American Institute of Physics: Physics of Fluids: Table of Contents
Table of Contents for Physics of Fluids. List of articles from both the latest and ahead of print issues.
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American Institute of Physics: Physics of Fluids: Table of Contents
American Institute of Physics
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Physics of Fluids
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An extensive scope of flow loops with a focus on particle transport
https://aip.scitation.org/doi/10.1063/5.0099309?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>All the oil and natural gas wells from underground reservoirs to the surface resemble flow loops. Over the past couple of decades, a variety of flow loops have been established in order to create real field environments in the laboratories. The flow loops help to further advance the efficiency of drilling, production, and hydraulic fracturing operations. This can be achieved by better understanding the mechanisms behind particle transport within the flow loops such as fluid velocity and rheology, particle size and concentration, acting forces on particles, and pressure drop. The objective of the current review is to critically investigate several flow loops with different applications addressing transportation of particles such as cuttings, sands, condensate droplets, hydrates, and proppants.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>All the oil and natural gas wells from underground reservoirs to the surface resemble flow loops. Over the past couple of decades, a variety of flow loops have been established in order to create real field environments in the laboratories. The flow loops help to further advance the efficiency of drilling, production, and hydraulic fracturing operations. This can be achieved by better understanding the mechanisms behind particle transport within the flow loops such as fluid velocity and rheology, particle size and concentration, acting forces on particles, and pressure drop. The objective of the current review is to critically investigate several flow loops with different applications addressing transportation of particles such as cuttings, sands, condensate droplets, hydrates, and proppants.
An extensive scope of flow loops with a focus on particle transport
10.1063/5.0099309
Physics of Fluids
20220802T01:38:27Z
© 2022 Author(s).
Nickolas Martin Brown
Morteza Dejam

High frequency Rayleigh scattering measurements of density fluctuations in highpressure premixed combustion
https://aip.scitation.org/doi/10.1063/5.0102330?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Measuring physical flow properties, such as density and temperature, at high frequency in high temperature and pressure environments is very challenging. Rapid fluctuations of these properties often have an impact on combustion efficiency and stability. We hereby attempt to measure density fluctuations in highpressure premixed combustion using high temporal resolution laser Rayleigh scattering. The Rayleigh scattering intensity was assessed by counting individual photons due to the low signal to noise ratio. The measurement system was first verified at various air pressures without combustion. Combustion experiments were then conducted at four different pressures, from 1 to 7 bar. The density fluctuations increased with pressure, but the dominant fluctuation frequency decreased. Proper orthogonal decomposition analysis of highspeed schlieren images of the flame front was used to verify the results.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Measuring physical flow properties, such as density and temperature, at high frequency in high temperature and pressure environments is very challenging. Rapid fluctuations of these properties often have an impact on combustion efficiency and stability. We hereby attempt to measure density fluctuations in highpressure premixed combustion using high temporal resolution laser Rayleigh scattering. The Rayleigh scattering intensity was assessed by counting individual photons due to the low signal to noise ratio. The measurement system was first verified at various air pressures without combustion. Combustion experiments were then conducted at four different pressures, from 1 to 7 bar. The density fluctuations increased with pressure, but the dominant fluctuation frequency decreased. Proper orthogonal decomposition analysis of highspeed schlieren images of the flame front was used to verify the results.
High frequency Rayleigh scattering measurements of density fluctuations in highpressure premixed combustion
10.1063/5.0102330
Physics of Fluids
20220808T10:28:26Z
© 2022 Author(s).

Optimizing the mean swimming velocity of a model twosphere swimmer
https://aip.scitation.org/doi/10.1063/5.0101459?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The swimming of a twosphere system oscillating in a viscous fluid is studied on the basis of simplified equations of motion which take account of both friction and inertial effects. In the model, the friction follows from an Oseen approximation to the mobility matrix, and the inertial effects follow from a dipole approximation to the added mass matrix. The resulting mean swimming velocity is evaluated analytically in a first harmonics approximation. For specific choices of the parameters, this is compared with the exact result following from a numerical calculation including higher harmonics. The Oseendipole model is compared with the simpler Oseen* model, in which the added mass effects are approximated by just the effective mass of the single spheres and dipole interactions are neglected. The expression for the mean swimming velocity can be reduced to a dimensionless scaling form. For given viscosity and mass density of the fluid, the frequency of the stroke and the ratio of radii can be chosen such that the swimming velocity is optimized.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The swimming of a twosphere system oscillating in a viscous fluid is studied on the basis of simplified equations of motion which take account of both friction and inertial effects. In the model, the friction follows from an Oseen approximation to the mobility matrix, and the inertial effects follow from a dipole approximation to the added mass matrix. The resulting mean swimming velocity is evaluated analytically in a first harmonics approximation. For specific choices of the parameters, this is compared with the exact result following from a numerical calculation including higher harmonics. The Oseendipole model is compared with the simpler Oseen* model, in which the added mass effects are approximated by just the effective mass of the single spheres and dipole interactions are neglected. The expression for the mean swimming velocity can be reduced to a dimensionless scaling form. For given viscosity and mass density of the fluid, the frequency of the stroke and the ratio of radii can be chosen such that the swimming velocity is optimized.
Optimizing the mean swimming velocity of a model twosphere swimmer
10.1063/5.0101459
Physics of Fluids
20220801T02:11:49Z
© 2022 Author(s).
B. U. Felderhof

Implications of dragonfly's muscle control on flapping kinematics and aerodynamics
https://aip.scitation.org/doi/10.1063/5.0097790?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this work, we designed and characterized a passive structural wing actuation setup that was able to realistically mimic the flapping and pitching kinematics of dragonflies. In this setup, an inelastic string limited the wing pitch that may be sufficiently simple for practical micro air vehicle applications. To further evaluate the dominance of inertial passive and active musclecontrolled pitch actuation in dragonfly flight, the flow fields and pitching angle variations of the naturally actuated wing of a tethered dragonfly were compared with that of the same wing artificially actuated via a proposed passive mechanism. We found that passive rotation characterizes most of the forewing flapping cycle except the upstroke reversal where the dragonfly uses its muscle movement to accelerate its forewing rotation. The measured flow fields show that accelerated wing rotation at the upstroke reversal will result in a stronger leading edge vortex during the downstroke, the additional force from which is estimated to account for 4.3% of the total cycle averaged force generated.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this work, we designed and characterized a passive structural wing actuation setup that was able to realistically mimic the flapping and pitching kinematics of dragonflies. In this setup, an inelastic string limited the wing pitch that may be sufficiently simple for practical micro air vehicle applications. To further evaluate the dominance of inertial passive and active musclecontrolled pitch actuation in dragonfly flight, the flow fields and pitching angle variations of the naturally actuated wing of a tethered dragonfly were compared with that of the same wing artificially actuated via a proposed passive mechanism. We found that passive rotation characterizes most of the forewing flapping cycle except the upstroke reversal where the dragonfly uses its muscle movement to accelerate its forewing rotation. The measured flow fields show that accelerated wing rotation at the upstroke reversal will result in a stronger leading edge vortex during the downstroke, the additional force from which is estimated to account for 4.3% of the total cycle averaged force generated.
Implications of dragonfly's muscle control on flapping kinematics and aerodynamics
10.1063/5.0097790
Physics of Fluids
20220808T10:28:49Z
© 2022 Author(s).
Di Liu
Csaba Hefler
Wei Shyy
Huihe Qiu

Numerical investigation of an insectscale flexible wing with a small amplitude flapping kinematics
https://aip.scitation.org/doi/10.1063/5.0098082?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>To maintain flight, insectscale air vehicles must adapt to their low Reynolds number flight conditions and generate sufficient aerodynamic force. Researchers conducted extensive studies to explore the mechanism of high aerodynamic efficiency on such a small scale. In this paper, a centimeterlevel flapping wing is used to investigate the mechanism and feasibility of whether a simple motion with a certain frequency can generate enough lift. The unsteady numerical simulations are based on the fluid structure interaction (FSI) method and dynamic mesh technology. The flapping motion is in a simple harmonic law of small amplitude with high frequency, which corresponds to the flapping wing driven by a piezoelectric actuator. The inertial and aerodynamic forces of the wing can cause chordwise torsion, thereby generating the vertical aerodynamic force. The concerned flapping frequency refers to the structural modal frequency and FSI modal frequency. According to the results, we find that under the condition that frequency ratio is 1.0, that is, when the wing flaps at the firstorder structural modal frequency, the deformation degree of the wing is the highest, but it does not produce good aerodynamic performance. However, under the condition that frequency ratio is 0.822, when the wing flaps at the firstorder FSI modal frequency, the aerodynamic efficiency achieve the highest and is equal to 0.273. Under the condition that frequency ratio is 0.6, that is, when the wing flaps at a frequency smaller than the firstorder FSI modal frequency, the flapping wing effectively utilizes the strain energy storage and release mechanism and produces the maximum vertical coefficient which is equal to 4.86. The study shows that this flapping motion can satisfy the requirements of lift to sustain the flight on this scale.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>To maintain flight, insectscale air vehicles must adapt to their low Reynolds number flight conditions and generate sufficient aerodynamic force. Researchers conducted extensive studies to explore the mechanism of high aerodynamic efficiency on such a small scale. In this paper, a centimeterlevel flapping wing is used to investigate the mechanism and feasibility of whether a simple motion with a certain frequency can generate enough lift. The unsteady numerical simulations are based on the fluid structure interaction (FSI) method and dynamic mesh technology. The flapping motion is in a simple harmonic law of small amplitude with high frequency, which corresponds to the flapping wing driven by a piezoelectric actuator. The inertial and aerodynamic forces of the wing can cause chordwise torsion, thereby generating the vertical aerodynamic force. The concerned flapping frequency refers to the structural modal frequency and FSI modal frequency. According to the results, we find that under the condition that frequency ratio is 1.0, that is, when the wing flaps at the firstorder structural modal frequency, the deformation degree of the wing is the highest, but it does not produce good aerodynamic performance. However, under the condition that frequency ratio is 0.822, when the wing flaps at the firstorder FSI modal frequency, the aerodynamic efficiency achieve the highest and is equal to 0.273. Under the condition that frequency ratio is 0.6, that is, when the wing flaps at a frequency smaller than the firstorder FSI modal frequency, the flapping wing effectively utilizes the strain energy storage and release mechanism and produces the maximum vertical coefficient which is equal to 4.86. The study shows that this flapping motion can satisfy the requirements of lift to sustain the flight on this scale.
Numerical investigation of an insectscale flexible wing with a small amplitude flapping kinematics
10.1063/5.0098082
Physics of Fluids
20220808T10:28:46Z
© 2022 Author(s).

Numerical investigation of centrifugetrapping technique for generating gas–liquid flows in microchannels
https://aip.scitation.org/doi/10.1063/5.0095472?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>We have recently presented a novel approach (called the centrifugetrapping method) based on a microfluidic structure for the generation of stratified flow and slug flow for biochemical applications based on centrifugal microfluidics. The technique relies on stratifying liquid into a spiral channel using centrifugal force and trapping bubbles between liquid plugs to form a slug flow. In this study, we comprehensively characterize the fluidic behavior of the system using a multiphase numerical model. The model is first validated by experiments and then used to evaluate the hydrodynamical effects of the system. Pressure fluctuation of the liquid plugs in the microchannel shows high stability of slug flow in rotational velocity ranging from 350 to 1000 RPM. The mixing efficiency of two liquids injected into the spiral channel is evaluated in generated stratified and slug flows. The results show that slug flow can be effectively utilized to enhance the mixing efficiency by more than 30% compared to singlephase or stratified flow. The formation of secondary flows into the liquid plugs is the main reason for elevated mixing.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>We have recently presented a novel approach (called the centrifugetrapping method) based on a microfluidic structure for the generation of stratified flow and slug flow for biochemical applications based on centrifugal microfluidics. The technique relies on stratifying liquid into a spiral channel using centrifugal force and trapping bubbles between liquid plugs to form a slug flow. In this study, we comprehensively characterize the fluidic behavior of the system using a multiphase numerical model. The model is first validated by experiments and then used to evaluate the hydrodynamical effects of the system. Pressure fluctuation of the liquid plugs in the microchannel shows high stability of slug flow in rotational velocity ranging from 350 to 1000 RPM. The mixing efficiency of two liquids injected into the spiral channel is evaluated in generated stratified and slug flows. The results show that slug flow can be effectively utilized to enhance the mixing efficiency by more than 30% compared to singlephase or stratified flow. The formation of secondary flows into the liquid plugs is the main reason for elevated mixing.
Numerical investigation of centrifugetrapping technique for generating gas–liquid flows in microchannels
10.1063/5.0095472
Physics of Fluids
20220801T02:11:53Z
© 2022 Author(s).
Maryam Maghazeh
Hossein Pishbin
Mahdi Navidbakhsh
Esmail Pishbin

A kinetic model for multicomponent gas transport in shale gas reservoirs and its applications
https://aip.scitation.org/doi/10.1063/5.0101272?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>An accurate gas transport model is of vital importance to the simulation and production optimization of unconventional gas reservoirs. Although great success has been achieved in the development of singlecomponent transport models, limited progress has been made in multicomponent systems. The major challenge of developing nonempirical multicomponent gas transport models lies in the absence of the quantification of the concentration impact on the fluid dynamic properties. To fill such a gap, this work presents a comprehensive transport model for multicomponent gas transport in shale and tight reservoirs. In developing the model, we first conducted molecular dynamic simulations to qualitatively understand the differential release of hydrocarbons from unconventional shale and tight reservoirs. It is found that the gas slippage, differential adsorption, and surface diffusion are the primary transport mechanisms in the working range of Knudsen number during reservoir production. Based on the molecular dynamic study, a quantitative transport model has been developed and validated, which extends existing models from singlecomponent systems to multiplecomponent systems. The kinetic theory of gases is adopted and modified to model the multicomponent slippage effect. A generalized Maxwell–Stefan formulation with extended Langmuir adsorption isotherm is used to model the multicomponent surface diffusion process. The accuracy of the proposed model is above 90% for low to moderate Knudsen numbers in modeling the differential release phenomenon in unconventional reservoirs.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>An accurate gas transport model is of vital importance to the simulation and production optimization of unconventional gas reservoirs. Although great success has been achieved in the development of singlecomponent transport models, limited progress has been made in multicomponent systems. The major challenge of developing nonempirical multicomponent gas transport models lies in the absence of the quantification of the concentration impact on the fluid dynamic properties. To fill such a gap, this work presents a comprehensive transport model for multicomponent gas transport in shale and tight reservoirs. In developing the model, we first conducted molecular dynamic simulations to qualitatively understand the differential release of hydrocarbons from unconventional shale and tight reservoirs. It is found that the gas slippage, differential adsorption, and surface diffusion are the primary transport mechanisms in the working range of Knudsen number during reservoir production. Based on the molecular dynamic study, a quantitative transport model has been developed and validated, which extends existing models from singlecomponent systems to multiplecomponent systems. The kinetic theory of gases is adopted and modified to model the multicomponent slippage effect. A generalized Maxwell–Stefan formulation with extended Langmuir adsorption isotherm is used to model the multicomponent surface diffusion process. The accuracy of the proposed model is above 90% for low to moderate Knudsen numbers in modeling the differential release phenomenon in unconventional reservoirs.
A kinetic model for multicomponent gas transport in shale gas reservoirs and its applications
10.1063/5.0101272
Physics of Fluids
20220801T02:12:01Z
© 2022 Author(s).

DropTrack—Automatic droplet tracking with YOLOv5 and DeepSORT for microfluidic applications
https://aip.scitation.org/doi/10.1063/5.0097597?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Deep neural networks are rapidly emerging as data analysis tools, often outperforming the conventional techniques used in complex microfluidic systems. One fundamental analysis frequently desired in microfluidic experiments is counting and tracking the droplets. Specifically, droplet tracking in dense emulsions is challenging due to inherently small droplets moving in tightly packed configurations. Sometimes, the individual droplets in these dense clusters are hard to resolve, even for a human observer. Here, two deep learningbased cuttingedge algorithms for object detection [you only look once (YOLO)] and object tracking (DeepSORT) are combined into a single image analysis tool, DropTrack, to track droplets in the microfluidic experiments. DropTrack analyzes input microfluidic experimental videos, extracts droplets' trajectories, and infers other observables of interest, such as droplet numbers. Training an object detector network for droplet recognition with manually annotated images is a laborintensive task and a persistent bottleneck. In this work, this problem is partly resolved by training many object detector networks (YOLOv5) with several hybrid datasets containing real and synthetic images. We present an analysis of a double emulsion experiment as a case study to measure DropTrack's performance. For our test case, the YOLO network trained by combining 40% real images and 60% synthetic images yields the best accuracy in droplet detection and droplet counting in real experimental videos. Also, this strategy reduces laborintensive image annotation work by 60%. DropTrack's performance is measured in terms of mean average precision of droplet detection, mean squared error in counting the droplets, and image analysis speed for inferring droplets' trajectories. The fastest configuration of DropTrack can detect and track the droplets at approximately 30 frames per second, well within the standards for a realtime image analysis.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Deep neural networks are rapidly emerging as data analysis tools, often outperforming the conventional techniques used in complex microfluidic systems. One fundamental analysis frequently desired in microfluidic experiments is counting and tracking the droplets. Specifically, droplet tracking in dense emulsions is challenging due to inherently small droplets moving in tightly packed configurations. Sometimes, the individual droplets in these dense clusters are hard to resolve, even for a human observer. Here, two deep learningbased cuttingedge algorithms for object detection [you only look once (YOLO)] and object tracking (DeepSORT) are combined into a single image analysis tool, DropTrack, to track droplets in the microfluidic experiments. DropTrack analyzes input microfluidic experimental videos, extracts droplets' trajectories, and infers other observables of interest, such as droplet numbers. Training an object detector network for droplet recognition with manually annotated images is a laborintensive task and a persistent bottleneck. In this work, this problem is partly resolved by training many object detector networks (YOLOv5) with several hybrid datasets containing real and synthetic images. We present an analysis of a double emulsion experiment as a case study to measure DropTrack's performance. For our test case, the YOLO network trained by combining 40% real images and 60% synthetic images yields the best accuracy in droplet detection and droplet counting in real experimental videos. Also, this strategy reduces laborintensive image annotation work by 60%. DropTrack's performance is measured in terms of mean average precision of droplet detection, mean squared error in counting the droplets, and image analysis speed for inferring droplets' trajectories. The fastest configuration of DropTrack can detect and track the droplets at approximately 30 frames per second, well within the standards for a realtime image analysis.
DropTrack—Automatic droplet tracking with YOLOv5 and DeepSORT for microfluidic applications
10.1063/5.0097597
Physics of Fluids
20220801T02:12:07Z
© 2022 Author(s).
Mihir Durve
Adriano Tiribocchi
Fabio Bonaccorso
Andrea Montessori
Marco Lauricella
Michał Bogdan
Jan Guzowski
Sauro Succi

A pressure compensation method for lattice Boltzmann simulation of particleladen flows in periodic geometries
https://aip.scitation.org/doi/10.1063/5.0094937?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>A simple and efficient boundary treatment is proposed for periodic boundary conditions in a lattice Boltzmann method for simulating fully developed, pressure driven particleladen flows in a complex geometry. The pressure driven effect is implemented by a simple pressure compensation method (PCM) using the pressure difference between the inlet and outlet boundaries. It eliminates the exchange of nonequilibrium distribution functions between inlet and outlet boundary nodes. It also eliminates the nonphysical oscillations of particle trajectory produced by a nonequilibrium extrapolation method when particles cross the periodic boundary. Simulation results show that the present PCM is equivalent to the body force method (BFM) for flow in a periodic straight channel with a uniform cross section. However, the BFM would significantly underestimate the fluid velocity for a flow and, hence, cannot accurately predict the particle trajectory in a periodic complex channel with a nonuniform cross section, especially at high Reynolds numbers.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>A simple and efficient boundary treatment is proposed for periodic boundary conditions in a lattice Boltzmann method for simulating fully developed, pressure driven particleladen flows in a complex geometry. The pressure driven effect is implemented by a simple pressure compensation method (PCM) using the pressure difference between the inlet and outlet boundaries. It eliminates the exchange of nonequilibrium distribution functions between inlet and outlet boundary nodes. It also eliminates the nonphysical oscillations of particle trajectory produced by a nonequilibrium extrapolation method when particles cross the periodic boundary. Simulation results show that the present PCM is equivalent to the body force method (BFM) for flow in a periodic straight channel with a uniform cross section. However, the BFM would significantly underestimate the fluid velocity for a flow and, hence, cannot accurately predict the particle trajectory in a periodic complex channel with a nonuniform cross section, especially at high Reynolds numbers.
A pressure compensation method for lattice Boltzmann simulation of particleladen flows in periodic geometries
10.1063/5.0094937
Physics of Fluids
20220802T01:38:31Z
© 2022 Author(s).

Modeling the squeeze flow of droplet over a step
https://aip.scitation.org/doi/10.1063/5.0098597?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this paper, we study the squeeze flow of a droplet confined between two plates in the presence of a step. Understanding this fluid mechanics problem is of the utmost importance particularly for nanoimprint lithography, wherein the photoresist droplets are dispensed on a substrate and imprinted and cured into a desired pattern. Often, the desired pattern includes various steps and trenches, and the droplets need to flow over. Here, we use the lubrication theory to find the instantaneous pressure and velocity fields. A volumeoffluid advection algorithm is also used for evolving the volume fraction in time. The obtained results reveal that for step sizes comparable to the gap between plates, the squeeze flow characteristics become quite distinct across the step. Under such circumstances, the fluid finds it less expensive to reverse its flow direction toward the deep region to pass through the lowresistance zone, which leads to a net mass flow rate across the step from a shallow to deep region. Such a mass transfer is found to be enhanced by applying larger squeezing forces. This phenomenon becomes less noticeable for liquid film thicknesses much larger than the step size. As a result, it takes large droplets a longer time to reach to the regime wherein a substantial mass flow rate occurs. In addition, the results suggest that the dimensionless characteristic features, such as the ratios of volume and area of liquid in the deep (or shallow) region to those of the total liquid, collapse onto their corresponding master curves.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this paper, we study the squeeze flow of a droplet confined between two plates in the presence of a step. Understanding this fluid mechanics problem is of the utmost importance particularly for nanoimprint lithography, wherein the photoresist droplets are dispensed on a substrate and imprinted and cured into a desired pattern. Often, the desired pattern includes various steps and trenches, and the droplets need to flow over. Here, we use the lubrication theory to find the instantaneous pressure and velocity fields. A volumeoffluid advection algorithm is also used for evolving the volume fraction in time. The obtained results reveal that for step sizes comparable to the gap between plates, the squeeze flow characteristics become quite distinct across the step. Under such circumstances, the fluid finds it less expensive to reverse its flow direction toward the deep region to pass through the lowresistance zone, which leads to a net mass flow rate across the step from a shallow to deep region. Such a mass transfer is found to be enhanced by applying larger squeezing forces. This phenomenon becomes less noticeable for liquid film thicknesses much larger than the step size. As a result, it takes large droplets a longer time to reach to the regime wherein a substantial mass flow rate occurs. In addition, the results suggest that the dimensionless characteristic features, such as the ratios of volume and area of liquid in the deep (or shallow) region to those of the total liquid, collapse onto their corresponding master curves.
Modeling the squeeze flow of droplet over a step
10.1063/5.0098597
Physics of Fluids
20220802T01:38:20Z
© 2022 Author(s).
Aryan Mehboudi
Shrawan Singhal
S. V. Sreenivasan

Enhanced droplet formation in a Tjunction microchannel using electric field: A lattice Boltzmann study
https://aip.scitation.org/doi/10.1063/5.0100312?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The electric fielddriven droplet formation technique can effectively improve the formation throughput and control the droplet size, which is important for the application of microscale droplets in biopharmaceuticals and chemical analysis. In this paper, the droplet formation characteristics in Tjunction microchannels under the action of electric field are investigated by coupling a threedimensional lattice Boltzmann method (3 D LBM) with the leaky dielectric model, focusing on the effects of electric capillary number, a flow ratio, and a viscosity ratio on the droplet size. It is shown that as the electrical capillary number increases, the nonuniformly distributed electric force stretches the dispersed phase to form a Taylor cone and increases shear force at the interface of the two liquids to overcome the surface tension force. This facilitates the transition from squeezing to dropping and reduces the droplet size. At high flow ratios, increasing the electric capillary number leads to a pinning effect between the dispersed phase and the wall, which intensifies the compression of continuous phase on the neck of dispersed phase, resulting in a significant decrease in the droplet size. As the viscosity ratio increases, the vortex resistance caused by electrical force decreases, and thus, the electric field effect will dominate the droplet formation process.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The electric fielddriven droplet formation technique can effectively improve the formation throughput and control the droplet size, which is important for the application of microscale droplets in biopharmaceuticals and chemical analysis. In this paper, the droplet formation characteristics in Tjunction microchannels under the action of electric field are investigated by coupling a threedimensional lattice Boltzmann method (3 D LBM) with the leaky dielectric model, focusing on the effects of electric capillary number, a flow ratio, and a viscosity ratio on the droplet size. It is shown that as the electrical capillary number increases, the nonuniformly distributed electric force stretches the dispersed phase to form a Taylor cone and increases shear force at the interface of the two liquids to overcome the surface tension force. This facilitates the transition from squeezing to dropping and reduces the droplet size. At high flow ratios, increasing the electric capillary number leads to a pinning effect between the dispersed phase and the wall, which intensifies the compression of continuous phase on the neck of dispersed phase, resulting in a significant decrease in the droplet size. As the viscosity ratio increases, the vortex resistance caused by electrical force decreases, and thus, the electric field effect will dominate the droplet formation process.
Enhanced droplet formation in a Tjunction microchannel using electric field: A lattice Boltzmann study
10.1063/5.0100312
Physics of Fluids
20220802T01:38:07Z
© 2022 Author(s).

Gaussian mixture models for diatomic gas−surface interactions under thermal nonequilibrium conditions
https://aip.scitation.org/doi/10.1063/5.0099863?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Scattering kernels are of paramount importance in modeling gas–surface interactions for rarefied gas flows. However, most existing empirical models need one or several accommodation coefficients (ACs) to be determined before applications. In this paper, an unsupervised machine learning technique, known as the Gaussian mixture (GM) model, is applied to establish a new scattering kernel based on the simulated data collected by molecular dynamics (MD) simulations. The main work is devoted to the scattering of diatomic molecules under thermal nonequilibrium conditions. Correspondingly, different MD simulations on the scattering process of nitrogen molecules from a platinum surface have been performed involving rotational and translational excitation. Here, we evaluate the performance of the GM and Cercignani–Lampis–Lord models against the MD approach by comparing the velocity correlation distributions and the relevant outgoing velocity probability density function as well as the computed ACs. The presented comparisons have demonstrated the superiority of the GM model in matching with MD results. Therefore, in the case of diatomic gases, the GM model can be employed as a promising strategy to derive the generalized boundary conditions.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Scattering kernels are of paramount importance in modeling gas–surface interactions for rarefied gas flows. However, most existing empirical models need one or several accommodation coefficients (ACs) to be determined before applications. In this paper, an unsupervised machine learning technique, known as the Gaussian mixture (GM) model, is applied to establish a new scattering kernel based on the simulated data collected by molecular dynamics (MD) simulations. The main work is devoted to the scattering of diatomic molecules under thermal nonequilibrium conditions. Correspondingly, different MD simulations on the scattering process of nitrogen molecules from a platinum surface have been performed involving rotational and translational excitation. Here, we evaluate the performance of the GM and Cercignani–Lampis–Lord models against the MD approach by comparing the velocity correlation distributions and the relevant outgoing velocity probability density function as well as the computed ACs. The presented comparisons have demonstrated the superiority of the GM model in matching with MD results. Therefore, in the case of diatomic gases, the GM model can be employed as a promising strategy to derive the generalized boundary conditions.
Gaussian mixture models for diatomic gas−surface interactions under thermal nonequilibrium conditions
10.1063/5.0099863
Physics of Fluids
20220804T11:52:25Z
© 2022 Author(s).

Theory of sphere motions in viscous fluids including elasticity and compressibility
https://aip.scitation.org/doi/10.1063/5.0098868?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>We study the motions of an elastic sphere and a compressible fluid sphere suspended in a compressible fluid. To this end, we use a scheme of a vector representation for the velocity in hydrodynamics and for the displacement in elasticity. First, we calculate the steadystate elastic displacement of a solid sphere under a gravity and a surfacetension gradient. Second, we examine the finitesize effects in a spherical container and find bulk acoustic resonance induced by an oscillating solid sphere. Third, applying periodic forces, we calculate the displacement, the velocity field, and the frequencydependent friction constant for an elastic sphere and a compressible fluid sphere. We find complex acoustic effects sensitively depending on the frequency.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>We study the motions of an elastic sphere and a compressible fluid sphere suspended in a compressible fluid. To this end, we use a scheme of a vector representation for the velocity in hydrodynamics and for the displacement in elasticity. First, we calculate the steadystate elastic displacement of a solid sphere under a gravity and a surfacetension gradient. Second, we examine the finitesize effects in a spherical container and find bulk acoustic resonance induced by an oscillating solid sphere. Third, applying periodic forces, we calculate the displacement, the velocity field, and the frequencydependent friction constant for an elastic sphere and a compressible fluid sphere. We find complex acoustic effects sensitively depending on the frequency.
Theory of sphere motions in viscous fluids including elasticity and compressibility
10.1063/5.0098868
Physics of Fluids
20220805T12:03:19Z
© 2022 Author(s).
Akira Onuki

Gas–surface interactions of a Couette–Poiseuille flow in a rectangular channel
https://aip.scitation.org/doi/10.1063/5.0099256?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Reduced mass flow rates of a rarefied Couette and Poiseuille flow in a long rectangular channel are calculated in the whole range of the gas rarefaction and a wide range of the width to height ratio. Furthermore, walls may be made of different materials so that different tangential momentum accommodation coefficients (TMACs) may be applied. Analytical solutions are given for the slip regime, where all four surrounding walls may have a different TMAC. Due to a simplified modeling assumption, these solutions can be used to correct the wellknown flow rates of a fully diffuse channel for different TMACs in the whole range of the gas rarefaction. If the slip solution and the diffuse solution are known, the procedure can principally be adapted for any channel shape. The results of the analytical model expressions are validated with simulation data of the plane Couette and Poiseuille flow and the Poiseuille flow through a pipe, which are found in the literature. In addition, the analytical solution is compared to results of the Direct Simulation Monte Carlo (DSMC) method of a Couette and a Poiseuille flow in a rectangular channel, which are provided as tabulated data for a variation of the gas rarefaction parameter at different aspect ratios and different combinations of TMACs. The procedure to calculate the mass flow rate of the certain flow as well as the application limits are discussed.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Reduced mass flow rates of a rarefied Couette and Poiseuille flow in a long rectangular channel are calculated in the whole range of the gas rarefaction and a wide range of the width to height ratio. Furthermore, walls may be made of different materials so that different tangential momentum accommodation coefficients (TMACs) may be applied. Analytical solutions are given for the slip regime, where all four surrounding walls may have a different TMAC. Due to a simplified modeling assumption, these solutions can be used to correct the wellknown flow rates of a fully diffuse channel for different TMACs in the whole range of the gas rarefaction. If the slip solution and the diffuse solution are known, the procedure can principally be adapted for any channel shape. The results of the analytical model expressions are validated with simulation data of the plane Couette and Poiseuille flow and the Poiseuille flow through a pipe, which are found in the literature. In addition, the analytical solution is compared to results of the Direct Simulation Monte Carlo (DSMC) method of a Couette and a Poiseuille flow in a rectangular channel, which are provided as tabulated data for a variation of the gas rarefaction parameter at different aspect ratios and different combinations of TMACs. The procedure to calculate the mass flow rate of the certain flow as well as the application limits are discussed.
Gas–surface interactions of a Couette–Poiseuille flow in a rectangular channel
10.1063/5.0099256
Physics of Fluids
20220805T01:37:19Z
© 2022 Author(s).
Heiko Pleskun
Andreas Brümmer

Predicting the lifetimes of evaporating droplets in ordered arrays
https://aip.scitation.org/doi/10.1063/5.0105243?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In many industrial processes, sessile droplets are well separated in an ordered array and evaporate to form various patterns. For an evaporating droplet in arrays, the presence of surrounding droplets causes a shielding effect that, in turn, leads to a decrease in the evaporation rate compared to the same droplet in isolation. Here, we demonstrate that, theoretically and experimentally, the shielding effect results in a significant increase in the lifetime of an evaporating droplet. Based on a recent theory, we determine the lifetimes of evaporating droplets in the ordered array. The theoretical prediction shows excellent agreement with our experimental data and even performs well outside its range of validity. These findings strengthen our fundamental understanding of interactions between evaporating droplets in arrays and provide new strategies for controlling the droplet evaporation.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In many industrial processes, sessile droplets are well separated in an ordered array and evaporate to form various patterns. For an evaporating droplet in arrays, the presence of surrounding droplets causes a shielding effect that, in turn, leads to a decrease in the evaporation rate compared to the same droplet in isolation. Here, we demonstrate that, theoretically and experimentally, the shielding effect results in a significant increase in the lifetime of an evaporating droplet. Based on a recent theory, we determine the lifetimes of evaporating droplets in the ordered array. The theoretical prediction shows excellent agreement with our experimental data and even performs well outside its range of validity. These findings strengthen our fundamental understanding of interactions between evaporating droplets in arrays and provide new strategies for controlling the droplet evaporation.
Predicting the lifetimes of evaporating droplets in ordered arrays
10.1063/5.0105243
Physics of Fluids
20220805T12:02:54Z
© 2022 Author(s).
Hao Chen
Qiaoru An
Hongya Zhang
Chengshuai Li
Haisheng Fang
Zhouping Yin

Fluid viscoelasticity suppresses chaotic convection and mixing due to electrokinetic instability
https://aip.scitation.org/doi/10.1063/5.0099481?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>When two fluids of different electrical conductivities are transported side by side in a microfluidic device under the influence of an electric field, an electrokinetic instability (EKI) is often generated after some critical values of the applied electric field strength and conductivity ratio. Many prior experimental and numerical studies show that this phenomenon results in a chaotic flow field inside a microdevice, thereby facilitating the mixing of two fluids if they are Newtonian in behavior. However, the present numerical study shows that this chaotic convection arising due to the electrokinetic instability can be suppressed if the fluids are viscoelastic instead of Newtonian ones. In particular, we observe that as the Weissenberg number (ratio of the elastic to that of the viscous forces) gradually increases and the polymer viscosity ratio (ratio of the solvent viscosity to that of the zeroshear rate viscosity of the polymeric solution) gradually decreases, the chaotic fluctuation inside a T microfluidic junction decreases within the present range of conditions encompassed in this study. We demonstrate that this suppression of the chaotic motion occurs due to the formation of a strand of high elastic stresses at the interface of the two fluids. We further show that this suppression of the chaotic fluctuation (particularly, the spanwise one) inhibits the mixing of two viscoelastic fluids. Therefore, one needs to be cautious when the EKI phenomenon is planned to use for mixing such viscoelastic fluids. Our observations are in line with that seen in limited experimental studies conducted for these kinds of viscoelastic fluids.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>When two fluids of different electrical conductivities are transported side by side in a microfluidic device under the influence of an electric field, an electrokinetic instability (EKI) is often generated after some critical values of the applied electric field strength and conductivity ratio. Many prior experimental and numerical studies show that this phenomenon results in a chaotic flow field inside a microdevice, thereby facilitating the mixing of two fluids if they are Newtonian in behavior. However, the present numerical study shows that this chaotic convection arising due to the electrokinetic instability can be suppressed if the fluids are viscoelastic instead of Newtonian ones. In particular, we observe that as the Weissenberg number (ratio of the elastic to that of the viscous forces) gradually increases and the polymer viscosity ratio (ratio of the solvent viscosity to that of the zeroshear rate viscosity of the polymeric solution) gradually decreases, the chaotic fluctuation inside a T microfluidic junction decreases within the present range of conditions encompassed in this study. We demonstrate that this suppression of the chaotic motion occurs due to the formation of a strand of high elastic stresses at the interface of the two fluids. We further show that this suppression of the chaotic fluctuation (particularly, the spanwise one) inhibits the mixing of two viscoelastic fluids. Therefore, one needs to be cautious when the EKI phenomenon is planned to use for mixing such viscoelastic fluids. Our observations are in line with that seen in limited experimental studies conducted for these kinds of viscoelastic fluids.
Fluid viscoelasticity suppresses chaotic convection and mixing due to electrokinetic instability
10.1063/5.0099481
Physics of Fluids
20220805T12:02:58Z
© 2022 Author(s).
C. Sasmal

Simplified method for wetting on curved boundaries in conservative phasefield latticeBoltzmann simulation of twophase flows with large density ratios
https://aip.scitation.org/doi/10.1063/5.0101291?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>A simplified method is proposed to implement a wetting boundary condition on curved surfaces within the conservative phasefield latticeBoltzmann (LB) simulation framework. It combines the idea of Huang et al. [“An alternative method to implement contact angle boundary condition and its application in hybrid latticeBoltzmann finitedifference simulations of twophase flows with immersed surfaces,” Eur. Phys. J. E 41, 17 (2018)] to find the order parameter on the other side of the wall with the conservative Allen–Cahn equation (CACE) for interface evolution solved by the LB equations. It inherits the advantage of the original method using the Cahn–Hilliard equation to avoid complicated interpolations under different geometries. By using the CACE, the boundary condition for the chemical potential is circumvented (making it more simplified), and the overshooting of the order parameter is also greatly suppressed, enabling it to simulate twophase flows with solid objects of various shapes and wettabilities at large density and viscosity ratios. Several twodimensional, axisymmetric, and threedimensional problems, including some previously studied by experiments, were simulated and the capability of the proposed method is demonstrated through its good agreement with theoretical predictions and/or experimental observations.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>A simplified method is proposed to implement a wetting boundary condition on curved surfaces within the conservative phasefield latticeBoltzmann (LB) simulation framework. It combines the idea of Huang et al. [“An alternative method to implement contact angle boundary condition and its application in hybrid latticeBoltzmann finitedifference simulations of twophase flows with immersed surfaces,” Eur. Phys. J. E 41, 17 (2018)] to find the order parameter on the other side of the wall with the conservative Allen–Cahn equation (CACE) for interface evolution solved by the LB equations. It inherits the advantage of the original method using the Cahn–Hilliard equation to avoid complicated interpolations under different geometries. By using the CACE, the boundary condition for the chemical potential is circumvented (making it more simplified), and the overshooting of the order parameter is also greatly suppressed, enabling it to simulate twophase flows with solid objects of various shapes and wettabilities at large density and viscosity ratios. Several twodimensional, axisymmetric, and threedimensional problems, including some previously studied by experiments, were simulated and the capability of the proposed method is demonstrated through its good agreement with theoretical predictions and/or experimental observations.
Simplified method for wetting on curved boundaries in conservative phasefield latticeBoltzmann simulation of twophase flows with large density ratios
10.1063/5.0101291
Physics of Fluids
20220802T01:38:35Z
© 2022 Author(s).

On the interfacial dynamics and capillary waves during impingement of a drop on liquid pool: A backgroundoriented schlieren study at low Weber numbers
https://aip.scitation.org/doi/10.1063/5.0098002?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Understanding the dynamics of a droplet impinging on a liquid pool and the associated phenomena have been of interest due to its prevalence in nature as well as in technical applications. This paper aims toward studying the characteristics of the capillary waves generated due to the low Weber number droplet interactions with the liquid pool. In this direction, experiments have been carried out for six different pool heights varying from h = 1.4 to 12 mm, encompassing thin liquid film, shallow pool, and deep pool regimes. Due to its wide usage, water has been chosen as the fluid of interest for droplet as well as for pool liquid. The study is focused on droplets impinging on the liquid pool at low Weber number ranging from 1 to 100. In order to characterize the postimpact perturbations in the liquid, background oriented schlieren (BOS) technique has been employed which offers realtime, nonintrusive wholefield measurements of the perturbations in the liquid pool. Measurements from BOS have been validated against the sideview projection of the impact. The transient variations of the air–water interface for different pool regimes and Weber numbers have been delineated. Results evince the formation of secondary wave at impact followed by the formation of primary wave after the crater retraction. The wave formation was faster and had higher amplitude in thin liquid regime for droplets with the same Weber number compared to the other regimes, but the perturbations were reduced through higher dissipation. The formation of the Worthington jet was seen in shallow and deep pool regimes for droplets with higher Weber number (We = 100), and its effect on the capillary wave is also discussed.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Understanding the dynamics of a droplet impinging on a liquid pool and the associated phenomena have been of interest due to its prevalence in nature as well as in technical applications. This paper aims toward studying the characteristics of the capillary waves generated due to the low Weber number droplet interactions with the liquid pool. In this direction, experiments have been carried out for six different pool heights varying from h = 1.4 to 12 mm, encompassing thin liquid film, shallow pool, and deep pool regimes. Due to its wide usage, water has been chosen as the fluid of interest for droplet as well as for pool liquid. The study is focused on droplets impinging on the liquid pool at low Weber number ranging from 1 to 100. In order to characterize the postimpact perturbations in the liquid, background oriented schlieren (BOS) technique has been employed which offers realtime, nonintrusive wholefield measurements of the perturbations in the liquid pool. Measurements from BOS have been validated against the sideview projection of the impact. The transient variations of the air–water interface for different pool regimes and Weber numbers have been delineated. Results evince the formation of secondary wave at impact followed by the formation of primary wave after the crater retraction. The wave formation was faster and had higher amplitude in thin liquid regime for droplets with the same Weber number compared to the other regimes, but the perturbations were reduced through higher dissipation. The formation of the Worthington jet was seen in shallow and deep pool regimes for droplets with higher Weber number (We = 100), and its effect on the capillary wave is also discussed.
On the interfacial dynamics and capillary waves during impingement of a drop on liquid pool: A backgroundoriented schlieren study at low Weber numbers
10.1063/5.0098002
Physics of Fluids
20220803T03:07:21Z
© 2022 Author(s).
Mohammad Autif Shahdhaar
Atul Srivastava
Suneet Singh

Numerical analysis of ligament instability and breakup in shear
flow
https://aip.scitation.org/doi/10.1063/5.0100511?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this study, we perform a numerical analysis of the instability of a ligament in a
shear flow and investigate the effects of air–liquid shear on the growth rate of the
ligament interface, breakup time, and droplet diameter formed by the breakup. The ligament
is stretched in the flow direction by the shearing of airflow. Furthermore, as the
influence of the shear flow increases, the ligament becomes deformed into a liquid sheet,
and a perforation forms at the center of the liquid sheet. The liquid sheet breaks up due
to the growth of the perforation and contracts under the influence of surface tension,
forming two ligaments with diameters smaller than that of the original ligament. The
shearing of the airflow causes the original ligament to elongate, and the cross section of
the ligament becomes elliptical, which increases instability. As a result, the growth rate
of the ligament exceeds the theoretical value and increases with increasing the wavenumber
of the initial disturbance. Therefore, the diameter of the formed droplets in the shear
flow decreases due to the increase in the wavenumber that governs the breakup of the
ligament, and as the growth rate increases, the breakup time for the ligament decreases.
As the velocity difference of the shear flow increases, constrictions of the ligament form
earlier and the diameter of the satellite droplet increases. As the diameter of the
satellite droplet increases and that of the main droplet decreases, the dispersion of the
droplet diameter decreases, making the diameter uniform.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this study, we perform a numerical analysis of the instability of a ligament in a
shear flow and investigate the effects of air–liquid shear on the growth rate of the
ligament interface, breakup time, and droplet diameter formed by the breakup. The ligament
is stretched in the flow direction by the shearing of airflow. Furthermore, as the
influence of the shear flow increases, the ligament becomes deformed into a liquid sheet,
and a perforation forms at the center of the liquid sheet. The liquid sheet breaks up due
to the growth of the perforation and contracts under the influence of surface tension,
forming two ligaments with diameters smaller than that of the original ligament. The
shearing of the airflow causes the original ligament to elongate, and the cross section of
the ligament becomes elliptical, which increases instability. As a result, the growth rate
of the ligament exceeds the theoretical value and increases with increasing the wavenumber
of the initial disturbance. Therefore, the diameter of the formed droplets in the shear
flow decreases due to the increase in the wavenumber that governs the breakup of the
ligament, and as the growth rate increases, the breakup time for the ligament decreases.
As the velocity difference of the shear flow increases, constrictions of the ligament form
earlier and the diameter of the satellite droplet increases. As the diameter of the
satellite droplet increases and that of the main droplet decreases, the dispersion of the
droplet diameter decreases, making the diameter uniform.
Numerical analysis of ligament instability and breakup in shear
flow
10.1063/5.0100511
Physics of Fluids
20220805T12:02:40Z
© 2022 Author(s).

Laser lightinduced deformation of free surface of oil due to thermocapillary Marangoni phenomenon: Experiment and computational fluid dynamics simulations
https://aip.scitation.org/doi/10.1063/5.0096610?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Remote lightinduced free liquid surface deformation has been studied in various systems for decades. One of the mechanisms able to do this task is driven by the thermocapillary Marangoni effect. The strength of the light–matter interaction, which is usually weak, here is amplified by the light absorption and heat production that changes surface tension. Here, we report on an experimental study aimed at dynamical aspects of the deformation induced under conditions of chopped laser excitation light. The lightinduced deformations are usually in the range of several micrometers. Therefore, we applied the interferometric technique to measure deformation profiles in real time. Experiments were performed in the shallow bath of the rapeseed oil with an azodye and excited with 514.5 nm and probed with 650 nm coherent laser beams, respectively. The mechanism of deformation driven by Marangoni effect was carefully modeled in 3D by computational fluid dynamic numerical simulations within the COMSOL Multiphysics package. The adaptive mesh technique used in the simulation together with solving the timedependent coupled Navier–Stokes and heat transport differential equations allowed us to replicate the experimental findings. A satisfactory agreement between the results of the simulations and those of the experiment in terms of the dynamics, shape, and depth of the deformation has been obtained. The toroidallike whirls accompanying the thermocapillary Marangoni effect were identified by the simulation results. We then experimentally proved that these toroidallike vortices, which accompany laser heating in dyed oil, formed a kind of novel hydrodynamic trap, in the center of their quiet zone, in which microcrystals can be trapped.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Remote lightinduced free liquid surface deformation has been studied in various systems for decades. One of the mechanisms able to do this task is driven by the thermocapillary Marangoni effect. The strength of the light–matter interaction, which is usually weak, here is amplified by the light absorption and heat production that changes surface tension. Here, we report on an experimental study aimed at dynamical aspects of the deformation induced under conditions of chopped laser excitation light. The lightinduced deformations are usually in the range of several micrometers. Therefore, we applied the interferometric technique to measure deformation profiles in real time. Experiments were performed in the shallow bath of the rapeseed oil with an azodye and excited with 514.5 nm and probed with 650 nm coherent laser beams, respectively. The mechanism of deformation driven by Marangoni effect was carefully modeled in 3D by computational fluid dynamic numerical simulations within the COMSOL Multiphysics package. The adaptive mesh technique used in the simulation together with solving the timedependent coupled Navier–Stokes and heat transport differential equations allowed us to replicate the experimental findings. A satisfactory agreement between the results of the simulations and those of the experiment in terms of the dynamics, shape, and depth of the deformation has been obtained. The toroidallike whirls accompanying the thermocapillary Marangoni effect were identified by the simulation results. We then experimentally proved that these toroidallike vortices, which accompany laser heating in dyed oil, formed a kind of novel hydrodynamic trap, in the center of their quiet zone, in which microcrystals can be trapped.
Laser lightinduced deformation of free surface of oil due to thermocapillary Marangoni phenomenon: Experiment and computational fluid dynamics simulations
10.1063/5.0096610
Physics of Fluids
20220805T12:03:03Z
© 2022 Author(s).
Monika Bełej
Katarzyna Grześkiewicz
Andrzej Miniewicz

Surface design of superhydrophobic parallel grooves for controllable petal bouncing and contact time reduction
https://aip.scitation.org/doi/10.1063/5.0102442?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>This study numerically investigates the bouncing characteristics of impacting droplets on superhydrophobic submillimeter parallel grooves by the levelset method. Once the Weber number (We) is increased to a critical value (Wec), a unique petallike droplet bouncing off the parallel grooves without horizontal retraction is found, dramatically reducing the contact time (tc) by up to ∼75%. Such a bouncing mode is attributed to the rectification of capillary energy stored in the penetrated liquids into upward motion. To achieve controllable petal bouncing, the coupling effects of impact velocity and surface geometric characteristics on tc and Wec are elucidated from the perspective of timescale, momentum, and energy. The numerical results indicate that narrowing the centertocenter spacing contributes to shortening tc and slowing down the growth of tc with We. In contrast, the effect of ridge height is negligible. By establishing the model of emptying time, the relationships of tc with impact velocity and geometric parameters are quantitatively identified. Furthermore, along with the strengthened anisotropic property, a large centertocenter spacing promotes the conversion of horizontal momentum into vertical momentum and suppresses the increment of surface energy, thus inducing the reduction in Wec. Distinct from known anisotropic surfaces in the previous work, the anisotropic property of parallelgrooved surface plays an opposite role in shortening tc. Finally, incorporating the energy balance approach, a semiempirical model is developed to predict Wec, exhibiting good agreement with present simulation. This work provides physical insights into petal bouncing and inspires the design of textured surfaces to reduce contact time.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>This study numerically investigates the bouncing characteristics of impacting droplets on superhydrophobic submillimeter parallel grooves by the levelset method. Once the Weber number (We) is increased to a critical value (Wec), a unique petallike droplet bouncing off the parallel grooves without horizontal retraction is found, dramatically reducing the contact time (tc) by up to ∼75%. Such a bouncing mode is attributed to the rectification of capillary energy stored in the penetrated liquids into upward motion. To achieve controllable petal bouncing, the coupling effects of impact velocity and surface geometric characteristics on tc and Wec are elucidated from the perspective of timescale, momentum, and energy. The numerical results indicate that narrowing the centertocenter spacing contributes to shortening tc and slowing down the growth of tc with We. In contrast, the effect of ridge height is negligible. By establishing the model of emptying time, the relationships of tc with impact velocity and geometric parameters are quantitatively identified. Furthermore, along with the strengthened anisotropic property, a large centertocenter spacing promotes the conversion of horizontal momentum into vertical momentum and suppresses the increment of surface energy, thus inducing the reduction in Wec. Distinct from known anisotropic surfaces in the previous work, the anisotropic property of parallelgrooved surface plays an opposite role in shortening tc. Finally, incorporating the energy balance approach, a semiempirical model is developed to predict Wec, exhibiting good agreement with present simulation. This work provides physical insights into petal bouncing and inspires the design of textured surfaces to reduce contact time.
Surface design of superhydrophobic parallel grooves for controllable petal bouncing and contact time reduction
10.1063/5.0102442
Physics of Fluids
20220808T10:28:41Z
© 2022 Author(s).

Water entry dynamics of rough microstructured spheres
https://aip.scitation.org/doi/10.1063/5.0102109?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this work, we proposed a facile underwater air cavity generation strategy based on rough microstructured spheres and explored its water entry dynamics and drag reduction characteristics. Under the assistance of microstructures, the threephase contact line is pinned near the sphere equator and inhibits the wetting of the liquid film along the sphere surface, so that leading the formation of air cavity. The water entry process is mainly divided into four stages: flow formation, cavity opening and stretching, cavity closure and entrapment, and cavity collapse. With the Froude number Fr, the pinchoff depth of air cavity obviously increases, and the pinchoff time is also delayed, which contributes to the formation of a longer bottom air cavity. In addition, the spheres with a larger impact velocity would fall faster in water during the initial falling period, while the terminal velocities are nearly the same for all the spheres when they are in a stable falling period. It is worth noting that for a same sphere, the larger impact velocity could not only contribute to the formation of a longer air cavity but also makes the generated air cavity keep in a stable and streamlined shape at different underwater depth, which is vitally important for achieving continuous drag reduction. Finally, we demonstrated numerically that the stable streamlined sphereincavity structure could reduce the hydrodynamic resistance levels up to 91.3% at Re ∼ 3.12 × 104, which is related to the boundary slip caused by an air layer trapped in the microstructures.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this work, we proposed a facile underwater air cavity generation strategy based on rough microstructured spheres and explored its water entry dynamics and drag reduction characteristics. Under the assistance of microstructures, the threephase contact line is pinned near the sphere equator and inhibits the wetting of the liquid film along the sphere surface, so that leading the formation of air cavity. The water entry process is mainly divided into four stages: flow formation, cavity opening and stretching, cavity closure and entrapment, and cavity collapse. With the Froude number Fr, the pinchoff depth of air cavity obviously increases, and the pinchoff time is also delayed, which contributes to the formation of a longer bottom air cavity. In addition, the spheres with a larger impact velocity would fall faster in water during the initial falling period, while the terminal velocities are nearly the same for all the spheres when they are in a stable falling period. It is worth noting that for a same sphere, the larger impact velocity could not only contribute to the formation of a longer air cavity but also makes the generated air cavity keep in a stable and streamlined shape at different underwater depth, which is vitally important for achieving continuous drag reduction. Finally, we demonstrated numerically that the stable streamlined sphereincavity structure could reduce the hydrodynamic resistance levels up to 91.3% at Re ∼ 3.12 × 104, which is related to the boundary slip caused by an air layer trapped in the microstructures.
Water entry dynamics of rough microstructured spheres
10.1063/5.0102109
Physics of Fluids
20220808T10:28:36Z
© 2022 Author(s).

The pappus angle as a key factor in the entire separation of a vortex ring from a dandelion seed’s pappus
https://aip.scitation.org/doi/10.1063/5.0098730?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>A separated vortex ring is the main aerodynamic feature of the dandelion seed which enables its flight. However, recent research on simply modeled dandelion seeds with a pappus angle of 180° (flat porous disk) shows that the vortex ring is not entirely separated from the pappus. This significantly differs from what is observed in a real dandelion seed and would prevent us from fully understanding its flight mechanism. In this work, the two key structural features of the pappus that are considered in the work of others, i.e., the circular disklike geometry and the porosity, were expected to be insufficient to form an entirely separated vortex ring. Therefore, refined numerical simulations were conducted on model pappi with different pappus angles, which are more similar to real dandelion seeds than flat porous disks. A stable and entirely separated vortex ring and a small vortex ring above the central disk were observed. The lower pappus angle increases the radial pressure gradient toward the central axis between the root of the filaments. When the net effect of this pressure gradient is large enough to overcome the effect of the pressure gradient away from the central axis below the pappus, the streamlines near the root of the filaments deflect to the central axis and the vortex ring is lifted and separates entirely. It is concluded that an appropriate pappus angle is another key feature for forming an entirely separated vortex ring. This more indepth flow mechanism is of considerable reference value for future research.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>A separated vortex ring is the main aerodynamic feature of the dandelion seed which enables its flight. However, recent research on simply modeled dandelion seeds with a pappus angle of 180° (flat porous disk) shows that the vortex ring is not entirely separated from the pappus. This significantly differs from what is observed in a real dandelion seed and would prevent us from fully understanding its flight mechanism. In this work, the two key structural features of the pappus that are considered in the work of others, i.e., the circular disklike geometry and the porosity, were expected to be insufficient to form an entirely separated vortex ring. Therefore, refined numerical simulations were conducted on model pappi with different pappus angles, which are more similar to real dandelion seeds than flat porous disks. A stable and entirely separated vortex ring and a small vortex ring above the central disk were observed. The lower pappus angle increases the radial pressure gradient toward the central axis between the root of the filaments. When the net effect of this pressure gradient is large enough to overcome the effect of the pressure gradient away from the central axis below the pappus, the streamlines near the root of the filaments deflect to the central axis and the vortex ring is lifted and separates entirely. It is concluded that an appropriate pappus angle is another key feature for forming an entirely separated vortex ring. This more indepth flow mechanism is of considerable reference value for future research.
The pappus angle as a key factor in the entire separation of a vortex ring from a dandelion seed’s pappus
10.1063/5.0098730
Physics of Fluids
20220801T02:11:43Z
© 2022 Author(s).

Nondimensional analysis of an unsteady flow in a magnetorheological damper
https://aip.scitation.org/doi/10.1063/5.0101569?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Theoretical modeling is often applied to study magnetorheological dampers (MRDs) with dimensional and nondimensional analyses. In contrast to dimensional models, nondimensional analyses can eliminate the influence of dimensionality and reduce the number of redundant parameters to simplify theoretical modeling and provide more universal applicability. However, most previous nondimensional analyses have been based on quasisteady flows that cannot reflect the transient response of an MRD because of the key assumption that the fluid velocity changes instantaneously. This study presents an investigation of the transient response of an MRD using a nondimensional analysis approach based on an unsteady model. We focus on the step response of the MRD with a step excitation of the piston speed, while the magnetic field is kept constant. For a comprehensive analysis, a set of dimensionless parameters are defined, including a nondimensional coordinate, a nondimensional time parameter, the Bingham number, a nondimensional preyield thickness, a damping coefficient, and a hydraulic amplification ratio. The relationships between these dimensionless numbers are analyzed. An unusual “concave area” is found in the velocity profile instead of a simple rigid flow during the transient process under a magnetic field. However, when the nondimensional time is 0.4, the delayed concave area disappears, and the rigid area fluid velocity reaches 98% of its stable value.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Theoretical modeling is often applied to study magnetorheological dampers (MRDs) with dimensional and nondimensional analyses. In contrast to dimensional models, nondimensional analyses can eliminate the influence of dimensionality and reduce the number of redundant parameters to simplify theoretical modeling and provide more universal applicability. However, most previous nondimensional analyses have been based on quasisteady flows that cannot reflect the transient response of an MRD because of the key assumption that the fluid velocity changes instantaneously. This study presents an investigation of the transient response of an MRD using a nondimensional analysis approach based on an unsteady model. We focus on the step response of the MRD with a step excitation of the piston speed, while the magnetic field is kept constant. For a comprehensive analysis, a set of dimensionless parameters are defined, including a nondimensional coordinate, a nondimensional time parameter, the Bingham number, a nondimensional preyield thickness, a damping coefficient, and a hydraulic amplification ratio. The relationships between these dimensionless numbers are analyzed. An unusual “concave area” is found in the velocity profile instead of a simple rigid flow during the transient process under a magnetic field. However, when the nondimensional time is 0.4, the delayed concave area disappears, and the rigid area fluid velocity reaches 98% of its stable value.
Nondimensional analysis of an unsteady flow in a magnetorheological damper
10.1063/5.0101569
Physics of Fluids
20220805T12:02:45Z
© 2022 Author(s).

Unsteady stretching of a glass tube with internal channel pressurization
https://aip.scitation.org/doi/10.1063/5.0096725?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Mathematical modeling is used to examine the unsteady problem of heating and pulling an axisymmetric cylindrical glass tube with an overpressure applied within the tube to form tapers with a near uniform bore and small wall thickness at the tip. To allow for the dependence of viscosity on temperature, a prescribed axially varying viscosity is assumed. Our motivation is the manufacture of emitter tips for mass spectrometry which provide a continuous fluid flow and do not become blocked. We demonstrate, for the first time, the feasibility of producing such emitters by this process and examine the influence of the process parameters, in particular the pulling force and overpressure, on the geometry. There is not a unique force and overpressure combination to achieve the desired geometry at the tip but smaller overpressure (hence force) yields a more uniform bore over the entire length of the emitter than does a larger overpressure (and force). However, the sensitivity of the geometry to small fluctuations in the parameters increases as the overpressure decreases. The best parameters depend on the accuracy of the puller used to manufacture the tapers and the permissible tolerances on the geometry. The model has wider application to the manufacture of other devices.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Mathematical modeling is used to examine the unsteady problem of heating and pulling an axisymmetric cylindrical glass tube with an overpressure applied within the tube to form tapers with a near uniform bore and small wall thickness at the tip. To allow for the dependence of viscosity on temperature, a prescribed axially varying viscosity is assumed. Our motivation is the manufacture of emitter tips for mass spectrometry which provide a continuous fluid flow and do not become blocked. We demonstrate, for the first time, the feasibility of producing such emitters by this process and examine the influence of the process parameters, in particular the pulling force and overpressure, on the geometry. There is not a unique force and overpressure combination to achieve the desired geometry at the tip but smaller overpressure (hence force) yields a more uniform bore over the entire length of the emitter than does a larger overpressure (and force). However, the sensitivity of the geometry to small fluctuations in the parameters increases as the overpressure decreases. The best parameters depend on the accuracy of the puller used to manufacture the tapers and the permissible tolerances on the geometry. The model has wider application to the manufacture of other devices.
Unsteady stretching of a glass tube with internal channel pressurization
10.1063/5.0096725
Physics of Fluids
20220805T12:02:49Z
© 2022 Author(s).
Gagani P. Ranathunga
Yvonne M. Stokes
Michael J. Chen

Shockdriven dispersal of a corrugated finitethickness particle layer
https://aip.scitation.org/doi/10.1063/5.0097596?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>A research area emerging in the multiphase flow community is the study of shockdriven multiphase instability (SDMI), a gas–particle analog of the traditional fluidfluid Richtmyer–Meshkov instability (RMI). In this work, we study the interaction of planar air shocks with corrugated glass particle curtains through the use of numerical simulations with an Eulerian–Lagrangian approach. One objective of this study is to compare the simulated particle curtains to a comparable set of shock tube experiments performed to analyze traditional RMI of a gas curtain. The simulations are set to match the experimental shock Mach numbers and perturbation wavelengths (3.6 and 7.2 mm) while also matching the Atwood number of the experiments to the multiphase Atwood number of the simulations. Varying particle diameters are tested in the simulations to explore the impact of particle diameter on the evolution of the particle curtain. This simulation setup allows for a onetoone comparison between RMI and SDMI under comparable conditions while also allowing for a separate study into the validity of the use of the multiphase Atwood number to compare the singlephase and multiphase instabilities. In particular, we show that the comparison depends on the diameter of the particles (thus, dependent on the Stokes number of the flow). A second objective of this study is to analyze the effect of the initial particle volume fraction on the evolution of the curtain and the behavior of the instability. This is done through analyzing the effect of the multiphase terms of the vorticity evolution equation on the vorticity deposition in SDMI. Also discussed is the effect of the particle diameter on the multiphase generation terms as well as in the baroclinic vorticity generation term in SDMI as the shock passes over the curtain.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>A research area emerging in the multiphase flow community is the study of shockdriven multiphase instability (SDMI), a gas–particle analog of the traditional fluidfluid Richtmyer–Meshkov instability (RMI). In this work, we study the interaction of planar air shocks with corrugated glass particle curtains through the use of numerical simulations with an Eulerian–Lagrangian approach. One objective of this study is to compare the simulated particle curtains to a comparable set of shock tube experiments performed to analyze traditional RMI of a gas curtain. The simulations are set to match the experimental shock Mach numbers and perturbation wavelengths (3.6 and 7.2 mm) while also matching the Atwood number of the experiments to the multiphase Atwood number of the simulations. Varying particle diameters are tested in the simulations to explore the impact of particle diameter on the evolution of the particle curtain. This simulation setup allows for a onetoone comparison between RMI and SDMI under comparable conditions while also allowing for a separate study into the validity of the use of the multiphase Atwood number to compare the singlephase and multiphase instabilities. In particular, we show that the comparison depends on the diameter of the particles (thus, dependent on the Stokes number of the flow). A second objective of this study is to analyze the effect of the initial particle volume fraction on the evolution of the curtain and the behavior of the instability. This is done through analyzing the effect of the multiphase terms of the vorticity evolution equation on the vorticity deposition in SDMI. Also discussed is the effect of the particle diameter on the multiphase generation terms as well as in the baroclinic vorticity generation term in SDMI as the shock passes over the curtain.
Shockdriven dispersal of a corrugated finitethickness particle layer
10.1063/5.0097596
Physics of Fluids
20220801T02:11:55Z
© 2022 Author(s).
Frederick Ouellet
Bertrand Rollin
Bradford Durant
Rahul Babu Koneru
S. Balachandar

Direct simulation Monte Carlo of the gasadsorption of particles in gas–particle flows
https://aip.scitation.org/doi/10.1063/5.0096606?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The acceleration and heating of particles in gas–particle flows represents a general question in fluid mechanics and has been widely studied since the 1920s. However, studies on the gas adsorption–desorption process of a particle in a gas–particle flow are still lacking. In this paper, we aim to introduce a framework with which to explain the gas adsorption–desorption kinetic equilibrium of a spherical particle in a gas–particle flow. We also explore the potential influences of gas adsorption and desorption on the heating and acceleration of a particle in such a flow. We study the gas adsorption–desorption process using the Langmuir adsorption model and the flow method presented by Brancher in a Direct Simulation Monte Carlo simulation. We develop a new model to consider the gas adsorption–desorption kinetic equilibrium on the surfaces of spherical particles in the calculation of the Thermal Accommodation Coefficient. We observe from the simulations that the trend of gas adsorption is opposite to that of the particle surface temperature. We also find that the efficiency of the convective heat transfer and acceleration of a particle is higher for a particle with a higher adsorption probability on its surface. This paper provides a new framework with which to study the gas adsorption–desorption processes of particles in gas–particle flows. It also inspires for future work on the potential applications of the particle gas adsorption–desorption model in astrophysics.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The acceleration and heating of particles in gas–particle flows represents a general question in fluid mechanics and has been widely studied since the 1920s. However, studies on the gas adsorption–desorption process of a particle in a gas–particle flow are still lacking. In this paper, we aim to introduce a framework with which to explain the gas adsorption–desorption kinetic equilibrium of a spherical particle in a gas–particle flow. We also explore the potential influences of gas adsorption and desorption on the heating and acceleration of a particle in such a flow. We study the gas adsorption–desorption process using the Langmuir adsorption model and the flow method presented by Brancher in a Direct Simulation Monte Carlo simulation. We develop a new model to consider the gas adsorption–desorption kinetic equilibrium on the surfaces of spherical particles in the calculation of the Thermal Accommodation Coefficient. We observe from the simulations that the trend of gas adsorption is opposite to that of the particle surface temperature. We also find that the efficiency of the convective heat transfer and acceleration of a particle is higher for a particle with a higher adsorption probability on its surface. This paper provides a new framework with which to study the gas adsorption–desorption processes of particles in gas–particle flows. It also inspires for future work on the potential applications of the particle gas adsorption–desorption model in astrophysics.
Direct simulation Monte Carlo of the gasadsorption of particles in gas–particle flows
10.1063/5.0096606
Physics of Fluids
20220802T01:38:11Z
© 2022 Author(s).

Simulating wetting phenomenon on curved surfaces based on the weightedorthogonal multiplerelaxationtime pseudopotential lattice Boltzmann model
https://aip.scitation.org/doi/10.1063/5.0101349?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this work, our recently developed weightedorthogonal multiplerelaxationtime pseudopotential lattice Boltzmann (PLB) model [J. Tang et al., “Multiphase flow simulation with threedimensional weightedorthogonal multiplerelaxationtime pseudopotential lattice Boltzmann model,” Phys. Fluids 33, 123305 (2021)] is further extended to simulate the complex wetting phenomenon on curved surfaces at large density ratios ([math]), where a new geometrical formulation scheme is proposed to characterize the wettability of the curved boundary. Compared with the existing geometrical formulation schemes, the significant advantage of the new scheme is that the characteristic vector representing the phase interface is no longer needed, and, thus, the complex calculations induced by the characteristic vector are avoided, which significantly simplifies computations and facilitates the implementation of the geometrical formulation scheme on curved boundaries. Meanwhile, it is applicable to both twodimensional and threedimensional (3D) simulations and maintains the feature of setting the contact angle explicitly. Furthermore, the numerical results of four classical wetting phenomenon benchmark cases at large density ratios predicted by the present model agree well with the analytical solutions, numerical results, or experimental results in the literature. It exhibits the capability of the present model coupled with the proposed scheme to simulate the wetting phenomenon involving curved surfaces with good numerical accuracy. Note that, to the author's knowledge, this is the first time that the geometrical formulation scheme has been successfully adopted in the 3D PLB model to simulate the wetting phenomenon on curved surfaces. We believe that this work lays the foundation for further application of the PLB model to the complex wetting phenomenon.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this work, our recently developed weightedorthogonal multiplerelaxationtime pseudopotential lattice Boltzmann (PLB) model [J. Tang et al., “Multiphase flow simulation with threedimensional weightedorthogonal multiplerelaxationtime pseudopotential lattice Boltzmann model,” Phys. Fluids 33, 123305 (2021)] is further extended to simulate the complex wetting phenomenon on curved surfaces at large density ratios ([math]), where a new geometrical formulation scheme is proposed to characterize the wettability of the curved boundary. Compared with the existing geometrical formulation schemes, the significant advantage of the new scheme is that the characteristic vector representing the phase interface is no longer needed, and, thus, the complex calculations induced by the characteristic vector are avoided, which significantly simplifies computations and facilitates the implementation of the geometrical formulation scheme on curved boundaries. Meanwhile, it is applicable to both twodimensional and threedimensional (3D) simulations and maintains the feature of setting the contact angle explicitly. Furthermore, the numerical results of four classical wetting phenomenon benchmark cases at large density ratios predicted by the present model agree well with the analytical solutions, numerical results, or experimental results in the literature. It exhibits the capability of the present model coupled with the proposed scheme to simulate the wetting phenomenon involving curved surfaces with good numerical accuracy. Note that, to the author's knowledge, this is the first time that the geometrical formulation scheme has been successfully adopted in the 3D PLB model to simulate the wetting phenomenon on curved surfaces. We believe that this work lays the foundation for further application of the PLB model to the complex wetting phenomenon.
Simulating wetting phenomenon on curved surfaces based on the weightedorthogonal multiplerelaxationtime pseudopotential lattice Boltzmann model
10.1063/5.0101349
Physics of Fluids
20220803T03:07:06Z
© 2022 Author(s).

Forming a composite model for nonBrownian suspensions
https://aip.scitation.org/doi/10.1063/5.0104540?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>We propose a twopart composite model to describe the rheology of nonBrownian suspensions. The stress response is composed of the sum of a matrix part (Tm) described by a multimode OldroydB model and a second component (To) which is assumed to be a Thompson–Souza Mendes model. We show how to determine the parameters to satisfy agreement with experiments in steady viscometric flows, uniaxial elongation flows, small to medium size sinusoidal strains, and reversing shear strain rates. Where possible, comparison is made with computations. Agreement with experiments and computations is reasonable, but more accurate computations and experiments would be welcome.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>We propose a twopart composite model to describe the rheology of nonBrownian suspensions. The stress response is composed of the sum of a matrix part (Tm) described by a multimode OldroydB model and a second component (To) which is assumed to be a Thompson–Souza Mendes model. We show how to determine the parameters to satisfy agreement with experiments in steady viscometric flows, uniaxial elongation flows, small to medium size sinusoidal strains, and reversing shear strain rates. Where possible, comparison is made with computations. Agreement with experiments and computations is reasonable, but more accurate computations and experiments would be welcome.
Forming a composite model for nonBrownian suspensions
10.1063/5.0104540
Physics of Fluids
20220803T03:07:24Z
© 2022 Author(s).
Roger I. Tanner
Shaocong Dai

Effects of liquid properties on atomization and spray characteristics studied by planar twophoton fluorescence
https://aip.scitation.org/doi/10.1063/5.0098922?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this work, planar twophoton laserinduced fluorescence (2pLIF) is applied for the first time to analyze the fluid dependent spray structure and atomization behavior of water and ethanol in a quantitative way. A commercial sixhole DISI (DirectInjection SparkIgnition) injector was studied at different injection pressures, operated with liquids containing the LIF dye fluorescein. Specifically for DISIinjectors, the fluiddependent atomization is very complex and not fully understood due to the cavitating, turbulent nozzle flow that dominates the spray formation. Optical access and analysis of the nearnozzle spray are often challenging due to multiple light scattering in dense regions which is reduced by 2pLIF measurements using a femtosecond laser. This allows highcontrast spray imaging close to the nozzle, resulting in an improved identification of single liquid structures of the spray. Thus, a higher accuracy of sizing is possible. Compared to water, the ethanol spray shape shows increased cone angles in the nozzle nearfield of about 6%, which cannot be explained by classical atomization theory based on aerodynamic breakup. The larger cone angle of ethanol was attributed to its larger viscosity, which could decelerate the flow at the wall of the injection hole, affecting the velocity profile of the emerging jet. The atomization shows a main jet breakup distance of 7–10 mm in which the structure sizes decreased drastically, specifically for water. For the size of the liquid structures in the nearnozzle region, which show dimensions of about 80–130 μm, ethanol exhibited about 2% smaller Feret's diameters than water for the tested time steps at 20 MPa. This effect is even more distinct for other injection pressures and positions at a further distance to the injector. For all investigated conditions and measurement positions downstream of the nozzle, ethanol showed on average about 24% smaller structures compared to the water spray. Although this trend is in accordance with the classical atomization theory based on the aerodynamic breakup mechanism, other effects, such as cavitation and nozzleflow induced breakup, contribute to this behavior.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this work, planar twophoton laserinduced fluorescence (2pLIF) is applied for the first time to analyze the fluid dependent spray structure and atomization behavior of water and ethanol in a quantitative way. A commercial sixhole DISI (DirectInjection SparkIgnition) injector was studied at different injection pressures, operated with liquids containing the LIF dye fluorescein. Specifically for DISIinjectors, the fluiddependent atomization is very complex and not fully understood due to the cavitating, turbulent nozzle flow that dominates the spray formation. Optical access and analysis of the nearnozzle spray are often challenging due to multiple light scattering in dense regions which is reduced by 2pLIF measurements using a femtosecond laser. This allows highcontrast spray imaging close to the nozzle, resulting in an improved identification of single liquid structures of the spray. Thus, a higher accuracy of sizing is possible. Compared to water, the ethanol spray shape shows increased cone angles in the nozzle nearfield of about 6%, which cannot be explained by classical atomization theory based on aerodynamic breakup. The larger cone angle of ethanol was attributed to its larger viscosity, which could decelerate the flow at the wall of the injection hole, affecting the velocity profile of the emerging jet. The atomization shows a main jet breakup distance of 7–10 mm in which the structure sizes decreased drastically, specifically for water. For the size of the liquid structures in the nearnozzle region, which show dimensions of about 80–130 μm, ethanol exhibited about 2% smaller Feret's diameters than water for the tested time steps at 20 MPa. This effect is even more distinct for other injection pressures and positions at a further distance to the injector. For all investigated conditions and measurement positions downstream of the nozzle, ethanol showed on average about 24% smaller structures compared to the water spray. Although this trend is in accordance with the classical atomization theory based on the aerodynamic breakup mechanism, other effects, such as cavitation and nozzleflow induced breakup, contribute to this behavior.
Effects of liquid properties on atomization and spray characteristics studied by planar twophoton fluorescence
10.1063/5.0098922
Physics of Fluids
20220804T11:52:28Z
© 2022 Author(s).
Hannah Ulrich
Bastian Lehnert
Diego Guénot
Kristoffer Svendsen
Olle Lundh
Michael Wensing
Edouard Berrocal
Lars Zigan

A pseudopotential lattice Boltzmann model for simulating mass transfer around a rising bubble under real buoyancy effect
https://aip.scitation.org/doi/10.1063/5.0098638?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>A pseudopotential multicomponent lattice Boltzmann (LB) model that can account for the real buoyancy effect is proposed to simulate the mass transfer process around a rising bubble. The density profiles at the equilibrium state are determined based on the hydrostatic condition and the zero diffusion flux condition (the balance of chemical potential). Compared with the LB models using effective buoyancy force, the proposed model has three advantages: (1) avoiding the unrealistic distribution of gas components within the bubble due to the upward effective buoyancy force, (2) removing the undesirable diffusion process due to the application of effective buoyancy force, and (3) considering the effect of the pressure gradient on the change of bubble size. In addition, Henry's law, which can be automatically recovered from the multicomponent LB equation, is reinterpreted from the perspective of the balance of chemical potential. Simulation results showed that the diffusion flux nonuniformly distributes over the surface of a rising bubble. The diffusion zone primarily occurs at the top and the lateral side of a rising bubble, whereas the diffusion transport just below the rising bubble is much less significant than its counterpart above the rising bubble. Various bubble shapes and their corresponding diffusion zones have been obtained. Moreover, the correlation between the Sherwood number and the Peclet number derived from the simulation results is consistent with those from previous numerical results. Thus, the proposed LB model is capable of conducting a quantitative analysis of the mass transfer around a rising bubble.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>A pseudopotential multicomponent lattice Boltzmann (LB) model that can account for the real buoyancy effect is proposed to simulate the mass transfer process around a rising bubble. The density profiles at the equilibrium state are determined based on the hydrostatic condition and the zero diffusion flux condition (the balance of chemical potential). Compared with the LB models using effective buoyancy force, the proposed model has three advantages: (1) avoiding the unrealistic distribution of gas components within the bubble due to the upward effective buoyancy force, (2) removing the undesirable diffusion process due to the application of effective buoyancy force, and (3) considering the effect of the pressure gradient on the change of bubble size. In addition, Henry's law, which can be automatically recovered from the multicomponent LB equation, is reinterpreted from the perspective of the balance of chemical potential. Simulation results showed that the diffusion flux nonuniformly distributes over the surface of a rising bubble. The diffusion zone primarily occurs at the top and the lateral side of a rising bubble, whereas the diffusion transport just below the rising bubble is much less significant than its counterpart above the rising bubble. Various bubble shapes and their corresponding diffusion zones have been obtained. Moreover, the correlation between the Sherwood number and the Peclet number derived from the simulation results is consistent with those from previous numerical results. Thus, the proposed LB model is capable of conducting a quantitative analysis of the mass transfer around a rising bubble.
A pseudopotential lattice Boltzmann model for simulating mass transfer around a rising bubble under real buoyancy effect
10.1063/5.0098638
Physics of Fluids
20220805T12:03:01Z
© 2022 Author(s).
Guanlong Guo
Pei Zhang
Liang Lei
S. A. GalindoTorres

Propagation of ionizing shock wave in a dusty gas medium under the influence of gravitational and azimuthal magnetic fields
https://aip.scitation.org/doi/10.1063/5.0094327?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this paper, a closedform solution for an ionizing spherical shock/blast wave in a dusty gas (a mixture of an ideal gas and solid particles of micrometer size) under the influence of gravitational and azimuthal magnetic fields is derived. In the dusty gas mixture, the solid particles are continuously distributed, and the equilibrium flow condition holds in the entire flow field region. Analytical solutions in the closed form for the firstorder approximation are derived for adiabatic and isothermal flows. Furthermore, for the second approximation, the set of ordinary differential equations is derived. The influence of problem parameters, such as the ratio of the density of the solid particles to the initial density of the ideal gas, the gravitational parameter, the solid particles mass concentration in the mixture, adiabatic index, and AlfvénMach number on the peak pressure on the blast wave, on physical variables and the damage radius of the blast wave is studied for the firstorder approximation. Our closedform solution for the firstorder approximation in the case of adiabatic flow is analogous to Taylor's solution in the case of a strong explosiongenerated blast wave. It is shown that the damage radius of the blast wave and the peak pressure on the blast wave both decrease with the addition of dust particles, and hence, the shock/blast wave strength decreases. It is observed that in the whole flow field region, the quantity [math] increases with an increase in the AlfvénMach number value, and hence, the shock decay with an increase in the AlfvénMach number.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this paper, a closedform solution for an ionizing spherical shock/blast wave in a dusty gas (a mixture of an ideal gas and solid particles of micrometer size) under the influence of gravitational and azimuthal magnetic fields is derived. In the dusty gas mixture, the solid particles are continuously distributed, and the equilibrium flow condition holds in the entire flow field region. Analytical solutions in the closed form for the firstorder approximation are derived for adiabatic and isothermal flows. Furthermore, for the second approximation, the set of ordinary differential equations is derived. The influence of problem parameters, such as the ratio of the density of the solid particles to the initial density of the ideal gas, the gravitational parameter, the solid particles mass concentration in the mixture, adiabatic index, and AlfvénMach number on the peak pressure on the blast wave, on physical variables and the damage radius of the blast wave is studied for the firstorder approximation. Our closedform solution for the firstorder approximation in the case of adiabatic flow is analogous to Taylor's solution in the case of a strong explosiongenerated blast wave. It is shown that the damage radius of the blast wave and the peak pressure on the blast wave both decrease with the addition of dust particles, and hence, the shock/blast wave strength decreases. It is observed that in the whole flow field region, the quantity [math] increases with an increase in the AlfvénMach number value, and hence, the shock decay with an increase in the AlfvénMach number.
Propagation of ionizing shock wave in a dusty gas medium under the influence of gravitational and azimuthal magnetic fields
10.1063/5.0094327
Physics of Fluids
20220808T10:28:17Z
© 2022 Author(s).
G. Nath

Experimental investigation on the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff hydrofoils
https://aip.scitation.org/doi/10.1063/5.0099776?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>We experimentally investigated the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff National Advisory Committee for Aeronautics 0015 hydrofoils in a lowpressure cavitation tunnel. We analyzed the cavitating dynamics by capturing the cavitation dynamics using two highspeed cameras at different cavitating regimes on the surface of the hydrofoils, made of polyvinyl chloride, brass, and aluminum. We then measured the associated structural deformations in specific cavitation regime such as cloud and partial cavitation dynamics, using a digital image correlation technique. The hydrofoil's angle of attack was set to 10°, and the flow's Reynolds number was adjusted to 0.6 × 106. Results showed that the cavity's shedding frequency on the flexible hydrofoil shifted faster to a higher frequency than on the stiff hydrofoils under similar cavitating conditions. The flexible hydrofoil underwent strong structural oscillations at the low cavitation number for the cloud cavitation regime. The associated amplitudes of the vibration were about 20 times higher than those of the hydrofoil made of brass. It was observed that the fluid–structure interaction can significantly affect the cavitationinduced vibration of the flexible hydrofoil.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>We experimentally investigated the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff National Advisory Committee for Aeronautics 0015 hydrofoils in a lowpressure cavitation tunnel. We analyzed the cavitating dynamics by capturing the cavitation dynamics using two highspeed cameras at different cavitating regimes on the surface of the hydrofoils, made of polyvinyl chloride, brass, and aluminum. We then measured the associated structural deformations in specific cavitation regime such as cloud and partial cavitation dynamics, using a digital image correlation technique. The hydrofoil's angle of attack was set to 10°, and the flow's Reynolds number was adjusted to 0.6 × 106. Results showed that the cavity's shedding frequency on the flexible hydrofoil shifted faster to a higher frequency than on the stiff hydrofoils under similar cavitating conditions. The flexible hydrofoil underwent strong structural oscillations at the low cavitation number for the cloud cavitation regime. The associated amplitudes of the vibration were about 20 times higher than those of the hydrofoil made of brass. It was observed that the fluid–structure interaction can significantly affect the cavitationinduced vibration of the flexible hydrofoil.
Experimental investigation on the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff hydrofoils
10.1063/5.0099776
Physics of Fluids
20220808T10:28:57Z
© 2022 Author(s).
Yuxing Lin
Ebrahim Kadivar
Ould el Moctar
Jens Neugebauer
Thomas E. Schellin

Energy extraction in the dynamic modes of flow for airfoil's laminar separation flutter
https://aip.scitation.org/doi/10.1063/5.0100195?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>This paper aims to gain new insight into the physical mechanism of laminar separation flutter (LSF) from the perspective of energy transfer and dynamic mode decomposition (DMD) modes of flow. An online DMD method accounting for the airfoil's pitch motion is developed, and the relationship between the topology of energy map and DMD modes is established. Simulation results indicate that there are two limit cycle branches in energy map, but only one branch is stable. The LSF time response state can be predicted accurately by the stable limit cycle branch. The topology of an energy map is dominated by the DMD mode corresponding to the airfoil's pitch frequency. The developed DMD method can extract the variation of flow structures effectively. The pressure distribution of DMD mode corresponding to the pitch frequency is dominated by the leadingedge suction and bubble's suction. The bubble's suction is induced by the trailingedge laminar separation bubble or laminar separation bubble (LSB). When the pitch amplitude is larger than 4°, the trailingedge laminar separation bubble transforms to LSB. The inherent mechanism is that increasing the trailingedge separation bubble's intensity promotes the energy extraction while the occurrence of LSB mitigates it.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>This paper aims to gain new insight into the physical mechanism of laminar separation flutter (LSF) from the perspective of energy transfer and dynamic mode decomposition (DMD) modes of flow. An online DMD method accounting for the airfoil's pitch motion is developed, and the relationship between the topology of energy map and DMD modes is established. Simulation results indicate that there are two limit cycle branches in energy map, but only one branch is stable. The LSF time response state can be predicted accurately by the stable limit cycle branch. The topology of an energy map is dominated by the DMD mode corresponding to the airfoil's pitch frequency. The developed DMD method can extract the variation of flow structures effectively. The pressure distribution of DMD mode corresponding to the pitch frequency is dominated by the leadingedge suction and bubble's suction. The bubble's suction is induced by the trailingedge laminar separation bubble or laminar separation bubble (LSB). When the pitch amplitude is larger than 4°, the trailingedge laminar separation bubble transforms to LSB. The inherent mechanism is that increasing the trailingedge separation bubble's intensity promotes the energy extraction while the occurrence of LSB mitigates it.
Energy extraction in the dynamic modes of flow for airfoil's laminar separation flutter
10.1063/5.0100195
Physics of Fluids
20220801T02:11:38Z
© 2022 Author(s).

Flow of a shallow film over a moving surface
https://aip.scitation.org/doi/10.1063/5.0099587?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The vonKarman–Pohlhausen averaging technique is employed to build a reduced model for the flow of a shallow film from a sluice gate or impacting jet over a moving surface. The viscous drag exerted on the film by the moving wall acts to arrest flow counter to the direction of the wall's motion, and force an adjustment toward the wall speed. For a (normally) impacting jet, this results in a range of wall speeds for which a steady state is reached in which all the fluid is eventually recirculated to flow along the wall, with a distinctive “heel” forming upstream of the impact region. For wall speeds below this range, the flow counter to the wall cannot be arrested, and unsteady states result. For wall speeds above this range, a different steady state emerges in which fluid is immediately diverted through and downstream of the impact region, eliminating any heel. The steady, heeled flow states predicted by the reduced model are in qualitative agreement with numerical simulations of the full twodimensional problem.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The vonKarman–Pohlhausen averaging technique is employed to build a reduced model for the flow of a shallow film from a sluice gate or impacting jet over a moving surface. The viscous drag exerted on the film by the moving wall acts to arrest flow counter to the direction of the wall's motion, and force an adjustment toward the wall speed. For a (normally) impacting jet, this results in a range of wall speeds for which a steady state is reached in which all the fluid is eventually recirculated to flow along the wall, with a distinctive “heel” forming upstream of the impact region. For wall speeds below this range, the flow counter to the wall cannot be arrested, and unsteady states result. For wall speeds above this range, a different steady state emerges in which fluid is immediately diverted through and downstream of the impact region, eliminating any heel. The steady, heeled flow states predicted by the reduced model are in qualitative agreement with numerical simulations of the full twodimensional problem.
Flow of a shallow film over a moving surface
10.1063/5.0099587
Physics of Fluids
20220802T01:38:24Z
© 2022 Author(s).
Sheldon Green
Boris Stoeber
Neil J. Balmforth

Solid bulk cargo liquefaction: Stability of liquid bridges
https://aip.scitation.org/doi/10.1063/5.0098834?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>This work focuses on investigating the correlation between the evolution of liquid bridges and moisture migration in solid bulk cargo liquefaction. We experimentally investigate the stability of liquid bridges in static and dynamic particles. The liquidholding capacity of static particles is determined, and the formation and fracturing of liquid bridges are related to the particle distribution and particle radius. The spacing of the bottom particles determines the maximum liquidholding capacity, while the space between the upper particles and the bottom particles determines the fracture position. The particles with larger radii have an increased liquidholding capacity and a low volumetric moisture content, which confirms that cargo that consists entirely of large particles would be apt to have seepage and would not liquefy. Moisture migration for pendular and funicular liquid bridges during stretching and squeezing is captured. We indicate that the fusion behavior of liquid bridges is an important inducement for moisture migration, and it dramatically decreases the liquidholding capacity. The findings suggest that cargo with low water content would still cause liquefaction, and that the water content should be reduced further for the safe transport of solid bulk cargo.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>This work focuses on investigating the correlation between the evolution of liquid bridges and moisture migration in solid bulk cargo liquefaction. We experimentally investigate the stability of liquid bridges in static and dynamic particles. The liquidholding capacity of static particles is determined, and the formation and fracturing of liquid bridges are related to the particle distribution and particle radius. The spacing of the bottom particles determines the maximum liquidholding capacity, while the space between the upper particles and the bottom particles determines the fracture position. The particles with larger radii have an increased liquidholding capacity and a low volumetric moisture content, which confirms that cargo that consists entirely of large particles would be apt to have seepage and would not liquefy. Moisture migration for pendular and funicular liquid bridges during stretching and squeezing is captured. We indicate that the fusion behavior of liquid bridges is an important inducement for moisture migration, and it dramatically decreases the liquidholding capacity. The findings suggest that cargo with low water content would still cause liquefaction, and that the water content should be reduced further for the safe transport of solid bulk cargo.
Solid bulk cargo liquefaction: Stability of liquid bridges
10.1063/5.0098834
Physics of Fluids
20220803T03:07:17Z
© 2022 Author(s).
Dracos Vassolos

Inertial instabilities of stratified jets: Linear stability theory
https://aip.scitation.org/doi/10.1063/5.0100979?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>This paper uses a linear stability analysis to investigate instabilities of barotropic and baroclinic jets that satisfy the necessary condition for inerital instabilities within the context of a rotating, stratified Boussinesq model. First, we review the different types of instabilities that can occur in these jets and the conditions that make the jet subject to inertial instability but stable to Rayleigh–Taylor instability. Second, we numerically solve onedimensional and twodimensional eigenvalue problems for the linear stability problems and examine the dependence of the growth rates on the Rossby number, Burger number, the aspect ratio, and the Reynolds number. We find that there are two critical Reynolds numbers where there is a transition between what type of instability has the largest growth rate. Finally, we examine the characteristics of inertial instabilities in more detail for three selected parameter sets: a low Reynolds number regime, a high Reynolds number regime, and a regime with low Reynolds number and larger aspect ratio. The most unstable mode in the low Reynolds number regime is a barotropic–baroclinic instability and has a barotropic spatial structure. In contrast, the most unstable mode in the high Reynolds number regime is an inertial instability and its spatial structure is independent of the alongflow direction. Modes with this property are commonly referred to as symmetric instabilities. In the intermediate regime, the flow can be unstable to both barotropic–baroclinic and inertial instabilities, possibly with comparable growth rates.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>This paper uses a linear stability analysis to investigate instabilities of barotropic and baroclinic jets that satisfy the necessary condition for inerital instabilities within the context of a rotating, stratified Boussinesq model. First, we review the different types of instabilities that can occur in these jets and the conditions that make the jet subject to inertial instability but stable to Rayleigh–Taylor instability. Second, we numerically solve onedimensional and twodimensional eigenvalue problems for the linear stability problems and examine the dependence of the growth rates on the Rossby number, Burger number, the aspect ratio, and the Reynolds number. We find that there are two critical Reynolds numbers where there is a transition between what type of instability has the largest growth rate. Finally, we examine the characteristics of inertial instabilities in more detail for three selected parameter sets: a low Reynolds number regime, a high Reynolds number regime, and a regime with low Reynolds number and larger aspect ratio. The most unstable mode in the low Reynolds number regime is a barotropic–baroclinic instability and has a barotropic spatial structure. In contrast, the most unstable mode in the high Reynolds number regime is an inertial instability and its spatial structure is independent of the alongflow direction. Modes with this property are commonly referred to as symmetric instabilities. In the intermediate regime, the flow can be unstable to both barotropic–baroclinic and inertial instabilities, possibly with comparable growth rates.
Inertial instabilities of stratified jets: Linear stability theory
10.1063/5.0100979
Physics of Fluids
20220803T03:07:11Z
© 2022 Author(s).
M. W. Harris
F. J. Poulin
K. G. Lamb

Instability of a salt jet emitted from a point source in an external electric field
https://aip.scitation.org/doi/10.1063/5.0098652?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The objective in the present work is to consider a simple example of instability of a conducting selfsimilar micro jet in the external electric field, which represents a prototype of some microfluidic instabilities. Salt from a point source is emitted into its own aquatic solution, which is subject to an external uniform velocity field together with an electrostatic field, and is convected downstream and diffused. The flow is considered in microscales so that, in contrast to the classical jets, the Reynolds numbers are practically zero, but the Péclet numbers are large. The parameters are found at which such a microjet is unstable. Along with the linear stability analysis, we have fulfilled the numerical simulations of the full nonlinear system of equations. The numerical simulation qualitatively confirmed the results of the linear stability and showed that this instability visually reminds classical instabilities of free jets and wakes.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The objective in the present work is to consider a simple example of instability of a conducting selfsimilar micro jet in the external electric field, which represents a prototype of some microfluidic instabilities. Salt from a point source is emitted into its own aquatic solution, which is subject to an external uniform velocity field together with an electrostatic field, and is convected downstream and diffused. The flow is considered in microscales so that, in contrast to the classical jets, the Reynolds numbers are practically zero, but the Péclet numbers are large. The parameters are found at which such a microjet is unstable. Along with the linear stability analysis, we have fulfilled the numerical simulations of the full nonlinear system of equations. The numerical simulation qualitatively confirmed the results of the linear stability and showed that this instability visually reminds classical instabilities of free jets and wakes.
Instability of a salt jet emitted from a point source in an external electric field
10.1063/5.0098652
Physics of Fluids
20220803T03:07:15Z
© 2022 Author(s).
S. Amiroudine
E. A. Demekhin
G. S. Ganchenko
V. S. Shelistov
E. A. Frants

Scattering of Mack modes by solidporous junctions in hypersonic boundary layers
https://aip.scitation.org/doi/10.1063/5.0106314?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Porous coating is regarded as an effective control strategy to delay the laminarturbulent transition in hypersonic boundary layers, whose effects include not only the successive distortion of the Mackmode growth rate but also the scattering effect appearing at the junctions between the solid and porous walls. The present paper focuses particularly on the latter mechanism by employing the harmonic linearized Navier–Stokes approach, which is confirmed to be sufficiently accurate as compared to the results obtained by direct numerical simulations. A transmission coefficient is introduced to quantitatively characterize the scattering effect, which is define as the ratio of the Mackinstability amplitude downstream of the junction to that upstream. For a solidtoporous junction, the scattering effect leads to a stabilization of the majority of the second mode but destabilizes slightly the first mode. In contrast, the poroustosolid junction plays the opposite role. The overall effect of a finitelength porous panel requires a combined consideration of the Mackmodegrowthrate distortion and the scattering effect of the two junctions at both boundaries of the panel, which shows a critical frequency separating the destabilizing and stabilizing frequency bands. Additionally, a decrease in the wall temperature leads to an enhanced scattering effect on the Mack modes.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Porous coating is regarded as an effective control strategy to delay the laminarturbulent transition in hypersonic boundary layers, whose effects include not only the successive distortion of the Mackmode growth rate but also the scattering effect appearing at the junctions between the solid and porous walls. The present paper focuses particularly on the latter mechanism by employing the harmonic linearized Navier–Stokes approach, which is confirmed to be sufficiently accurate as compared to the results obtained by direct numerical simulations. A transmission coefficient is introduced to quantitatively characterize the scattering effect, which is define as the ratio of the Mackinstability amplitude downstream of the junction to that upstream. For a solidtoporous junction, the scattering effect leads to a stabilization of the majority of the second mode but destabilizes slightly the first mode. In contrast, the poroustosolid junction plays the opposite role. The overall effect of a finitelength porous panel requires a combined consideration of the Mackmodegrowthrate distortion and the scattering effect of the two junctions at both boundaries of the panel, which shows a critical frequency separating the destabilizing and stabilizing frequency bands. Additionally, a decrease in the wall temperature leads to an enhanced scattering effect on the Mack modes.
Scattering of Mack modes by solidporous junctions in hypersonic boundary layers
10.1063/5.0106314
Physics of Fluids
20220805T12:02:52Z
© 2022 Author(s).

Fragmentation of inviscid liquid and destination of satellite droplets
https://aip.scitation.org/doi/10.1063/5.0102220?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The breakup process of the inviscid liquid bridge sandwiched between two coaxial and equalsized rods is investigated by tracking its profile. Here, the focus is on the quasistatic profile of the liquid bridge close to rupture and its influence on the subsequent dynamic breakup behaviors. With the increasing distance between the two rods, the profile of the liquid bridge close to rupture undergoes a transition from symmetry to asymmetry. We found there exists a critical slenderness above which the liquid bridge will be asymmetric and present a profile that can be well fitted by one cycle of the sine wave. It is demonstrated both experimentally and theoretically that the ratio of the length of the bridge to its equivalent radius, defined as geometric mean of the radii at the peak and trough of the bridge, is always [math] for the asymmetric bridge close to rupture. Different with the symmetric evolution of the short bridge, the long asymmetric bridge pinches off first from the side near the bigger sessile drop and then from the other side, which endows the satellite droplet with a lateral momentum, resulting in the satellite recollected by the sessile drop. The influence of the slenderness on the time interval among the asymmetric pinchoff, velocity, destination, and size of the satellite was investigated. A scaling law was proposed to describe the relationship between the lateral momentum of the satellite and the time interval between two pinchoff. This work is expected to benefit the utilizing or suppressing the satellite in practice.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The breakup process of the inviscid liquid bridge sandwiched between two coaxial and equalsized rods is investigated by tracking its profile. Here, the focus is on the quasistatic profile of the liquid bridge close to rupture and its influence on the subsequent dynamic breakup behaviors. With the increasing distance between the two rods, the profile of the liquid bridge close to rupture undergoes a transition from symmetry to asymmetry. We found there exists a critical slenderness above which the liquid bridge will be asymmetric and present a profile that can be well fitted by one cycle of the sine wave. It is demonstrated both experimentally and theoretically that the ratio of the length of the bridge to its equivalent radius, defined as geometric mean of the radii at the peak and trough of the bridge, is always [math] for the asymmetric bridge close to rupture. Different with the symmetric evolution of the short bridge, the long asymmetric bridge pinches off first from the side near the bigger sessile drop and then from the other side, which endows the satellite droplet with a lateral momentum, resulting in the satellite recollected by the sessile drop. The influence of the slenderness on the time interval among the asymmetric pinchoff, velocity, destination, and size of the satellite was investigated. A scaling law was proposed to describe the relationship between the lateral momentum of the satellite and the time interval between two pinchoff. This work is expected to benefit the utilizing or suppressing the satellite in practice.
Fragmentation of inviscid liquid and destination of satellite droplets
10.1063/5.0102220
Physics of Fluids
20220808T10:28:38Z
© 2022 Author(s).

Intrinsic lowdimensional manifold (ILDM)based concept for the coupling of turbulent mixing with manifoldbased simplified chemistry for the turbulent flame simulation
https://aip.scitation.org/doi/10.1063/5.0098974?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Manifold based simplified chemistry is an efficient reduction technique for the chemical kinetics, which aims to reduce the computational effort in numerical simulations. While the concept of reduced chemistry has been used for decades and various models have been developed up to now, their coupling with turbulent physical processes (e.g., mixing processes) has not been investigated extensively. This is attributed to the fact that the turbulent physical processes act as perturbation to the chemistry which pulls the thermokinetic states away from the manifold, and these states must relax back onto the manifold again. The present work gives insight into the coupling of reduced kinetic and the turbulent mixing processes. Accordingly, a strategy based on the Intrinsic LowDimensional Manifold concept is proposed. This coupling strategy is validated through the wellknown Sandia Flame series. It is shown that the numerical results agree very well with those using detailed chemistry (no coupling model required) and experimental measurement. The suggested coupling strategy can be used for any manifold based simplified chemistry.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Manifold based simplified chemistry is an efficient reduction technique for the chemical kinetics, which aims to reduce the computational effort in numerical simulations. While the concept of reduced chemistry has been used for decades and various models have been developed up to now, their coupling with turbulent physical processes (e.g., mixing processes) has not been investigated extensively. This is attributed to the fact that the turbulent physical processes act as perturbation to the chemistry which pulls the thermokinetic states away from the manifold, and these states must relax back onto the manifold again. The present work gives insight into the coupling of reduced kinetic and the turbulent mixing processes. Accordingly, a strategy based on the Intrinsic LowDimensional Manifold concept is proposed. This coupling strategy is validated through the wellknown Sandia Flame series. It is shown that the numerical results agree very well with those using detailed chemistry (no coupling model required) and experimental measurement. The suggested coupling strategy can be used for any manifold based simplified chemistry.
Intrinsic lowdimensional manifold (ILDM)based concept for the coupling of turbulent mixing with manifoldbased simplified chemistry for the turbulent flame simulation
10.1063/5.0098974
Physics of Fluids
20220801T02:11:45Z
© 2022 Author(s).
Prashant Shrotriya
Ulrich Maas

Wall heat transfer in highenthalpy hypersonic turbulent boundary layers
https://aip.scitation.org/doi/10.1063/5.0100416?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this paper, we investigate the differences in wall heat transfer between the low and highenthalpy turbulent boundary layers by exploiting direct numerical simulation databases of hypersonic turbulent boundary layers at the freestream Mach number of 4.5 and the friction Reynolds number of 800. For that purpose, we refine the integral formula of decomposing the wall heat flux proposed by Sun et al. [“A decomposition formula for the wall heat flux of a compressible boundary layer,” Adv. Aerodyn. 4, 1–13 (2022)], enabling us to scrutinize the contribution of different physical processes. Statistical results show that the mean wall heat transfer is primarily contributed by the heat conduction, the turbulent heat transfer, viscous dissipation of mean kinetic energy, and turbulent kinetic energy production. Among these processes, the contribution of the turbulent heat flux in the highenthalpy case is 10% higher than that in the lowenthalpy case. Such discrepancy is caused by the turbulent–chemistry interaction consisting of velocity and species mass fraction fluctuations. Coherent structures in the conditionally averaged fields related to this process reveal that the sweep in the viscous sublayer and ejection in the logarithmic layer bringing the hot fluid downward and upward, respectively, significantly alter the distribution of the species mass fraction. The wall heat flux fluctuations are slightly enhanced in the highenthalpy flows, which is ascribed to be the intensification of traveling wave packets.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this paper, we investigate the differences in wall heat transfer between the low and highenthalpy turbulent boundary layers by exploiting direct numerical simulation databases of hypersonic turbulent boundary layers at the freestream Mach number of 4.5 and the friction Reynolds number of 800. For that purpose, we refine the integral formula of decomposing the wall heat flux proposed by Sun et al. [“A decomposition formula for the wall heat flux of a compressible boundary layer,” Adv. Aerodyn. 4, 1–13 (2022)], enabling us to scrutinize the contribution of different physical processes. Statistical results show that the mean wall heat transfer is primarily contributed by the heat conduction, the turbulent heat transfer, viscous dissipation of mean kinetic energy, and turbulent kinetic energy production. Among these processes, the contribution of the turbulent heat flux in the highenthalpy case is 10% higher than that in the lowenthalpy case. Such discrepancy is caused by the turbulent–chemistry interaction consisting of velocity and species mass fraction fluctuations. Coherent structures in the conditionally averaged fields related to this process reveal that the sweep in the viscous sublayer and ejection in the logarithmic layer bringing the hot fluid downward and upward, respectively, significantly alter the distribution of the species mass fraction. The wall heat flux fluctuations are slightly enhanced in the highenthalpy flows, which is ascribed to be the intensification of traveling wave packets.
Wall heat transfer in highenthalpy hypersonic turbulent boundary layers
10.1063/5.0100416
Physics of Fluids
20220802T01:38:17Z
© 2022 Author(s).

Conditioned structure functions in turbulent hydrogen/air flames
https://aip.scitation.org/doi/10.1063/5.0096509?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Direct numerical simulation data obtained from two turbulent, lean hydrogen–air flames propagating in a box are analyzed to explore the influence of combustioninduced thermal expansion on turbulence in unburned gas. For this purpose, Helmholtz–Hodge decomposition is applied to the computed velocity fields. Subsequently, the secondorder structure functions conditioned to unburned reactants are sampled from divergencefree solenoidal velocity field or irrotational potential velocity field, yielded by the decomposition. Results show that thermal expansion significantly affects the conditioned potential structure functions not only inside the mean flame brushes but also upstream of them. Upstream of the flames, first, transverse structure functions for transverse potential velocities grow with distance [math] between sampling points more slowly when compared to the counterpart structure functions sampled from the entire or solenoidal velocity field. Second, the former growth rate depends substantially on the distance from the flamebrush leading edge, even at small [math]. Third, potential root mean square (rms) velocities increase with the decrease in distance from the flamebrush leading edge and are comparable with solenoidal rms velocities near the leading edge. Fourth, although the conditioned axial and transverse potential rms velocities are always close to one another, thus implying isotropy of the potential velocity field in unburned reactants, the potential structure functions exhibit a high degree of anisotropy. Fifth, thermal expansion effects are substantial even for the solenoidal structure functions and even upstream of a highly turbulent flame. These findings call for development of advanced models of turbulence in flames, which allow for the discussed thermal expansion effects.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Direct numerical simulation data obtained from two turbulent, lean hydrogen–air flames propagating in a box are analyzed to explore the influence of combustioninduced thermal expansion on turbulence in unburned gas. For this purpose, Helmholtz–Hodge decomposition is applied to the computed velocity fields. Subsequently, the secondorder structure functions conditioned to unburned reactants are sampled from divergencefree solenoidal velocity field or irrotational potential velocity field, yielded by the decomposition. Results show that thermal expansion significantly affects the conditioned potential structure functions not only inside the mean flame brushes but also upstream of them. Upstream of the flames, first, transverse structure functions for transverse potential velocities grow with distance [math] between sampling points more slowly when compared to the counterpart structure functions sampled from the entire or solenoidal velocity field. Second, the former growth rate depends substantially on the distance from the flamebrush leading edge, even at small [math]. Third, potential root mean square (rms) velocities increase with the decrease in distance from the flamebrush leading edge and are comparable with solenoidal rms velocities near the leading edge. Fourth, although the conditioned axial and transverse potential rms velocities are always close to one another, thus implying isotropy of the potential velocity field in unburned reactants, the potential structure functions exhibit a high degree of anisotropy. Fifth, thermal expansion effects are substantial even for the solenoidal structure functions and even upstream of a highly turbulent flame. These findings call for development of advanced models of turbulence in flames, which allow for the discussed thermal expansion effects.
Conditioned structure functions in turbulent hydrogen/air flames
10.1063/5.0096509
Physics of Fluids
20220803T03:07:02Z
© 2022 Author(s).
Vladimir A. Sabelnikov
Andrei N. Lipatnikov
Nikolay V. Nikitin
Francisco E. HernándezPérez
Hong G. Im

Scaling laws of statistics of wallbounded turbulence at supercritical pressure: Evaluation and mechanism
https://aip.scitation.org/doi/10.1063/5.0101889?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>A growing body of studies support that the real fluid effects related to the abrupt density changes in supercritical fluids significantly affect statistical properties of turbulence, yet developing appropriate scaling laws for wallbounded turbulence at supercritical state is still difficult. In the present study, we conduct direct numerical simulations on channel flows of supercritical fluids to evaluate the usefulness of classical scaling developed for variableproperty flows. We find that the expressions based on semilocal scaling [[math] and [math]] fail to collapse the statistical profiles at supercritical pressure. We analyze the mechanism of the failure of semilocal scaling by quantifying the modulations of turbulent structures of supercritical fluids due to changes in fluid properties. The intensified ejection and sweep of lowspeed streaks destabilize the streamwise streaks and reduce the streamwise coherence, changing the statistics and affecting the usefulness of semilocal scaling. To shed light on the scaling laws of fluctuating velocities, we finally examine the hypotheses in Townsend wallattached eddy theory in the context of flows at a supercritical state. It is found that the attached eddies are selfsimilar nearwall structures, which result in the logarithmic profiles of streamwise and spanwise velocity fluctuations; the population density of the attached eddies can be well approximated by an exponential scaling.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>A growing body of studies support that the real fluid effects related to the abrupt density changes in supercritical fluids significantly affect statistical properties of turbulence, yet developing appropriate scaling laws for wallbounded turbulence at supercritical state is still difficult. In the present study, we conduct direct numerical simulations on channel flows of supercritical fluids to evaluate the usefulness of classical scaling developed for variableproperty flows. We find that the expressions based on semilocal scaling [[math] and [math]] fail to collapse the statistical profiles at supercritical pressure. We analyze the mechanism of the failure of semilocal scaling by quantifying the modulations of turbulent structures of supercritical fluids due to changes in fluid properties. The intensified ejection and sweep of lowspeed streaks destabilize the streamwise streaks and reduce the streamwise coherence, changing the statistics and affecting the usefulness of semilocal scaling. To shed light on the scaling laws of fluctuating velocities, we finally examine the hypotheses in Townsend wallattached eddy theory in the context of flows at a supercritical state. It is found that the attached eddies are selfsimilar nearwall structures, which result in the logarithmic profiles of streamwise and spanwise velocity fluctuations; the population density of the attached eddies can be well approximated by an exponential scaling.
Scaling laws of statistics of wallbounded turbulence at supercritical pressure: Evaluation and mechanism
10.1063/5.0101889
Physics of Fluids
20220804T11:52:16Z
© 2022 Author(s).

Treebased machine learning models for prediction of bed elevation around bridge piers
https://aip.scitation.org/doi/10.1063/5.0098394?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Scouring around bridge piers is a highly nonlinear process making its prediction by deterministic and stochastic models challenging. This study explores the application of inferential models for predictions of bed elevations around bridge piers. The objective is to get a generalized machine learning model with an interpretable structure. The historical data comprise a detailed record of streamflow and bed elevations that were captured by sensors installed at the 5th Street Bridge piers over Ocmulgee River at Macon, GA. We investigate the accuracy and efficiency of various treebased machine learning algorithms, including a single tree as well as homogeneous ensemble models for simultaneous predictions of bed elevation at multiple sensors installed at piers. The ensemble models were based on bagging and boosting techniques. Special attention is given to balancing between overfitting and underfitting without compromise on the model's robustness. Observation of the performance metrics showed that treebased models have excellent predictive capacity. It was observed that boosting models, including a gradient based regression model, and adaptive boosting outperformed the bagging model. Among all the models investigated in this study, the adaptive boosting method was observed to be most generalizable. The performance of developed models shows the potential of treebased ensemble models in providing rapid and robust predictions for complex nonlinear fluid flows.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Scouring around bridge piers is a highly nonlinear process making its prediction by deterministic and stochastic models challenging. This study explores the application of inferential models for predictions of bed elevations around bridge piers. The objective is to get a generalized machine learning model with an interpretable structure. The historical data comprise a detailed record of streamflow and bed elevations that were captured by sensors installed at the 5th Street Bridge piers over Ocmulgee River at Macon, GA. We investigate the accuracy and efficiency of various treebased machine learning algorithms, including a single tree as well as homogeneous ensemble models for simultaneous predictions of bed elevation at multiple sensors installed at piers. The ensemble models were based on bagging and boosting techniques. Special attention is given to balancing between overfitting and underfitting without compromise on the model's robustness. Observation of the performance metrics showed that treebased models have excellent predictive capacity. It was observed that boosting models, including a gradient based regression model, and adaptive boosting outperformed the bagging model. Among all the models investigated in this study, the adaptive boosting method was observed to be most generalizable. The performance of developed models shows the potential of treebased ensemble models in providing rapid and robust predictions for complex nonlinear fluid flows.
Treebased machine learning models for prediction of bed elevation around bridge piers
10.1063/5.0098394
Physics of Fluids
20220805T12:03:01Z
© 2022 Author(s).
Khawar Rehman
YungChieh Wang
Muhammad Waseem
Seung Ho Hong

Turbulent channel flow controlled by travelingwavelike body force mimicking oscillating thin films
https://aip.scitation.org/doi/10.1063/5.0096823?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>To improve energy efficiency, flow control techniques for skinfriction drag and heat transfer with regard to wall turbulence are essential. This study performs direct numerical simulation of turbulent channel flows. The travelingwavelike body force is employed as the flow control technique to break the similarity between momentum and heat transfer. The traveling wave control mimics the selfexcited thin film in the corresponding experimental study. When the wave traveled slowly along the downstream direction, the skinfriction drag, heat transfer, and analogy factor were found to increase. Moreover, these parameters increased with an increase in the reference height of the traveling wave (hw). Flow visualization shows turbulence enhancement owing to the increase in hw. Threecomponent decomposition elucidates the difference between the control effect on the Reynolds shear stress and the turbulent heat flux.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>To improve energy efficiency, flow control techniques for skinfriction drag and heat transfer with regard to wall turbulence are essential. This study performs direct numerical simulation of turbulent channel flows. The travelingwavelike body force is employed as the flow control technique to break the similarity between momentum and heat transfer. The traveling wave control mimics the selfexcited thin film in the corresponding experimental study. When the wave traveled slowly along the downstream direction, the skinfriction drag, heat transfer, and analogy factor were found to increase. Moreover, these parameters increased with an increase in the reference height of the traveling wave (hw). Flow visualization shows turbulence enhancement owing to the increase in hw. Threecomponent decomposition elucidates the difference between the control effect on the Reynolds shear stress and the turbulent heat flux.
Turbulent channel flow controlled by travelingwavelike body force mimicking oscillating thin films
10.1063/5.0096823
Physics of Fluids
20220808T10:28:53Z
© 2022 Author(s).

Effect of gravityinduced fluid inertia on the accumulation and dispersion of motile plankton settling weakly in turbulence
https://aip.scitation.org/doi/10.1063/5.0101142?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>We investigate the effect of gravityinduced fluid inertia on motile plankton cells settling weakly through isotropic turbulence using direct numerical simulations. Gyrotaxis arises from the gravityinduced fluid inertial torque, leading to upward migration of the settling elongated microorganisms when their swimming speed exceeds the settling speed. Preferential sampling and smallscale fractal clustering of plankton cells are studied over a wide range of swimming speeds and aspect ratios. It is found that orientation fluctuation induced by the effect of the fluid inertia and preferential alignment with turbulent strain are the most important factors affecting the statistics, which are responsible for determining the optimal shape. For strong gyrotaxis, the organisms tend to form noticeable clusters in the vertical direction. An investigation of the dispersion reveals that the fluid inertial effects contribute to the enhancement of the longtime vertical dispersion of the organisms by increasing their rootmeansquared velocity. Our results show how the fluid inertial effects can influence clustering and dispersion statistics of the organisms in turbulence, which turns out to provide an environment conducive to their survival.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>We investigate the effect of gravityinduced fluid inertia on motile plankton cells settling weakly through isotropic turbulence using direct numerical simulations. Gyrotaxis arises from the gravityinduced fluid inertial torque, leading to upward migration of the settling elongated microorganisms when their swimming speed exceeds the settling speed. Preferential sampling and smallscale fractal clustering of plankton cells are studied over a wide range of swimming speeds and aspect ratios. It is found that orientation fluctuation induced by the effect of the fluid inertia and preferential alignment with turbulent strain are the most important factors affecting the statistics, which are responsible for determining the optimal shape. For strong gyrotaxis, the organisms tend to form noticeable clusters in the vertical direction. An investigation of the dispersion reveals that the fluid inertial effects contribute to the enhancement of the longtime vertical dispersion of the organisms by increasing their rootmeansquared velocity. Our results show how the fluid inertial effects can influence clustering and dispersion statistics of the organisms in turbulence, which turns out to provide an environment conducive to their survival.
Effect of gravityinduced fluid inertia on the accumulation and dispersion of motile plankton settling weakly in turbulence
10.1063/5.0101142
Physics of Fluids
20220808T10:28:22Z
© 2022 Author(s).

Propeller wake instabilities under turbulentinflow conditions
https://aip.scitation.org/doi/10.1063/5.0101977?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The wake instabilities of a propeller operating under turbulentinflow conditions were studied by the improved delayed detached eddy simulation method on an unstructured mesh consisting of almost 82.5 × 106 cells, capturing propeller wakes extending to the downstream distance of 9D (where D is the propeller diameter). Two turbulentinflow cases with the turbulence intensity of 5% and 20% were considered. The mean loads and phaseaveraged flow field show good agreement with experiments. As the propeller blade interacts with the turbulent inflow, a wide peak extending approximately ±10 Hz in the power spectral density of the time histories of the thrust and torque coefficient. Simulation results reveal wake instability mechanisms of the propeller operating under different turbulentinflow conditions. The turbulence added to the inlet boundary interacts with the tip vortices, which accelerates the destabilization processes of the tip vortex system from two aspects. First, the interaction between the inflow turbulence and the tip vortex promotes the diffusion of tip vortices. Second, the interaction between the inflow turbulence and the tip vortices magnifies the instability motion of the tip vortex. The wake vortex system of the highturbulence inflow condition loses its stability after 2.2D downstream, while the initial instability behaviors for the lowturbulence inflow condition are observed at the location of 3.4D downstream. The present study presents a deeper insight into the flow physics driving the tip vortex pairing process for a propeller operating under turbulentinflow conditions.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The wake instabilities of a propeller operating under turbulentinflow conditions were studied by the improved delayed detached eddy simulation method on an unstructured mesh consisting of almost 82.5 × 106 cells, capturing propeller wakes extending to the downstream distance of 9D (where D is the propeller diameter). Two turbulentinflow cases with the turbulence intensity of 5% and 20% were considered. The mean loads and phaseaveraged flow field show good agreement with experiments. As the propeller blade interacts with the turbulent inflow, a wide peak extending approximately ±10 Hz in the power spectral density of the time histories of the thrust and torque coefficient. Simulation results reveal wake instability mechanisms of the propeller operating under different turbulentinflow conditions. The turbulence added to the inlet boundary interacts with the tip vortices, which accelerates the destabilization processes of the tip vortex system from two aspects. First, the interaction between the inflow turbulence and the tip vortex promotes the diffusion of tip vortices. Second, the interaction between the inflow turbulence and the tip vortices magnifies the instability motion of the tip vortex. The wake vortex system of the highturbulence inflow condition loses its stability after 2.2D downstream, while the initial instability behaviors for the lowturbulence inflow condition are observed at the location of 3.4D downstream. The present study presents a deeper insight into the flow physics driving the tip vortex pairing process for a propeller operating under turbulentinflow conditions.
Propeller wake instabilities under turbulentinflow conditions
10.1063/5.0101977
Physics of Fluids
20220808T10:33:17Z
© 2022 Author(s).

Prediction of broadband noise from rotating blade elements with serrated trailing edges
https://aip.scitation.org/doi/10.1063/5.0094423?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>This paper conducts a theoretical investigation into the prediction of broadband trailingedge noise for rotating serrated blades. Lyu's semianalytical noise prediction model for isolated flat plates is extended to rotating blades using Schlinker and Amiet's approach and applied to three test applications including a wind turbine, a cooling fan, and an open propeller. The model is validated by comparing the straight edge results with that presented in the work of Sinayoko et al., which shows an excellent agreement. The noise spectra obtained using differentorder approximations show that the secondorder solution yields a converged result. It is found that trailingedge serrations can lead to noise reduction in the intermediate and highfrequency ranges at an observer angle of 45° at low Mach numbers but may lead to noise increase in the intermediatefrequency range at high Mach numbers. The results show that the directivity patterns change due to the use of trailingedge serrations and the directivity peaks are observed at high frequencies. A detailed analysis on the effects of rotation shows that for lowMach number applications, the Doppler effect is weak and the peaky directivity pattern is mainly affected by the nonuniform directivity of an isolated flat plate at high frequencies. However, for highMach number applications, the Doppler effect is significant and also contributes to the final directivity pattern of rotating blades.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>This paper conducts a theoretical investigation into the prediction of broadband trailingedge noise for rotating serrated blades. Lyu's semianalytical noise prediction model for isolated flat plates is extended to rotating blades using Schlinker and Amiet's approach and applied to three test applications including a wind turbine, a cooling fan, and an open propeller. The model is validated by comparing the straight edge results with that presented in the work of Sinayoko et al., which shows an excellent agreement. The noise spectra obtained using differentorder approximations show that the secondorder solution yields a converged result. It is found that trailingedge serrations can lead to noise reduction in the intermediate and highfrequency ranges at an observer angle of 45° at low Mach numbers but may lead to noise increase in the intermediatefrequency range at high Mach numbers. The results show that the directivity patterns change due to the use of trailingedge serrations and the directivity peaks are observed at high frequencies. A detailed analysis on the effects of rotation shows that for lowMach number applications, the Doppler effect is weak and the peaky directivity pattern is mainly affected by the nonuniform directivity of an isolated flat plate at high frequencies. However, for highMach number applications, the Doppler effect is significant and also contributes to the final directivity pattern of rotating blades.
Prediction of broadband noise from rotating blade elements with serrated trailing edges
10.1063/5.0094423
Physics of Fluids
20220808T10:28:55Z
© 2022 Author(s).

Trajectoryoptimized clusterbased network model for the sphere wake
https://aip.scitation.org/doi/10.1063/5.0098655?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>We propose a novel trajectoryoptimized clusterbased network model (tCNM) for nonlinear model order reduction from timeresolved data following Li et al. [“Clusterbased network model,” J. Fluid Mech. 906, A21 (2021)] and improving the accuracy for a given number of centroids. The starting point is kmeans++ clustering, which minimizes the representation error of the snapshots by their closest centroids. The dynamics is presented by “flights” between the centroids. The proposed trajectoryoptimized clustering aims to reduce the kinematic representation error further by shifting the centroids closer to the snapshot trajectory and refining state propagation with trajectory support points. Thus, curved trajectories are better resolved. The resulting tCNM is demonstrated for the sphere wake for three flow regimes, including the periodic, quasiperiodic, and chaotic dynamics. The representation error of tCNM is five times smaller as compared to the approximation by the closest centroid. Thus, the error is at the same level as proper orthogonal decomposition (POD) of same order. Yet, tCNM has distinct advantages over POD modeling: it is human interpretable by representing dynamics by a handful of coherent structures and their transitions; it shows robust dynamics by design, i.e., stable longtime behavior; and its development is fully automatable, i.e., it does not require tunable auxiliary closure and other models.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>We propose a novel trajectoryoptimized clusterbased network model (tCNM) for nonlinear model order reduction from timeresolved data following Li et al. [“Clusterbased network model,” J. Fluid Mech. 906, A21 (2021)] and improving the accuracy for a given number of centroids. The starting point is kmeans++ clustering, which minimizes the representation error of the snapshots by their closest centroids. The dynamics is presented by “flights” between the centroids. The proposed trajectoryoptimized clustering aims to reduce the kinematic representation error further by shifting the centroids closer to the snapshot trajectory and refining state propagation with trajectory support points. Thus, curved trajectories are better resolved. The resulting tCNM is demonstrated for the sphere wake for three flow regimes, including the periodic, quasiperiodic, and chaotic dynamics. The representation error of tCNM is five times smaller as compared to the approximation by the closest centroid. Thus, the error is at the same level as proper orthogonal decomposition (POD) of same order. Yet, tCNM has distinct advantages over POD modeling: it is human interpretable by representing dynamics by a handful of coherent structures and their transitions; it shows robust dynamics by design, i.e., stable longtime behavior; and its development is fully automatable, i.e., it does not require tunable auxiliary closure and other models.
Trajectoryoptimized clusterbased network model for the sphere wake
10.1063/5.0098655
Physics of Fluids
20220808T10:28:32Z
© 2022 Author(s).
Bernd R. Noack

Direct numerical simulation of shockwave/boundary layer interaction controlled with convergent–divergent riblets
https://aip.scitation.org/doi/10.1063/5.0102261?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this paper, a section of convergent–divergent (C–D) riblets is applied upstream of a compression ramp in a supersonic turbulent boundary layer in a Mach 2.9 flow with a Reynolds number of [math]. Direct numerical simulations are undertaken to examine the impact of C–D riblets on the shock wave/boundary layer interaction and the feasibility of using them to mitigate flow separation. Over the riblet section, a largescale secondary roll mode is produced by C–D riblets with the downwelling motion occurring around the diverging region and upwelling motion near the converging region. This consequently leads to a spanwise heterogeneity in mean quantities and turbulent structures over the riblet section and also in the interaction zone. Compared with the baseline case, the area of the separation zone for the riblet case experiences a dramatic local reduction of 92% in the diverging region, owing to the downwelling motion that injects the highmomentum fluid toward the wall and the nearwall spanwise velocity that transports the lowmomentum fluid away. The enhanced upwelling motion around the converging region induced by C–D riblets, on the one hand, contributes to the decrease of the nearwall momentum and subsequently the increase of the local separation area. On the other hand, the upwelling motion effectively reduces the incoming Mach number upstream of the compression corner. This appears to reduce the strength of the separation shock, leading to a more gradual compression of the incoming flow that helps ease the enlargement of the separation area nearby. Overall, the area of the mean flow separation is reduced by 56%, indicating an effective flow separation control by the C–D riblets.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this paper, a section of convergent–divergent (C–D) riblets is applied upstream of a compression ramp in a supersonic turbulent boundary layer in a Mach 2.9 flow with a Reynolds number of [math]. Direct numerical simulations are undertaken to examine the impact of C–D riblets on the shock wave/boundary layer interaction and the feasibility of using them to mitigate flow separation. Over the riblet section, a largescale secondary roll mode is produced by C–D riblets with the downwelling motion occurring around the diverging region and upwelling motion near the converging region. This consequently leads to a spanwise heterogeneity in mean quantities and turbulent structures over the riblet section and also in the interaction zone. Compared with the baseline case, the area of the separation zone for the riblet case experiences a dramatic local reduction of 92% in the diverging region, owing to the downwelling motion that injects the highmomentum fluid toward the wall and the nearwall spanwise velocity that transports the lowmomentum fluid away. The enhanced upwelling motion around the converging region induced by C–D riblets, on the one hand, contributes to the decrease of the nearwall momentum and subsequently the increase of the local separation area. On the other hand, the upwelling motion effectively reduces the incoming Mach number upstream of the compression corner. This appears to reduce the strength of the separation shock, leading to a more gradual compression of the incoming flow that helps ease the enlargement of the separation area nearby. Overall, the area of the mean flow separation is reduced by 56%, indicating an effective flow separation control by the C–D riblets.
Direct numerical simulation of shockwave/boundary layer interaction controlled with convergent–divergent riblets
10.1063/5.0102261
Physics of Fluids
20220801T02:11:57Z
© 2022 Author(s).

Study on the mechanism of shock wave and boundary layer interaction control using highfrequency pulsed arc discharge plasma
https://aip.scitation.org/doi/10.1063/5.0095487?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>This paper studies the response characteristics of shock wave and boundary layer interaction (SWBLI) controlled by highfrequency pulsed arc discharge (PAD) in a Mach 2.5 flow. The dynamic evolution of SWBLI disturbed by arc plasma energy deposition was captured, and the controlling mechanism under different exciting power and frequency was explored. The results showed that the blast wave induced by PADs had a strong impact on SWBLI structures and distorted the separation shock wave. During the downstream propagation, the controlling gas bubbles (CGBs) delivered a continuous thermal excitation to the boundary layer and reached the maximum penetration depth near the semicylinder. The arc discharge in the SWBLI region induced larger energy deposition, which made the heating zone obtain the highest initial temperature and longest heating duration. Under the plasma condition of 1 × 1011 W/m3/15 kHz, both the upstream part of the shear layer and the foot portion of the reattachment shock wave were removed. When setting the excitation to 2.5 × 1010 W/m3/60 kHz, a thermal exciting surface of merged CGBs was formed and the separation shock wave was completely replaced by an equivalent compressionwave system. A better dragreduction effect on the flow field would be produced by the actuator with an increased operating power or frequency, and a drag reduction rate of nearly 25.5% was achieved under the 2.5 × 1010 W/m3/60 kHz control condition.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>This paper studies the response characteristics of shock wave and boundary layer interaction (SWBLI) controlled by highfrequency pulsed arc discharge (PAD) in a Mach 2.5 flow. The dynamic evolution of SWBLI disturbed by arc plasma energy deposition was captured, and the controlling mechanism under different exciting power and frequency was explored. The results showed that the blast wave induced by PADs had a strong impact on SWBLI structures and distorted the separation shock wave. During the downstream propagation, the controlling gas bubbles (CGBs) delivered a continuous thermal excitation to the boundary layer and reached the maximum penetration depth near the semicylinder. The arc discharge in the SWBLI region induced larger energy deposition, which made the heating zone obtain the highest initial temperature and longest heating duration. Under the plasma condition of 1 × 1011 W/m3/15 kHz, both the upstream part of the shear layer and the foot portion of the reattachment shock wave were removed. When setting the excitation to 2.5 × 1010 W/m3/60 kHz, a thermal exciting surface of merged CGBs was formed and the separation shock wave was completely replaced by an equivalent compressionwave system. A better dragreduction effect on the flow field would be produced by the actuator with an increased operating power or frequency, and a drag reduction rate of nearly 25.5% was achieved under the 2.5 × 1010 W/m3/60 kHz control condition.
Study on the mechanism of shock wave and boundary layer interaction control using highfrequency pulsed arc discharge plasma
10.1063/5.0095487
Physics of Fluids
20220808T10:28:28Z
© 2022 Author(s).

Spacestreamlinebased method of characteristics for inverse design of threedimensional super/hypersonic flows
https://aip.scitation.org/doi/10.1063/5.0098428?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>The inverse design of super/hypersonic flows is widely utilized in aerospace, especially in waveriders, inlets, and nozzles. However, most of the existing methods are intended for the twodimensional (2D) problem. The inverse method for generalized threedimensional (3D) supersonic flows is still immature and is the main purpose of the spacestreamlinebased method of characteristics (SMOC) presented in this paper. The key of SMOC is to integrate an additional Euler predictor–corrector algorithm for pressure gradients in the unit calculation process. In this way, the temporary orientation of the osculating plane (OP) of the space streamline is determined, and the conventional 2D axisymmetric method of characteristics can be adopted in the OP. Three common unit processes of SMOC and the posedness are introduced, and the astringency is demonstrated by corresponding algebraic calculations. With this method, inviscid super/hypersonic flows can be solved on the basis of specified flow features, such as a 3D shock surface or a 3D wall pressure distribution. The accuracy and efficiency of SMOC are verified by using an inverse design example, that is, the flow produced by an elliptic conical surface at a freestream Mach number of 6. The numerical simulation of the inverse design result indicates that the 3D shock wave geometry and the 3D wall pressure distribution match the targets completely. The relative rootmeansquared error of the surface geometry is 10−3 magnitude, and the computation time cost of the inverse design is less than that of the general direct Euler solver.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>The inverse design of super/hypersonic flows is widely utilized in aerospace, especially in waveriders, inlets, and nozzles. However, most of the existing methods are intended for the twodimensional (2D) problem. The inverse method for generalized threedimensional (3D) supersonic flows is still immature and is the main purpose of the spacestreamlinebased method of characteristics (SMOC) presented in this paper. The key of SMOC is to integrate an additional Euler predictor–corrector algorithm for pressure gradients in the unit calculation process. In this way, the temporary orientation of the osculating plane (OP) of the space streamline is determined, and the conventional 2D axisymmetric method of characteristics can be adopted in the OP. Three common unit processes of SMOC and the posedness are introduced, and the astringency is demonstrated by corresponding algebraic calculations. With this method, inviscid super/hypersonic flows can be solved on the basis of specified flow features, such as a 3D shock surface or a 3D wall pressure distribution. The accuracy and efficiency of SMOC are verified by using an inverse design example, that is, the flow produced by an elliptic conical surface at a freestream Mach number of 6. The numerical simulation of the inverse design result indicates that the 3D shock wave geometry and the 3D wall pressure distribution match the targets completely. The relative rootmeansquared error of the surface geometry is 10−3 magnitude, and the computation time cost of the inverse design is less than that of the general direct Euler solver.
Spacestreamlinebased method of characteristics for inverse design of threedimensional super/hypersonic flows
10.1063/5.0098428
Physics of Fluids
20220808T10:33:25Z
© 2022 Author(s).

Plasma flow control of the tip vortices over a very low aspectratio wing
https://aip.scitation.org/doi/10.1063/5.0101110?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Flow control of the tip vortices over a very low aspectratio wing was carried out using the dielectricbarrierdischarge plasma actuators. The results indicate a large change in the aerodynamic forces by plasma flow control, where the lift coefficient is increased by the blowing plasma actuator by 23% and is reduced by the suction plasma actuator by 30%. The change in the drag coefficient is less than 10%. The blowing plasma moves the tip vortex outboard away from the wing tip, increasing the streamwise vorticity as well as the turbulence intensities and the Reynolds stress. With the suction plasma, the tip vortex is shifted inboard closer to the wing tip. Coflowing with the tip vortex, the blowing plasma increases the tip vortex circulation, while it is reduced by the counterflowing suction plasma.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Flow control of the tip vortices over a very low aspectratio wing was carried out using the dielectricbarrierdischarge plasma actuators. The results indicate a large change in the aerodynamic forces by plasma flow control, where the lift coefficient is increased by the blowing plasma actuator by 23% and is reduced by the suction plasma actuator by 30%. The change in the drag coefficient is less than 10%. The blowing plasma moves the tip vortex outboard away from the wing tip, increasing the streamwise vorticity as well as the turbulence intensities and the Reynolds stress. With the suction plasma, the tip vortex is shifted inboard closer to the wing tip. Coflowing with the tip vortex, the blowing plasma increases the tip vortex circulation, while it is reduced by the counterflowing suction plasma.
Plasma flow control of the tip vortices over a very low aspectratio wing
10.1063/5.0101110
Physics of Fluids
20220801T02:11:51Z
© 2022 Author(s).
KwingSo Choi

Numerical investigation of flow around two cylinders in tandem above a scoured bed
https://aip.scitation.org/doi/10.1063/5.0098470?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Flow mechanisms around two cylinders in tandem arrangement above a scoured bed have been investigated using the threedimensional unsteady Navier–Stokes equations with the Spalart–Allmaras improved delayed detachededdy simulation model. A turbulent inlet boundary layer generation method is adopted to obtain more realistic inlet boundary conditions. First, uniform flow over a single cylinder at Re = 3900 and flow over a single cylinder above scoured beds at Re = 6000 were simulated to validate the numerical model, boundary layer generation method, and mesh density effect. Second, two cylinders in the tandem arrangement above scoured beds with six different pitch ratios L/D are investigated numerically in terms of instantaneous vortex characteristics, the hydrodynamic force, and timeaveraged flow fields. The simulation results in scoured beds are compared with simulations under the near flat wall and wallfree conditions. The major findings can be summarized as follows. (1) When [math], the wake of two tandem cylinders is dominated by the intermittent shedding, and the downstream sand dune in the scoured bed hinders the Kármán vortex formation at the rear of the downstream cylinder. Lift force fluctuations of the two cylinders have small amplitudes, and their spectra show a multipeak distribution and no dominant peak frequency in the spectrum of the upstream cylinder. A squarish cavitylike recirculation zone is formed between two cylinders at [math]. (2) When [math], the periodic vortex shedding is evident in the wake of the upstream cylinder, and the small sand berm between two cylinders has an impact on the bottom shear layer of the upstream cylinder. The downstream cylinder is periodically impacted by the vortices shed from the upstream cylinder. Lift force spectra of the upstream and downstream cylinders have the same peak frequency. (3) Due to the influence of scoured bed and the inlet boundary layer, the timeaveraged lift coefficient of the upstream cylinder remains negative when [math], and the critical spacing for drag inversion is relatively smaller compared with under wallfree conditions. The negative pressure coefficient values of the upstream cylinder are smaller than the values in near flat wall and wallfree conditions.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Flow mechanisms around two cylinders in tandem arrangement above a scoured bed have been investigated using the threedimensional unsteady Navier–Stokes equations with the Spalart–Allmaras improved delayed detachededdy simulation model. A turbulent inlet boundary layer generation method is adopted to obtain more realistic inlet boundary conditions. First, uniform flow over a single cylinder at Re = 3900 and flow over a single cylinder above scoured beds at Re = 6000 were simulated to validate the numerical model, boundary layer generation method, and mesh density effect. Second, two cylinders in the tandem arrangement above scoured beds with six different pitch ratios L/D are investigated numerically in terms of instantaneous vortex characteristics, the hydrodynamic force, and timeaveraged flow fields. The simulation results in scoured beds are compared with simulations under the near flat wall and wallfree conditions. The major findings can be summarized as follows. (1) When [math], the wake of two tandem cylinders is dominated by the intermittent shedding, and the downstream sand dune in the scoured bed hinders the Kármán vortex formation at the rear of the downstream cylinder. Lift force fluctuations of the two cylinders have small amplitudes, and their spectra show a multipeak distribution and no dominant peak frequency in the spectrum of the upstream cylinder. A squarish cavitylike recirculation zone is formed between two cylinders at [math]. (2) When [math], the periodic vortex shedding is evident in the wake of the upstream cylinder, and the small sand berm between two cylinders has an impact on the bottom shear layer of the upstream cylinder. The downstream cylinder is periodically impacted by the vortices shed from the upstream cylinder. Lift force spectra of the upstream and downstream cylinders have the same peak frequency. (3) Due to the influence of scoured bed and the inlet boundary layer, the timeaveraged lift coefficient of the upstream cylinder remains negative when [math], and the critical spacing for drag inversion is relatively smaller compared with under wallfree conditions. The negative pressure coefficient values of the upstream cylinder are smaller than the values in near flat wall and wallfree conditions.
Numerical investigation of flow around two cylinders in tandem above a scoured bed
10.1063/5.0098470
Physics of Fluids
20220801T02:11:34Z
© 2022 Author(s).
Kraposhin Matvey
Andrey Epikhin

Mechanism of the rotor−stator interaction in a centrifugal pump with guided vanes based on dynamic mode decomposition
https://aip.scitation.org/doi/10.1063/5.0098193?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>In this paper, the mechanism of the rotor–stator interaction in a centrifugal pump with guide vanes is studied numerically and theoretically. The dynamic mode decomposition method is employed to decouple and reconstruct the unsteady flow. A diametrical mode theory suitable for centrifugal pumps with guided vanes is proposed to determine the source of harmonics with higher amplitudes quickly. The results show that the dominant frequencies of the pressure pulsation in the volute and guide vanes are the blade passing frequency and its harmonic frequencies, and the corresponding flow structure is stable and has higher modal energy. The rotor–stator interaction effect around the impeller outlet is most pronounced. The potential flow effect works on the impeller and guide vanes but decays rapidly. The pressure pulsation caused by the wake effect propagates downstream and persists for long distances, which is the main reason for forming the modal pressure field in the volute. The modal reconstruction can reproduce the dynamic evolution process of the pressure field at the characteristic frequencies. The propagation characteristics of the modal pressure field in the volute can be accurately predicted by theoretical analysis. This research can provide an essential reference for fault diagnosis and vibration control of the centrifugal pump.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>In this paper, the mechanism of the rotor–stator interaction in a centrifugal pump with guide vanes is studied numerically and theoretically. The dynamic mode decomposition method is employed to decouple and reconstruct the unsteady flow. A diametrical mode theory suitable for centrifugal pumps with guided vanes is proposed to determine the source of harmonics with higher amplitudes quickly. The results show that the dominant frequencies of the pressure pulsation in the volute and guide vanes are the blade passing frequency and its harmonic frequencies, and the corresponding flow structure is stable and has higher modal energy. The rotor–stator interaction effect around the impeller outlet is most pronounced. The potential flow effect works on the impeller and guide vanes but decays rapidly. The pressure pulsation caused by the wake effect propagates downstream and persists for long distances, which is the main reason for forming the modal pressure field in the volute. The modal reconstruction can reproduce the dynamic evolution process of the pressure field at the characteristic frequencies. The propagation characteristics of the modal pressure field in the volute can be accurately predicted by theoretical analysis. This research can provide an essential reference for fault diagnosis and vibration control of the centrifugal pump.
Mechanism of the rotor−stator interaction in a centrifugal pump with guided vanes based on dynamic mode decomposition
10.1063/5.0098193
Physics of Fluids
20220802T01:38:14Z
© 2022 Author(s).

Modeling of the compartmentalization effect induced by leadingedge tubercles
https://aip.scitation.org/doi/10.1063/5.0098400?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>As a passive flow control technique, the use of leadingedge tubercles inspired by humpback whale flippers has attracted much interest. It is believed that one of the flow control mechanisms of leadingedge tubercles is compartmentalization, which is similar to the way in which wing fences act. However, to date, there has been no direct evidence for this belief. In view of this, the present work aims to verify and quantitatively describe the compartmentalization effect induced by leadingedge tubercles. Numerical simulation is performed to investigate the flow structures on a wavy airfoil with leadingedge tubercles, and the results reveal the presence of typical biperiodic flow patterns when a critical angle of attack is exceeded. Based on the flow characteristics of the wavy airfoil, special fences paired in a diverging configuration are designed and positioned on the baseline airfoil. A modeling method is developed to determine the main parameters of the fence configurations. It is found that the fenced airfoils designed using this method are able to reproduce the typical flow characteristics of the wavy airfoil under different inflow conditions. The spanwise distributions of the sectional airfoil performance under flow control by leadingedge tubercles and by the specially designed fences are very similar. A combined mechanism mainly including the liftingline theory and the compartmentalization theory is proposed to provide a more comprehensive picture of the flow dynamic of leadingedge tubercles. This work provides strong evidence to confirm the compartmentalization mechanism of action of leadingedge tubercles, as well as developing a quantitative modeling method, both of which are important for fully understanding the underlying mechanism and guiding further optimization of this passive flow control technique.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>As a passive flow control technique, the use of leadingedge tubercles inspired by humpback whale flippers has attracted much interest. It is believed that one of the flow control mechanisms of leadingedge tubercles is compartmentalization, which is similar to the way in which wing fences act. However, to date, there has been no direct evidence for this belief. In view of this, the present work aims to verify and quantitatively describe the compartmentalization effect induced by leadingedge tubercles. Numerical simulation is performed to investigate the flow structures on a wavy airfoil with leadingedge tubercles, and the results reveal the presence of typical biperiodic flow patterns when a critical angle of attack is exceeded. Based on the flow characteristics of the wavy airfoil, special fences paired in a diverging configuration are designed and positioned on the baseline airfoil. A modeling method is developed to determine the main parameters of the fence configurations. It is found that the fenced airfoils designed using this method are able to reproduce the typical flow characteristics of the wavy airfoil under different inflow conditions. The spanwise distributions of the sectional airfoil performance under flow control by leadingedge tubercles and by the specially designed fences are very similar. A combined mechanism mainly including the liftingline theory and the compartmentalization theory is proposed to provide a more comprehensive picture of the flow dynamic of leadingedge tubercles. This work provides strong evidence to confirm the compartmentalization mechanism of action of leadingedge tubercles, as well as developing a quantitative modeling method, both of which are important for fully understanding the underlying mechanism and guiding further optimization of this passive flow control technique.
Modeling of the compartmentalization effect induced by leadingedge tubercles
10.1063/5.0098400
Physics of Fluids
20220802T01:38:38Z
© 2022 Author(s).

Threedimensional rogue waves and dustacoustic dark soliton collisions in degenerate ultradense magnetoplasma in the presence of dust pressure anisotropy
https://aip.scitation.org/doi/10.1063/5.0096990?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>A threedimensional Thomas–Fermi dense anisotropic magnetized plasma having Fermi–Dirac distributed ions and electrons as well as classical fluid negative dust impurities is considered to analyze oblique modulational instability (MI) and headon collisions among dustacoustic dark solitons. The Chew–Golberger–Low description is employed to define the anisotropic dust pressure. The linear analysis is investigated. It is found that for larger wavelengths, the pressure anisotropy has a strong effect on the wave frequency. Following the multiscale reductive perturbation technique, a (3 + 1)dimensional nonlinear Schrödinger equation is derived. Also, the MI criterion is identified, and the regions of (un)stable modulated waves are determined precisely. In addition to that, (un)stable domains of the modulated structures as well as the profile of the dustacoustic rogue waves are found to be strongly affected by dust grain density, pressure anisotropy, and the strength of the magnetic field. In the stable regions, the facetoface dark soliton collision and their phase shifts as well as their analytical trajectories are reported by applying the extended Poincare–Lighthill–Kuo method. Numerical analysis reveals that the phase shifts increase with dust concentration but decrease with dust pressure anisotropy. The present results may be applicable in exploring the nonlinear wave dynamics and solitary wave interactions in dense astrophysical plasmas especially to white dwarfs, interiors of the neutron stars, and magnet stars.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>A threedimensional Thomas–Fermi dense anisotropic magnetized plasma having Fermi–Dirac distributed ions and electrons as well as classical fluid negative dust impurities is considered to analyze oblique modulational instability (MI) and headon collisions among dustacoustic dark solitons. The Chew–Golberger–Low description is employed to define the anisotropic dust pressure. The linear analysis is investigated. It is found that for larger wavelengths, the pressure anisotropy has a strong effect on the wave frequency. Following the multiscale reductive perturbation technique, a (3 + 1)dimensional nonlinear Schrödinger equation is derived. Also, the MI criterion is identified, and the regions of (un)stable modulated waves are determined precisely. In addition to that, (un)stable domains of the modulated structures as well as the profile of the dustacoustic rogue waves are found to be strongly affected by dust grain density, pressure anisotropy, and the strength of the magnetic field. In the stable regions, the facetoface dark soliton collision and their phase shifts as well as their analytical trajectories are reported by applying the extended Poincare–Lighthill–Kuo method. Numerical analysis reveals that the phase shifts increase with dust concentration but decrease with dust pressure anisotropy. The present results may be applicable in exploring the nonlinear wave dynamics and solitary wave interactions in dense astrophysical plasmas especially to white dwarfs, interiors of the neutron stars, and magnet stars.
Threedimensional rogue waves and dustacoustic dark soliton collisions in degenerate ultradense magnetoplasma in the presence of dust pressure anisotropy
10.1063/5.0096990
Physics of Fluids
20220804T11:52:21Z
© 2022 Author(s).
D. V. Douanla
C. G. L. Tiofack
Alim
M. Aboubakar
A. Mohamadou
Wedad Albalawi
S. A. ElTantawy
L. S. ElSherif

Transport properties for neutral C, H, N, O, and Sicontaining species and mixtures from the Gordon and McBride thermodynamic database
https://aip.scitation.org/doi/10.1063/5.0098060?af=R&feed=mostrecent
Physics of Fluids, <a href="https://aip.scitation.org/toc/phf/34/8">Volume 34, Issue 8</a>, August 2022. <br/>Accurate transport properties of nonionized gas mixtures of C, H, O, N, and Sicontaining species at temperatures up to 4000 K are essential in many scientific fields. Mixture transport properties are computed through the solution of linear transport systems, requiring collision integrals as functions of temperature for each binary collision pair in the mixture. Due to the dimensionality of the problem, no such database exists for all the 180 hydrocarbons and silicon species detailed in the ninecoefficient polynomial thermodynamic database of Gordon and McBride, widely used in many applications. This constraint was overcome by using a phenomenological intermolecular potential energy surface suitable for transport properties, which describes the pair interaction approximated with two fundamental species physical properties, namely the dipole electric polarizability and the number of effective electrons participating in the interaction. These two parameters were calculated with ab initio quantum chemistry calculations, since they were not always available in literature. The studied methodology was verified and validated against other approaches at a species and collision integral level. Transport properties for a variety of equilibrium mixtures, including planetary atmospheres and chemical compositions of thermal protection materials relevant to aerospace applications, were calculated, assessing the predictive capabilities of this new database.
Physics of Fluids, Volume 34, Issue 8, August 2022. <br/>Accurate transport properties of nonionized gas mixtures of C, H, O, N, and Sicontaining species at temperatures up to 4000 K are essential in many scientific fields. Mixture transport properties are computed through the solution of linear transport systems, requiring collision integrals as functions of temperature for each binary collision pair in the mixture. Due to the dimensionality of the problem, no such database exists for all the 180 hydrocarbons and silicon species detailed in the ninecoefficient polynomial thermodynamic database of Gordon and McBride, widely used in many applications. This constraint was overcome by using a phenomenological intermolecular potential energy surface suitable for transport properties, which describes the pair interaction approximated with two fundamental species physical properties, namely the dipole electric polarizability and the number of effective electrons participating in the interaction. These two parameters were calculated with ab initio quantum chemistry calculations, since they were not always available in literature. The studied methodology was verified and validated against other approaches at a species and collision integral level. Transport properties for a variety of equilibrium mixtures, including planetary atmospheres and chemical compositions of thermal protection materials relevant to aerospace applications, were calculated, assessing the predictive capabilities of this new database.
Transport properties for neutral C, H, N, O, and Sicontaining species and mixtures from the Gordon and McBride thermodynamic database
10.1063/5.0098060
Physics of Fluids
20220805T12:03:05Z
© 2022 Author(s).
Georgios Bellas Chatzigeorgis
Justin B. Haskins
James B. Scoggins