Pub Date : 2024-08-20DOI: 10.1103/physrevfluids.9.084302
Rui Zhu, Zhiguo He, Eckart Meiburg
We investigate the collapse of submerged cohesive granular columns as a function of their packing density and the cohesive force strength, via grain-resolving direct numerical simulations. The cohesive force acts to reduce the final runout distance of the collapsing columns. In addition, it significantly accelerates the initial contraction for loosely packed columns and decelerates the dilation for densely packed columns, leading to a larger or smaller excess pore pressure, respectively. Early on, the collapsing column exhibits distinct planar failure surfaces, whose angle with the horizontal increases with the packing density. We employ a network science-based approach to analyze the cohesive and contact force chains. Pronounced force-chain network structures form preferentially in the failure region. They tend to be larger for higher packing density, which induces a larger macroscopic cohesive resistance. The cohesive force tends to reduce the normal contact force, which results in shorter contact force chains.
{"title":"Effects of initial packing density and cohesion on submerged granular collapse","authors":"Rui Zhu, Zhiguo He, Eckart Meiburg","doi":"10.1103/physrevfluids.9.084302","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084302","url":null,"abstract":"We investigate the collapse of submerged cohesive granular columns as a function of their packing density and the cohesive force strength, via grain-resolving direct numerical simulations. The cohesive force acts to reduce the final runout distance of the collapsing columns. In addition, it significantly accelerates the initial contraction for loosely packed columns and decelerates the dilation for densely packed columns, leading to a larger or smaller excess pore pressure, respectively. Early on, the collapsing column exhibits distinct planar failure surfaces, whose angle with the horizontal increases with the packing density. We employ a network science-based approach to analyze the cohesive and contact force chains. Pronounced force-chain network structures form preferentially in the failure region. They tend to be larger for higher packing density, which induces a larger macroscopic cohesive resistance. The cohesive force tends to reduce the normal contact force, which results in shorter contact force chains.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"18 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210875","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 : 2024-08-19DOI: 10.1103/physrevfluids.9.l081501
M. Magnani, S. Musacchio, A. Provenzale, G. Boffetta
Permafrost thaw is a major concern raised by the ongoing climate change. An understudied phenomenon possibly affecting the pace of permafrost thaw is the onset of convective motions within the active layer caused by the density anomaly of water. Here we explore the effects of groundwater convection on permafrost thawing using a model that accounts for ice-water phase transitions, coupled with the dynamics of the temperature field transported by the Darcy's flow across a porous matrix. Numerical simulations of this model show that ice thawing in the presence of convection is much faster than in the diffusive case and deepens at a constant velocity proportional to the soil permeability. A scaling argument is able to predict correctly the asymptotic velocity. Since in the convective regime the heat transport is mediated by the coherent motion of thermal plumes across the thawed layer, we find that the depth of the thawing interface becomes highly heterogeneous.
{"title":"Convection in the active layer speeds up permafrost thaw in coarse-grained soils","authors":"M. Magnani, S. Musacchio, A. Provenzale, G. Boffetta","doi":"10.1103/physrevfluids.9.l081501","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.l081501","url":null,"abstract":"Permafrost thaw is a major concern raised by the ongoing climate change. An understudied phenomenon possibly affecting the pace of permafrost thaw is the onset of convective motions within the active layer caused by the density anomaly of water. Here we explore the effects of groundwater convection on permafrost thawing using a model that accounts for ice-water phase transitions, coupled with the dynamics of the temperature field transported by the Darcy's flow across a porous matrix. Numerical simulations of this model show that ice thawing in the presence of convection is much faster than in the diffusive case and deepens at a constant velocity proportional to the soil permeability. A scaling argument is able to predict correctly the asymptotic velocity. Since in the convective regime the heat transport is mediated by the coherent motion of thermal plumes across the thawed layer, we find that the depth of the thawing interface becomes highly heterogeneous.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"62 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210858","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 : 2024-08-19DOI: 10.1103/physrevfluids.9.083605
Dongdong Liu, Hongdong Yin, Zeyu Wu, Xiang Luo
Droplet impact on rotating surfaces experiences the tangential shear force from the rotating surface, generating a centrifugal force that either enhances the spreading or destabilizes the expanding lamella. In this study, we experimentally characterize the impact of a water droplet on rotating surfaces with various wettabilities, and theoretically analyze the observed impacting dynamics, including the enhanced spreading and the transition to the destabilization of the expanding lamella. Liquids with a wide range of viscosity are tested to explore the effect of liquid viscosity on the impacting dynamics. We propose a simplified approach to predict the tangential velocity induced by the surface's tangential shear force, and validate the predicted velocity by flow field measurement. We further deduce a quantitative description for the maximum spreading factor in the spreading regime, and derive the critical condition for the destabilization of the lamella for a water droplet. Good agreements are found between the predicted values and the measured ones for the impact on the rotating surfaces with various wettabilities.
{"title":"Droplet impact on rotating surfaces: The effect of centrifugal force and wettability on spreading dynamics","authors":"Dongdong Liu, Hongdong Yin, Zeyu Wu, Xiang Luo","doi":"10.1103/physrevfluids.9.083605","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083605","url":null,"abstract":"Droplet impact on rotating surfaces experiences the tangential shear force from the rotating surface, generating a centrifugal force that either enhances the spreading or destabilizes the expanding lamella. In this study, we experimentally characterize the impact of a water droplet on rotating surfaces with various wettabilities, and theoretically analyze the observed impacting dynamics, including the enhanced spreading and the transition to the destabilization of the expanding lamella. Liquids with a wide range of viscosity are tested to explore the effect of liquid viscosity on the impacting dynamics. We propose a simplified approach to predict the tangential velocity induced by the surface's tangential shear force, and validate the predicted velocity by flow field measurement. We further deduce a quantitative description for the maximum spreading factor in the spreading regime, and derive the critical condition for the destabilization of the lamella for a water droplet. Good agreements are found between the predicted values and the measured ones for the impact on the rotating surfaces with various wettabilities.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"16 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210857","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 : 2024-08-16DOI: 10.1103/physrevfluids.9.084101
Andrew J. Fox, Michael D. Graham
The dynamics of flexible filaments entrained in flow, important for understanding many biological and industrial processes, are computationally expensive to model with full physics simulations. In this paper, we describe a data-driven technique to create high-fidelity low-dimensional models of flexible fiber dynamics using machine learning; the technique is applied to sedimentation in a quiescent, viscous Newtonian fluid, using results from detailed simulations as the dataset. The approach combines an autoencoder neural network architecture to learn a low-dimensional latent representation of the filament shape, with a neural ordinary differential equation that learns the evolution of the particle in the latent state. The model was designed to model filaments of varying flexibility, characterized by an elastogravitational number , and was trained on a dataset containing the evolution of fibers beginning at set angles of inclination. For the range of considered here (100–10 000), the filament shape dynamics can be represented with high accuracy with only four degrees of freedom, in contrast with the 93 present in the original bead-spring model used to generate the dynamic trajectories. We predict the evolution of fibers set at arbitrary angles and demonstrate that our data-driven model can accurately forecast the evolution of a fiber at both trained and untrained elastogravitational numbers.
柔性细丝在流动过程中的动力学对理解许多生物和工业过程非常重要，但用完整的物理模拟来建立模型的计算成本很高。在本文中，我们介绍了一种利用机器学习创建高保真低维柔性纤维动力学模型的数据驱动技术；该技术被应用于静态粘性牛顿流体中的沉降，并将详细模拟的结果作为数据集。该方法将自动编码器神经网络架构与神经常微分方程相结合，前者用于学习长丝形状的低维潜在表示，后者用于学习颗粒在潜在状态下的演变。该模型设计用于模拟不同柔韧性的丝状物，以弹性重力数 B 为特征，并在包含从设定倾斜角度开始的纤维演变的数据集上进行了训练。在本文所考虑的 B 范围内（100-10000），只需四个自由度就能高精度地表示出纤维的形状动态，而用于生成动态轨迹的原始珠弹簧模型则需要 93 个自由度。我们预测了设定为任意角度的纤维的演变，并证明我们的数据驱动模型可以准确预测纤维在训练有素和未经训练的弹力数下的演变。
{"title":"Data-driven low-dimensional model of a sedimenting flexible fiber","authors":"Andrew J. Fox, Michael D. Graham","doi":"10.1103/physrevfluids.9.084101","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084101","url":null,"abstract":"The dynamics of flexible filaments entrained in flow, important for understanding many biological and industrial processes, are computationally expensive to model with full physics simulations. In this paper, we describe a data-driven technique to create high-fidelity low-dimensional models of flexible fiber dynamics using machine learning; the technique is applied to sedimentation in a quiescent, viscous Newtonian fluid, using results from detailed simulations as the dataset. The approach combines an autoencoder neural network architecture to learn a low-dimensional latent representation of the filament shape, with a neural ordinary differential equation that learns the evolution of the particle in the latent state. The model was designed to model filaments of varying flexibility, characterized by an elastogravitational number <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi mathvariant=\"script\">B</mi></math>, and was trained on a dataset containing the evolution of fibers beginning at set angles of inclination. For the range of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi mathvariant=\"script\">B</mi></math> considered here (100–10 000), the filament shape dynamics can be represented with high accuracy with only four degrees of freedom, in contrast with the 93 present in the original bead-spring model used to generate the dynamic trajectories. We predict the evolution of fibers set at arbitrary angles and demonstrate that our data-driven model can accurately forecast the evolution of a fiber at both trained and untrained elastogravitational numbers.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"30 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210873","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 : 2024-08-15DOI: 10.1103/physrevfluids.9.084607
Serge Mora, Martine Le Berre, Yves Pomeau
In flood events caused by a gradual increase in the flow rate of a watercourse, the rise in water level is often abrupt, while the fall in level is delayed. We show that such behavior can be demonstrated by considering stationary flows at a high Reynolds number in a prismatic open channel: Several geometries of the channel cross section lead to a subcritical instability that results in a discontinuous rise in the level when the flow rate exceeds a critical value , and in a fall, also discontinuous, when the flow rate returns below a value lower than . This hysteretic behavior originates from the interplay between gravity, which drives the flow downstream, and turbulent friction with the channel wall. The potential existence of several solutions arising from this bifurcation requires careful consideration in flood simulations.
在由河道流速逐渐增加引起的洪水事件中，水位的上升往往是突然的，而水位的下降则是延迟的。我们通过考虑棱柱形明渠中高雷诺数下的静止水流，证明了这种行为：水道横截面的几种几何形状会导致亚临界不稳定性，当流速超过临界值 Fi 时，水位会不连续地上升；当流速回到低于 Fi 的值 Fo 时，水位会不连续地下降。这种滞后行为源于推动水流向下游的重力和水流与河道壁的湍流摩擦力之间的相互作用。在洪水模拟中，需要仔细考虑这种分叉可能产生的几种解决方案。
{"title":"Flooding as a sub-critical instability in open channels","authors":"Serge Mora, Martine Le Berre, Yves Pomeau","doi":"10.1103/physrevfluids.9.084607","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084607","url":null,"abstract":"In flood events caused by a gradual increase in the flow rate of a watercourse, the rise in water level is often abrupt, while the fall in level is delayed. We show that such behavior can be demonstrated by considering stationary flows at a high Reynolds number in a prismatic open channel: Several geometries of the channel cross section lead to a subcritical instability that results in a discontinuous rise in the level when the flow rate exceeds a critical value <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>F</mi><mi>i</mi></msub></math>, and in a fall, also discontinuous, when the flow rate returns below a value <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>F</mi><mi mathvariant=\"script\">o</mi></msub></math> lower than <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>F</mi><mi>i</mi></msub></math>. This hysteretic behavior originates from the interplay between gravity, which drives the flow downstream, and turbulent friction with the channel wall. The potential existence of several solutions arising from this bifurcation requires careful consideration in flood simulations.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"100 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210863","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 : 2024-08-15DOI: 10.1103/physrevfluids.9.083604
S. Danial Naghib, Matin Mirbaha, Kristina Logushkova, Jérôme Bibette, Nicolas Bremond
Calibrated gel microspheres are used in several life-science applications, from embolization to DNA barcoding and drug delivery. Along with selecting or designing specific materials that depend on the application, various processes have been developed to produce such hydrogel particles. Here, we report a high throughput strategy that is based on the controlled fragmentation of an aqueous jet in air that results in droplets of monomer solution, their entry and collection in an oil bath, followed by polymerization of the emulsion droplets which thus turn into gel beads. Each step of the process is detailed and the operating conditions are optimized to obtain homogeneous polyacrylamide gel microspheres. The impact area of the stream of droplets at the free surface, that can be tuned with the help of an electric field, plays a major role in minimizing coalescence of droplets as well as mass transport between the dispersed phase and the continuous phase which is correlated to the sedimentation flow features of the dilute emulsion.
{"title":"Coupling atomization, emulsification, and polymerization steps for creating gel microspheres","authors":"S. Danial Naghib, Matin Mirbaha, Kristina Logushkova, Jérôme Bibette, Nicolas Bremond","doi":"10.1103/physrevfluids.9.083604","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083604","url":null,"abstract":"Calibrated gel microspheres are used in several life-science applications, from embolization to DNA barcoding and drug delivery. Along with selecting or designing specific materials that depend on the application, various processes have been developed to produce such hydrogel particles. Here, we report a high throughput strategy that is based on the controlled fragmentation of an aqueous jet in air that results in droplets of monomer solution, their entry and collection in an oil bath, followed by polymerization of the emulsion droplets which thus turn into gel beads. Each step of the process is detailed and the operating conditions are optimized to obtain homogeneous polyacrylamide gel microspheres. The impact area of the stream of droplets at the free surface, that can be tuned with the help of an electric field, plays a major role in minimizing coalescence of droplets as well as mass transport between the dispersed phase and the continuous phase which is correlated to the sedimentation flow features of the dilute emulsion.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"12 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210859","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 : 2024-08-15DOI: 10.1103/physrevfluids.9.083702
Vladislav Eltishchev, Gennadiy Losev, Peter Frick
The circular surface wave (CSW) of a low-temperature gallium alloy in the immovable cell with a central bottom electrode and an upper ring electrode exposed to axial magnetic fields is studied experimentally. It is shown that, depending on the force parameter and geometrical characteristics of the cell [cell radius, height of the liquid metal (LM) layer, and position of the circular electrode], three modes can occur in the cell: rest, CSW, or axial rotation with a deep funnel on the surface providing the circular contact of the LM with the electrode. A mode map showing the boundaries of the CSW existence domain is plotted on the parameter plane. The mechanism which provides the existence of a stable CSW is suggested. It is shown that the CSW is a superposition of two intense large-scale vortices. The main vortex, whose axis coincides with the axis of the cylindrical cell, is generated by the Lorentz force localized near the bottom electrode and arising from the interaction of the divergent electric current with the vertical magnetic field. The intensity of the second vortex is an order of magnitude less, and the axis of rotation is directed to the contact area of the liquid metal with the ring electrode, which appears near the crest of the wave. Similar to the main vortex, it exists due to the interaction of the current converging to the contact area with the superimposed magnetic field. The second vortex provides the lifting of the LM ahead of the incoming wave. The intensity of both vortices is proportional to the product of the external field by the total current, which explains the linear relationship between the relative frequency of surface oscillations and their amplitude.
{"title":"Maintenance mechanism of a circular surface wave in a magnetohydrodynamic cell and limits of its existence","authors":"Vladislav Eltishchev, Gennadiy Losev, Peter Frick","doi":"10.1103/physrevfluids.9.083702","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083702","url":null,"abstract":"The circular surface wave (CSW) of a low-temperature gallium alloy in the immovable cell with a central bottom electrode and an upper ring electrode exposed to axial magnetic fields is studied experimentally. It is shown that, depending on the force parameter and geometrical characteristics of the cell [cell radius, height of the liquid metal (LM) layer, and position of the circular electrode], three modes can occur in the cell: rest, CSW, or axial rotation with a deep funnel on the surface providing the circular contact of the LM with the electrode. A mode map showing the boundaries of the CSW existence domain is plotted on the parameter plane. The mechanism which provides the existence of a stable CSW is suggested. It is shown that the CSW is a superposition of two intense large-scale vortices. The main vortex, whose axis coincides with the axis of the cylindrical cell, is generated by the Lorentz force localized near the bottom electrode and arising from the interaction of the divergent electric current with the vertical magnetic field. The intensity of the second vortex is an order of magnitude less, and the axis of rotation is directed to the contact area of the liquid metal with the ring electrode, which appears near the crest of the wave. Similar to the main vortex, it exists due to the interaction of the current converging to the contact area with the superimposed magnetic field. The second vortex provides the lifting of the LM ahead of the incoming wave. The intensity of both vortices is proportional to the product of the external field by the total current, which explains the linear relationship between the relative frequency of surface oscillations and their amplitude.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"406 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210861","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 : 2024-08-14DOI: 10.1103/physrevfluids.9.083302
Byjesh N. Radhakrishnan, Ahana Purushothaman, Ranabir Dey, Sumesh P. Thampi
We study the trajectories of a model microorganism inside three-dimensional channels with square and rectangular cross sections. Using (1) numerical simulations based on the lattice-Boltzmann method and (2) analytical expressions using far-field hydrodynamic approximations and the method of images we systematically investigate the role of the strength and finite-size of the squirmer, confinement dimensions, and initial conditions in determining the three-dimensional trajectories of microswimmers. Our results indicate that the hydrodynamic interactions with the confining walls of the channel significantly affect the swimming speed and trajectory of the model microswimmer. Specifically, pullers always display sliding motion inside the channel: weak pullers slide through the channel center line, while strong pullers slide through a path close to any of the walls. Pushers generally follow helical motion in a square channel. Unlike pullers and pushers, the trajectories of neutral swimmers are not easy to generalize and are sensitive to the initial conditions. Despite this diversity in the trajectories, the far-field expressions capture the essential features of channel-confined swimmers. Finally, we propose a method based on the principle of superposition to understand the origin of the three-dimensional trajectories of channel confined swimmers. Such construction allows us to predict and justify the origin of apparently complex three-dimensional trajectories generated by different types of swimmers in channels with square and rectangular cross sections.
{"title":"Confinement induced three-dimensional trajectories of microswimmers in rectangular channels","authors":"Byjesh N. Radhakrishnan, Ahana Purushothaman, Ranabir Dey, Sumesh P. Thampi","doi":"10.1103/physrevfluids.9.083302","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083302","url":null,"abstract":"We study the trajectories of a model microorganism inside three-dimensional channels with square and rectangular cross sections. Using (1) numerical simulations based on the lattice-Boltzmann method and (2) analytical expressions using far-field hydrodynamic approximations and the method of images we systematically investigate the role of the strength and finite-size of the squirmer, confinement dimensions, and initial conditions in determining the three-dimensional trajectories of microswimmers. Our results indicate that the hydrodynamic interactions with the confining walls of the channel significantly affect the swimming speed and trajectory of the model microswimmer. Specifically, pullers always display sliding motion inside the channel: weak pullers slide through the channel center line, while strong pullers slide through a path close to any of the walls. Pushers generally follow helical motion in a square channel. Unlike pullers and pushers, the trajectories of neutral swimmers are not easy to generalize and are sensitive to the initial conditions. Despite this diversity in the trajectories, the far-field expressions capture the essential features of channel-confined swimmers. Finally, we propose a method based on the principle of superposition to understand the origin of the three-dimensional trajectories of channel confined swimmers. Such construction allows us to predict and justify the origin of apparently complex three-dimensional trajectories generated by different types of swimmers in channels with square and rectangular cross sections.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"19 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210865","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 : 2024-08-14DOI: 10.1103/physrevfluids.9.084606
Min Lu, Zixuan Yang, Guowei He, Lian Shen
Heat transfer in wind turbulence over breaking waves is studied through direct numerical simulations. The air-water system is simulated on an Eulerian grid with the interface between the two phases captured by a coupled level set and volume-of-fluid method. To examine the effect of wave age, different cases representing slow, intermediate, and fast waves are considered for the scenario of air temperature being higher than the water temperature. The results show that the evolution of mean temperature profile responds nonmonotonically to the increasing wave age. At a small wave age, the mean temperature near the water surface increases after wave breaking. At intermediate and large wave ages, however, the temperature decreases after wave breaking, while the decrement magnitude is larger at the intermediate wave age. An investigation of the temperature fluctuation flux indicates that a combined effect of wave-coherent flux and turbulence-induced flux leads to a large magnitude of temperature decrement at the intermediate wave age. A further analysis of the production term in the transport equation of the turbulence-induced temperature flux elucidates the mechanism underlying the generation of the turbulence-induced flux at the intermediate wave age. The findings of the present study suggest that temperature responds in a more complex manner to wave age than velocity does and this phenomenon should be considered in models for air-sea interaction and weather forecasting.
{"title":"Numerical investigation on the heat transfer in wind turbulence over breaking waves","authors":"Min Lu, Zixuan Yang, Guowei He, Lian Shen","doi":"10.1103/physrevfluids.9.084606","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084606","url":null,"abstract":"Heat transfer in wind turbulence over breaking waves is studied through direct numerical simulations. The air-water system is simulated on an Eulerian grid with the interface between the two phases captured by a coupled level set and volume-of-fluid method. To examine the effect of wave age, different cases representing slow, intermediate, and fast waves are considered for the scenario of air temperature being higher than the water temperature. The results show that the evolution of mean temperature profile responds nonmonotonically to the increasing wave age. At a small wave age, the mean temperature near the water surface increases after wave breaking. At intermediate and large wave ages, however, the temperature decreases after wave breaking, while the decrement magnitude is larger at the intermediate wave age. An investigation of the temperature fluctuation flux indicates that a combined effect of wave-coherent flux and turbulence-induced flux leads to a large magnitude of temperature decrement at the intermediate wave age. A further analysis of the production term in the transport equation of the turbulence-induced temperature flux elucidates the mechanism underlying the generation of the turbulence-induced flux at the intermediate wave age. The findings of the present study suggest that temperature responds in a more complex manner to wave age than velocity does and this phenomenon should be considered in models for air-sea interaction and weather forecasting.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"3 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142227808","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 experimentally investigate the morphology and breakup of a droplet as it descends freely from a height and encounters an airstream. The size distributions of the child droplets are analyzed using high-speed shadowgraphy and in-line holography techniques. We find that a droplet falling from various heights exhibits shape oscillations due to the intricate interplay between inertia and surface tension forces, leading to significant variations in the radial deformation of the droplet, influencing the breakup dynamics under an identical airstream condition. Specifically, the droplet undergoes vibrational breakup when introduced at a location slightly above the air nozzle. In contrast, as the release height of the droplet increases, keeping the Weber number defined based on the velocity of the airstream fixed, a dynamic interplay between the inertia of the droplet and the aerodynamic flow field comes into play, resulting in a sequence of breakup modes transitioning from vibrational breakup to retracting bag breakup, bag breakup, bag-stamen breakup, retracting bag-stamen breakup, and eventually returning to vibrational breakup. Our experiments also reveal that the size distribution resulting from retracting bag breakup primarily arises from rim and node fragmentation, leading to a bimodal distribution. In contrast, bag and bag-stamen breakups yield a trimodal size distribution due to the combined contributions of bag, rim, and node breakup mechanisms. Furthermore, we utilize a theoretical model that incorporates the effective Weber number, considering different release heights. This model accurately predicts the size distribution of the child droplets resulting from the various breakup modes observed in our experiments.
{"title":"Droplet breakup and size distribution in an airstream: Effect of inertia","authors":"Someshwar Sanjay Ade, Pavan Kumar Kirar, Lakshmana Dora Chandrala, Kirti Chandra Sahu","doi":"10.1103/physrevfluids.9.084004","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084004","url":null,"abstract":"We experimentally investigate the morphology and breakup of a droplet as it descends freely from a height and encounters an airstream. The size distributions of the child droplets are analyzed using high-speed shadowgraphy and in-line holography techniques. We find that a droplet falling from various heights exhibits shape oscillations due to the intricate interplay between inertia and surface tension forces, leading to significant variations in the radial deformation of the droplet, influencing the breakup dynamics under an identical airstream condition. Specifically, the droplet undergoes vibrational breakup when introduced at a location slightly above the air nozzle. In contrast, as the release height of the droplet increases, keeping the Weber number defined based on the velocity of the airstream fixed, a dynamic interplay between the inertia of the droplet and the aerodynamic flow field comes into play, resulting in a sequence of breakup modes transitioning from vibrational breakup to retracting bag breakup, bag breakup, bag-stamen breakup, retracting bag-stamen breakup, and eventually returning to vibrational breakup. Our experiments also reveal that the size distribution resulting from retracting bag breakup primarily arises from rim and node fragmentation, leading to a bimodal distribution. In contrast, bag and bag-stamen breakups yield a trimodal size distribution due to the combined contributions of bag, rim, and node breakup mechanisms. Furthermore, we utilize a theoretical model that incorporates the effective Weber number, considering different release heights. This model accurately predicts the size distribution of the child droplets resulting from the various breakup modes observed in our experiments.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"55 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210868","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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