Pub Date : 2024-09-13DOI: 10.1103/physrevfluids.9.094004
Palas Kumar Farsoiya, Stéphane Popinet, Howard A. Stone, Luc Deike
Improved numerical methods are needed to understand the effect of surfactants in interfacial fluid mechanics, with various applications including thin films, inkjet printing, and ocean-atmosphere interactions. We provide a three-dimensional coupled volume of fluid (VoF) and phase field numerical approach to simulate the effects of insoluble surfactant-laden flows. The framework is validated against analytical cases for surfactant transport and Marangoni stresses. We then systematically investigate a single surfactant-laden rising bubble. The characteristics of a clean bubble rising in a quiescent liquid are governed by nondimensional numbers, i.e., the Galileo number , which compares inertial and viscous effects, and the Bond number , which compares gravitational and surface tension stresses. The effect of insoluble surfactants introduces an additional independent parameter, the Marangoni number , comparing the change in surface tension forces due to gradients in surfactants concentration with viscous forces. We apply our numerical methods to investigate the influence of surfactants (through the Marangoni number) on rising bubbles in otherwise quiescent fluids. We observe that an increase in the Marangoni number first decreases the rise velocity before reaching a limiting value at high . The value of necessary to observe a significant slowdown increases with . We discuss the associated surfactant accumulation and the vortical dynamics when a steady state is reached. Finally, we perform three-dimensional simulations and demonstrate that Marangoni effects can induce a change in the rise trajectory from spiraling to zigzagging for set values of Bo and Ga, consistent with experimental results.
需要改进数值方法来了解表面活性剂在界面流体力学中的影响，其应用领域包括薄膜、喷墨打印和海洋-大气相互作用。我们提供了一种三维耦合流体体积（VoF）和相场数值方法，用于模拟含有不溶性表面活性剂的流动的影响。根据表面活性剂传输和马兰戈尼应力的分析案例对该框架进行了验证。然后，我们系统地研究了单个含表面活性剂的上升气泡。在静止液体中上升的清洁气泡的特性受非量纲数的制约，即伽利略数 Ga（比较惯性效应和粘性效应）和邦德数 Bo（比较重力应力和表面张力应力）。不溶性表面活性剂的影响引入了一个额外的独立参数，即马兰戈尼数 Ma，用于比较表面活性剂浓度梯度引起的表面张力变化与粘性力。我们运用数值方法研究了表面活性剂（通过马兰戈尼数）对静止流体中上升气泡的影响。我们观察到，马兰戈尼数的增加首先会降低上升速度，然后在高 Ma 值时达到极限值。我们讨论了相关的表面活性剂积累以及达到稳定状态时的涡旋动力学。最后，我们进行了三维模拟，并证明马兰戈尼效应可以诱导上升轨迹从螺旋上升到设定 Bo 和 Ga 值的之字形上升，这与实验结果一致。
{"title":"Coupled volume of fluid and phase field method for direct numerical simulation of insoluble surfactant-laden interfacial flows and application to rising bubbles","authors":"Palas Kumar Farsoiya, Stéphane Popinet, Howard A. Stone, Luc Deike","doi":"10.1103/physrevfluids.9.094004","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094004","url":null,"abstract":"Improved numerical methods are needed to understand the effect of surfactants in interfacial fluid mechanics, with various applications including thin films, inkjet printing, and ocean-atmosphere interactions. We provide a three-dimensional coupled volume of fluid (VoF) and phase field numerical approach to simulate the effects of insoluble surfactant-laden flows. The framework is validated against analytical cases for surfactant transport and Marangoni stresses. We then systematically investigate a single surfactant-laden rising bubble. The characteristics of a clean bubble rising in a quiescent liquid are governed by nondimensional numbers, i.e., the Galileo number <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Ga</mtext></math>, which compares inertial and viscous effects, and the Bond number <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Bo</mtext></math>, which compares gravitational and surface tension stresses. The effect of insoluble surfactants introduces an additional independent parameter, the Marangoni number <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Ma</mtext></math>, comparing the change in surface tension forces due to gradients in surfactants concentration with viscous forces. We apply our numerical methods to investigate the influence of surfactants (through the Marangoni number) on rising bubbles in otherwise quiescent fluids. We observe that an increase in the Marangoni number first decreases the rise velocity before reaching a limiting value at high <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Ma</mtext></math>. The value of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Ma</mtext></math> necessary to observe a significant slowdown increases with <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Ga</mtext></math>. We discuss the associated surfactant accumulation and the vortical dynamics when a steady state is reached. Finally, we perform three-dimensional simulations and demonstrate that Marangoni effects can induce a change in the rise trajectory from spiraling to zigzagging for set values of Bo and Ga, consistent with experimental results.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142227825","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":null,"pages":null},"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":null,"pages":null},"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.9.094003
Chunheng Zhao, Taehun Lee, Andreas Carlson
We use the conservative phase-field lattice Boltzmann method to investigate the dynamics when a Newtonian droplet comes in contact with an immiscible viscoelastic liquid film. The dynamics of the three liquid phases are explored through numerical simulations, with a focus on illustrating the contact line dynamics and the viscoelastic effects described by the Oldroyd-B model. The droplet dynamics are contrasted with the case of a Newtonian fluid film. The simulations demonstrate that when the film is viscoelastic, the droplet dynamics become insensitive to the film thickness when the polymer viscosity and relaxation time are large. A viscoelastic ridge forms at the moving contact line, which evolves with a power-law dependence on time. By rescaling the interface profile of the ridge using its height and width, it appears to collapse onto a similar shape. Our findings reveal a strong correlation between the viscoelastic stress and the interface shape near the contact line.
{"title":"Spreading and engulfment of a viscoelastic film onto a Newtonian droplet","authors":"Chunheng Zhao, Taehun Lee, Andreas Carlson","doi":"10.1103/physrevfluids.9.094003","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094003","url":null,"abstract":"We use the conservative phase-field lattice Boltzmann method to investigate the dynamics when a Newtonian droplet comes in contact with an immiscible viscoelastic liquid film. The dynamics of the three liquid phases are explored through numerical simulations, with a focus on illustrating the contact line dynamics and the viscoelastic effects described by the Oldroyd-B model. The droplet dynamics are contrasted with the case of a Newtonian fluid film. The simulations demonstrate that when the film is viscoelastic, the droplet dynamics become insensitive to the film thickness when the polymer viscosity and relaxation time are large. A viscoelastic ridge forms at the moving contact line, which evolves with a power-law dependence on time. By rescaling the interface profile of the ridge using its height and width, it appears to collapse onto a similar shape. Our findings reveal a strong correlation between the viscoelastic stress and the interface shape near the contact line.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210883","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.9.094802
Raphael Stuhlmeier, Conor Heffernan, Alberto Alberello, Emilian Părău
This paper sets out to explore the modulational (or Benjamin-Feir) instability of a monochromatic wave propagating in the presence of damping such as that induced by sea ice on the ocean surface. The fundamental wave motion is modelled using the spatial Zakharov equation, to which either uniform or nonuniform (frequency-dependent) damping is added. By means of mode truncation the spatial analog of the classical Benjamin-Feir instability can be studied analytically using dynamical systems techniques. The formulation readily yields the free surface and its envelope, giving insight into the physical implications of damping on the modulational instability. The evolution of an initially unstable mode is also studied numerically by integrating the damped, spatial Zakharov equation, in order to complement the analytical theory. This sheds light on the effects of damping on spectral broadening arising from this instability.
{"title":"Modulational instability of nonuniformly damped, broad-banded waves: Applications to waves in sea ice","authors":"Raphael Stuhlmeier, Conor Heffernan, Alberto Alberello, Emilian Părău","doi":"10.1103/physrevfluids.9.094802","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094802","url":null,"abstract":"This paper sets out to explore the modulational (or Benjamin-Feir) instability of a monochromatic wave propagating in the presence of damping such as that induced by sea ice on the ocean surface. The fundamental wave motion is modelled using the spatial Zakharov equation, to which either uniform or nonuniform (frequency-dependent) damping is added. By means of mode truncation the spatial analog of the classical Benjamin-Feir instability can be studied analytically using dynamical systems techniques. The formulation readily yields the free surface and its envelope, giving insight into the physical implications of damping on the modulational instability. The evolution of an initially unstable mode is also studied numerically by integrating the damped, spatial Zakharov equation, in order to complement the analytical theory. This sheds light on the effects of damping on spectral broadening arising from this instability.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210887","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":null,"pages":null},"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":null,"pages":null},"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}
Pub Date : 2024-09-09DOI: 10.1103/physrevfluids.9.l091601
Kenneth R. Langley, Tariq Alghamdi, Andres A. Aguirre-Pablo, Nathan B. Speirs, S. T. Thoroddsen, Peter Taborek
High-speed videos in an optical cryostat, with frame rates up to fps, are used to study the dynamics of laser-induced cavitation in helium near the critical point and in the supercritical region. The propagation of strong shock waves are observed in both regimes. The time dependence of the cavitation bubble radius as well as the acoustic pressure field outside the bubble are described by standard compressible flow models. In the temperature range , a symmetric cloud of micron-scale bubbles are observed outside the main cavitation bubble as it approaches its maximum radius which is due to homogeneous nucleation and spinodal decomposition in the low-pressure fluid outside the bubble. Nucleation of secondary bubbles is also observed far below the critical point, but this requires large negative pressures that can be generated by shock waves that reflect from the primary bubble.
{"title":"Laser-induced cavitation in liquid He4 near the liquid-vapor critical point","authors":"Kenneth R. Langley, Tariq Alghamdi, Andres A. Aguirre-Pablo, Nathan B. Speirs, S. T. Thoroddsen, Peter Taborek","doi":"10.1103/physrevfluids.9.l091601","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.l091601","url":null,"abstract":"High-speed videos in an optical cryostat, with frame rates up to <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>5</mn><mo>×</mo><msup><mn>10</mn><mn>6</mn></msup></mrow></math> fps, are used to study the dynamics of laser-induced cavitation in helium near the critical point and in the supercritical region. The propagation of strong shock waves are observed in both regimes. The time dependence of the cavitation bubble radius as well as the acoustic pressure field outside the bubble are described by standard compressible flow models. In the temperature range <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>4</mn><mi>K</mi><mo><</mo><mi>T</mi><mo><</mo><mn>5.2</mn><mi>K</mi></mrow></math>, a symmetric cloud of micron-scale bubbles are observed outside the main cavitation bubble as it approaches its maximum radius which is due to homogeneous nucleation and spinodal decomposition in the low-pressure fluid outside the bubble. Nucleation of secondary bubbles is also observed far below the critical point, but this requires large negative pressures that can be generated by shock waves that reflect from the primary bubble.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-09-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210885","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":null,"pages":null},"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}
Pub Date : 2024-09-05DOI: 10.1103/physrevfluids.9.094301
Harkirat Singh, David L. Henann
Many dense granular systems are non-monodisperse, consisting of particles of different sizes, and will segregate based on size during flow. This phenomenon is an important aspect of many industrial and geophysical processes, necessitating predictive continuum models. This paper systematically studies a key aspect of the three-dimensional nature of segregation and diffusion in flowing, dense, bidisperse granular mixtures—namely, segregation and diffusion acting along the direction perpendicular to the plane of shearing, which we refer to as the anti-plane modes of segregation and diffusion. To this end, we consider discrete-element method (DEM) simulations of flows of dense, bidisperse mixtures of frictional spheres in an idealized configuration that isolates anti-plane segregation and diffusion. We find that previously developed constitutive equations, calibrated to DEM simulation results from flows in which both the segregation and diffusion processes occur within the plane of shearing, do not capture aspects of the anti-plane segregation dynamics. Accordingly, we utilize DEM simulation results to inform and calibrate constitutive equations for the segregation and diffusion fluxes in their anti-plane modes. Predictions of the resulting continuum model for the anti-plane segregation dynamics are tested against additional DEM simulation results across different cases, while parameters such as the shear strain rate and mixture composition are varied, and we find that the calibrated model predictions match well with the DEM simulation results. Finally, we suggest a strategy for generalizing the constitutive forms for the segregation and diffusion fluxes to obtain three-dimensional constitutive equations that account for both the in-plane and the anti-plane modes of the segregation and diffusion processes.
许多致密颗粒系统都是非单分散的，由不同大小的颗粒组成，在流动过程中会根据大小发生分离。这种现象是许多工业和地球物理过程的一个重要方面，需要建立预测性连续模型。本文系统地研究了流动、致密、双分散粒状混合物中偏析和扩散三维性质的一个关键方面，即沿垂直于剪切平面方向的偏析和扩散作用，我们将其称为偏析和扩散的反平面模式。为此，我们采用离散元素法（DEM）模拟了摩擦球双向分散的致密混合物在理想化配置下的流动，该配置隔离了反平面偏析和扩散。我们发现，根据 DEM 模拟结果校准的先前开发的构成方程无法捕捉到反平面偏析动力学的各个方面，而偏析和扩散过程均发生在剪切平面内。因此，我们利用 DEM 模拟结果，为反面模式的偏析和扩散通量提供信息，并校准其构成方程。在改变剪切应变率和混合物成分等参数的同时，我们还根据不同情况下的其他 DEM 模拟结果，测试了由此产生的反面偏析动力学连续模型的预测结果，结果发现校准后的模型预测结果与 DEM 模拟结果非常吻合。最后，我们提出了一种对偏析和扩散通量的构成形式进行概括的策略，以获得同时考虑偏析和扩散过程的平面内和反面模式的三维构成方程。
{"title":"Anti-plane segregation and diffusion in dense, bidisperse granular shear flow","authors":"Harkirat Singh, David L. Henann","doi":"10.1103/physrevfluids.9.094301","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.094301","url":null,"abstract":"Many dense granular systems are non-monodisperse, consisting of particles of different sizes, and will segregate based on size during flow. This phenomenon is an important aspect of many industrial and geophysical processes, necessitating predictive continuum models. This paper systematically studies a key aspect of the three-dimensional nature of segregation and diffusion in flowing, dense, bidisperse granular mixtures—namely, segregation and diffusion acting along the direction perpendicular to the plane of shearing, which we refer to as the anti-plane modes of segregation and diffusion. To this end, we consider discrete-element method (DEM) simulations of flows of dense, bidisperse mixtures of frictional spheres in an idealized configuration that isolates anti-plane segregation and diffusion. We find that previously developed constitutive equations, calibrated to DEM simulation results from flows in which both the segregation and diffusion processes occur within the plane of shearing, do not capture aspects of the anti-plane segregation dynamics. Accordingly, we utilize DEM simulation results to inform and calibrate constitutive equations for the segregation and diffusion fluxes in their anti-plane modes. Predictions of the resulting continuum model for the anti-plane segregation dynamics are tested against additional DEM simulation results across different cases, while parameters such as the shear strain rate and mixture composition are varied, and we find that the calibrated model predictions match well with the DEM simulation results. Finally, we suggest a strategy for generalizing the constitutive forms for the segregation and diffusion fluxes to obtain three-dimensional constitutive equations that account for both the in-plane and the anti-plane modes of the segregation and diffusion processes.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-09-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210860","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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