicator to identify troubled cells, which forms a highorder limiting procedure. A highorder MRWENO limiter is used for both two and threedimensional cases to reconstruct different degrees of freedom on adaptive Cartesian grids. If the mesh is refined or coarsened, the details of the implementation algorithm are presented to determine how the hanging nodes are modulated and how the numerical solutions are redefined on such adaptive Cartesian grids. The parallelization of this method can be achieved by linking to the octreebased adaptive mesh refinement library called p4est. Finally, the low dissipation, shock capture ability, and load balancing of the highorder DG method with an MRWENO limiter may enhance the resolutions of nearby strong discontinuities in adaptive Cartesian grids.
A highorder nonlinear limiter for discontinuous Galerkin method on parallel adaptive Cartesian grids
10.1063/5.0138993
Physics of Fluids
20230313T11:26:04Z
© 2023 Author(s).

Effect of surface radiation on the surface heat flux of twodimensional hypersonic panel flow
https://aip.scitation.org/doi/10.1063/5.0139774?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This article investigates the effect of surface radiation on the surface heat flux of a twodimensional hypersonic panel flow by comparing the surface heat fluxes of penetratingradiativeequilibrium cases with those of constantwalltemperature cases. A Du Fort–Frankeltype difference scheme is applied to cases with different external flow properties and is verified by comparing the results under a constantwalltemperature boundary condition with selfsimilar solutions. Both laminar and turbulent flows are considered, and turbulence is modeled using a Baldwin–Lomax turbulence model with the assumption that fullscale turbulence is reached at the leading edge. The results show that the surface heat fluxes for laminar penetratingradiativeequilibrium cases are greater by the order of 10% than those for constantwalltemperature cases with the same temperature value at the corresponding points. Though turbulent instances are substantially more difficult, surface heat fluxes of penetratingradiativeequilibrium cases are fairly similar to those of constantwalltemperature cases. These results serve as the foundation for a brief discussion of how parameters affect the outcome and the proposal of a modified reference enthalpy method that can be used to predict heat flux when surface radiation causes a streamwise variation in wall temperature.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This article investigates the effect of surface radiation on the surface heat flux of a twodimensional hypersonic panel flow by comparing the surface heat fluxes of penetratingradiativeequilibrium cases with those of constantwalltemperature cases. A Du Fort–Frankeltype difference scheme is applied to cases with different external flow properties and is verified by comparing the results under a constantwalltemperature boundary condition with selfsimilar solutions. Both laminar and turbulent flows are considered, and turbulence is modeled using a Baldwin–Lomax turbulence model with the assumption that fullscale turbulence is reached at the leading edge. The results show that the surface heat fluxes for laminar penetratingradiativeequilibrium cases are greater by the order of 10% than those for constantwalltemperature cases with the same temperature value at the corresponding points. Though turbulent instances are substantially more difficult, surface heat fluxes of penetratingradiativeequilibrium cases are fairly similar to those of constantwalltemperature cases. These results serve as the foundation for a brief discussion of how parameters affect the outcome and the proposal of a modified reference enthalpy method that can be used to predict heat flux when surface radiation causes a streamwise variation in wall temperature.
Effect of surface radiation on the surface heat flux of twodimensional hypersonic panel flow
10.1063/5.0139774
Physics of Fluids
20230313T11:26:16Z
© 2023 Author(s).

Flow control effect of pulsed arc discharge plasma actuation on impinging shock wave/boundary layer interaction
https://aip.scitation.org/doi/10.1063/5.0140098?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The current study investigates the control effect of the pulsed arc discharge plasma on the impinging shock wave and boundary layer interaction (SWBLI) generated by a 14° wedge in a Mach 2.5 flow. The response characteristics of SWBLI on pulsed arc discharge actuation were illustrated, and the controlling mechanism of shockinduced flow separation under different plasma power settings was revealed. The results, which were well validated by the relative published experiment, showed that when setting the exciting power density ph as 1.0 × 1011 W/m3, the oblique shock wave obtained an obvious fluctuation, and the foot of the reattachment shock wave was partially removed. In addition, as the controlling gas bubble passed through the interaction region, the reverse flow zone was enlarged, and the separation shock wave was shifted upward. When ph was set to 4.8 × 109 W/m3, the flow separation induced by SWBLI was effectively suppressed and the size of the reverse flow zone was significantly reduced. Moreover, as the energy input was increased, the arcinduced blast wave (BW) velocity was obviously enhanced. Additionally, it is further found that the arc plasma energy deposition density in the discharge region was the determining factor for SWBLI control, even for a relatively small exciting energy input. Better drag reduction of the flow field would be achieved with the pulsed arc discharge plasma of higher power density, and a drag reduction rate of nearly 10.05% was obtained at ph = 1 × 1011 W/m3 control condition.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The current study investigates the control effect of the pulsed arc discharge plasma on the impinging shock wave and boundary layer interaction (SWBLI) generated by a 14° wedge in a Mach 2.5 flow. The response characteristics of SWBLI on pulsed arc discharge actuation were illustrated, and the controlling mechanism of shockinduced flow separation under different plasma power settings was revealed. The results, which were well validated by the relative published experiment, showed that when setting the exciting power density ph as 1.0 × 1011 W/m3, the oblique shock wave obtained an obvious fluctuation, and the foot of the reattachment shock wave was partially removed. In addition, as the controlling gas bubble passed through the interaction region, the reverse flow zone was enlarged, and the separation shock wave was shifted upward. When ph was set to 4.8 × 109 W/m3, the flow separation induced by SWBLI was effectively suppressed and the size of the reverse flow zone was significantly reduced. Moreover, as the energy input was increased, the arcinduced blast wave (BW) velocity was obviously enhanced. Additionally, it is further found that the arc plasma energy deposition density in the discharge region was the determining factor for SWBLI control, even for a relatively small exciting energy input. Better drag reduction of the flow field would be achieved with the pulsed arc discharge plasma of higher power density, and a drag reduction rate of nearly 10.05% was obtained at ph = 1 × 1011 W/m3 control condition.
Flow control effect of pulsed arc discharge plasma actuation on impinging shock wave/boundary layer interaction
10.1063/5.0140098
Physics of Fluids
20230313T11:25:58Z
© 2023 Author(s).

Interaction of shock train with cavity shear layer in a scramjet isolator
https://aip.scitation.org/doi/10.1063/5.0137481?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The interaction between the selfexcited shock train flow and the cavity shear layer in a scramjet isolator is investigated numerically using detachededdy simulations. The effect of changing the position of the shock train by controlling the back pressure ratio and the effect of changing the cavity front wall angle are analyzed using unsteady statistics and modal analysis. The propagation mechanism of the pressure disturbance was investigated by spatiotemporal crosscorrelation coefficient analysis. In the present numerical study, a constant isolator section with a cavity front wall (θ = 90° and 60°) was considered, followed by a diffuser section simulated at Mach number 2.2 with three different back pressure ratios (pb/p∞ = 0.7, 5.0, and 6.0). The change in back pressure provides three different conditions (i.e., no shock train, shock train ends before the leading edge of the cavity, and shock train present above the cavity). To understand the unsteady dynamics of the interaction of the shear layer with the shock train, the spatiotemporal trajectory of the wall pressure and the centerline pressure distribution, the spatiotemporal crosscorrelation coefficient, and the modal analysis by dynamic mode decomposition are obtained. The results show that the lowfrequency shock train oscillation dominates the selfsustained cavity oscillation. The spatiotemporal crosscorrelation between the wall surface and the center of the cavity bottom wall indicates the propagation of local disturbances originating from the separated boundary layer caused by the shock and the recirculation zone in the corners of the cavity. Dynamic mode decomposition analysis shows the shear layer at the leading edge of the cavity and the downstream propagation of large eddies from the cavity. It also shows the pairing of coherent structures between the shock train and the recirculation zone of the cavity.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The interaction between the selfexcited shock train flow and the cavity shear layer in a scramjet isolator is investigated numerically using detachededdy simulations. The effect of changing the position of the shock train by controlling the back pressure ratio and the effect of changing the cavity front wall angle are analyzed using unsteady statistics and modal analysis. The propagation mechanism of the pressure disturbance was investigated by spatiotemporal crosscorrelation coefficient analysis. In the present numerical study, a constant isolator section with a cavity front wall (θ = 90° and 60°) was considered, followed by a diffuser section simulated at Mach number 2.2 with three different back pressure ratios (pb/p∞ = 0.7, 5.0, and 6.0). The change in back pressure provides three different conditions (i.e., no shock train, shock train ends before the leading edge of the cavity, and shock train present above the cavity). To understand the unsteady dynamics of the interaction of the shear layer with the shock train, the spatiotemporal trajectory of the wall pressure and the centerline pressure distribution, the spatiotemporal crosscorrelation coefficient, and the modal analysis by dynamic mode decomposition are obtained. The results show that the lowfrequency shock train oscillation dominates the selfsustained cavity oscillation. The spatiotemporal crosscorrelation between the wall surface and the center of the cavity bottom wall indicates the propagation of local disturbances originating from the separated boundary layer caused by the shock and the recirculation zone in the corners of the cavity. Dynamic mode decomposition analysis shows the shear layer at the leading edge of the cavity and the downstream propagation of large eddies from the cavity. It also shows the pairing of coherent structures between the shock train and the recirculation zone of the cavity.
Interaction of shock train with cavity shear layer in a scramjet isolator
10.1063/5.0137481
Physics of Fluids
20230314T12:08:28Z
© 2023 Author(s).
Vignesh Ram Petha Sethuraman
Yosheph Yang

Characteristics of reattached oblique detonation induced by a double wedge
https://aip.scitation.org/doi/10.1063/5.0140177?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The stationary characteristics of the oblique detonation wave (ODW) induced by the double wedge with an expansion corner are investigated using twodimensional Navier–Stokes equations along with a twostep inductionexothermic kinetic model. The results show that the detached ODW can be reattached by expansion waves induced by the double wedge so that the standing window of ODW can be expanded. The restanding position of ODW depends on the location and strength of the expansion waves, which are governed by the first wedge length L and the corner angle between the first and second wedge surface θC. There is a critical angle reattachment that determines whether the ODW can be reattached by expansion waves, and this critical angle increases as wedge length increases. However, the detached ODW cannot be reattached when the wedge length is increased to a critical value regardless of the wedge corner. The restanding position moves downstream with the increment of θC until the last Mach wave tangent to the subsonic zone behind the strong overdriven ODW because no more Mach waves interact with the initiation zone. Moreover, the comparison of viscous and inviscid fields demonstrates that a shorter wedge length is necessary for the viscous field to reattach the ODW because the recirculation zone forms a gas wedge that extends the first wedge surface.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The stationary characteristics of the oblique detonation wave (ODW) induced by the double wedge with an expansion corner are investigated using twodimensional Navier–Stokes equations along with a twostep inductionexothermic kinetic model. The results show that the detached ODW can be reattached by expansion waves induced by the double wedge so that the standing window of ODW can be expanded. The restanding position of ODW depends on the location and strength of the expansion waves, which are governed by the first wedge length L and the corner angle between the first and second wedge surface θC. There is a critical angle reattachment that determines whether the ODW can be reattached by expansion waves, and this critical angle increases as wedge length increases. However, the detached ODW cannot be reattached when the wedge length is increased to a critical value regardless of the wedge corner. The restanding position moves downstream with the increment of θC until the last Mach wave tangent to the subsonic zone behind the strong overdriven ODW because no more Mach waves interact with the initiation zone. Moreover, the comparison of viscous and inviscid fields demonstrates that a shorter wedge length is necessary for the viscous field to reattach the ODW because the recirculation zone forms a gas wedge that extends the first wedge surface.
Characteristics of reattached oblique detonation induced by a double wedge
10.1063/5.0140177
Physics of Fluids
20230315T11:59:09Z
© 2023 Author(s).

Investigation of flow control methods for reducing heat flux on a Vshaped blunt leading edge under real gas effects
https://aip.scitation.org/doi/10.1063/5.0142156?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Complex shock interactions and severe aerothermal loads are often encountered on the lips of threedimensional inwardturning inlets, which presents significant challenges to the performance and safety of hypersonic flight vehicles. However, there have been few investigations on reducing the heat flux of the lips, especially when considering real gas effects. It is, therefore, necessary to investigate flow control methods that are suitable for the lips under real gas effects. Three flow control methods are implemented in this work—a passive method with the shock control bump and stagnation bulge, an active method with counterflow jet, and a combined method. The lip is simplified as a Vshaped blunt leading edge to eliminate the influence of other structures. Numerical simulations are performed at freestream Mach numbers ranging from 6.0 to 12.0. The principles and abilities of different flow control methods for reducing heat flux are compared and analyzed. Although the passive and active methods can reduce the heat flux by more than 40% at low Mach numbers, they have an apparent deficiency under strong real gas effects at high Mach numbers. Moreover, the active method causes new heat flux peaks near the nozzle and at the reattachment position of the flow separation zone. Therefore, a combined method is proposed for further reducing the heat flux. By coupling the passive and active methods, the combined method can reduce the heat flux by nearly 60%. In general, the flow control methods investigated in this work can achieve satisfactory heat flux reduction abilities.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Complex shock interactions and severe aerothermal loads are often encountered on the lips of threedimensional inwardturning inlets, which presents significant challenges to the performance and safety of hypersonic flight vehicles. However, there have been few investigations on reducing the heat flux of the lips, especially when considering real gas effects. It is, therefore, necessary to investigate flow control methods that are suitable for the lips under real gas effects. Three flow control methods are implemented in this work—a passive method with the shock control bump and stagnation bulge, an active method with counterflow jet, and a combined method. The lip is simplified as a Vshaped blunt leading edge to eliminate the influence of other structures. Numerical simulations are performed at freestream Mach numbers ranging from 6.0 to 12.0. The principles and abilities of different flow control methods for reducing heat flux are compared and analyzed. Although the passive and active methods can reduce the heat flux by more than 40% at low Mach numbers, they have an apparent deficiency under strong real gas effects at high Mach numbers. Moreover, the active method causes new heat flux peaks near the nozzle and at the reattachment position of the flow separation zone. Therefore, a combined method is proposed for further reducing the heat flux. By coupling the passive and active methods, the combined method can reduce the heat flux by nearly 60%. In general, the flow control methods investigated in this work can achieve satisfactory heat flux reduction abilities.
Investigation of flow control methods for reducing heat flux on a Vshaped blunt leading edge under real gas effects
10.1063/5.0142156
Physics of Fluids
20230315T11:49:53Z
© 2023 Author(s).

A scaleaware dispersionrelationpreserving finite difference scheme for computational aeroacoustics
https://aip.scitation.org/doi/10.1063/5.0138462?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The numerical schemes for computational aeroacoustics (CAA) should have minimal dispersion and proper dissipation in order to accurately capture the amplitude and phase of waves. In this paper, we propose a scaleaware dispersionrelationpreserving (SADRP) finite difference scheme based on an improved scale sensor and a new dispersion control strategy. The scale sensor quantifies the local length scale of the solution in the form of the effective scaled wavenumber. The new feature of this scale sensor is the accurate prediction of the wavenumber for a pure sine wave. The new dispersion control strategy determines the dispersion parameter of the scheme in terms of the scale sensor. In contrast to the traditional dispersionrelationpreserving (DRP) scheme that minimizes the integral dispersion error, the new strategy directly solves the dispersion parameter by requiring the numerical dispersion relation to be equal to the exact one. As a result, precise dispersion relation can be realized within a very broad wavenumber range. The approximate dispersion relation analysis shows that the SADRP scheme maintains an accurate dispersion relation up to the scaled wavenumber k = 2.5. Moreover, the overshoot in the dispersion relation of the DRP scheme is not presented in that of the SADRP scheme. To suppress nonphysical oscillations, we also add proper dissipation that is adjusted automatically according to the effective scaled wavenumber. Several CAA benchmark test cases are presented to demonstrate the higher resolution and higher efficiency achieved by the proposed scheme compared with the conventional spectrally optimized schemes.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The numerical schemes for computational aeroacoustics (CAA) should have minimal dispersion and proper dissipation in order to accurately capture the amplitude and phase of waves. In this paper, we propose a scaleaware dispersionrelationpreserving (SADRP) finite difference scheme based on an improved scale sensor and a new dispersion control strategy. The scale sensor quantifies the local length scale of the solution in the form of the effective scaled wavenumber. The new feature of this scale sensor is the accurate prediction of the wavenumber for a pure sine wave. The new dispersion control strategy determines the dispersion parameter of the scheme in terms of the scale sensor. In contrast to the traditional dispersionrelationpreserving (DRP) scheme that minimizes the integral dispersion error, the new strategy directly solves the dispersion parameter by requiring the numerical dispersion relation to be equal to the exact one. As a result, precise dispersion relation can be realized within a very broad wavenumber range. The approximate dispersion relation analysis shows that the SADRP scheme maintains an accurate dispersion relation up to the scaled wavenumber k = 2.5. Moreover, the overshoot in the dispersion relation of the DRP scheme is not presented in that of the SADRP scheme. To suppress nonphysical oscillations, we also add proper dissipation that is adjusted automatically according to the effective scaled wavenumber. Several CAA benchmark test cases are presented to demonstrate the higher resolution and higher efficiency achieved by the proposed scheme compared with the conventional spectrally optimized schemes.
A scaleaware dispersionrelationpreserving finite difference scheme for computational aeroacoustics
10.1063/5.0138462
Physics of Fluids
20230315T11:50:11Z
© 2023 Author(s).

Development of a thermopressure acoustic model and its application in modeling threedimensional acoustofluidic systems
https://aip.scitation.org/doi/10.1063/5.0140656?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Theoretical modeling of acoustofluidic systems faces extreme challenges as the thickness of the thermoviscous boundary layer is very small compared to the microscale fluid dimensions. The classical pressure acoustic model overcomes these difficulties and is extensively used in simulating threedimensional (3D) or large twodimensional (2D) acoustofluidic systems. However, this model cannot be applied to thermoviscous acoustofluidics, as it does not consider energy conservation. Modeling thermoviscous acoustofluidic systems is, therefore, difficult and restricted to small 2D systems only. Here, we have developed a thermopressure acoustic model that can effectively simulate thermoviscous acoustofluidic systems. The model has been validated with the full model by performing numerical simulations for a small 2D acoustofluidic system for which capturing the acoustic boundary layer effect is feasible using the full model. After successful validation, we demonstrate that the thermopressure acoustic model can also be applied to studying 3D acoustofluidic systems.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Theoretical modeling of acoustofluidic systems faces extreme challenges as the thickness of the thermoviscous boundary layer is very small compared to the microscale fluid dimensions. The classical pressure acoustic model overcomes these difficulties and is extensively used in simulating threedimensional (3D) or large twodimensional (2D) acoustofluidic systems. However, this model cannot be applied to thermoviscous acoustofluidics, as it does not consider energy conservation. Modeling thermoviscous acoustofluidic systems is, therefore, difficult and restricted to small 2D systems only. Here, we have developed a thermopressure acoustic model that can effectively simulate thermoviscous acoustofluidic systems. The model has been validated with the full model by performing numerical simulations for a small 2D acoustofluidic system for which capturing the acoustic boundary layer effect is feasible using the full model. After successful validation, we demonstrate that the thermopressure acoustic model can also be applied to studying 3D acoustofluidic systems.
Development of a thermopressure acoustic model and its application in modeling threedimensional acoustofluidic systems
10.1063/5.0140656
Physics of Fluids
20230315T11:49:55Z
© 2023 Author(s).
Pradipta Kr. Das
Venkat R. Bhethanabotla

Mach cutoff phenomenon of sonic boom in temperature inversion layers
https://aip.scitation.org/doi/10.1063/5.0143378?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The Mach cutoff of sonic boom occurs when shock waves propagate from supersonic to subsonic regions in a stratified atmosphere. In this study, numerical simulations are performed to clarify Mach cutoff behaviors in atmospheric temperature inversion layers, which have not yet been investigated. The twodimensional Euler equations with a gravitational source term are solved in a horizontally stratified atmosphere. Surface and nonsurface temperature inversion layers include three and four supersonic–subsonic mixing layers, respectively. Computations are made to analyze the propagation characteristics of a conventional Nshaped waveform. The results show that multiple upward and downward cutoffs occur owing to atmospheric temperature inversions, with multiple outgoing and evanescent waves, and multiple shock wave focusing. The upward and downward cutoffs act similarly. The positive and negativepressure regions are inversed when a cutoff occurs twice; thus, each cutoff shifts a phase by π/2, as expressed in the linear theory of caustics. A waveform transition occurs owing to wave splitting in a cutoff region, in addition to the phase shift. For surface temperature inversion, upward cutoffs make the waves between the cutoff and ground surfaces persist, resulting in rumbling. For nonsurface temperature inversion, upward and downward cutoffs occur alternately, and the waves between them persist as well. Therefore, multiple cutoffs in temperature inversion layers play an important role in sonic boom evaluation during low supersonic flight.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The Mach cutoff of sonic boom occurs when shock waves propagate from supersonic to subsonic regions in a stratified atmosphere. In this study, numerical simulations are performed to clarify Mach cutoff behaviors in atmospheric temperature inversion layers, which have not yet been investigated. The twodimensional Euler equations with a gravitational source term are solved in a horizontally stratified atmosphere. Surface and nonsurface temperature inversion layers include three and four supersonic–subsonic mixing layers, respectively. Computations are made to analyze the propagation characteristics of a conventional Nshaped waveform. The results show that multiple upward and downward cutoffs occur owing to atmospheric temperature inversions, with multiple outgoing and evanescent waves, and multiple shock wave focusing. The upward and downward cutoffs act similarly. The positive and negativepressure regions are inversed when a cutoff occurs twice; thus, each cutoff shifts a phase by π/2, as expressed in the linear theory of caustics. A waveform transition occurs owing to wave splitting in a cutoff region, in addition to the phase shift. For surface temperature inversion, upward cutoffs make the waves between the cutoff and ground surfaces persist, resulting in rumbling. For nonsurface temperature inversion, upward and downward cutoffs occur alternately, and the waves between them persist as well. Therefore, multiple cutoffs in temperature inversion layers play an important role in sonic boom evaluation during low supersonic flight.
Mach cutoff phenomenon of sonic boom in temperature inversion layers
10.1063/5.0143378
Physics of Fluids
20230316T02:04:39Z
© 2023 Author(s).
Rei Yamashita
Yoshikazu Makino

A parametric study on control authority and vorticity transport in a compressor airfoil with plasma actuation at low Reynolds number
https://aip.scitation.org/doi/10.1063/5.0141480?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In order to obtain the general criterion of control authority for suppressing flow separation in the compressor airfoil at low Reynolds number by nanosecond pulsed dielectric barrier discharge plasma actuation with different actuation parameters, a parametric study on the actuation power and actuation frequency is performed using a large eddy simulation. A nondimensional actuation power is proposed to reveal the relationship between the actuation power and the characteristic power of the baseline flow field, in order to provide a reference for obtaining a balance between energy consumption and flow control. With different actuation powers, the mechanism behind the evolution process of vortex structures induced by the plasma actuation is revealed through a decomposition of the vorticity according to the vorticity transport equation and the analysis of the nondimensional circulation according to the Q criterion. It is found that the evolution shows a relatively consistent tendency and can be divided into three stages corresponding to different disturbance processes induced by Kelvin–Helmholtz instabilities. Finally, a criterion for the effective actuation frequency based on the continuous generation of induced vortex structures is established considering different actuation powers and simplified to specific parameters within the flow field.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In order to obtain the general criterion of control authority for suppressing flow separation in the compressor airfoil at low Reynolds number by nanosecond pulsed dielectric barrier discharge plasma actuation with different actuation parameters, a parametric study on the actuation power and actuation frequency is performed using a large eddy simulation. A nondimensional actuation power is proposed to reveal the relationship between the actuation power and the characteristic power of the baseline flow field, in order to provide a reference for obtaining a balance between energy consumption and flow control. With different actuation powers, the mechanism behind the evolution process of vortex structures induced by the plasma actuation is revealed through a decomposition of the vorticity according to the vorticity transport equation and the analysis of the nondimensional circulation according to the Q criterion. It is found that the evolution shows a relatively consistent tendency and can be divided into three stages corresponding to different disturbance processes induced by Kelvin–Helmholtz instabilities. Finally, a criterion for the effective actuation frequency based on the continuous generation of induced vortex structures is established considering different actuation powers and simplified to specific parameters within the flow field.
A parametric study on control authority and vorticity transport in a compressor airfoil with plasma actuation at low Reynolds number
10.1063/5.0141480
Physics of Fluids
20230317T03:03:41Z
© 2023 Author(s).

Flow in multilayered vegetated compound channels with different bank slopes
https://aip.scitation.org/doi/10.1063/5.0142400?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Bank angle and floodplain vegetation emergence determine the flow nature in a compound channel. Two sets of [math] and [math] bank angle compound channel is considered in the present work. Each set considers three cases of vegetation arrangements: no vegetation, multilayered fully submerged, and multilayered partially emergent. The flow characteristics like velocity, Reynolds shear stress (RSS), and turbulent kinetic energy (TKE) do not vary much in the cross section in the absence of vegetation. However, with vegetation, the slopes and nearby region are affected the most as it acts as an intermediary region between the main channel and floodplain. An analysis of the anisotropic invariant map shows the dominance of the transverse component in the slopes compared to the main channel and floodplain. The velocity in and around the slopes is higher for steep slopes ([math]) compared to a gradual slope ([math]) compound channel. The streamwise RSS and bursting events also show higher magnitude near the channel bed in and around the sloping region. This indicates the instability of the steep banks compared to gradual bank slopes. The increase in floodplain vegetation emergence also affects the slopes. The magnitude of RSS and TKE in the slopes is higher with greater vegetation emergence in the floodplain. This shows the higher vulnerability of the slopes in the presence of higher vegetation emergence. From the hydraulic engineering perspective, this study will be helpful in the field of understanding the failure of banks and ways to maintain their stability.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Bank angle and floodplain vegetation emergence determine the flow nature in a compound channel. Two sets of [math] and [math] bank angle compound channel is considered in the present work. Each set considers three cases of vegetation arrangements: no vegetation, multilayered fully submerged, and multilayered partially emergent. The flow characteristics like velocity, Reynolds shear stress (RSS), and turbulent kinetic energy (TKE) do not vary much in the cross section in the absence of vegetation. However, with vegetation, the slopes and nearby region are affected the most as it acts as an intermediary region between the main channel and floodplain. An analysis of the anisotropic invariant map shows the dominance of the transverse component in the slopes compared to the main channel and floodplain. The velocity in and around the slopes is higher for steep slopes ([math]) compared to a gradual slope ([math]) compound channel. The streamwise RSS and bursting events also show higher magnitude near the channel bed in and around the sloping region. This indicates the instability of the steep banks compared to gradual bank slopes. The increase in floodplain vegetation emergence also affects the slopes. The magnitude of RSS and TKE in the slopes is higher with greater vegetation emergence in the floodplain. This shows the higher vulnerability of the slopes in the presence of higher vegetation emergence. From the hydraulic engineering perspective, this study will be helpful in the field of understanding the failure of banks and ways to maintain their stability.
Flow in multilayered vegetated compound channels with different bank slopes
10.1063/5.0142400
Physics of Fluids
20230310T12:33:05Z
© 2023 Author(s).
Jyotirmoy Barman
Bimlesh Kumar

Experimental study on the evolution of pore structure of coal samples under freeze–thaw
https://aip.scitation.org/doi/10.1063/5.0145187?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>To investigate the effect of freeze–thaw on damage and pore structure characteristics, some coal samples were freezethawed by freeze–thaw test machines. The evolution law of the pore structure of coal samples under freeze–thaw cycles, the porosity, and the bound and free fluid volume of coal samples under freeze–thaw were studied by low nuclear magnetic resonance, the complexity of the pore structure of coal samples under different freeze–thaw cycles was analyzed by fractal dimension, and the influence mechanism of freeze–thaw on the pore structure of coal samples was discussed. The results indicate that: (1) freeze–thaw could damage the pore and fracture structure and reduce the wave velocity of coal samples, and the wave velocity of coal samples after 120 freeze–thaw cycles decreased 66.5% compared with that of before the freeze–thaw cycle. (2) Freeze–thaw can effectively promote the development of pore and fracture. With the increase in freeze–thaw cycles, the proportion of micropores of coal samples decreases, while the proportion of mesopores and macropores gradually increases. (3) The freeze–thaw cycle can promote the increase in the coal sample porosity, and the increment of total, residual, and effective porosity is 3.47%, 1.94%, and 1.53%, respectively, after 120 freeze–thaw. (4) The fractal dimension of total, adsorption, and seepage pore of coal samples all decrease with the increase in freeze–thaw cycles, indicating that liquid nitrogen can reduce the complexity of the pore structure and weaken the heterogeneity of the pore structure.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>To investigate the effect of freeze–thaw on damage and pore structure characteristics, some coal samples were freezethawed by freeze–thaw test machines. The evolution law of the pore structure of coal samples under freeze–thaw cycles, the porosity, and the bound and free fluid volume of coal samples under freeze–thaw were studied by low nuclear magnetic resonance, the complexity of the pore structure of coal samples under different freeze–thaw cycles was analyzed by fractal dimension, and the influence mechanism of freeze–thaw on the pore structure of coal samples was discussed. The results indicate that: (1) freeze–thaw could damage the pore and fracture structure and reduce the wave velocity of coal samples, and the wave velocity of coal samples after 120 freeze–thaw cycles decreased 66.5% compared with that of before the freeze–thaw cycle. (2) Freeze–thaw can effectively promote the development of pore and fracture. With the increase in freeze–thaw cycles, the proportion of micropores of coal samples decreases, while the proportion of mesopores and macropores gradually increases. (3) The freeze–thaw cycle can promote the increase in the coal sample porosity, and the increment of total, residual, and effective porosity is 3.47%, 1.94%, and 1.53%, respectively, after 120 freeze–thaw. (4) The fractal dimension of total, adsorption, and seepage pore of coal samples all decrease with the increase in freeze–thaw cycles, indicating that liquid nitrogen can reduce the complexity of the pore structure and weaken the heterogeneity of the pore structure.
Experimental study on the evolution of pore structure of coal samples under freeze–thaw
10.1063/5.0145187
Physics of Fluids
20230314T12:08:27Z
© 2023 Author(s).

An improved gas–liquid–solid coupling model with plastic failure for hydraulic flushing in gassy coal seam and application in borehole arrangement
https://aip.scitation.org/doi/10.1063/5.0144786?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Hydraulic flushing can increase the efficiency of gas extraction by artificially modifying the coal reservoir. Considering the plastic failure of coal mass, an improved gas–liquid–solid coupling model for hydraulic flushing and gas extraction is constructed. The parameter evolution in the hydraulic flushing process was numerically investigated to determine the optimal borehole arrangement of hydraulic flushing. The results show that the relative permeability of gas gradually increases with the initial dewatering. The gas rates of both regular extraction and hydraulic flushing enhanced extraction show an increasing–decreasing trend. An increased and delayed peak gas rate is observed comparing with the regular extraction, caused by the hydraulic flushing induced new fractures. The area around of borehole is divided into the failure zone, the plastic softening zone, and the elastic zone after hydraulic flushing. The failure zone has the greatest increase in coal permeability, followed by the plastic softening zone, while the elastic zone keeps no significant change. The larger difference between the horizontal stress and vertical stress, the more obvious the elliptical shape of the permeability change area near the borehole, as well as the pressure drop in the elliptical zone. With the increase in the hydraulic flushing radius, the permeability increasing zone and gas pressure decreasing zone gradually increase. Subsequently, the equivalent effective radius and equivalent influencing radius were obtained, as well as the optimal borehole spacing for hydraulic flushing by crosslayer drilling. Finally, the optimal borehole spacing is obtained for different borehole diameters and efficient extraction times. These provide a theoretical guidance for field application of hydraulic flushing in a lowpermeable coal seam.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Hydraulic flushing can increase the efficiency of gas extraction by artificially modifying the coal reservoir. Considering the plastic failure of coal mass, an improved gas–liquid–solid coupling model for hydraulic flushing and gas extraction is constructed. The parameter evolution in the hydraulic flushing process was numerically investigated to determine the optimal borehole arrangement of hydraulic flushing. The results show that the relative permeability of gas gradually increases with the initial dewatering. The gas rates of both regular extraction and hydraulic flushing enhanced extraction show an increasing–decreasing trend. An increased and delayed peak gas rate is observed comparing with the regular extraction, caused by the hydraulic flushing induced new fractures. The area around of borehole is divided into the failure zone, the plastic softening zone, and the elastic zone after hydraulic flushing. The failure zone has the greatest increase in coal permeability, followed by the plastic softening zone, while the elastic zone keeps no significant change. The larger difference between the horizontal stress and vertical stress, the more obvious the elliptical shape of the permeability change area near the borehole, as well as the pressure drop in the elliptical zone. With the increase in the hydraulic flushing radius, the permeability increasing zone and gas pressure decreasing zone gradually increase. Subsequently, the equivalent effective radius and equivalent influencing radius were obtained, as well as the optimal borehole spacing for hydraulic flushing by crosslayer drilling. Finally, the optimal borehole spacing is obtained for different borehole diameters and efficient extraction times. These provide a theoretical guidance for field application of hydraulic flushing in a lowpermeable coal seam.
An improved gas–liquid–solid coupling model with plastic failure for hydraulic flushing in gassy coal seam and application in borehole arrangement
10.1063/5.0144786
Physics of Fluids
20230317T02:43:27Z
© 2023 Author(s).

Wronskian solutions and Pfaffianization for a (3 + 1)dimensional generalized variablecoefficient KadomtsevPetviashvili equation in a fluid or plasma
https://aip.scitation.org/doi/10.1063/5.0141559?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this paper, we investigate a (3 + 1)dimensional generalized variablecoefficient KadomtsevPetviashvili (GVCKP) equation in a fluid or plasma. The Nthorder Wronskian solutions for that equation are derived and proved under certain variablecoefficient constraints, where N is a positive integer. One, two, and threesoliton solutions in the Wronskian for that equation are given. By means of the Pfaffianization procedure, a coupled (3 + 1)dimensional GVCKP system is constructed from that equation. Bilinear form for that coupled system is exported. Under certain variablecoefficient constraints, those Wronskitype and Grammtype Pfaffian solutions for that coupled system are obtained and proved with the help of the Pfaffian identities.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this paper, we investigate a (3 + 1)dimensional generalized variablecoefficient KadomtsevPetviashvili (GVCKP) equation in a fluid or plasma. The Nthorder Wronskian solutions for that equation are derived and proved under certain variablecoefficient constraints, where N is a positive integer. One, two, and threesoliton solutions in the Wronskian for that equation are given. By means of the Pfaffianization procedure, a coupled (3 + 1)dimensional GVCKP system is constructed from that equation. Bilinear form for that coupled system is exported. Under certain variablecoefficient constraints, those Wronskitype and Grammtype Pfaffian solutions for that coupled system are obtained and proved with the help of the Pfaffian identities.
Wronskian solutions and Pfaffianization for a (3 + 1)dimensional generalized variablecoefficient KadomtsevPetviashvili equation in a fluid or plasma
10.1063/5.0141559
Physics of Fluids
20230301T01:53:35Z
© 2023 Author(s).

Extension of the Shakhov Bhatnagar–Gross–Krook model for nonequilibrium gas flows
https://aip.scitation.org/doi/10.1063/5.0139635?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Bhatnagar–Gross–Krook (BGK) models are widely used to study rarefied gas dynamics. However, as simplified versions of the Boltzmann collision model, their performances are uncertain and need to be carefully investigated in highly nonequilibrium flows. In this study, several common BGK models, such as the ellipsoidal statistical BGK (ESBGK) and Shakhov BGK (SBGK) models, are theoretically analyzed using their moment equations. Then, numerical comparisons are performed between the Boltzmann collision model and BGK models based on various benchmarks, such as Fourier flow, Couette flow, and shock wave. The prediction performance of the ESBGK model is better than that of the SBGK model in Fourier flow, while prediction performance of the SBGK model is better than that of the ESBGK model in Couette flow and shock wave. However, with increasing Knudsen number or Mach number, the results of both ESBGK and SBGK deviate from the Boltzmann solutions. These phenomena are attributed to the incorrect governing equations of highorder moments of BGK models. To improve the performance of the current BGK models, the SBGK model is extended by adding more highorder moments into the target distribution function of the original one. Our analytical and numerical results demonstrate that the extended SBGK (SBGK+) model provides the same relaxation coefficients as the Boltzmann collision model for the production terms of highorder moment equations. Compared with the other BGK models, the proposed SBGK+ model exhibits better performance for various flow regimes.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Bhatnagar–Gross–Krook (BGK) models are widely used to study rarefied gas dynamics. However, as simplified versions of the Boltzmann collision model, their performances are uncertain and need to be carefully investigated in highly nonequilibrium flows. In this study, several common BGK models, such as the ellipsoidal statistical BGK (ESBGK) and Shakhov BGK (SBGK) models, are theoretically analyzed using their moment equations. Then, numerical comparisons are performed between the Boltzmann collision model and BGK models based on various benchmarks, such as Fourier flow, Couette flow, and shock wave. The prediction performance of the ESBGK model is better than that of the SBGK model in Fourier flow, while prediction performance of the SBGK model is better than that of the ESBGK model in Couette flow and shock wave. However, with increasing Knudsen number or Mach number, the results of both ESBGK and SBGK deviate from the Boltzmann solutions. These phenomena are attributed to the incorrect governing equations of highorder moments of BGK models. To improve the performance of the current BGK models, the SBGK model is extended by adding more highorder moments into the target distribution function of the original one. Our analytical and numerical results demonstrate that the extended SBGK (SBGK+) model provides the same relaxation coefficients as the Boltzmann collision model for the production terms of highorder moment equations. Compared with the other BGK models, the proposed SBGK+ model exhibits better performance for various flow regimes.
Extension of the Shakhov Bhatnagar–Gross–Krook model for nonequilibrium gas flows
10.1063/5.0139635
Physics of Fluids
20230301T01:51:55Z
© 2023 Author(s).

Modified nonlinear Schrödinger equation for gravity waves with the influence of wind, currents, and dissipation
https://aip.scitation.org/doi/10.1063/5.0137966?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A new modified nonlinear Schrödinger (MNLS) equation is derived for gravity waves with the presence of wind, dissipation, and shear currents in finite water depth. Horizontal surface currents are assumed stationary and slowly varying spatially. Using the MNLS equation, the modulational instability (MI) of deepwater gravity wave trains affected by wind and dissipation is considered. It was demonstrated that the modulational perturbation of waves is unstable or becomes unstable after several wave periods, whereas the dissipation will suppress the MI. Then, a new theoretical formula for predicting the maximum amplitude is derived to take into account the effect of vorticity, dissipation, and wind. The effect of dissipation becomes significant in strong currents, while wind can increase the height amplification. Furthermore, an explicit analytical Peregrine breather (PB) solution that considers the effect of vorticity, dissipation, and wind is presented. Opposing currents and winds will increase the height of PB. However, following currents and dissipation have opposite effects. The effects of the shear current, dissipation, and wind on nondimensional maximum amplitudes during the evolution of the Akhmediev breather are similar to PB solution.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A new modified nonlinear Schrödinger (MNLS) equation is derived for gravity waves with the presence of wind, dissipation, and shear currents in finite water depth. Horizontal surface currents are assumed stationary and slowly varying spatially. Using the MNLS equation, the modulational instability (MI) of deepwater gravity wave trains affected by wind and dissipation is considered. It was demonstrated that the modulational perturbation of waves is unstable or becomes unstable after several wave periods, whereas the dissipation will suppress the MI. Then, a new theoretical formula for predicting the maximum amplitude is derived to take into account the effect of vorticity, dissipation, and wind. The effect of dissipation becomes significant in strong currents, while wind can increase the height amplification. Furthermore, an explicit analytical Peregrine breather (PB) solution that considers the effect of vorticity, dissipation, and wind is presented. Opposing currents and winds will increase the height of PB. However, following currents and dissipation have opposite effects. The effects of the shear current, dissipation, and wind on nondimensional maximum amplitudes during the evolution of the Akhmediev breather are similar to PB solution.
Modified nonlinear Schrödinger equation for gravity waves with the influence of wind, currents, and dissipation
10.1063/5.0137966
Physics of Fluids
20230302T02:38:42Z
© 2023 Author(s).
Marc Perlin

Numerical study on vortexinduced vibrations of a flexible cylinder subjected to multidirectional flows
https://aip.scitation.org/doi/10.1063/5.0138063?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Vortexinduced vibrations (VIVs) of a flexible cylinder subjected to multidirectional flows have been studied based on a wake oscillator model. The multidirectional flow comprises two slabs of flows in different directions, with each slab having a uniform unidirectional profile. The dynamics of the flexible cylinder is described based on the linear Euler–Bernoulli beam theory, and a wake oscillator model is uniformly distributed along the cylinder to model the hydrodynamic force acting on it. The dynamics of the coupled system has been solved numerically using the finite element method, and simulations have been conducted with the cylinder subjected to multidirectional flows with different angles between the two slabs. A large number of different initial conditions have been applied, and more than one steadystate response has been captured. The steadystate responses exhibit two different patterns: one is characterized by two waves traveling in opposite directions, while the other is dominated by a single traveling wave. The crossflow VIV primarily occurs in the local crossflow direction, and a transition of its vibrating direction happens at the interface of the two flows. Such transition is not observed in the inline VIV, and significant vibrations at the double frequency appear in both local crossflow and inline directions. Energy analysis shows that this transition is boosted by a specific energy transfer pattern between the structure and the flow, which excites the vibration of the cylinder in some directions while damps it in others.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Vortexinduced vibrations (VIVs) of a flexible cylinder subjected to multidirectional flows have been studied based on a wake oscillator model. The multidirectional flow comprises two slabs of flows in different directions, with each slab having a uniform unidirectional profile. The dynamics of the flexible cylinder is described based on the linear Euler–Bernoulli beam theory, and a wake oscillator model is uniformly distributed along the cylinder to model the hydrodynamic force acting on it. The dynamics of the coupled system has been solved numerically using the finite element method, and simulations have been conducted with the cylinder subjected to multidirectional flows with different angles between the two slabs. A large number of different initial conditions have been applied, and more than one steadystate response has been captured. The steadystate responses exhibit two different patterns: one is characterized by two waves traveling in opposite directions, while the other is dominated by a single traveling wave. The crossflow VIV primarily occurs in the local crossflow direction, and a transition of its vibrating direction happens at the interface of the two flows. Such transition is not observed in the inline VIV, and significant vibrations at the double frequency appear in both local crossflow and inline directions. Energy analysis shows that this transition is boosted by a specific energy transfer pattern between the structure and the flow, which excites the vibration of the cylinder in some directions while damps it in others.
Numerical study on vortexinduced vibrations of a flexible cylinder subjected to multidirectional flows
10.1063/5.0138063
Physics of Fluids
20230303T12:57:58Z
© 2023 Author(s).

The fixed points and the manifolds in a second order Stokes wave
https://aip.scitation.org/doi/10.1063/5.0139906?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Here, we present an analysis of the flow properties of second order Stokes waves in water. The description of the flow field is developed using the concept of fixed points and manifolds, which is commonly employed for the analysis of a nonlinear dynamic system. We find that the components of the velocity field are related to each other by an elliptic correlation, where the center of the ellipse represents the fixed points. Since an ellipse is not likely to pass through its center, the estimation of the fixed points in a second order Stokes wave seems challenging. However, we find that the fixed points can be found out in a degenerate case of the ellipse; such a case is observed at the bottom surface that is found to host all the fixed points. The vertical lines passing through the fixed points represent the manifolds. We find that, interestingly, the fixed points and the corresponding manifolds are not fixed but rather move with a speed that equals the wave celerity. Here, we show that the deformation of the free surface requires straining. The flow field evolves in a manner to sustain such straining. Despite the rigid nature, the flow straining is also observed at the bottom surface. Such straining is found to be generated by the fixed points at the bottom surface. The vertically oriented manifolds are found acting as the guides to mediate such flow and straining exchange between the free and bottom surface.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Here, we present an analysis of the flow properties of second order Stokes waves in water. The description of the flow field is developed using the concept of fixed points and manifolds, which is commonly employed for the analysis of a nonlinear dynamic system. We find that the components of the velocity field are related to each other by an elliptic correlation, where the center of the ellipse represents the fixed points. Since an ellipse is not likely to pass through its center, the estimation of the fixed points in a second order Stokes wave seems challenging. However, we find that the fixed points can be found out in a degenerate case of the ellipse; such a case is observed at the bottom surface that is found to host all the fixed points. The vertical lines passing through the fixed points represent the manifolds. We find that, interestingly, the fixed points and the corresponding manifolds are not fixed but rather move with a speed that equals the wave celerity. Here, we show that the deformation of the free surface requires straining. The flow field evolves in a manner to sustain such straining. Despite the rigid nature, the flow straining is also observed at the bottom surface. Such straining is found to be generated by the fixed points at the bottom surface. The vertically oriented manifolds are found acting as the guides to mediate such flow and straining exchange between the free and bottom surface.
The fixed points and the manifolds in a second order Stokes wave
10.1063/5.0139906
Physics of Fluids
20230303T12:57:50Z
© 2023 Author(s).
Anjanee Kumar
Kaustav Chaudhury

Heat transfer modulation in Rayleigh–Bénard convection by an oscillatory bottom plate
https://aip.scitation.org/doi/10.1063/5.0138407?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this paper, we consider a heat transfer modulation in Rayleigh–Bénard convection by imposing a periodic sinusoidal oscillation to the bottom hot plate parallel to itself. Twodimensional numerical simulations are carried out under lateral periodic conditions, over a Rayleigh number range of [math] and for a fixed Prandtl number of [math] = 7.1. For a given Rayleigh number, it is found that the Nusselt number, characterizing the global heat transfer efficiency of the system, shows a counterintuitive initial drop and subsequent rise behavior, as the characteristic oscillatory velocity [math] increases. Accordingly, taking the classical Rayleigh–Bénard convection as a reference, a heat transfer reduction regime for low [math] and a heat transfer enhancement regime for high [math] are recognized. The reduction regime is resulted from the thickening of the thermal boundary layer due to the amplified viscous effect by the oscillation, which increases the thermal resistance of the system. In addition to thickening the thermal boundary layer, a stronger oscillation could also trigger a thermal boundary layer instability, inducing massive emission of the thermal plumes and eventually giving rise to a significant global heat transfer enhancement. Moreover, the combined effect of thickening and destabilizing of the thermal boundary layer leads to a temporal periodic evolution of the Nusselt number at the bottom plate in the enhancement regime. A critical oscillatory velocity Vc is selected at the crossover between two regimes, and it is found decreasing with an increasing [math] as [math]. Through dimensional analysis, we provide a physical explanation for this dependence.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this paper, we consider a heat transfer modulation in Rayleigh–Bénard convection by imposing a periodic sinusoidal oscillation to the bottom hot plate parallel to itself. Twodimensional numerical simulations are carried out under lateral periodic conditions, over a Rayleigh number range of [math] and for a fixed Prandtl number of [math] = 7.1. For a given Rayleigh number, it is found that the Nusselt number, characterizing the global heat transfer efficiency of the system, shows a counterintuitive initial drop and subsequent rise behavior, as the characteristic oscillatory velocity [math] increases. Accordingly, taking the classical Rayleigh–Bénard convection as a reference, a heat transfer reduction regime for low [math] and a heat transfer enhancement regime for high [math] are recognized. The reduction regime is resulted from the thickening of the thermal boundary layer due to the amplified viscous effect by the oscillation, which increases the thermal resistance of the system. In addition to thickening the thermal boundary layer, a stronger oscillation could also trigger a thermal boundary layer instability, inducing massive emission of the thermal plumes and eventually giving rise to a significant global heat transfer enhancement. Moreover, the combined effect of thickening and destabilizing of the thermal boundary layer leads to a temporal periodic evolution of the Nusselt number at the bottom plate in the enhancement regime. A critical oscillatory velocity Vc is selected at the crossover between two regimes, and it is found decreasing with an increasing [math] as [math]. Through dimensional analysis, we provide a physical explanation for this dependence.
Heat transfer modulation in Rayleigh–Bénard convection by an oscillatory bottom plate
10.1063/5.0138407
Physics of Fluids
20230303T04:38:20Z
© 2023 Author(s).

Research on wake and potential flow effects of rotor–stator interaction in a centrifugal pump with guided vanes
https://aip.scitation.org/doi/10.1063/5.0138867?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this paper, the wake and potential flow effects of the rotor–stator interaction in a centrifugal pump with guide vanes are investigated from the view of the separation of turbulent and acoustic pressure fluctuations. The highest vibration levels in pumps are, in general, originated in the potential flow and wake effects. However, it is challenging to distinguish their effects on flow evolution. The pellicular mode decomposition method is applied to innovatively separate potential flow and wake disturbances in a centrifugal pump. By pellicular, we mean an infinitely thin layer of air located on the monitoring surface. The pellicular modes are a set of acoustic modes, with which a set of normalized orthogonal basis can be constructed. The impacts of potential flow and wake disturbances are visualized and evaluated quantitatively. The results show that only a very limited region is where the potential flow disturbance works. The higher the harmonics, the smaller the disturbance range. The wake disturbance is responsible for the modal pressure field. Modal pressure fields with low diametrical nodes decay more slowly than those with low harmonics. In addition, special attention is paid to the impact of the geometric asymmetry of the volute on the impeller force. The circular volute with a twostage pressure drop improves the radial force of the impeller. More deep understandings on the mechanism of the rotor–stator interaction are reached by decoupling the potential flow and wake disturbances. This work serves as a guide for further research in fault diagnosis and vibration control of centrifugal pumps.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this paper, the wake and potential flow effects of the rotor–stator interaction in a centrifugal pump with guide vanes are investigated from the view of the separation of turbulent and acoustic pressure fluctuations. The highest vibration levels in pumps are, in general, originated in the potential flow and wake effects. However, it is challenging to distinguish their effects on flow evolution. The pellicular mode decomposition method is applied to innovatively separate potential flow and wake disturbances in a centrifugal pump. By pellicular, we mean an infinitely thin layer of air located on the monitoring surface. The pellicular modes are a set of acoustic modes, with which a set of normalized orthogonal basis can be constructed. The impacts of potential flow and wake disturbances are visualized and evaluated quantitatively. The results show that only a very limited region is where the potential flow disturbance works. The higher the harmonics, the smaller the disturbance range. The wake disturbance is responsible for the modal pressure field. Modal pressure fields with low diametrical nodes decay more slowly than those with low harmonics. In addition, special attention is paid to the impact of the geometric asymmetry of the volute on the impeller force. The circular volute with a twostage pressure drop improves the radial force of the impeller. More deep understandings on the mechanism of the rotor–stator interaction are reached by decoupling the potential flow and wake disturbances. This work serves as a guide for further research in fault diagnosis and vibration control of centrifugal pumps.
Research on wake and potential flow effects of rotor–stator interaction in a centrifugal pump with guided vanes
10.1063/5.0138867
Physics of Fluids
20230303T12:58:06Z
© 2023 Author(s).

Accurate storm surge prediction using a multirecurrent neural network structure
https://aip.scitation.org/doi/10.1063/5.0137792?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This paper considers storm surge prediction using a neural network and considering multiple physical characteristics. Based on the factors that influence storm surges and historical observation data, we divide the input to the neural network into time features extracted from the prediction target and the auxiliary features that affect storm surges, and construct a feature gate within multiple recurrent neural network (RNN) cells. Historical hurricane data are used to assess the effectiveness and accuracy of the proposed model. Comparative analysis against a long shortterm memory (LSTM) storm surge prediction model is conducted to verify the prediction performance of the proposed method. The comparison results show that the multiRNN model is superior to the LSTM model in terms of four evaluation metrics and for all lead times. In particular, the multiRNN model accurately predicts the maximum storm surge water level, and the prediction results are more consistent with the rise and fall of the water. A comparison of the storm surge forecasts using inputs from different time intervals under different evaluation indices confirms the generalization and stability of our proposed model. The experiments of storm surge prediction at six stations further confirm the wide applicability of the model.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This paper considers storm surge prediction using a neural network and considering multiple physical characteristics. Based on the factors that influence storm surges and historical observation data, we divide the input to the neural network into time features extracted from the prediction target and the auxiliary features that affect storm surges, and construct a feature gate within multiple recurrent neural network (RNN) cells. Historical hurricane data are used to assess the effectiveness and accuracy of the proposed model. Comparative analysis against a long shortterm memory (LSTM) storm surge prediction model is conducted to verify the prediction performance of the proposed method. The comparison results show that the multiRNN model is superior to the LSTM model in terms of four evaluation metrics and for all lead times. In particular, the multiRNN model accurately predicts the maximum storm surge water level, and the prediction results are more consistent with the rise and fall of the water. A comparison of the storm surge forecasts using inputs from different time intervals under different evaluation indices confirms the generalization and stability of our proposed model. The experiments of storm surge prediction at six stations further confirm the wide applicability of the model.
Accurate storm surge prediction using a multirecurrent neural network structure
10.1063/5.0137792
Physics of Fluids
20230306T10:56:08Z
© 2023 Author(s).

Mach reflection of detonation wave on porous wall
https://aip.scitation.org/doi/10.1063/5.0140347?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This study reports the Mach reflection of gaseous detonation on porous wedges experimentally, in which the porous wall is consisted of equidistant inline square columns. The smoked foil technique was utilized to monitor the evolution of the triplepoint trajectory and detonation cells in the Mach stem region. In addition to the wedge angle and initial pressure of gaseous mixture, this paper also focuses on the effect of porosity and pore size on the Mach reflection and its mechanism. The results show that the strength of the Mach stem is significantly weakened by the porous media compared with that on the smooth wedge, which is ascribed to the diffraction and reflection waves generated by the interaction of the Mach stem front with the pore. Furthermore, the onset of the triplepoint trajectory is delayed, the angle of which is decreased. The porosity and pore size present distinct influence on the Mach stem height. With the increase in the porosity and the decrease in the pore size, the Mach stem region is attenuated more dramatically and the Mach stem is harder to be formed or even could not be observed. Furthermore, the triplepoint trajectory on the porous wall exhibits local selfsimilarity and satisfies the frozen limit in the near field and the equilibrium limit in the far field. However, the lengths of the existence of the frozen limit and the transition to the equilibrium limit on the porous wall are found to be much shorter than the hydrodynamic thickness, and the recovery of the selfsimilarity depends largely on the porosity and pore size.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This study reports the Mach reflection of gaseous detonation on porous wedges experimentally, in which the porous wall is consisted of equidistant inline square columns. The smoked foil technique was utilized to monitor the evolution of the triplepoint trajectory and detonation cells in the Mach stem region. In addition to the wedge angle and initial pressure of gaseous mixture, this paper also focuses on the effect of porosity and pore size on the Mach reflection and its mechanism. The results show that the strength of the Mach stem is significantly weakened by the porous media compared with that on the smooth wedge, which is ascribed to the diffraction and reflection waves generated by the interaction of the Mach stem front with the pore. Furthermore, the onset of the triplepoint trajectory is delayed, the angle of which is decreased. The porosity and pore size present distinct influence on the Mach stem height. With the increase in the porosity and the decrease in the pore size, the Mach stem region is attenuated more dramatically and the Mach stem is harder to be formed or even could not be observed. Furthermore, the triplepoint trajectory on the porous wall exhibits local selfsimilarity and satisfies the frozen limit in the near field and the equilibrium limit in the far field. However, the lengths of the existence of the frozen limit and the transition to the equilibrium limit on the porous wall are found to be much shorter than the hydrodynamic thickness, and the recovery of the selfsimilarity depends largely on the porosity and pore size.
Mach reflection of detonation wave on porous wall
10.1063/5.0140347
Physics of Fluids
20230306T10:56:09Z
© 2023 Author(s).

Object tracking based droplet characterization of high flowrate electrospray
https://aip.scitation.org/doi/10.1063/5.0139222?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Electrospray was applied to the wet electrostatic precipitator to reduce the water consumption of highefficiency fine dust collection. The size of droplets must be large to avoid evaporating quickly under high temperature exhaust gas conditions, so a high flow rate condition of several milliliters/min is used, which is relatively high compared with previous applications. Because a high flow rate electrospray has a wide spray range and a low spray density, imagebased droplet size measurement was used. A bias in the probability in the distribution occurs because of the difference in velocity between the droplets. In this study, an approach with object tracking was suggested to eliminate the bias from velocity differences. High flow rate electrospray droplets under various voltage conditions were visualized with a highspeed camera. Based on the image processing, the corrected distribution was characterized, and the effect of the bias was established through comparison with the general distribution. In addition, the spray pattern and the droplet distribution according to the applied voltage of three spraying modes were analyzed. Improved analysis of the actual spray using this approach will guide the selection of operating conditions that optimize dust collection efficiency.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Electrospray was applied to the wet electrostatic precipitator to reduce the water consumption of highefficiency fine dust collection. The size of droplets must be large to avoid evaporating quickly under high temperature exhaust gas conditions, so a high flow rate condition of several milliliters/min is used, which is relatively high compared with previous applications. Because a high flow rate electrospray has a wide spray range and a low spray density, imagebased droplet size measurement was used. A bias in the probability in the distribution occurs because of the difference in velocity between the droplets. In this study, an approach with object tracking was suggested to eliminate the bias from velocity differences. High flow rate electrospray droplets under various voltage conditions were visualized with a highspeed camera. Based on the image processing, the corrected distribution was characterized, and the effect of the bias was established through comparison with the general distribution. In addition, the spray pattern and the droplet distribution according to the applied voltage of three spraying modes were analyzed. Improved analysis of the actual spray using this approach will guide the selection of operating conditions that optimize dust collection efficiency.
Object tracking based droplet characterization of high flowrate electrospray
10.1063/5.0139222
Physics of Fluids
20230307T12:16:26Z
© 2023 Author(s).

Intelligent controller for unmanned surface vehicles by deep reinforcement learning
https://aip.scitation.org/doi/10.1063/5.0139568?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>With the development of the applications of unmanned surface vehicles (USVs), USV automation technologies are attracting increasing attention. In the industry, through the subtask division, it is generally believed that coursekeeping is a critical basic subsystem in a series of complex automation systems and affects USV automation performance to a great extent. By coursekeeping, we mean USV adjusts its angle to the desired angle and keeps it. In recent decades, coursekeeping has been mainly achieved through classical first principles technologies, such as proportion–integral–differential (PID) controllers, leading to extremely laborious parameter tuning, especially in changeable wave environments. With the emergence and extensive application of datadriven technologies, deep reinforcement learning is conspicuous in sequential decisionmaking tasks, but it introduces a lack of explainability and physical meaning. To take full advantage of the datadriven and first principles paradigm and easily extend to the industry, in this paper, we propose an intelligent adaptive PID controller enhanced by proximal policy optimization (PPO) to achieve USV highlevel automation. We then further verify its performance in pathfollowing tasks compared with the PID controller. The results demonstrate that the proposed controller inherits the merits of explainability from PID and excellent sequential decision making from PPO and possesses excellent disturbance rejection performance when facing the disturbance of a changeable wave environment.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>With the development of the applications of unmanned surface vehicles (USVs), USV automation technologies are attracting increasing attention. In the industry, through the subtask division, it is generally believed that coursekeeping is a critical basic subsystem in a series of complex automation systems and affects USV automation performance to a great extent. By coursekeeping, we mean USV adjusts its angle to the desired angle and keeps it. In recent decades, coursekeeping has been mainly achieved through classical first principles technologies, such as proportion–integral–differential (PID) controllers, leading to extremely laborious parameter tuning, especially in changeable wave environments. With the emergence and extensive application of datadriven technologies, deep reinforcement learning is conspicuous in sequential decisionmaking tasks, but it introduces a lack of explainability and physical meaning. To take full advantage of the datadriven and first principles paradigm and easily extend to the industry, in this paper, we propose an intelligent adaptive PID controller enhanced by proximal policy optimization (PPO) to achieve USV highlevel automation. We then further verify its performance in pathfollowing tasks compared with the PID controller. The results demonstrate that the proposed controller inherits the merits of explainability from PID and excellent sequential decision making from PPO and possesses excellent disturbance rejection performance when facing the disturbance of a changeable wave environment.
Intelligent controller for unmanned surface vehicles by deep reinforcement learning
10.1063/5.0139568
Physics of Fluids
20230307T11:18:59Z
© 2023 Author(s).

Decrypting the mechanisms of wicking and evaporation heat transfer on micropillars during the pool boiling of water using highresolution infrared thermometry
https://aip.scitation.org/doi/10.1063/5.0135110?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Surfaces with micrometerscale pillars have shown great potential in delaying the boiling crisis and enhancing the critical heat flux (CHF). However, physical mechanisms enabling this enhancement remain unclear. This knowledge gap is due to a lack of diagnostics that allow elucidating how micropillars affect thermal transport phenomena on the engineered surface. In this study, for the first time, we are able to measure timedependent temperature and heat flux distributions on a boiling surface with engineered micropillars using infrared thermometry. Using these data, we reveal the presence of an intrapillar liquid layer, created by the nucleation of bubbles and partially refilled by capillary effects. However, contrarily to conventional wisdom, the energy removed by the evaporation of this liquid cannot explain the observed CHF enhancement. Yet, predicting its dry out is the key to delaying the boiling crisis. We achieve this goal using simple analytic models and demonstrate that this process is driven by conduction effects in the boiling substrates and, importantly, in the intrapillar liquid layer itself. Importantly, these effects also control the wicking flow rate and its penetration length. The boiling crisis occurs when, by coalescing, the size of the intrapillar liquid layer becomes too large for the wicking flow to reach its innermost region. Our study reveals and quantifies unidentified physical aspects, key to the performance optimization of boiling surfaces for cooling applications.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Surfaces with micrometerscale pillars have shown great potential in delaying the boiling crisis and enhancing the critical heat flux (CHF). However, physical mechanisms enabling this enhancement remain unclear. This knowledge gap is due to a lack of diagnostics that allow elucidating how micropillars affect thermal transport phenomena on the engineered surface. In this study, for the first time, we are able to measure timedependent temperature and heat flux distributions on a boiling surface with engineered micropillars using infrared thermometry. Using these data, we reveal the presence of an intrapillar liquid layer, created by the nucleation of bubbles and partially refilled by capillary effects. However, contrarily to conventional wisdom, the energy removed by the evaporation of this liquid cannot explain the observed CHF enhancement. Yet, predicting its dry out is the key to delaying the boiling crisis. We achieve this goal using simple analytic models and demonstrate that this process is driven by conduction effects in the boiling substrates and, importantly, in the intrapillar liquid layer itself. Importantly, these effects also control the wicking flow rate and its penetration length. The boiling crisis occurs when, by coalescing, the size of the intrapillar liquid layer becomes too large for the wicking flow to reach its innermost region. Our study reveals and quantifies unidentified physical aspects, key to the performance optimization of boiling surfaces for cooling applications.
Decrypting the mechanisms of wicking and evaporation heat transfer on micropillars during the pool boiling of water using highresolution infrared thermometry
10.1063/5.0135110
Physics of Fluids
20230308T12:22:49Z
© 2023 Author(s).
Md Mahamudur Rahman
Matteo Bucci

A Chebyshev–Tau spectral method for coupled modes of underwater sound propagation in rangedependent ocean environments
https://aip.scitation.org/doi/10.1063/5.0138012?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>A coupledmode model is a classic approach for solving rangedependent sound propagations and is often used to provide benchmark solutions in comparison with other numerical models because of its high accuracy. Existing coupledmode programs have disadvantages such as high computational cost, weak adaptability to complex ocean environments, and numerical instability. In this paper, a new algorithm that uses an improved range normalization of a “stairstep” and global matrix approach to address range dependence in ocean environments is designed. This algorithm uses the Chebyshev–Tau spectral method to solve the eigenpairs in the rangeindependent segments. The Chebyshev–Tau spectral method can converge rapidly, and the rate of convergence depends on the smoothness of the sound speed and density profiles. The main steps of the algorithm are parallelized, so parallel computing technologies are also applied for further acceleration. Based on this algorithm, an efficient program is implemented, and numerical simulations verify that this algorithm is reliable, accurate, and capable. Compared with the existing coupledmode programs, the newly developed program is more stable and efficient with comparable accuracy and can simulate waveguides in more complex and realistic ocean environments.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>A coupledmode model is a classic approach for solving rangedependent sound propagations and is often used to provide benchmark solutions in comparison with other numerical models because of its high accuracy. Existing coupledmode programs have disadvantages such as high computational cost, weak adaptability to complex ocean environments, and numerical instability. In this paper, a new algorithm that uses an improved range normalization of a “stairstep” and global matrix approach to address range dependence in ocean environments is designed. This algorithm uses the Chebyshev–Tau spectral method to solve the eigenpairs in the rangeindependent segments. The Chebyshev–Tau spectral method can converge rapidly, and the rate of convergence depends on the smoothness of the sound speed and density profiles. The main steps of the algorithm are parallelized, so parallel computing technologies are also applied for further acceleration. Based on this algorithm, an efficient program is implemented, and numerical simulations verify that this algorithm is reliable, accurate, and capable. Compared with the existing coupledmode programs, the newly developed program is more stable and efficient with comparable accuracy and can simulate waveguides in more complex and realistic ocean environments.
A Chebyshev–Tau spectral method for coupled modes of underwater sound propagation in rangedependent ocean environments
10.1063/5.0138012
Physics of Fluids
20230310T12:19:47Z
© 2023 Author(s).

Experimental assessment of Theodorsen's function for uncoupled pitch–plunge motion
https://aip.scitation.org/doi/10.1063/5.0139918?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The accuracy of Theodorsen's lift model for purepitch, pureplunge and combined pitch–plunge oscillations of a twodimensional model is compared with wind tunnel results. The reduced frequency of the oscillation was in the range of [math], and the freestream Reynolds number was in the range of [math]. The lift response to an uncoupled combined pitch–plunge motion (for which the frequency of pitch and plunge motions were not the same) is discussed using experimental results. The Theodorsen’s lift model is rewritten for the general uncoupled pitch–plunge motions by a linear superposition of all components of the airfoil bound circulation. Both amplitude and phase from the Theodorsen's function are compared with those of the wind tunnel data, and the results are discussed. The Theodorsen’s function is found to be a good estimator for both purepitch and pureplunge motions. It further appropriately estimates the lift amplitude for the case of coupled pitch–plunge motion; however, the prediction is not accurate for the uncoupled pitch–plunge motion. A motion amplitude ratio is defined, which shows the level of aperiodicity of the motion. Discrepancy between experimental and analytical results increase with the reduction of the lift amplitude ratio and with the deviation of frequency ratio from unity.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The accuracy of Theodorsen's lift model for purepitch, pureplunge and combined pitch–plunge oscillations of a twodimensional model is compared with wind tunnel results. The reduced frequency of the oscillation was in the range of [math], and the freestream Reynolds number was in the range of [math]. The lift response to an uncoupled combined pitch–plunge motion (for which the frequency of pitch and plunge motions were not the same) is discussed using experimental results. The Theodorsen’s lift model is rewritten for the general uncoupled pitch–plunge motions by a linear superposition of all components of the airfoil bound circulation. Both amplitude and phase from the Theodorsen's function are compared with those of the wind tunnel data, and the results are discussed. The Theodorsen’s function is found to be a good estimator for both purepitch and pureplunge motions. It further appropriately estimates the lift amplitude for the case of coupled pitch–plunge motion; however, the prediction is not accurate for the uncoupled pitch–plunge motion. A motion amplitude ratio is defined, which shows the level of aperiodicity of the motion. Discrepancy between experimental and analytical results increase with the reduction of the lift amplitude ratio and with the deviation of frequency ratio from unity.
Experimental assessment of Theodorsen's function for uncoupled pitch–plunge motion
10.1063/5.0139918
Physics of Fluids
20230313T11:26:21Z
© 2023 Author(s).
A. Daliri
M. J. Maghrebi
M. R. Soltani

Experimental investigation on the vortexinduced vibration of an inclined flexible pipe and the evaluation of the independence principle
https://aip.scitation.org/doi/10.1063/5.0138364?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>This paper reports the experimental results of the vortexinduced vibration (VIV) response of an inclined flexible pipe with various oblique angles (θ) ranging from 0° to 60°. The flexible pipe with an aspect ratio of 75 was fixed at both ends. The VIV response was examined in the reduced velocity range of 4.02–17.55 to evaluate the IP (independence principle) validity. The experimental results illustrate that the spatial distribution of response amplitudes and associated modal weights varies with θ, and the differences are enlarged in the mode transition cases. With increasing the θ, the onset normal reduced velocity (Urn) of mode transition shifts to a lower value and the Urn range of mode transition is narrowed gradually. Additionally, the mode competition is enhanced, resulting in pronounced traveling waves with accelerated propagation speed. The vortex shedding pattern varies along the span, presenting the 2S (two vortices are shed per cycle) and P + S (a pair of vortices and a single vortex are shed from two sides of the cylinder in one cycle) patterns at the upstream and downstream pipe segments, respectively. The streamwise distance between two adjacent vortices is enlarged with the increase in θ. The pipe placed with different inclined angles presents different partitions of coupling pattern, and the Urn range of each subregion is different. By comprehensive consideration of the spatial–temporal evolution of VIV response, the mode transition features, the coupling pattern, and vortex shedding characteristics, the IP is invalid in predicting the VIV of flexible pipe.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>This paper reports the experimental results of the vortexinduced vibration (VIV) response of an inclined flexible pipe with various oblique angles (θ) ranging from 0° to 60°. The flexible pipe with an aspect ratio of 75 was fixed at both ends. The VIV response was examined in the reduced velocity range of 4.02–17.55 to evaluate the IP (independence principle) validity. The experimental results illustrate that the spatial distribution of response amplitudes and associated modal weights varies with θ, and the differences are enlarged in the mode transition cases. With increasing the θ, the onset normal reduced velocity (Urn) of mode transition shifts to a lower value and the Urn range of mode transition is narrowed gradually. Additionally, the mode competition is enhanced, resulting in pronounced traveling waves with accelerated propagation speed. The vortex shedding pattern varies along the span, presenting the 2S (two vortices are shed per cycle) and P + S (a pair of vortices and a single vortex are shed from two sides of the cylinder in one cycle) patterns at the upstream and downstream pipe segments, respectively. The streamwise distance between two adjacent vortices is enlarged with the increase in θ. The pipe placed with different inclined angles presents different partitions of coupling pattern, and the Urn range of each subregion is different. By comprehensive consideration of the spatial–temporal evolution of VIV response, the mode transition features, the coupling pattern, and vortex shedding characteristics, the IP is invalid in predicting the VIV of flexible pipe.
Experimental investigation on the vortexinduced vibration of an inclined flexible pipe and the evaluation of the independence principle
10.1063/5.0138364
Physics of Fluids
20230313T11:26:10Z
© 2023 Author(s).

Dynamics of two interacting acoustic bubbles at short separation distances
https://aip.scitation.org/doi/10.1063/5.0135370?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>We investigate experimentally the attraction between two closely spaced, oscillating microbubbles. Above a certain value of the applied acoustic field, the bubbles jump to a new equilibrium location, where they are separated by a thin fluid layer whose the thickness is much smaller than the bubble radii. We demonstrate that this new equilibrium is caused by the sign reversal of the radiation interaction force acting between the two bubbles, attributed to the multiple rescattering effects of the waves emitted by the bubbles. Theoretical investigation reveals that a new stable equilibrium appears at short distances, resulting in a quasicontacting bubble pair.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>We investigate experimentally the attraction between two closely spaced, oscillating microbubbles. Above a certain value of the applied acoustic field, the bubbles jump to a new equilibrium location, where they are separated by a thin fluid layer whose the thickness is much smaller than the bubble radii. We demonstrate that this new equilibrium is caused by the sign reversal of the radiation interaction force acting between the two bubbles, attributed to the multiple rescattering effects of the waves emitted by the bubbles. Theoretical investigation reveals that a new stable equilibrium appears at short distances, resulting in a quasicontacting bubble pair.
Dynamics of two interacting acoustic bubbles at short separation distances
10.1063/5.0135370
Physics of Fluids
20230314T10:16:40Z
© 2023 Author(s).
Gabriel Regnault
Alexander A. Doinikov
Cyril Mauger
Philippe BlancBenon
Claude Inserra

Pretrained combustion model and transfer learning in thermoacoustic instability
https://aip.scitation.org/doi/10.1063/5.0142378?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this paper, deep learning is involved to comprehend thermoacoustic instability more deeply and achieve early warning more reliably. Flame images and pressure series are acquired in model combustors. A total of seven data domains are obtained by changing the combustor structural parameters. Then, the pretrained model TIPE (Thermoacoustic ImagePressure Encoder), containing an image encoder with ResNet architecture and a pressure encoder with transformer architecture, is trained through the contrastive selfsupervised task of aligning the image and pressure signals in the embedding space. Furthermore, transfer learning in thermoacoustic instability prediction is performed based on knearest neighbors. Results show that the pretrained model can better resist the negative effect caused by class imbalance. The weighted F1 score of the pretrained model is 6.72% and 2.61% larger than supervised models in zeroshot transfer and fewshot transfer, respectively. It is inferred that the more generic features encoded by TIPE result in superior generalization in comparison with traditional supervised methods. Moreover, our proposed method is insensitive to the thresholds of determining thermoacoustic states. Principal component analysis reveals the physical interpretability preliminarily through the connection between feature principal components and pressure fluctuation amplitudes. Finally, the key spatial region of flame images and temporal interval of pressure series are visualized by class activation map and global attention scores.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this paper, deep learning is involved to comprehend thermoacoustic instability more deeply and achieve early warning more reliably. Flame images and pressure series are acquired in model combustors. A total of seven data domains are obtained by changing the combustor structural parameters. Then, the pretrained model TIPE (Thermoacoustic ImagePressure Encoder), containing an image encoder with ResNet architecture and a pressure encoder with transformer architecture, is trained through the contrastive selfsupervised task of aligning the image and pressure signals in the embedding space. Furthermore, transfer learning in thermoacoustic instability prediction is performed based on knearest neighbors. Results show that the pretrained model can better resist the negative effect caused by class imbalance. The weighted F1 score of the pretrained model is 6.72% and 2.61% larger than supervised models in zeroshot transfer and fewshot transfer, respectively. It is inferred that the more generic features encoded by TIPE result in superior generalization in comparison with traditional supervised methods. Moreover, our proposed method is insensitive to the thresholds of determining thermoacoustic states. Principal component analysis reveals the physical interpretability preliminarily through the connection between feature principal components and pressure fluctuation amplitudes. Finally, the key spatial region of flame images and temporal interval of pressure series are visualized by class activation map and global attention scores.
Pretrained combustion model and transfer learning in thermoacoustic instability
10.1063/5.0142378
Physics of Fluids
20230314T10:16:37Z
© 2023 Author(s).

Experimental study on the interactions between wave groups in doublewavegroup focusing
https://aip.scitation.org/doi/10.1063/5.0142042?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Nonlinear interactions in wavegroup focusing are regarded as one of the main mechanisms in the generation of destructive extreme waves. In real seas, the wideband bimodal state is a typical configuration, containing interactions within a single wave group and between different wave groups. The former has been well uncovered under the assumption of narrow bandwidth, but the latter is poorly understood. In this paper, physical experiments are conducted to reveal the physics of doublewavegroup focusing considering various energy distributions. Superposed wavemaker signals generated by the iteration method are applied to produce a doublewavegroup focusing with the interactions being decomposed. Results of the waveletbased bicoherence spectrum show that doublewavegroup focusing is distinguished from the linear superposition of two singlewavegroup focusing mainly in the nonlinear interactions induced by the secondorder sum harmonics. Under the assumption of equivalent energy, interactions of the secondorder sum harmonics between the lower frequency group and higher frequency group cannot be ignored in swelldominated states, and lesser linear interactions and stronger nonlinear interactions are observed while the spectral distribution of the doublewavegroup is more asymmetrical. This work is anticipated to contribute to the understanding of the generation mechanism of extreme waves driven by strong nonlinearity.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Nonlinear interactions in wavegroup focusing are regarded as one of the main mechanisms in the generation of destructive extreme waves. In real seas, the wideband bimodal state is a typical configuration, containing interactions within a single wave group and between different wave groups. The former has been well uncovered under the assumption of narrow bandwidth, but the latter is poorly understood. In this paper, physical experiments are conducted to reveal the physics of doublewavegroup focusing considering various energy distributions. Superposed wavemaker signals generated by the iteration method are applied to produce a doublewavegroup focusing with the interactions being decomposed. Results of the waveletbased bicoherence spectrum show that doublewavegroup focusing is distinguished from the linear superposition of two singlewavegroup focusing mainly in the nonlinear interactions induced by the secondorder sum harmonics. Under the assumption of equivalent energy, interactions of the secondorder sum harmonics between the lower frequency group and higher frequency group cannot be ignored in swelldominated states, and lesser linear interactions and stronger nonlinear interactions are observed while the spectral distribution of the doublewavegroup is more asymmetrical. This work is anticipated to contribute to the understanding of the generation mechanism of extreme waves driven by strong nonlinearity.
Experimental study on the interactions between wave groups in doublewavegroup focusing
10.1063/5.0142042
Physics of Fluids
20230314T10:16:39Z
© 2023 Author(s).

Exploring hidden flow structures from sparse data through deeplearningstrengthened proper orthogonal decomposition
https://aip.scitation.org/doi/10.1063/5.0138287?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>Proper orthogonal decomposition (POD) enables complex flow fields to be decomposed into linear modes according to their energy, allowing the key features of the flow to be extracted. However, traditional POD requires highquality inputs, namely, highresolution spatiotemporal data. To alleviate the dependence of traditional POD on the quality and quantity of data, this paper presents a POD method that is strengthened by a physicsinformed neural network (PINN) with an overlapping domain decomposition strategy. The loss function and convergence of modes are considered simultaneously to determine the convergence of the PINNPOD model. The proposed framework is applied to the flow past a twodimensional circular cylinder at Reynolds numbers ranging from 100 to 10 000 and achieves accurate and robust extraction of flow structures from spatially sparse observation data. The spatial structures and dominant frequency can also be extracted under highlevel noise. These results demonstrate that the proposed PINNPOD method is a reliable tool for extracting the key features from sparse observation data of flow fields, potentially shedding light on the datadriven discovery of hidden fluid dynamics.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>Proper orthogonal decomposition (POD) enables complex flow fields to be decomposed into linear modes according to their energy, allowing the key features of the flow to be extracted. However, traditional POD requires highquality inputs, namely, highresolution spatiotemporal data. To alleviate the dependence of traditional POD on the quality and quantity of data, this paper presents a POD method that is strengthened by a physicsinformed neural network (PINN) with an overlapping domain decomposition strategy. The loss function and convergence of modes are considered simultaneously to determine the convergence of the PINNPOD model. The proposed framework is applied to the flow past a twodimensional circular cylinder at Reynolds numbers ranging from 100 to 10 000 and achieves accurate and robust extraction of flow structures from spatially sparse observation data. The spatial structures and dominant frequency can also be extracted under highlevel noise. These results demonstrate that the proposed PINNPOD method is a reliable tool for extracting the key features from sparse observation data of flow fields, potentially shedding light on the datadriven discovery of hidden fluid dynamics.
Exploring hidden flow structures from sparse data through deeplearningstrengthened proper orthogonal decomposition
10.1063/5.0138287
Physics of Fluids
20230315T11:50:11Z
© 2023 Author(s).

Dynamics of the restricted vortex problem with a honeycomb configuration
https://aip.scitation.org/doi/10.1063/5.0143647?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>In this paper, we have investigated the restricted 14vortex problem with a honeycomb configuration similar to the football surface pattern. First, we give a sufficient condition for the existence of honeycomb configurations and obtain the instability of configurations. Then, we establish the equation of motion of the tracer particle for the restricted 14vortex problem and analyze the stability and distribution of equilibrium points and singular points. As can be seen from the global phase diagram of system, there are only four types of orbits: equilibrium points, homoclinic orbits, heteroclinic orbits, and periodic orbits.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>In this paper, we have investigated the restricted 14vortex problem with a honeycomb configuration similar to the football surface pattern. First, we give a sufficient condition for the existence of honeycomb configurations and obtain the instability of configurations. Then, we establish the equation of motion of the tracer particle for the restricted 14vortex problem and analyze the stability and distribution of equilibrium points and singular points. As can be seen from the global phase diagram of system, there are only four types of orbits: equilibrium points, homoclinic orbits, heteroclinic orbits, and periodic orbits.
Dynamics of the restricted vortex problem with a honeycomb configuration
10.1063/5.0143647
Physics of Fluids
20230315T11:49:57Z
© 2023 Author(s).

Effect of surface wettability on evaporation rate of droplet array
https://aip.scitation.org/doi/10.1063/5.0137614?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The evaporation of droplets in an array is hindered by adjacent droplets because of vapormediated interactions. Existing theoretical models for predicting the evaporation rate of droplets in the array neglect the important factor of surface wettability. In this work, we developed a model involving a contact angle function to accurately predict the evaporation rate of droplets with an arbitrary contact angle in the array. Fick's first and second laws were solved for evaporating droplets in the array by using steadystate threedimensional numerical simulations, to derive the contact angle function. The proposed model was experimentally validated for arrayed droplets evaporating on flat hydrophilic and hydrophobic surfaces. We show that the contact angle function approaches unity on hydrophilic surfaces, which implies that the proposed model coincides with Wray et al.'s model. On the other hand, the contact angle function is much lower than unity on hydrophobic surfaces, indicating a low evaporation rate of droplets in the array. The findings of this study are expected to advance our understanding of droplet evaporation in arrays in a wide range of scientific and engineering applications.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The evaporation of droplets in an array is hindered by adjacent droplets because of vapormediated interactions. Existing theoretical models for predicting the evaporation rate of droplets in the array neglect the important factor of surface wettability. In this work, we developed a model involving a contact angle function to accurately predict the evaporation rate of droplets with an arbitrary contact angle in the array. Fick's first and second laws were solved for evaporating droplets in the array by using steadystate threedimensional numerical simulations, to derive the contact angle function. The proposed model was experimentally validated for arrayed droplets evaporating on flat hydrophilic and hydrophobic surfaces. We show that the contact angle function approaches unity on hydrophilic surfaces, which implies that the proposed model coincides with Wray et al.'s model. On the other hand, the contact angle function is much lower than unity on hydrophobic surfaces, indicating a low evaporation rate of droplets in the array. The findings of this study are expected to advance our understanding of droplet evaporation in arrays in a wide range of scientific and engineering applications.
Effect of surface wettability on evaporation rate of droplet array
10.1063/5.0137614
Physics of Fluids
20230315T11:50:14Z
© 2023 Author(s).
M. Mohib Ur Rehman
Alexandros Askounis

An investigation of the flow structure beneath solitary waves with constant vorticity on a conducting fluid under normal electric fields
https://aip.scitation.org/doi/10.1063/5.0142779?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>The motion of an interface separating two fluids under the effect of electric fields is a subject that has picked the attention of researchers from different areas. While there is an abundance of studies investigating the free surface wave properties, very few works have examined the associated velocity field within the bulk of the fluid. Therefore, in this paper, we investigate numerically the flow structure beneath solitary waves with constant vorticity on an inviscid conducting fluid bounded above by a dielectric gas under normal electric fields in the framework of a weakly nonlinear theory. Elevation and depression solitary waves with constant vorticity are computed by a pseudospectral method, and a parameter sweep on the intensity of the electric field is carried out to study its role in the appearance of stagnation points. We find that for elevation solitary waves, the location of stagnation points does not change significantly with a variation of the electric field. For depression solitary waves, on the other hand, the electric field acts as a catalyzer that makes possible the appearance of stagnation points. In the sense that in its absence, there are no stagnation points.
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>The motion of an interface separating two fluids under the effect of electric fields is a subject that has picked the attention of researchers from different areas. While there is an abundance of studies investigating the free surface wave properties, very few works have examined the associated velocity field within the bulk of the fluid. Therefore, in this paper, we investigate numerically the flow structure beneath solitary waves with constant vorticity on an inviscid conducting fluid bounded above by a dielectric gas under normal electric fields in the framework of a weakly nonlinear theory. Elevation and depression solitary waves with constant vorticity are computed by a pseudospectral method, and a parameter sweep on the intensity of the electric field is carried out to study its role in the appearance of stagnation points. We find that for elevation solitary waves, the location of stagnation points does not change significantly with a variation of the electric field. For depression solitary waves, on the other hand, the electric field acts as a catalyzer that makes possible the appearance of stagnation points. In the sense that in its absence, there are no stagnation points.
An investigation of the flow structure beneath solitary waves with constant vorticity on a conducting fluid under normal electric fields
10.1063/5.0142779
Physics of Fluids
20230317T03:03:49Z
© 2023 Author(s).
M. V. Flamarion
T. Gao
R. RibeiroJr

Erratum: “Flow control of wake around a wallmounted cube using a horizontal hole of different diameters” [Phys. Fluids 34, 035127 (2022)]
https://aip.scitation.org/doi/10.1063/5.0146087?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>
Erratum: “Flow control of wake around a wallmounted cube using a horizontal hole of different diameters” [Phys. Fluids 34, 035127 (2022)]
10.1063/5.0146087
Physics of Fluids
20230301T02:12:10Z
© 2023 Author(s).

Publisher's Note: “Analytical model for predicting the length scale of shock/boundary layer interaction with curvature” [Phys. Fluids 34, 111701 (2022)]
https://aip.scitation.org/doi/10.1063/5.0147923?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>
Publisher's Note: “Analytical model for predicting the length scale of shock/boundary layer interaction with curvature” [Phys. Fluids 34, 111701 (2022)]
10.1063/5.0147923
Physics of Fluids
20230306T10:55:59Z
© 2023 Author(s).

Erratum: “An asymptotic expansion method vs a selfsimilar solution for convective heat transfer in rotating conedisk systems” [Phys. Fluids 34, 103610 (2022)]
https://aip.scitation.org/doi/10.1063/5.0143943?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/35/3">Volume 35, Issue 3</a>, March 2023. <br/>
Physics of Fluids, Volume 35, Issue 3, March 2023. <br/>
Erratum: “An asymptotic expansion method vs a selfsimilar solution for convective heat transfer in rotating conedisk systems” [Phys. Fluids 34, 103610 (2022)]
10.1063/5.0143943
Physics of Fluids
20230316T02:04:38Z
© 2023 Author(s).
Igor V. Shevchuk