arXiv: Fluid Dynamics最新文献_第4页

Subcritical dynamos in rapidly rotating planar convection 快速旋转平面对流中的亚临界发电机

arXiv: Fluid Dynamics Pub Date : 2020-11-02 DOI: 10.1103/physrevfluids.5.113702

R. G. Cooper, P. Bushby, C. Guervilly

{"title":"Subcritical dynamos in rapidly rotating planar convection","authors":"R. G. Cooper, P. Bushby, C. Guervilly","doi":"10.1103/physrevfluids.5.113702","DOIUrl":"https://doi.org/10.1103/physrevfluids.5.113702","url":null,"abstract":"We study dynamo action using numerical simulations of planar Boussinesq convection at rapid rotation (low Ekman numbers, Ek), focusing on subcritical dynamo action in which the dynamo is sustained for Rayleigh numbers, Ra, below the critical Rayleigh number for the onset of nonmagnetic convection, Ra$_c$. These solutions are found by first investigating the supercritical regime, in which the dynamo is able to generate a large-scale magnetic field that significantly influences the convective motions, with an associated Elsasser number of order Ek$^{1/3}$. Subcritical solutions are then found by tracking this solution branch into the subcritical regime, taking a supercritical solution and then gradually lowering the corresponding Rayleigh number. We show that decreasing the Ekman number leads to an extension of the subcritical range of Ra/Ra$_c$, down to an optimal value of Ek$=10^{-5}$. For magnetic Prandtl numbers of order unity, subcriticality is then hampered by the emergence of a large-scale mode at lower Ekman numbers when the dynamo driven by the smaller scale convection generates relatively stronger large-scale magnetic field. The inability of the large-scale mode to sustain dynamo action leads to an intermittent behaviour that appears to inhibit subcriticality. The subcritical solutions are also sensitive to the value of the magnetic Reynolds number (or equivalently, the magnetic Prandtl number, Pm), as values of the magnetic Reynolds number greater than 70 are required to produce dynamo action, but large values lead to fluctuations that are able to push the system too far from the subcritical branch and towards the trivial conducting state.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"93 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125004219","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: 3

How to build coarse-grain transport models consistent from the kinetic to fluid regimes 如何建立从动力学到流体相一致的粗粒输运模型

arXiv: Fluid Dynamics Pub Date : 2020-11-01 DOI: 10.1063/5.0037133

E. Torres, Georgios Bellas Chatzigeorgis, T. Magin

{"title":"How to build coarse-grain transport models consistent from the kinetic to fluid regimes","authors":"E. Torres, Georgios Bellas Chatzigeorgis, T. Magin","doi":"10.1063/5.0037133","DOIUrl":"https://doi.org/10.1063/5.0037133","url":null,"abstract":"In this paper, we examine how to build coarse-grain transport models consistently from the kinetic to fluid regimes. The internal energy of the gas particles is described through a state-to-state approach. A kinetic equation allows us to study transport phenomena in phase space for a non-homogeneous gas mixture. Internal energy excitation is modeled using a binary collision operator, whereas the gas chemical processes rely on a reactive collision operator. We obtain an asymptotic fluid model by means of a Chapman-Enskog perturbative solution to the Boltzmann equation in the Maxwellian reaction regime. The macroscopic conservation equations of species mass, mixture momentum, and energy are given, as well as expressions of the transport properties. Reversibility relations for elementary processes are formulated in the coarse-grain model at the kinetic level and are enforced in the collision routines of the direct simulation Monte Carlo method used to solve the kinetic equation. Furthermore, respecting these reversibility relations is key to deriving a fluid model that is well-posed and compatible with the second law of thermodynamics. Consistency between the kinetic and fluid simulations is assessed for the simulation of a shock wave in a nitrogen gas using the Uniform RoVibrational Collisional coarse-grain model. The kinetic and fluid simulations show good agreement for the macroscopic properties and transport fluxes.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"15 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123034183","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: 6

Realization of incompressible Navier–Stokes flow as superposition of transport processes for Clebsch potentials 不可压缩Navier-Stokes流作为克莱施势输运过程叠加的实现

arXiv: Fluid Dynamics Pub Date : 2020-10-28 DOI: 10.1063/5.0035339

N. Sato

{"title":"Realization of incompressible Navier–Stokes flow as superposition of transport processes for Clebsch potentials","authors":"N. Sato","doi":"10.1063/5.0035339","DOIUrl":"https://doi.org/10.1063/5.0035339","url":null,"abstract":"In ideal fluids, Clebsch potentials occur as paired canonical variables associated with the Hamiltonian description of the Euler equations. This paper explores the properties of the incompressible Navier-Stokes equations when the velocity field is expressed through a complete set of paired Clebsch potentials. First, it is shown that the incompressible Navier-Stokes equations can be cast as a system of transport (convection-diffusion) equations where each Clebsch potential plays the role of a generalized distribution function. The diffusion operator associated with each Clebsch potential departs from the standard Laplacian due to a term depending on the Lie-bracket of the corresponding Clebsch pair. It is further shown that the Clebsch potentials can be used to define a Shannon-type entropy measure, i.e. a functional, different from energy and enstrophy, whose growth rate is non-negative. As a consequence, the flow must vanish at equilibrium. This functional can be interpreted as a measure of the topological complexity of the velocity field. In addition, the Clebsch parametrization enables the identification of a class of flows, larger than the class of two dimensional flows, possessing the property that the vortex stretching term identically vanishes and the growth rate of entrophy is non-positive.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"33 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-10-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122686435","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: 1

General method for determining the boundary layer thickness in nonequilibrium flows 确定非平衡流动边界层厚度的一般方法

arXiv: Fluid Dynamics Pub Date : 2020-10-27 DOI: 10.1103/PHYSREVFLUIDS.6.024608

K. Griffin, Lin Fu, P. Moin

{"title":"General method for determining the boundary layer thickness in nonequilibrium flows","authors":"K. Griffin, Lin Fu, P. Moin","doi":"10.1103/PHYSREVFLUIDS.6.024608","DOIUrl":"https://doi.org/10.1103/PHYSREVFLUIDS.6.024608","url":null,"abstract":"While the computation of the boundary-layer thickness is straightforward for canonical equilibrium flows, there are no established definitions for general non-equilibrium flows. In this work, a method is developed based on a local reconstruction of the \"inviscid\" velocity profile $U_I[y]$ resulting from the application of the Bernoulli equation in the wall-normal direction. The boundary-layer thickness $delta_{99}$ is then defined as the location where $U/U_I = 0.99$, which is consistent with its classical definition for the zero-pressure-gradient boundary layers (ZPGBLs). The proposed local-reconstruction method is parameter free and can be deployed for both internal and external flows without resorting to an iterative procedure, numerical integration, or numerical differentiation. The superior performance of the local-reconstruction method over various existing methods is demonstrated by applying the methods to laminar and turbulent boundary layers and two flows over airfoils. Numerical experiments reveal that the local-reconstruction method is more accurate and more robust than existing methods, and it is applicable for flows over a wide range of Reynolds numbers.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"80 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122145283","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: 25

Supergranule aggregation for constant heat flux-driven turbulent convection 恒热流驱动湍流对流的超颗粒聚集

arXiv: Fluid Dynamics Pub Date : 2020-10-26 DOI: 10.1103/PHYSREVRESEARCH.3.013231

Philipp P. Vieweg, J. Scheel, J. Schumacher

{"title":"Supergranule aggregation for constant heat flux-driven turbulent convection","authors":"Philipp P. Vieweg, J. Scheel, J. Schumacher","doi":"10.1103/PHYSREVRESEARCH.3.013231","DOIUrl":"https://doi.org/10.1103/PHYSREVRESEARCH.3.013231","url":null,"abstract":"Turbulent convection processes in nature are often found to be organized in a hierarchy of plume structures and flow patterns. The gradual aggregation of convection cells or granules to a supergranule which eventually fills the whole horizontal layer is reported and analysed in spectral element direct numerical simulations of three-dimensional turbulent Rayleigh-Benard convection at an aspect ratio of 60. The formation proceeds over a time span of more than $10^4$ convective time units for the largest accessible Rayleigh number and occurs only when the turbulence is driven by a constant heat flux which is imposed at the bottom and top planes enclosing the convection layer. The resulting gradual inverse cascade process is observed for both temperature variance and turbulent kinetic energy. An additional analysis of the leading Lyapunov vector field for the full turbulent flow trajectory in its high-dimensional phase space demonstrates that turbulent flow modes at a certain scale continue to give rise locally to modes with longer wavelength in the turbulent case. As a consequence successively larger convection patterns grow until the horizontal extension of the layer is reached. This instability mechanism, which is known to exist near the onset of constant heat flux-driven convection, is shown here to persist into the fully developed turbulent flow regime thus connecting weakly nonlinear pattern formation with the one in fully developed turbulence. We discuss possible implications of our study for observed, but not yet consistently numerically reproducible, solar supergranulation which could lead to improved simulation models of surface convection in the Sun.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"19 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-10-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124152821","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

Dynamics of a reactive spherical particle falling in a linearly stratified fluid 反应性球形颗粒在线性分层流体中下落的动力学

arXiv: Fluid Dynamics Pub Date : 2020-10-23 DOI: 10.1103/physrevfluids.5.114803

L. Huguet, Victor Barge-Zwick, M. L. Bars

{"title":"Dynamics of a reactive spherical particle falling in a linearly stratified fluid","authors":"L. Huguet, Victor Barge-Zwick, M. L. Bars","doi":"10.1103/physrevfluids.5.114803","DOIUrl":"https://doi.org/10.1103/physrevfluids.5.114803","url":null,"abstract":"Motivated by numerous geophysical applications, we have carried out laboratory experiments of a reactive (i.e. melting) solid sphere freely falling by gravity in a stratified environment, in the regime of large Reynolds ($Re$) and Froude numbers. We compare our results to non-reactive spheres in the same regime. First, we confirm for larger values of $Re$, the stratification drag enhancement previously observed for low and moderate Re (Magnaudet and Mercier, 2020). We also show an even more significant drag enhancement due to melting, much larger than the stratification-induced one. We argue that the mechanism for both enhancements is similar, due to the specific structure of the vorticity field sets by buoyancy effects and associated baroclinic torques, as deciphered for stratification by Zhang et al. (2019). Using particle image velocimetry, we then characterize the long-term evolution (at time $t gg 1/N$ with $N$ the Br\"unt-V\"ais\"al\"a frequency) of the internal wave field generated by the wake of the spheres. Measured wave field is similar for both reactive and inert spheres: indeed, each sphere fall might be considered as a quasi impulsive source of energy in time and the horizontal direction, as the falling time (resp. the sphere radius) is much smaller than $N$ (resp. than the tank width). Internal gravity waves are generated by wake turbulence over a broad spectrum, with the least damped component being at the Br\"unt-V\"ais\"al\"a frequency and the largest admissible horizontal wavelength. About 1% of the initial potential energy of each sphere is converted in to kinetic energy in the internal waves, with no significant dependence on the Froude number over the explored range.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"35 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-10-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115851966","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: 0

Intermittent unsteady propulsion with a combined heaving and pitching foil 起伏和俯仰相结合的间歇非定常推进

arXiv: Fluid Dynamics Pub Date : 2020-10-20 DOI: 10.1103/PHYSREVFLUIDS.6.043101

E. Akoz, A. Mivehchi, K. Moored

{"title":"Intermittent unsteady propulsion with a combined heaving and pitching foil","authors":"E. Akoz, A. Mivehchi, K. Moored","doi":"10.1103/PHYSREVFLUIDS.6.043101","DOIUrl":"https://doi.org/10.1103/PHYSREVFLUIDS.6.043101","url":null,"abstract":"Inviscid computations are presented of a self-propelled virtual body connected to a combined heaving and pitching foil that uses continuous and intermittent motions. It is determined that intermittent swimming can improve efficiency when the dimensionless heave ratio is $h^* 0.5$) motions. Based on insight derived from classic unsteady thin airfoil theory, it is discovered that pitch-dominated motions are driven by added-mass-based thrust production where self-propelled efficiency is maximized for high reduced frequencies, while heave-dominated motions are driven by circulatory-based thrust production where self-propelled efficiency is maximized by low reduced frequencies. Regardless of the dimensionless heave ratio the reduced frequency is high for small amplitude motions, high Lighthill numbers, and low duty cycles and vice versa. Moreover, during intermittent swimming, the stopping vortex that is shed at the junction of the bursting and coasting phases becomes negligibly weak for $h^* < 0.5$ and small amplitude motions of $A^* = 0.4$. This study provides insight into the mechanistic trade-offs that occur when biological or bio-inspired swimmers continuously or intermittently use combined heaving and pitching hydrofoils.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"41 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123363393","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: 3

Thixotropy in viscoplastic drop impact on thin films 触变性在粘塑性滴对薄膜的影响

arXiv: Fluid Dynamics Pub Date : 2020-10-14 DOI: 10.1103/PHYSREVFLUIDS.6.043301

S. Sen, Anthony G. Morales, R. Ewoldt

{"title":"Thixotropy in viscoplastic drop impact on thin films","authors":"S. Sen, Anthony G. Morales, R. Ewoldt","doi":"10.1103/PHYSREVFLUIDS.6.043301","DOIUrl":"https://doi.org/10.1103/PHYSREVFLUIDS.6.043301","url":null,"abstract":"We use high-speed imaging to study the effect of thixotropic aging in drop impact of yield-stress fluids on pre-coated substrates. Our results reveal that drop splashing is suppressed for \"aged\" compared to \"unaged\" samples, indicating that thixotropic breakdown timescales during impact are long enough to affect the dynamics. We propose and test several hypotheses for modifying the dimensionless group ${rm IF}(D/t)$ [1,2] to account for thixotropic aging. The main challenge is that the steady flow properties (Herschel-Bulkley model parameters) used in the current dimensionless group cannot be defined or measured for thixotropically aged samples, because any deformation inherently rejuvenates and breaks down the microstructure. We find the most suitable hypothesis is to only increase the yield stress ($sigma_{rm y}$, plastic component) based on the storage modulus of aged samples, while keeping the viscous parameters ($K$ or $eta_{infty}$) constant. The work reveals fundamental insight into rarely studied short-timescale flow conditions with thixotropic effects. These results are important for applications such as fire suppression or spray coating that involve complex fluids of varying degrees of thixotropic aging.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"92 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-10-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133861451","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: 13

Effects of aspect ratio on rolling and twisting foils 长径比对轧制和扭转箔的影响

arXiv: Fluid Dynamics Pub Date : 2020-10-14 DOI: 10.1103/PHYSREVFLUIDS.6.013101

A. N. Zurman-Nasution, B. Ganapathisubramani, G. Weymouth

引用次数: 3

Overstable rotating convection in the presence of a vertical magnetic field 在垂直磁场存在下的超稳定旋转对流

arXiv: Fluid Dynamics Pub Date : 2020-10-13 DOI: 10.1063/5.0035555

Ankan Banerjee, M. Ghosh, Lekha Sharma, P. Pal

{"title":"Overstable rotating convection in the presence of a vertical magnetic field","authors":"Ankan Banerjee, M. Ghosh, Lekha Sharma, P. Pal","doi":"10.1063/5.0035555","DOIUrl":"https://doi.org/10.1063/5.0035555","url":null,"abstract":"We present the results of our investigation on nonlinear overstable rotating magnetoconvection (RMC) in presence of vertical external magnetic field. We focus on the dynamics appearing near the onset of convection by varying the system control parameters, namely, the Taylor number ($mathrm{Ta}$), the Chandrasekhar number ($mathrm{Q}$) and the Prandtl number ($mathrm{Pr}$) in the ranges $750leqmathrm{Ta}leq10^6$, $0 < mathrm{Q} leq 10^3$ and $0 < mathrm{Pr} leq 0.5$. Three dimensional (3D) direct numerical simulations (DNS) of the governing equations and low-dimensional modeling of the system are performed for this purpose. Extensive DNS in the specified parameter space shows two qualitatively different onsets depending on $mathrm{Ta}$, $mathrm{Q}$ and $mathrm{Pr}$. In the first one, bistability appears at the onset, where both subcritical and supercritical convection coexist, while only supercritical convection is observed in the second one. Analysis of the low-dimensional model reveals that a supercritical Hopf bifurcation is responsible for the supercritical onset and a subcritical pitchfork bifurcation is responsible for the subcritical onset. It is also observed that appearance of subcritical convection at the onset has strong dependence on all three control parameters $mathrm{Ta}$, $mathrm{Q}$ and $mathrm{Pr}$. The scenario of subcritical convection is found to disappear as $mathrm{Pr}$ is increased for fixed $mathrm{Ta}$ and $mathrm{Q}$. However, most striking findings of the investigation is that the increment in $mathrm{Ta}$ for fixed $mathrm{Q}$ and $mathrm{Pr}$ opposes the subcritical convection, whereas the increment in $mathrm{Q}$ for fixed $mathrm{Ta}$ and $mathrm{Pr}$ favors it. This is in sharp contrast with the earlier results reported in RMC.","PeriodicalId":328276,"journal":{"name":"arXiv: Fluid Dynamics","volume":"03 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-10-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130551569","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: 0