Pub Date : 2024-09-19DOI: 10.1103/physrevfluids.9.094803
Z. Taebel, M. L. McAllister, A. Scotti, M. Onorato, T. S. van den Bremer
The statistical treatment of random weakly nonlinear interactions between waves, called wave turbulence (WT), is fundamental to understanding the development of the ocean surface. For gravity waves, wave turbulence predicts a dual (direct and inverse) cascade of energy and wave action, which yield power-law solutions for the energy spectrum. While energy cascades were predicted more than 50 years ago, observing them in the laboratory with mechanical forcing remains a challenge. Here, we present experiments in which we attempted to reproduce both direct and inverse cascades in a large circular wave tank. The geometry of the wave tank allows for the creation of isotropically spread surface waves, which is an assumption that underlies WT theory. Although we did see evidence of a direct cascade of energy, we did not observe an inverse cascade of wave action. We discuss the competing effects of dissipation and intermittency, which may dominate or obscure the weakly nonlinear dynamics.
{"title":"Laboratory study of wave turbulence under isotropic forcing","authors":"Z. Taebel, M. L. McAllister, A. Scotti, M. Onorato, T. S. van den Bremer","doi":"10.1103/physrevfluids.9.094803","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094803","url":null,"abstract":"The statistical treatment of random weakly nonlinear interactions between waves, called wave turbulence (WT), is fundamental to understanding the development of the ocean surface. For gravity waves, wave turbulence predicts a dual (direct and inverse) cascade of energy and wave action, which yield power-law solutions for the energy spectrum. While energy cascades were predicted more than 50 years ago, observing them in the laboratory with mechanical forcing remains a challenge. Here, we present experiments in which we attempted to reproduce both direct and inverse cascades in a large circular wave tank. The geometry of the wave tank allows for the creation of isotropically spread surface waves, which is an assumption that underlies WT theory. Although we did see evidence of a direct cascade of energy, we did not observe an inverse cascade of wave action. We discuss the competing effects of dissipation and intermittency, which may dominate or obscure the weakly nonlinear dynamics.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"26 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142251968","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-19DOI: 10.1103/physrevfluids.9.093604
Yuxue Zhong, Jingzhu Wang, Jianlin Huang, Yiwei Wang
The collapse of a spark-induced bubble results in a high-speed jet, and when that jet impacts an elastic membrane, the latter deforms considerably and is accompanied by secondary cavitation. The cavitation bubble then moves away from the membrane with subsequent oscillation. To study the mechanism for this cavitation, experiments involving diffuse light and particle image velocimetry were conducted to capture the behaviors of the cavitation bubble and flow field resulting from the rapid deformation of the membrane. Analyzing the velocity and pressure fields reveals secondary cavitation induced by a sudden acceleration rather than a large velocity. Secondary cavitation occurs when the dimensionless inertial force overcomes the dimensionless pressure difference required for cavitation; subjecting these dimensionless quantities to a parametric study shows that the secondary cavitation occurs when the former is greater than the latter. During the collapse of the cavitation bubble, a thin jet points toward the membrane with an impact velocity of approximately 35 m/s. The subsequent oscillation of the cavitation bubble results in vortex flow moving away from the membrane. These findings provide insights into engineering applications such as bioengineering and mixing.
{"title":"Cavitation caused by an elastic membrane deforming under the jetting of a spark-induced bubble","authors":"Yuxue Zhong, Jingzhu Wang, Jianlin Huang, Yiwei Wang","doi":"10.1103/physrevfluids.9.093604","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.093604","url":null,"abstract":"The collapse of a spark-induced bubble results in a high-speed jet, and when that jet impacts an elastic membrane, the latter deforms considerably and is accompanied by secondary cavitation. The cavitation bubble then moves away from the membrane with subsequent oscillation. To study the mechanism for this cavitation, experiments involving diffuse light and particle image velocimetry were conducted to capture the behaviors of the cavitation bubble and flow field resulting from the rapid deformation of the membrane. Analyzing the velocity and pressure fields reveals secondary cavitation induced by a sudden acceleration rather than a large velocity. Secondary cavitation occurs when the dimensionless inertial force overcomes the dimensionless pressure difference required for cavitation; subjecting these dimensionless quantities to a parametric study shows that the secondary cavitation occurs when the former is greater than the latter. During the collapse of the cavitation bubble, a thin jet points toward the membrane with an impact velocity of approximately 35 m/s. The subsequent oscillation of the cavitation bubble results in vortex flow moving away from the membrane. These findings provide insights into engineering applications such as bioengineering and mixing.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"5 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142251967","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-17DOI: 10.1103/physrevfluids.9.093603
Kyle I. McKee, Bauyrzhan K. Primkulov, Kotaro Hashimoto, Yoshiyuki Tagawa, John W. M. Bush
In recent experiments, [E. Sawaguchi et al., J. Fluid Mech.862, 261 (2019)] directly probed the lubrication layer of air beneath a droplet levitating inside a rotating cylindrical drum. For small rotation rates of the drum, the lubrication film beneath the drop adopted a steady shape, while at higher rotation rates, traveling waves propagated along the drop's lower surface with roughly half the wall velocity. Here, we rationalize the physical origin of these waves. We begin with a simplified model of the lubrication flow beneath the droplet, and examine the linear stability of this base state to perturbations of the Tollmien-Schlichting type. Our developments lead to the Orr-Sommerfeld equation (OSE), whose eigenvalues give the growth rates and phase speeds of the perturbations. By considering wavelengths long relative to the lubrication film thickness, we solve the OSE perturbatively and so deduce the wavelength and phase velocity of the most unstable mode. We find satisfactory agreement between experiment and theory over the parameter regime considered in the laboratory.
{"title":"Waves beneath a drop levitating over a moving wall","authors":"Kyle I. McKee, Bauyrzhan K. Primkulov, Kotaro Hashimoto, Yoshiyuki Tagawa, John W. M. Bush","doi":"10.1103/physrevfluids.9.093603","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.093603","url":null,"abstract":"In recent experiments, [E. Sawaguchi <i>et al.</i>, <span>J. Fluid Mech.</span> <b>862</b>, 261 (2019)] directly probed the lubrication layer of air beneath a droplet levitating inside a rotating cylindrical drum. For small rotation rates of the drum, the lubrication film beneath the drop adopted a steady shape, while at higher rotation rates, traveling waves propagated along the drop's lower surface with roughly half the wall velocity. Here, we rationalize the physical origin of these waves. We begin with a simplified model of the lubrication flow beneath the droplet, and examine the linear stability of this base state to perturbations of the Tollmien-Schlichting type. Our developments lead to the Orr-Sommerfeld equation (OSE), whose eigenvalues give the growth rates and phase speeds of the perturbations. By considering wavelengths long relative to the lubrication film thickness, we solve the OSE perturbatively and so deduce the wavelength and phase velocity of the most unstable mode. We find satisfactory agreement between experiment and theory over the parameter regime considered in the laboratory.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"36 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142268723","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-16DOI: 10.1103/physrevfluids.9.093602
Huiyong Feng, Haibo Huang, Jian Hou, Chao Li, Bei Wei
This work explores the variation of viscosity of capsule suspension during the process of capsule rupture and polymer release using the immersed-boundary lattice Boltzmann method. The variation of viscosity is classified into three stages in the rupture process: the deformation stage, the rupture stage, and the stable stage. In the process of polymer release, two new stages of the variation of viscosity emerge: the diffusion stage and the dilution stage. Furthermore, the influence of viscosity ratio () on the viscosity is investigated. We find that the effective viscosity grows with and approaches the solid particle limit for very large , reflecting a similar behavior in the capsule shape. Finally, an available law that relates suspension viscosity to , capillary number (), and volume fraction () is established. The findings of this research have potential applications in fields such as oil exploration and capsule transportation.
{"title":"Viscosity of capsule suspensions: Effects of internal-external viscosity ratio and capsule rupture release","authors":"Huiyong Feng, Haibo Huang, Jian Hou, Chao Li, Bei Wei","doi":"10.1103/physrevfluids.9.093602","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.093602","url":null,"abstract":"This work explores the variation of viscosity of capsule suspension during the process of capsule rupture and polymer release using the immersed-boundary lattice Boltzmann method. The variation of viscosity is classified into three stages in the rupture process: the deformation stage, the rupture stage, and the stable stage. In the process of polymer release, two new stages of the variation of viscosity emerge: the diffusion stage and the dilution stage. Furthermore, the influence of viscosity ratio (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>λ</mi></math>) on the viscosity is investigated. We find that the effective viscosity grows with <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>λ</mi></math> and approaches the solid particle limit for very large <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>λ</mi></math>, reflecting a similar behavior in the capsule shape. Finally, an available law that relates suspension viscosity to <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>λ</mi></math>, capillary number (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>Ca</mi></math>), and volume fraction (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>ϕ</mi></math>) is established. The findings of this research have potential applications in fields such as oil exploration and capsule transportation.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"7 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142251969","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-16DOI: 10.1103/physrevfluids.9.093903
Basheer A. Khan, Shai Arogeti, Alexander Yakhot
The crisis (or critical) Reynolds number () is established at 1870, describing the threshold beyond which the lifetimes of turbulent puffs prior to the relaminarization extend from time units (), where and denote the pipe diameter and mean velocity, respectively. To analyze the role of inplane motion for sustaining turbulence, fully resolved direct numerical simulations have been performed to generate a localized, equilibrium turbulent puff at . Employing our approach based on proper orthogonal decomposition, the research confirms that azimuthal motion significantly contributes to the transition to turbulence. Notably, at supercritical Reynolds numbers () ranging from to , reducing azimuthal motion energy by 80% substantially shortens the lifetime of turbulent puffs. It has been shown that the relaminarization of turbulent puffs at subcritical Reynolds numbers, , clearly implies an exponential time decay of turbulence energy. The expression for the decay rate was obtained as a best-fit curve of direct numerical simulations.
危机（或临界）雷诺数（Rec）确定为 1870，描述了湍流涌在再层流化之前的寿命从 O(104)-O(106) 个时间单位（D/Um）（其中 D 和 Um 分别表示管道直径和平均速度）所超过的临界值。为了分析平面内运动对维持湍流的作用，我们进行了完全解析的直接数值模拟，以产生 Re=1920 的局部平衡湍流泡。采用我们基于适当正交分解的方法，研究证实方位运动对湍流的过渡有显著作用。值得注意的是，在 Re=1920 到 Re=2100 的超临界雷诺数（Re>Rec）范围内，减少 80% 的方位角运动能量会大大缩短湍流泡的寿命。研究表明，在次临界雷诺数（Re=1720-1840）下，湍流泡的再层流化明显意味着湍流能量的指数时间衰减。衰减率的表达式是通过直接数值模拟得到的最佳拟合曲线。
{"title":"Examination of the onset and decay of turbulence in pipe flow","authors":"Basheer A. Khan, Shai Arogeti, Alexander Yakhot","doi":"10.1103/physrevfluids.9.093903","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.093903","url":null,"abstract":"The crisis (or critical) Reynolds number (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mtext>Re</mtext><mi>c</mi></msub></math>) is established at 1870, describing the threshold beyond which the lifetimes of turbulent puffs prior to the relaminarization extend from <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mi>O</mi><mrow><mo>(</mo><msup><mn>10</mn><mn>4</mn></msup><mo>)</mo></mrow><mspace width=\"0.16em\"></mspace><mtext>to</mtext><mspace width=\"0.16em\"></mspace><mi>O</mi><mrow><mo>(</mo><msup><mn>10</mn><mn>6</mn></msup><mo>)</mo></mrow></mrow></math> time units (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mi>D</mi><mo>/</mo><msub><mi>U</mi><mi>m</mi></msub></mrow></math>), where <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>D</mi></math> and <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>U</mi><mi>m</mi></msub></math> denote the pipe diameter and mean velocity, respectively. To analyze the role of inplane motion for sustaining turbulence, fully resolved direct numerical simulations have been performed to generate a localized, equilibrium turbulent puff at <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Re</mtext><mo>=</mo><mn>1920</mn></mrow></math>. Employing our approach based on proper orthogonal decomposition, the research confirms that azimuthal motion significantly contributes to the transition to turbulence. Notably, at supercritical Reynolds numbers (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Re</mtext><mo>></mo><msub><mtext>Re</mtext><mi>c</mi></msub></mrow></math>) ranging from <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Re</mtext><mo>=</mo><mn>1920</mn></mrow></math> to <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Re</mtext><mo>=</mo><mn>2100</mn></mrow></math>, reducing azimuthal motion energy by 80% substantially shortens the lifetime of turbulent puffs. It has been shown that the relaminarization of turbulent puffs at subcritical Reynolds numbers, <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Re</mtext><mo>=</mo><mn>1720</mn><mtext>–</mtext><mn>1840</mn></mrow></math>, clearly implies an exponential time decay of turbulence energy. The expression for the decay rate was obtained as a best-fit curve of direct numerical simulations.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"14 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142268450","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-13DOI: 10.1103/physrevfluids.9.094604
Sukhdev Mouraya, Nandita Pan, Supratik Banerjee
The nonlinear transfer rate of the total energy (transfer rate of kinetic energy transfer rate due to the work done by the magnetization) for an incompressible turbulent ferrofluid system is studied under the assumption of statistical homogeneity. Using the formalism of the two-point correlators, an exact relation connecting the second-order statistical moments to the average energy injection rate is derived for the scale-to-scale transfer of the total energy. We validate the universality of the exact relation through direct numerical simulations for stationary and nonstationary cascade regimes. For a weak external magnetic field, both kinetic and the total energy cascade with nearly the same cascade rate. A stationary cascade regime is achieved, and hence a good agreement between the exact energy transfer rate and the average energy injection is found. Due to the rapid alignment of the ferrofluid particles in the presence of strong external fields, the turbulence dynamics becomes nonstationary. Interestingly, there too, both kinetic and the total energy exhibit inertial range cascades but with different cascade rates which can be explained using the nonstationary form of our derived exact relation.
{"title":"Stationary and nonstationary energy cascades in homogeneous ferrofluid turbulence","authors":"Sukhdev Mouraya, Nandita Pan, Supratik Banerjee","doi":"10.1103/physrevfluids.9.094604","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094604","url":null,"abstract":"The nonlinear transfer rate of the total energy (transfer rate of kinetic energy <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mo>+</mo></math> transfer rate due to the work done by the magnetization) for an incompressible turbulent ferrofluid system is studied under the assumption of statistical homogeneity. Using the formalism of the two-point correlators, an exact relation connecting the second-order statistical moments to the average energy injection rate is derived for the scale-to-scale transfer of the total energy. We validate the universality of the exact relation through direct numerical simulations for stationary and nonstationary cascade regimes. For a weak external magnetic field, both kinetic and the total energy cascade with nearly the same cascade rate. A stationary cascade regime is achieved, and hence a good agreement between the exact energy transfer rate and the average energy injection is found. Due to the rapid alignment of the ferrofluid particles in the presence of strong external fields, the turbulence dynamics becomes nonstationary. Interestingly, there too, both kinetic and the total energy exhibit inertial range cascades but with different cascade rates which can be explained using the nonstationary form of our derived exact relation.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"49 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210889","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-13DOI: 10.1103/physrevfluids.9.094605
A. Mohamed, A. Delache, F. S. Godeferd, J. Liu, M. Oberlack, Y. Wang
We study the propagation of inertial waves (IWs) generated by an axisymmetric torus oscillating at frequency in a rotating fluid. Inertial waves are emitted from the torus and propagate at an angle that depends on the ratio of the rotation frequency of the fluid to the forcing frequency of the torus. The waves focus in a neighborhood of the apex of the propagation cone. Using direct numerical simulations, we characterize the flow in this region, within a linear approximation or in the regime where nonlinear interactions between waves produce a turbulent patch. Forcing by the torus is modeled in two ways. The first model represents the effect of the oscillating torus as a local volume force in the form of a Dirac delta function, called the Dirac ring. The second approach aims at a more realistic three-dimensional model of a torus represented by a volume penalization technique. We observe the appearance of a mean flow composed of a central vortex produced by the nonlinear interaction of the IWs. We show that this phenomenon is in agreement with the theory of Davidson et al. [J. Fluid Mech.557, 135 (2006)] for a rotating fluid. Using Dirac ring forcing in the linear regime, we obtain the dependence on the propagation angle of the vertical kinetic energy at the focal point, which reaches a maximum for , in agreement with the linear theory developed by Liu et al. [Phys. Fluids34, 086601 (2022)]. A similar angle is observed in the 3D torus forcing case for both linear and nonlinear simulations: the angle maximizes the vertical velocity and dissipation, attesting an optimal energy transfer from the oscillating source to the focal region. In the nonlinear regime, we obtain the detailed spectral distribution of the kinetic energy in the focal zone, and we develop a spatiotemporal analysis of the velocity field that shows a wide presence of IWs in the flow. Moreover, we identify triadic resonances of IWs that lead to the production of the turbulent patch and of a large-scale mode similar to the geostrophic mean flow.
{"title":"Maximization of inertial waves focusing in linear and nonlinear regimes","authors":"A. Mohamed, A. Delache, F. S. Godeferd, J. Liu, M. Oberlack, Y. Wang","doi":"10.1103/physrevfluids.9.094605","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094605","url":null,"abstract":"We study the propagation of inertial waves (IWs) generated by an axisymmetric torus oscillating at frequency <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>ω</mi><mi>f</mi></msub></math> in a rotating fluid. Inertial waves are emitted from the torus and propagate at an angle <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>θ</mi><mi>f</mi></msub></math> that depends on the ratio of the rotation frequency of the fluid to the forcing frequency of the torus. The waves focus in a neighborhood of the apex of the propagation cone. Using direct numerical simulations, we characterize the flow in this region, within a linear approximation or in the regime where nonlinear interactions between waves produce a turbulent patch. Forcing by the torus is modeled in two ways. The first model represents the effect of the oscillating torus as a local volume force in the form of a Dirac delta function, called the Dirac ring. The second approach aims at a more realistic three-dimensional model of a torus represented by a volume penalization technique. We observe the appearance of a mean flow composed of a central vortex produced by the nonlinear interaction of the IWs. We show that this phenomenon is in agreement with the theory of Davidson <i>et al.</i> [<span>J. Fluid Mech.</span> <b>557</b>, 135 (2006)] for a rotating fluid. Using Dirac ring forcing in the linear regime, we obtain the dependence on the propagation angle of the vertical kinetic energy at the focal point, which reaches a maximum for <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><msub><mi>θ</mi><mi>f</mi></msub><mo>=</mo><msup><mn>35</mn><mo>∘</mo></msup></mrow></math>, in agreement with the linear theory developed by Liu <i>et al.</i> [<span>Phys. Fluids</span> <b>34</b>, 086601 (2022)]. A similar angle is observed in the 3D torus forcing case for both linear and nonlinear simulations: the angle <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><msub><mi>θ</mi><mi>f</mi></msub><mo>=</mo><msup><mn>30</mn><mo>∘</mo></msup></mrow></math> maximizes the vertical velocity and dissipation, attesting an optimal energy transfer from the oscillating source to the focal region. In the nonlinear regime, we obtain the detailed spectral distribution of the kinetic energy in the focal zone, and we develop a spatiotemporal analysis of the velocity field that shows a wide presence of IWs in the flow. Moreover, we identify triadic resonances of IWs that lead to the production of the turbulent patch and of a large-scale mode similar to the geostrophic mean flow.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"6 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210879","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-12DOI: 10.1103/physrevfluids.9.093902
Diederik Beckers, Jeff D. Eldredge
This study explores the application of deep reinforcement learning (RL) to design an airfoil pitch controller capable of minimizing lift variations in randomly disturbed flows. The controller, treated as an agent in a partially observable Markov decision process, receives non-Markovian observations from the environment, simulating practical constraints where flow information is limited to force and pressure sensors. Deep RL, particularly the TD3 algorithm, is used to approximate an optimal control policy under such conditions. Testing is conducted for a flat plate airfoil in two environments: a classical unsteady environment with vertical acceleration disturbances (i.e., a Wagner setup) and a viscous flow model with pulsed point force disturbances. In both cases, augmenting observations of the lift, pitch angle, and angular velocity with extra wake information (e.g., from pressure sensors) and retaining memory of past observations enhances RL control performance. Results demonstrate the capability of RL control to match or exceed standard linear controllers in minimizing lift variations. Special attention is given to the choice of training data and the generalization to unseen disturbances.
{"title":"Deep reinforcement learning of airfoil pitch control in a highly disturbed environment using partial observations","authors":"Diederik Beckers, Jeff D. Eldredge","doi":"10.1103/physrevfluids.9.093902","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.093902","url":null,"abstract":"This study explores the application of deep reinforcement learning (RL) to design an airfoil pitch controller capable of minimizing lift variations in randomly disturbed flows. The controller, treated as an agent in a partially observable Markov decision process, receives non-Markovian observations from the environment, simulating practical constraints where flow information is limited to force and pressure sensors. Deep RL, particularly the TD3 algorithm, is used to approximate an optimal control policy under such conditions. Testing is conducted for a flat plate airfoil in two environments: a classical unsteady environment with vertical acceleration disturbances (i.e., a Wagner setup) and a viscous flow model with pulsed point force disturbances. In both cases, augmenting observations of the lift, pitch angle, and angular velocity with extra wake information (e.g., from pressure sensors) and retaining memory of past observations enhances RL control performance. Results demonstrate the capability of RL control to match or exceed standard linear controllers in minimizing lift variations. Special attention is given to the choice of training data and the generalization to unseen disturbances.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"1 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210880","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-12DOI: 10.1103/physrevfluids.9.094002
Vincent Gourmandie, Juliette Pierre, Valentin Leroy, Caroline Derec
In this study, we systematically investigate the effect of surface tension on bubble entrapment after drop impact in the pinching regime. Experiments are conducted using three different systems: pure water, aqueous solutions with ethanol, or with surfactant molecules, both at various concentrations. Results are compiled for a large set of values of the surface tension and the drop impact velocity . Across all solutions, the cavity development dynamics exhibit similarity and are effectively characterized by dimensionless gravito-capillary parameters. Whatever the surface tension, our measurements indicate that only of the impact energy is converted into potential energy of the cavity. However, a notable distinction arises when considering bubble entrapment. We have constructed a bubbling diagram in the () plane, and observed that the conditions for bubble entrapment are altered with changing surface tension in water-ethanol mixtures. More intriguingly, these conditions are modified in a distinctly different manner for surfactant solutions. To interpret our experimental findings, we compile a comprehensive set of experimental and numerical results from the literature. We demonstrate the possibility of unifying results across all systems and our water-ethanol mixtures through an empirical law including the influence of surface tension and viscosity. Although no physical justification exists at this stage, this empirical law suggests the significant role of capillary waves traveling along the cavity interface in bubble entrapment. Within this context, the behavior of surfactant-laden solutions aligns with other homogeneous solutions by considering the elastic properties conferred upon the interfaces by surfactant molecules.
{"title":"Bubble entrapment by drop impact: Combined effect of surface tension and viscosity","authors":"Vincent Gourmandie, Juliette Pierre, Valentin Leroy, Caroline Derec","doi":"10.1103/physrevfluids.9.094002","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094002","url":null,"abstract":"In this study, we systematically investigate the effect of surface tension on bubble entrapment after drop impact in the pinching regime. Experiments are conducted using three different systems: pure water, aqueous solutions with ethanol, or with surfactant molecules, both at various concentrations. Results are compiled for a large set of values of the surface tension <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>γ</mi></math> and the drop impact velocity <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>U</mi></math>. Across all solutions, the cavity development dynamics exhibit similarity and are effectively characterized by dimensionless gravito-capillary parameters. Whatever the surface tension, our measurements indicate that only <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>40</mn><mo>%</mo></mrow></math> of the impact energy is converted into potential energy of the cavity. However, a notable distinction arises when considering bubble entrapment. We have constructed a <i>bubbling diagram</i> in the (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>U</mi><mo>,</mo><mi>γ</mi></math>) plane, and observed that the conditions for bubble entrapment are altered with changing surface tension in water-ethanol mixtures. More intriguingly, these conditions are modified in a distinctly different manner for surfactant solutions. To interpret our experimental findings, we compile a comprehensive set of experimental and numerical results from the literature. We demonstrate the possibility of unifying results across all systems and our water-ethanol mixtures through an empirical law including the influence of surface tension and viscosity. Although no physical justification exists at this stage, this empirical law suggests the significant role of capillary waves traveling along the cavity interface in bubble entrapment. Within this context, the behavior of surfactant-laden solutions aligns with other homogeneous solutions by considering the elastic properties conferred upon the interfaces by surfactant molecules.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"16 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210892","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We study the stability of gravity-driven viscous liquid films flowing down a vertical cylinder that is uniformly coated with a thin layer of elastic solids. Combining the gravity-driven viscous flows with the elastic deformation of the coated soft layer, we formulate a long-wave model to describe the evolution of a film flow-soft structure coupled system. Based on the model, we systematically examine the impact of the coating properties, including the elasticity and thickness on the temporal and spatiotemporal stability. Temporal stability analysis shows that the soft layer plays a dual role, namely, the elasticity acts as a destabilizing factor, leading to large deformations of both film interface and soft surface. However, due to the geometrical effect, increasing the layer thickness stabilizes the Rayleigh-Plateau instability. By contrast, the linear phase speed is always enhanced with increasing the elasticity or the thickness of the coated layer. We then analyze the spatiotemporal nature of free-surface instabilities and find that the elasticity can trigger the film flows from being absolutely unstable to convectively unstable. Transient numerical solutions of the full asymptotic model further verify the predictions from linear stability analysis, and more importantly, reveal the nonlinear effect of the softness. Compared to liquid films falling down the cylinder with rigid walls, the soft surface can enhance the coalescence of faster, larger sliding droplets with preceding slower, smaller sliding ones, thus resulting in a more unstable system. Our study highlights the potential of coating a thin layer of soft materials onto the walls of substrate to regulate the dynamics of liquid film systems, and may have implications for the emerging bioinspired applications; for instance, the large-scale collection and transport of water on flexible microfiber arrays.
{"title":"Stability of gravity-driven viscous films flowing down a soft cylinder","authors":"Youchuang Chao, Lailai Zhu, Zijing Ding, Tiantian Kong, Juntao Chang, Ziao Wang","doi":"10.1103/physrevfluids.9.094001","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094001","url":null,"abstract":"We study the stability of gravity-driven viscous liquid films flowing down a vertical cylinder that is uniformly coated with a thin layer of elastic solids. Combining the gravity-driven viscous flows with the elastic deformation of the coated soft layer, we formulate a long-wave model to describe the evolution of a film flow-soft structure coupled system. Based on the model, we systematically examine the impact of the coating properties, including the elasticity and thickness on the temporal and spatiotemporal stability. Temporal stability analysis shows that the soft layer plays a dual role, namely, the elasticity acts as a destabilizing factor, leading to large deformations of both film interface and soft surface. However, due to the geometrical effect, increasing the layer thickness stabilizes the Rayleigh-Plateau instability. By contrast, the linear phase speed is always enhanced with increasing the elasticity or the thickness of the coated layer. We then analyze the spatiotemporal nature of free-surface instabilities and find that the elasticity can trigger the film flows from being absolutely unstable to convectively unstable. Transient numerical solutions of the full asymptotic model further verify the predictions from linear stability analysis, and more importantly, reveal the nonlinear effect of the softness. Compared to liquid films falling down the cylinder with rigid walls, the soft surface can enhance the coalescence of faster, larger sliding droplets with preceding slower, smaller sliding ones, thus resulting in a more unstable system. Our study highlights the potential of coating a thin layer of soft materials onto the walls of substrate to regulate the dynamics of liquid film systems, and may have implications for the emerging bioinspired applications; for instance, the large-scale collection and transport of water on flexible microfiber arrays.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"6 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-09-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210884","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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