A point force acting on a Brinkman fluid in confinement is always counterbalanced by the force on the porous medium, the force on the walls and the stress at open boundaries. We discuss the distribution of those forces in different geometries: a long pipe, a medium with a single no-slip planar boundary, a porous sphere with an open boundary and a porous sphere with a no-slip wall. We determine the forces using the Lorentz reciprocal theorem and additionally validate the results with explicit analytical flow solutions. We discuss the relevance of our findings for cellular processes such as cytoplasmic streaming and centrosome positioning.
{"title":"On force balance in Brinkman fluids under confinement","authors":"Abdallah Daddi-Moussa-Ider, Andrej Vilfan","doi":"arxiv-2409.10183","DOIUrl":"https://doi.org/arxiv-2409.10183","url":null,"abstract":"A point force acting on a Brinkman fluid in confinement is always\u0000counterbalanced by the force on the porous medium, the force on the walls and\u0000the stress at open boundaries. We discuss the distribution of those forces in\u0000different geometries: a long pipe, a medium with a single no-slip planar\u0000boundary, a porous sphere with an open boundary and a porous sphere with a\u0000no-slip wall. We determine the forces using the Lorentz reciprocal theorem and\u0000additionally validate the results with explicit analytical flow solutions. We\u0000discuss the relevance of our findings for cellular processes such as\u0000cytoplasmic streaming and centrosome positioning.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267536","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Vincent Bertin, Alexandros T. Oratis, Jacco H. Snoeijer
The adhesion between dry solid surfaces is typically governed by contact forces, involving surface forces and elasticity. For surfaces immersed in a fluid, out-of-contact adhesion arises due to the viscous resistance to the opening of the liquid gap. While the adhesion between dry solids is described by the classical JKR theory, there is no equivalent framework for the wet adhesion of soft solids. Here, we investigate theoretically the viscous adhesion emerging during the separation of a sphere from an elastic substrate. The suction pressure within the thin viscous film between the solids induces significant elastic displacements. Unexpectedly, the elastic substrate closely follows the motion of the sphere, leading to a sticking without contact. The initial dynamics is described using similarity solutions, resulting in a nonlinear adhesion force that grows in time as F ~ t^(2/3). When elastic displacements become large enough, another similarity solution emerges that leads to a violent snap-off of the adhesive contact through a finite-time singularity. The observed phenomenology bears a strong resemblance with JKR theory, and is relevant for a wide range of applications involving viscous adhesion.
干燥固体表面之间的粘附力通常由接触力决定，涉及表面力和弹性。对于浸入液体中的表面，由于液体间隙打开时的粘性阻力，会产生非接触粘附力。虽然干固体之间的粘附可以用经典的 JKR 理论来描述，但对于软固体的湿粘附却没有相应的框架。在这里，我们从理论上研究了球体与弹性基体分离过程中出现的粘性粘附力。固体之间的粘性薄膜内的吸力会引起显著的弹性位移。出乎意料的是，弹性基体紧跟球体运动，导致无接触粘连。初始动力学是用相似解来描述的，结果产生了一个非线性粘附力，该粘附力随时间 F ~ t^(2/3) 而增长。当弹性位移变得足够大时，就会出现另一种相似解，通过有限次奇异性导致粘着接触的剧烈断裂。观察到的现象与 JKR 理论非常相似，并且与涉及粘性粘附的广泛应用相关。
{"title":"Sticking without contact: Elastohydrodynamic adhesion","authors":"Vincent Bertin, Alexandros T. Oratis, Jacco H. Snoeijer","doi":"arxiv-2409.10723","DOIUrl":"https://doi.org/arxiv-2409.10723","url":null,"abstract":"The adhesion between dry solid surfaces is typically governed by contact\u0000forces, involving surface forces and elasticity. For surfaces immersed in a\u0000fluid, out-of-contact adhesion arises due to the viscous resistance to the\u0000opening of the liquid gap. While the adhesion between dry solids is described\u0000by the classical JKR theory, there is no equivalent framework for the wet\u0000adhesion of soft solids. Here, we investigate theoretically the viscous\u0000adhesion emerging during the separation of a sphere from an elastic substrate.\u0000The suction pressure within the thin viscous film between the solids induces\u0000significant elastic displacements. Unexpectedly, the elastic substrate closely\u0000follows the motion of the sphere, leading to a sticking without contact. The\u0000initial dynamics is described using similarity solutions, resulting in a\u0000nonlinear adhesion force that grows in time as F ~ t^(2/3). When elastic\u0000displacements become large enough, another similarity solution emerges that\u0000leads to a violent snap-off of the adhesive contact through a finite-time\u0000singularity. The observed phenomenology bears a strong resemblance with JKR\u0000theory, and is relevant for a wide range of applications involving viscous\u0000adhesion.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"20 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267534","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Subhajit Kar, Roy Barkan, James C. McWilliams, M. Jeroen Molemaker
The spontaneous emission of internal waves (IWs) from balanced mesoscale eddies has been previously proposed to provide a source of oceanic IW kinetic energy (KE). This study examines the mechanisms leading to the spontaneous emission of spiral-shaped IWs from an anticyclonic eddy with an order-one Rossby number, using a high-resolution numerical simulation of a flat-bottomed, wind-forced, reentrant channel flow configured to resemble the Antarctic Circumpolar Current. It is demonstrated that IWs are spontaneously generated as a result of a loss of balance process that is concentrated at the eddy edge, and then radiate radially outward. A 2D linear stability analysis of the eddy shows that the spontaneous emission arises from a radiative instability which involves an interaction between a vortex Rossby wave supported by the radial gradient of potential vorticity and an outgoing IWs. This particular instability occurs when the perturbation frequency is superinertial. This finding is supported by a KE analysis of the unstable modes and the numerical solution, where it is shown that the horizontal shear production provides the source of perturbation KE. Furthermore, the horizontal length scale and frequency of the most unstable mode from the stability analysis agree well with those of the spontaneously emitted IWs in the numerical solution.
以前曾有人提出，平衡中尺度涡的自发内波（IWs）是海洋内波动能（KE）的来源。本研究使用高分辨率数值模拟了一个平底、风力强迫、重入式通道流，其配置类似于南极环极流，研究了导致螺旋形内波从一个具有一阶罗斯比数的反气旋涡中自发发射的机制。结果表明，IWs 是由集中在漩涡边缘的失去平衡过程自发产生的，然后径向向外辐射。对涡流的二维线性稳定性分析表明，自发辐射源于一种辐射不稳定性，它涉及到由潜在涡度径向梯度支持的涡旋罗斯比波与外向 IWs 之间的相互作用。这种特殊的不稳定性发生在扰动频率为超惯性时。对不稳定模式的 KE 分析和数值求解支持了这一发现，表明水平剪切力的产生提供了扰动 KE 的来源。此外，稳定性分析得出的最不稳定模式的水平长度尺度和频率与数值解中自发发射的 IWs 的水平长度尺度和频率非常吻合。
{"title":"Spontaneous emission of internal waves by a radiative instability","authors":"Subhajit Kar, Roy Barkan, James C. McWilliams, M. Jeroen Molemaker","doi":"arxiv-2409.10758","DOIUrl":"https://doi.org/arxiv-2409.10758","url":null,"abstract":"The spontaneous emission of internal waves (IWs) from balanced mesoscale\u0000eddies has been previously proposed to provide a source of oceanic IW kinetic\u0000energy (KE). This study examines the mechanisms leading to the spontaneous\u0000emission of spiral-shaped IWs from an anticyclonic eddy with an order-one\u0000Rossby number, using a high-resolution numerical simulation of a flat-bottomed,\u0000wind-forced, reentrant channel flow configured to resemble the Antarctic\u0000Circumpolar Current. It is demonstrated that IWs are spontaneously generated as\u0000a result of a loss of balance process that is concentrated at the eddy edge,\u0000and then radiate radially outward. A 2D linear stability analysis of the eddy\u0000shows that the spontaneous emission arises from a radiative instability which\u0000involves an interaction between a vortex Rossby wave supported by the radial\u0000gradient of potential vorticity and an outgoing IWs. This particular\u0000instability occurs when the perturbation frequency is superinertial. This\u0000finding is supported by a KE analysis of the unstable modes and the numerical\u0000solution, where it is shown that the horizontal shear production provides the\u0000source of perturbation KE. Furthermore, the horizontal length scale and\u0000frequency of the most unstable mode from the stability analysis agree well with\u0000those of the spontaneously emitted IWs in the numerical solution.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"11 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267530","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Fluid dynamics, and flight in particular, is a domain where organisms challenge our understanding of its physics. Integrating the current knowledge of animal flight, we propose to revisit the use of live animals to study physical phenomena. After a short description of the physics of flight, we examine the broad literature on animal flight focusing on studies of living animals. We start out reviewing the diverse animal species studied so far and then focus on the experimental techniques used to study them quantitatively. Our network analysis reveals how the three clades of animals performing powered flight - insects, birds and bats - are studied using substantially different combinations of measurement techniques. We then combine these insights with a new paradigm for increasing our physical understanding of flight. This paradigm relies on the concept of Animal Learning, where animals are used as probes to study fluid phenomena and variables involved in flight, harnessing their natural adaptability.
{"title":"Advancing flight physics through natural adaptation and animal learning","authors":"Ariane Gayout, David Lentink","doi":"arxiv-2409.10067","DOIUrl":"https://doi.org/arxiv-2409.10067","url":null,"abstract":"Fluid dynamics, and flight in particular, is a domain where organisms\u0000challenge our understanding of its physics. Integrating the current knowledge\u0000of animal flight, we propose to revisit the use of live animals to study\u0000physical phenomena. After a short description of the physics of flight, we\u0000examine the broad literature on animal flight focusing on studies of living\u0000animals. We start out reviewing the diverse animal species studied so far and\u0000then focus on the experimental techniques used to study them quantitatively.\u0000Our network analysis reveals how the three clades of animals performing powered\u0000flight - insects, birds and bats - are studied using substantially different\u0000combinations of measurement techniques. We then combine these insights with a\u0000new paradigm for increasing our physical understanding of flight. This paradigm\u0000relies on the concept of Animal Learning, where animals are used as probes to\u0000study fluid phenomena and variables involved in flight, harnessing their\u0000natural adaptability.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"2020 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267537","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Nikos Vasileiadis, Giorgos Tatsios, Craig White, Duncan A. Lockerby, Matthew K. Borg, Livio Gibelli
This paper presents uniGasFoam, an open-source particle-based solver for multiscale rarefied gas flow simulations, which has been developed within the well-established OpenFOAM framework, and is an extension of the direct simulation Monte Carlo (DSMC) solver dsmcFoam+. The developed solver addresses the coupling challenges inherent in hybrid continuum-particle methods, originating from the disparate nature of finite-volume (FV) solvers found in computational fluid dynamics (CFD) software and DSMC particle solvers. This is achieved by employing alternative stochastic particle methods, resembling DSMC, to tackle the continuum limit. The uniGasFoam particle-particle coupling produces a numerical implementation that is simpler and more robust, faster in many steady-state flows, and more scalable for transient flows compared to conventional continuum-particle coupling. The presented framework is unified and generic, and can couple DSMC with stochastic particle (SP) and unified stochastic particle (USP) methods, or be employed for pure DSMC, SP, and USP gas simulations. To enhance user experience, optimise computational resources and minimise user error, advanced adaptive algorithms such as transient adaptive sub-cells, non-uniform cell weighting, and adaptive global time stepping have been integrated into uniGasFoam. In this paper, the hybrid USP-DSMC module of uniGasFoam is rigorously validated through multiple benchmark cases, consistently showing excellent agreement with pure DSMC, hybrid CFD-DSMC, and literature results. Notably, uniGasFoam achieves significant computational gains compared to pure dsmcFoam+ simulations, rendering it a robust computational tool well-suited for addressing multiscale rarefied gas flows of engineering importance.
{"title":"uniGasFoam: a particle-based OpenFOAM solver for multiscale rarefied gas flows","authors":"Nikos Vasileiadis, Giorgos Tatsios, Craig White, Duncan A. Lockerby, Matthew K. Borg, Livio Gibelli","doi":"arxiv-2409.10288","DOIUrl":"https://doi.org/arxiv-2409.10288","url":null,"abstract":"This paper presents uniGasFoam, an open-source particle-based solver for\u0000multiscale rarefied gas flow simulations, which has been developed within the\u0000well-established OpenFOAM framework, and is an extension of the direct\u0000simulation Monte Carlo (DSMC) solver dsmcFoam+. The developed solver addresses\u0000the coupling challenges inherent in hybrid continuum-particle methods,\u0000originating from the disparate nature of finite-volume (FV) solvers found in\u0000computational fluid dynamics (CFD) software and DSMC particle solvers. This is\u0000achieved by employing alternative stochastic particle methods, resembling DSMC,\u0000to tackle the continuum limit. The uniGasFoam particle-particle coupling\u0000produces a numerical implementation that is simpler and more robust, faster in\u0000many steady-state flows, and more scalable for transient flows compared to\u0000conventional continuum-particle coupling. The presented framework is unified\u0000and generic, and can couple DSMC with stochastic particle (SP) and unified\u0000stochastic particle (USP) methods, or be employed for pure DSMC, SP, and USP\u0000gas simulations. To enhance user experience, optimise computational resources\u0000and minimise user error, advanced adaptive algorithms such as transient\u0000adaptive sub-cells, non-uniform cell weighting, and adaptive global time\u0000stepping have been integrated into uniGasFoam. In this paper, the hybrid\u0000USP-DSMC module of uniGasFoam is rigorously validated through multiple\u0000benchmark cases, consistently showing excellent agreement with pure DSMC,\u0000hybrid CFD-DSMC, and literature results. Notably, uniGasFoam achieves\u0000significant computational gains compared to pure dsmcFoam+ simulations,\u0000rendering it a robust computational tool well-suited for addressing multiscale\u0000rarefied gas flows of engineering importance.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"14 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267535","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Cytoplasmic streaming, the persistent flow of fluid inside a cell, induces intracellular transport, which plays a key role in fundamental biological processes. In meiosis II mouse oocytes (developing egg cells) awaiting fertilisation, the spindle, which is the protein structure responsible for dividing genetic material in a cell, must maintain its position near the cell cortex (the thin actin network bound to the cell membrane) for many hours. However, the cytoplasmic streaming that accompanies this stable positioning would intuitively appear to destabilise the spindle position. Here, through a combination of numerical and analytical modelling, we reveal a new, hydrodynamic mechanism for stable spindle positioning beneath the cortical cap. We show that this stability depends critically on the spindle size and the active driving from the cortex, and demonstrate that stable spindle positioning can result purely from a hydrodynamic suction force exerted on the spindle by the cytoplasmic flow. Our findings show that local fluid dynamic forces can be sufficient to stabilise the spindle, explaining robustness against perturbations not only perpendicular but also parallel to the cortex. Our results shed light on the importance of cytoplasmic streaming in mammalian meiosis.
{"title":"Hydrodynamic mechanism for stable spindle positioning in meiosis II oocytes","authors":"Weida Liao, Eric Lauga","doi":"arxiv-2409.10401","DOIUrl":"https://doi.org/arxiv-2409.10401","url":null,"abstract":"Cytoplasmic streaming, the persistent flow of fluid inside a cell, induces\u0000intracellular transport, which plays a key role in fundamental biological\u0000processes. In meiosis II mouse oocytes (developing egg cells) awaiting\u0000fertilisation, the spindle, which is the protein structure responsible for\u0000dividing genetic material in a cell, must maintain its position near the cell\u0000cortex (the thin actin network bound to the cell membrane) for many hours.\u0000However, the cytoplasmic streaming that accompanies this stable positioning\u0000would intuitively appear to destabilise the spindle position. Here, through a\u0000combination of numerical and analytical modelling, we reveal a new,\u0000hydrodynamic mechanism for stable spindle positioning beneath the cortical cap.\u0000We show that this stability depends critically on the spindle size and the\u0000active driving from the cortex, and demonstrate that stable spindle positioning\u0000can result purely from a hydrodynamic suction force exerted on the spindle by\u0000the cytoplasmic flow. Our findings show that local fluid dynamic forces can be\u0000sufficient to stabilise the spindle, explaining robustness against\u0000perturbations not only perpendicular but also parallel to the cortex. Our\u0000results shed light on the importance of cytoplasmic streaming in mammalian\u0000meiosis.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"52 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267542","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Nanofluids are suspensions of nanoscale particles (such as metals and their oxides) in base fluids (such as water, oil, or alcohol), which can significantly enhance the heat transfer performance of the base fluid. However, when nanofluids are applied to heat pipes, it is common for nanoparticles to accumulate within the heat pipe's capillary wick, clogging it and increasing thermal resistance. This paper investigates the phenomenon of boiling of water and nanofluids enhanced by copper foam through experimental methods. When the liquid is injected into copper foam placed on a heating plate, some of the liquid is squeezed out along the boundary of the heated surface of the copper foam during boiling. This phenomenon is independent of gravity but related to the hydrophilicity or hydrophobicity of the heating surface. Based on these properties, we design a device to guide the squeezed-out liquid to other locations, offering a promising solution to the problem of nanoparticle accumulation in the heat pipe's capillary wick.
{"title":"Experimental Study on Boiling of Nanofluids in Copper Foam","authors":"Kai-Xin Hu, Jing-Han Pan","doi":"arxiv-2409.09995","DOIUrl":"https://doi.org/arxiv-2409.09995","url":null,"abstract":"Nanofluids are suspensions of nanoscale particles (such as metals and their\u0000oxides) in base fluids (such as water, oil, or alcohol), which can\u0000significantly enhance the heat transfer performance of the base fluid. However,\u0000when nanofluids are applied to heat pipes, it is common for nanoparticles to\u0000accumulate within the heat pipe's capillary wick, clogging it and increasing\u0000thermal resistance. This paper investigates the phenomenon of boiling of water\u0000and nanofluids enhanced by copper foam through experimental methods. When the\u0000liquid is injected into copper foam placed on a heating plate, some of the\u0000liquid is squeezed out along the boundary of the heated surface of the copper\u0000foam during boiling. This phenomenon is independent of gravity but related to\u0000the hydrophilicity or hydrophobicity of the heating surface. Based on these\u0000properties, we design a device to guide the squeezed-out liquid to other\u0000locations, offering a promising solution to the problem of nanoparticle\u0000accumulation in the heat pipe's capillary wick.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"33 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267538","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Arnab Moitro, Sai Sandeep Dammati, Alexei Y. Poludnenko
Direct numerical simulations (DNS) are one of the main ab initio tools to study turbulent flows. However, due to their considerable computational cost, DNS are primarily restricted to canonical flows at moderate Reynolds numbers, in which turbulence is isolated from the realistic, large-scale flow dynamics. In contrast, lower fidelity techniques, such as large eddy simulations (LES), are employed for modelling real-life systems. Such approaches rely on closure models that make multiple assumptions, including turbulent equilibrium, small-scale universality, etc., which require prior knowledge of the flow and can be violated. We propose a method, which couples a lower-fidelity, unresolved, time-dependent calculation of an entire system (LES) with an embedded Small-Eddy Simulation (SES) that provides a high-fidelity, fully resolved solution in a sub-region of interest of the LES. Such coupling is achieved by continuous replacement of the large SES scales with a low-pass filtered LES velocity field. The method is formulated in physical space, makes no assumptions of equilibrium, small-scale structure, and boundary conditions. A priori tests of both steady and unsteady homogeneous, isotropic turbulence are used to demonstrate the method accuracy in recovering turbulence properties, including spectra, probability density functions of the intermittent quantities, and sub-grid dissipation. Finally, SES is compared with two alternative approaches: one embedding a high-resolution region through static mesh refinement and a generalization of the traditional volumetric spectral forcing. Unlike these methods, SES is shown to achieve DNS-level accuracy at a fraction of the cost of the full DNS, thus opening the possibility to study high-Re flows.
直接数值模拟（DNS）是研究湍流的主要原初工具之一。然而，由于计算成本高昂，直接数值模拟主要局限于中等雷诺数的典型流动，在这种流动中，湍流与现实的大尺度流动动力学相隔离。相反，低保真度技术，如大涡模拟（LES），则被用于模拟现实系统。这些方法依赖于闭合模型，而闭合模型需要多种假设，包括湍流平衡、小尺度普遍性等，这些假设需要事先了解流动情况，并且可能会被违反。我们提出了一种方法，将整个系统的低保真、未解析、随时间变化的计算（LES）与嵌入式小型埃迪模拟（SES）相结合，后者在 LES 的相关子区域提供高保真、全解析的解决方案。这种耦合是通过用低通滤波 LES 速度场连续替换大 SES 尺度来实现的。该方法是在物理空间中制定的，不假定平衡、小尺度结构和边界条件。对稳定和非稳定的均质各向同性湍流进行了先验测试，以证明该方法在恢复湍流特性（包括频谱、间歇量的概率密度函数和子网格耗散）方面的准确性。最后，将 SES 与两种替代方法进行了比较：一种是通过静态网格细化嵌入高分辨率区域的方法，另一种是对传统容积谱强迫进行概括的方法。与这些方法不同的是，SES 只需全 DNS 的一小部分成本就能达到 DNS 水平的精度，从而为研究高 Re 流体提供了可能。
{"title":"Large/small eddy simulations: A high-fidelity method for studying high-Reynolds number turbulent flows","authors":"Arnab Moitro, Sai Sandeep Dammati, Alexei Y. Poludnenko","doi":"arxiv-2409.09901","DOIUrl":"https://doi.org/arxiv-2409.09901","url":null,"abstract":"Direct numerical simulations (DNS) are one of the main ab initio tools to\u0000study turbulent flows. However, due to their considerable computational cost,\u0000DNS are primarily restricted to canonical flows at moderate Reynolds numbers,\u0000in which turbulence is isolated from the realistic, large-scale flow dynamics.\u0000In contrast, lower fidelity techniques, such as large eddy simulations (LES),\u0000are employed for modelling real-life systems. Such approaches rely on closure\u0000models that make multiple assumptions, including turbulent equilibrium,\u0000small-scale universality, etc., which require prior knowledge of the flow and\u0000can be violated. We propose a method, which couples a lower-fidelity,\u0000unresolved, time-dependent calculation of an entire system (LES) with an\u0000embedded Small-Eddy Simulation (SES) that provides a high-fidelity, fully\u0000resolved solution in a sub-region of interest of the LES. Such coupling is\u0000achieved by continuous replacement of the large SES scales with a low-pass\u0000filtered LES velocity field. The method is formulated in physical space, makes\u0000no assumptions of equilibrium, small-scale structure, and boundary conditions.\u0000A priori tests of both steady and unsteady homogeneous, isotropic turbulence\u0000are used to demonstrate the method accuracy in recovering turbulence\u0000properties, including spectra, probability density functions of the\u0000intermittent quantities, and sub-grid dissipation. Finally, SES is compared\u0000with two alternative approaches: one embedding a high-resolution region through\u0000static mesh refinement and a generalization of the traditional volumetric\u0000spectral forcing. Unlike these methods, SES is shown to achieve DNS-level\u0000accuracy at a fraction of the cost of the full DNS, thus opening the\u0000possibility to study high-Re flows.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"103 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267539","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Steady-state perturbations to a stagnation point flow of the form ${bf U}=(0,A'y,-A'z)$ are known which consist of a periodic array of counter-rotating vortices whose axes are parallel to the $y$-axis and which lie in the plane $z=0$. A new understanding of how these vortices depend on the supply of incoming vorticity from afar has lead to the discovery of new families of steady-state periodic vortices that can exist in a stagnation point flow. These new flows have a greater variety of structures than those previously known. An understanding of the linkage between the vortices and the weak inflow of vorticity can have important implications for situations where such vortices are observed.
{"title":"Periodic Steady Vortices in a Stagnation Point Flow II","authors":"Oliver S. Kerr","doi":"arxiv-2409.09695","DOIUrl":"https://doi.org/arxiv-2409.09695","url":null,"abstract":"Steady-state perturbations to a stagnation point flow of the form ${bf\u0000U}=(0,A'y,-A'z)$ are known which consist of a periodic array of\u0000counter-rotating vortices whose axes are parallel to the $y$-axis and which lie\u0000in the plane $z=0$. A new understanding of how these vortices depend on the\u0000supply of incoming vorticity from afar has lead to the discovery of new\u0000families of steady-state periodic vortices that can exist in a stagnation point\u0000flow. These new flows have a greater variety of structures than those\u0000previously known. An understanding of the linkage between the vortices and the weak inflow of\u0000vorticity can have important implications for situations where such vortices\u0000are observed.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"39 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267540","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We theoretically illustrate how complex fluids flowing over superhydrophobic surfaces may exhibit giant flow enhancements in the double limit of small solid fractions ($epsilonll1$) and strong shear thinning ($betall1$, $beta$ being the ratio of the viscosity at infinite shear rate to that at zero shear rate). Considering a Carreau liquid within the canonical scenario of longitudinal shear-driven flow over a grooved superhydrophobic surface, we show that, as $beta$ is decreased, the scaling of the effective slip length at small solid fractions is enhanced from the logarithmic scaling $ln(1/epsilon)$ for Newtonian fluids to the algebraic scaling $1/epsilon^{frac{1-n}{n}}$, attained for $beta=mathcal{O}(epsilon^{frac{1-n}{n}})$, $nin(0,1)$ being the exponent in the Carreau model. We illuminate this scaling enhancement and the geometric-rheological mechanism underlying it through asymptotic arguments and numerical simulations.
{"title":"Giant superhydrophobic slip of shear-thinning liquids","authors":"Ory Schnitzer, Prasun K. Ray","doi":"arxiv-2409.09374","DOIUrl":"https://doi.org/arxiv-2409.09374","url":null,"abstract":"We theoretically illustrate how complex fluids flowing over superhydrophobic\u0000surfaces may exhibit giant flow enhancements in the double limit of small solid\u0000fractions ($epsilonll1$) and strong shear thinning ($betall1$, $beta$\u0000being the ratio of the viscosity at infinite shear rate to that at zero shear\u0000rate). Considering a Carreau liquid within the canonical scenario of\u0000longitudinal shear-driven flow over a grooved superhydrophobic surface, we show\u0000that, as $beta$ is decreased, the scaling of the effective slip length at\u0000small solid fractions is enhanced from the logarithmic scaling\u0000$ln(1/epsilon)$ for Newtonian fluids to the algebraic scaling\u0000$1/epsilon^{frac{1-n}{n}}$, attained for\u0000$beta=mathcal{O}(epsilon^{frac{1-n}{n}})$, $nin(0,1)$ being the exponent\u0000in the Carreau model. We illuminate this scaling enhancement and the\u0000geometric-rheological mechanism underlying it through asymptotic arguments and\u0000numerical simulations.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"9 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142267541","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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