arXiv: Fluid Dynamics最新文献_第6页

Hydrodynamics of tandem flapping pectoral fins with varying stroke phase offsets 不同冲程相位偏移的串联扑动胸鳍流体力学

arXiv: Fluid Dynamics Pub Date : 2020-09-22 DOI: 10.1103/PhysRevFluids.5.094101

K. Sampath, J. Geder, R. Ramamurti, M. Pruessner, Raymond Koehler

{"title":"Hydrodynamics of tandem flapping pectoral fins with varying stroke phase offsets","authors":"K. Sampath, J. Geder, R. Ramamurti, M. Pruessner, Raymond Koehler","doi":"10.1103/PhysRevFluids.5.094101","DOIUrl":"https://doi.org/10.1103/PhysRevFluids.5.094101","url":null,"abstract":"We show how phasing between tandem bioinspired fins flapping at high-stroke amplitudes modulates rear fin thrust production and wake characteristics. Load cell thrust measurements show that the rear fin generates 25% more thrust than the front fin when it lags the latter by a quarter cycle, and performs 8% worse when it leads the front fin by the same amount. The flow interactions between the fins responsible for these observations are analyzed using two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics simulations. Distributions of velocity elucidate variations in the effective flow induced on the rear fin for different phase offsets. Vortex structure interactions and particle rakes reveal the contributions of the leading- and trailing-edge vortices shed by the front fin in modulating the suction at the rear fin leading edge. Furthermore, the wake structure far downstream of the fins changes in its coherence, axial and radial extents for the different phase offsets. These findings are relevant for the design and performance optimization of various unmanned underwater vehicles that utilize such tandem systems.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-09-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129849062","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}

引用次数: 9

Speed–direction description of turbulent flows 紊流的速度方向描述

arXiv: Fluid Dynamics Pub Date : 2020-09-22 DOI: 10.1063/5.0031218

M. Olshanskii

引用次数: 0

No Vortex in Straight Flows: On the Eigen-Representations of Velocity Gradient ∇v 直线流动中无涡:速度梯度∇v的本征表示

arXiv: Fluid Dynamics Pub Date : 2020-09-21 DOI: 10.1007/978-3-030-70217-5_5

Xiangyang Xu, Zhiwen Xu, Changxin Tang, Xiaohang Zhang, W. Zou

引用次数: 1

Late-time description of immiscible Rayleigh–Taylor instability: A lattice Boltzmann study 非混相瑞利-泰勒不稳定性的后期描述:晶格玻尔兹曼研究

arXiv: Fluid Dynamics Pub Date : 2020-09-20 DOI: 10.1063/5.0057269

H. Liang, Zhenhua Xia, H. Huang

{"title":"Late-time description of immiscible Rayleigh–Taylor instability: A lattice Boltzmann study","authors":"H. Liang, Zhenhua Xia, H. Huang","doi":"10.1063/5.0057269","DOIUrl":"https://doi.org/10.1063/5.0057269","url":null,"abstract":"The late-time growth of single-mode immiscible Rayleigh-Taylor instability is investigated over a comprehensive range of the Reynolds numbers ($1leq Re leq 10000$) and Atwood numbers $(0.05 leq A leq 0.7)$ using an improved lattice Boltzmann multiphase method. We first reported that the instability with a moderately high Atwood number of 0.7 undergoes a sequence of distinguishing stages at high Reynolds numbers, named as the linear growth, saturated velocity growth, reacceleration and chaotic development stages. The spike and bubble at the secondary stage evolve with the constant velocities and their values agree well with the potential flow theory. Owing to the increasing strengths of the vortices, the spike and bubble are accelerated with velocities exceeding than the asymptotic values and the evolution of the instability enters into the reacceleration stage. Lastly, the curves for the spike and bubble velocities have some fluctuations at the chaotic stage and also a complex interfacial structure with large topological change is observed, while it still preserves the symmetry property. To determine the nature of the late-time growth, we also calculated the spike and bubble growth rates by using five popular statistical methods and two comparative techniques are recommended, resulting in the spike and bubble growth rates of about 0.13 and 0.022, respectively. When the Reynolds number is gradually reduced, some later stages cannot be reached successively and the structure of the evolutional interface becomes relatively smooth. Moreover, we observe that the spike growth rate shows an overall increase with the Atwood number, while it has little influence on the bubble growth rate being approximately 0.0215.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"49 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-09-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115533614","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}

引用次数: 18

Effect of imposed shear on the dynamics of a contaminated two-layer film flow down a slippery incline 施加剪切对污染双层膜沿滑坡流动动力学的影响

arXiv: Fluid Dynamics Pub Date : 2020-09-17 DOI: 10.1063/5.0024201

M. Sani, S. A. Selvan, Sukhendu Ghosh, H. Behera

{"title":"Effect of imposed shear on the dynamics of a contaminated two-layer film flow down a slippery incline","authors":"M. Sani, S. A. Selvan, Sukhendu Ghosh, H. Behera","doi":"10.1063/5.0024201","DOIUrl":"https://doi.org/10.1063/5.0024201","url":null,"abstract":"The linear instability of a surfactant-laden two-layer falling film over an inclined slippery wall is analyzed under an influence of external shear which is imposed on the top surface of the flow. The free surface of the flow as well as the interface among the fluids are contaminated by insoluble surfactants. Dynamics of both the layers are governed by the Navier--Stokes equations, and the surfactant transport equation regulates the motion of the insoluble surfactants at the interface and free surface. Instability mechanisms are compared by imposing the external shear along and opposite to the flow direction. A coupled Orr--Sommerfeld system of equations for the considered problem is derived using the perturbation technique and normal mode analysis. The eigenmodes corresponding to the Orr--Sommerfeld eigenvalue problem are obtained by employing the spectral collocation method. The numerical results imply that the stronger external shear destabilizes the interface mode instability. However, a stabilizing impact of the external shear on the surface mode is noticed if the shear is imposed in the flow direction, which is in contrast to the role of imposed external shear on the surface mode for a surfactant laden single layer falling film. Moreover, the impression of shear mode on the primary instability is analyzed in the high Reynolds number regime with sufficiently low inclination angle. Under such configuration, dominance of the shear mode over the surface mode is observed due to the weaker impact of the gravitation force on the surface instability. The shear mode can also be stabilized by applying the external shear in the counter direction of the streamwise flow. Conclusively, the extra imposed shear on the stratified two-layer falling film plays an active role to control the attitude of the instabilities.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"28 3 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129407748","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}

引用次数: 8

Correlation of internal flow structure with heat transfer efficiency in turbulent Rayleigh–Bénard convection 紊流rayleigh - bsamadard对流内部流动结构与换热效率的关系

arXiv: Fluid Dynamics Pub Date : 2020-09-16 DOI: 10.1063/5.0024408

A. Xu, Xin Chen, Feng Wang, H. Xi

{"title":"Correlation of internal flow structure with heat transfer efficiency in turbulent Rayleigh–Bénard convection","authors":"A. Xu, Xin Chen, Feng Wang, H. Xi","doi":"10.1063/5.0024408","DOIUrl":"https://doi.org/10.1063/5.0024408","url":null,"abstract":"To understand how internal flow structures manifest themselves in the global heat transfer, we study the correlation between different flow modes and the instantaneous Nusselt number ($Nu$) in a two-dimensional square Rayleigh-Benard convection cell. High-resolution and long-time direct numerical simulations are carried out for Rayleigh numbers between $10^{7}$ and $10^{9}$ and a Prandtl number of 5.3. The investigated Nusselt numbers include the volume-averaged $Nu_{text{vol}}$, the wall-averaged $Nu_{text{wall}}$, the kinetic energy dissipation based $Nu_{text{kinetic}}$, and the thermal energy dissipation based $Nu_{text{thermal}}$. The Fourier mode decomposition and proper orthogonal decomposition are adopted to extract the coherent flow structure. Our results show that the single-roll mode, the horizontally stacked double-roll mode, and the quadrupolar flow mode are more efficient for heat transfer on average. In contrast, the vertically stacked double-roll mode is inefficient for heat transfer on average. The volume-averaged $Nu_{text{vol}}$ and the kinetic energy dissipation based $Nu_{text{kinetic}}$ can better reproduce the correlation of internal flow structures with heat transfer efficiency than that of the wall-averaged $Nu_{text{wall}}$ and the thermal energy dissipation based $Nu_{text{thermal}}$, even though these four Nusselt numbers give consistent time-averaged mean values. The ensemble-averaged time trace of $Nu$ during flow reversal shows that only the volume-averaged $Nu_{text{vol}}$ can reproduce the overshoot phenomena that is observed in the previous experimental study. Our results reveal that the proper choice of $Nu$ is critical to obtain a meaningful interpretation.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"12 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-09-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123803893","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}

引用次数: 21

Experimental investigation of the return flow instability in magnetized spherical Couette flows 磁化球形库埃特流回流不稳定性的实验研究

arXiv: Fluid Dynamics Pub Date : 2020-09-15 DOI: 10.1063/5.0029570

J. Ogbonna, Ferran Garcia, T. Gundrum, M. Seilmayer, Frank Stefani

引用次数: 9

Simulations of Intracycle Angular Velocity Control for a Crossflow Turbine 横流涡轮循环内角速度控制仿真研究

arXiv: Fluid Dynamics Pub Date : 2020-09-13 DOI: 10.2514/1.J059797

Mukul Dave, B. Strom, Abigale Snortland, Owen J. H. Williams, B. Polagye, Jennifer A. Franck University of Wisconsin-Madison, XFlow Energy Company, U. Washington

{"title":"Simulations of Intracycle Angular Velocity Control for a Crossflow Turbine","authors":"Mukul Dave, B. Strom, Abigale Snortland, Owen J. H. Williams, B. Polagye, Jennifer A. Franck University of Wisconsin-Madison, XFlow Energy Company, U. Washington","doi":"10.2514/1.J059797","DOIUrl":"https://doi.org/10.2514/1.J059797","url":null,"abstract":"Straight-bladed cross-flow turbines are computationally explored for harvesting energy in wind and water currents. One challenge for cross-flow turbines is the transient occurrence of high apparent angles of attack on the blades that reduces efficiency due to flow separation. This paper explores kinematic manipulation of the apparent angle of attack through intracycle control of the angular velocity. Using an unsteady Reynolds-averaged Navier-Stokes (URANS) model at moderate Reynolds numbers, the kinematics and associated flow physics are explored for confined and unconfined configurations. The computations demonstrate an increase in turbine efficiency up to 54%, very closely matching the benefits shown by previous intracycle control experiments. Simulations display the time-evolution of angle of attack and flow velocity relative to the blade, which are modified with sinusoidal angular velocity such that the peak torque generation aligns with the peak angular velocity. With optimal kinematics in a confined flow there is minimal flow separation during peak power generation, however there is a large trailing edge vortex (TEV) shed as the torque decreases. The unconfined configuration has more prominent flow separation and is more susceptible to Reynolds number, resulting in a 41% increase in power generation under the same kinematic conditions as the confined flow.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"43 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121446812","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}

引用次数: 8

Self-sustaining and propagating mechanism of localized wave packet in plane-Poiseuille flow 平面泊泽维尔流中局部波包的自持与传播机制

arXiv: Fluid Dynamics Pub Date : 2020-09-11 DOI: 10.1063/5.0042346

Yue Xiao, J. Tao, Linsen Zhang

引用次数: 7

Wind- and gravity-forced drop depinning 风和重力迫使滴剥落

arXiv: Fluid Dynamics Pub Date : 2020-09-09 DOI: 10.1103/PHYSREVFLUIDS.6.023601

E. White, Jason A. Schmucker

引用次数: 7