摘要:

The phenomenon of Brinkman screening, whereby the fluid velocity disturbance produced by each particle in a fixed bed is attenuated by the forces that the fluid exerts on surrounding particles, plays a crucial role in limiting the range of velocity correlations in porous media and fixed beds. Koch and Brady [J. Fluid Mech.154, 399 (1985)] showed theoretically that Brinkman screening leads to a finite hydrodynamic diffusion coefficient for fluid phase tracers in dilute fixed beds. In this Letter, we present the results of two simulation techniques (lattice-Boltzmann method and a multipole method) confirming the screening of the fluid velocity covariance that underlies the Koch and Brady theory. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869830

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The impact of a water drop on a thin layer of glycerin leads to the formation of an intricate flower-like pattern. We show that these leaf-like forms are generated by a surface-tension instability at the air–liquid interface along which there exists variable concentration of glycerin and water. Spatial variations of surface tension drive intense vortices inside the water layer, which interact with the glycerin–water concentration at the surface. Horizontal bending of these vortices is reinforced by the resulting enhancement of the surface-tension gradients. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869831

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The dynamic procedure is applied to the problem of modeling the subgrid-scale variance and dissipation rate of a conserved scalar in large eddy simulations of turbulent reacting flows. A simple scaling relation for the subgrid-scale variance is proposed, and the coefficient of the scaling law is obtained using the dynamic procedure. The variance dissipation rate is modeled by assuming equilibrium with the local variance production rate, which is obtained using a dynamic model. Example model predictions are obtained using actual large eddy simulation data, and the subgrid variance predicted by the dynamic model is compared to results obtained using a scale similarity model. Generalization of the approach to multiple scalars and nonconserved scalars is briefly discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869832

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We investigate the centrifugal instability of a pair of radially stratified immiscible liquids in the annular gap between concentric, corotating cylinders: two-fluid Taylor–Couette flow. Experiments show that a two-layer flow with a well-defined interface and Taylor vortices in each phase can be obtained. The experimental results are in good agreement with predictions of inviscid arguments based on a two-phase extension of Rayleigh’s criterion, as well as with detailed linear stability calculations. For a given geometry, the most stable configuration occurs for fluids of roughly (exactly in the inviscid limit) equal dynamic viscosities. For fluids with sufficiently low viscosity, we also experimentally observe another instability that is evidently the two-fluid analog of the instability of a thin film of liquid coating the inside of a rotating drum. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869833

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The flow of two viscous liquids is investigated numerically with a volume of fluid scheme. The scheme incorporates a semi-implicit Stokes solver to enable computations at low Reynolds numbers, and a second-order velocity interpolation. The code is validated against linear theory for the stability of two-layer Couette flow, and weakly nonlinear theory for a Hopf bifurcation. Examples of long-time wave saturation are shown. The formation of fingers for relatively small initial amplitudes as well as larger amplitudes are presented in two and three dimensions as initial-value problems. Fluids of different viscosity and density are considered, with an emphasis on the effect of the viscosity difference. Results at low Reynolds numbers show elongated fingers in two dimensions that break in three dimensions to form drops, while different topological changes take place at higher Reynolds numbers. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869834

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The granular flow through an open silo is investigated experimentally. A mechanism based both on the dilation of the granular medium and an interaction with the interstitial gas causes the flow to stop at regular intervals. The experiments are carried out at different surrounding pressuresP0,and it is found that the intermittent flow becomes continuous at sufficiently lowP0,showing that the intermittency is linked to the interaction the gas. The scaling of the average flow rate with particle size further supports our view of the gas-grain interaction. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869835

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We consider small disturbances to the steady viscous flow between concentric rotating circular cylinders. It is shown that as the dimensionless gap width &dgr; approaches zero the terms from ∂/∂&thgr; in the governing equations areO(&dgr;1/2)and so do not appear in the first approximation. This is in contrast to the classical small-gap approximation, in which terms from ∂/∂&thgr; do appear in the first approximation. Here &thgr; is the usual azimuthal angle. Let &egr; be a measure of the size of the small perturbation. In Sec. III it is argued that the asymptotic relation&egr;∼&dgr;2is needed to obtain a correct approximation to weakly nonlinear axisymmetric (Taylor-vortex) equilibrium flow for small values of &dgr;. We use this relation between &egr; and &dgr; to obtain a unified amplitude equation to describe both axisymmetric modes and those with a general dependence on the azimuthal angle, taking into account weakly nonlinear effects. This allows a description of single-mode equilibrium states and of their stability. Some unexpected results are obtained about the stability of these modes. The mode withm=0is shown to be stable for all relevant values of the Taylor numberT. This is to be expected under the particular limiting process used. However the modes withm=1,2, and 3 are shown to become stable for successivelyhighervalues ofT. It is also shown that these nonaxisymmetric equilibrium states may be obtained by the solution of some initial value problems, thus leaving open the possibility of attaining wavy disturbances by a different method than the usual one of instability of Taylor vortices. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869836

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The linear stability of a dielectric fluid filling a cylindrical capacitor is analyzed. The outer or the inner cylinders forming the capacitor may rotate with fixed angular velocities, thus generating a Taylor-Couette flow inside the fluid. A weak, moderate or strong injection of charge is assumed to occur at the anode when a high voltage is applied to the electrodes. The physical mechanisms associated to the instability of the steady flow are discussed. Special attention is paid to the case when the mobility of ions is similar to or smaller than the hydrodynamic mobility of the fluid. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869837

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The formation two-dimensional dipolar vortices by the interaction between two shielded monopolar vortices with opposite vorticity, as shown in a numerical study by Couder and Basdevant,1is investigated in detail, both experimentally, in a nonrotating stratified fluid and numerically by direct solutions of the two-dimensional Navier–Stokes equations. A comparative study between the laboratory experiments and numerical simulations is performed. The vorticity distribution measured in the early stage of the evolution in the laboratory is used as initial data for the simulations, and an additional damping term in the Navier–Stokes equations, that accounts for the vertical diffusion in the laboratory experiments, is used. The results show that, depending on the initial separation between the vortices, the shields of the monopoles are peeled off and indeed a compact dipole with a linear (&ohgr;,&psgr;)-relationship is formed, or when the monopoles are further apart the shields of the monopoles are perturbed and two tripoles are formed. The characteristics of the emerged dipole are analyzed and a dye visualization of the dipole formation is performed. A second, more general numerical study yields a relationship between the formation time of the dipole and the initial separation distance between the monopoles and it shows that the deshielding process can be explained by the domination of strain over vorticity. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869838

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present a numerical investigation of the linear response of a turbulent flow to a perturbation at large scale. In the incompressible case, we show that part of the small scale action corresponds to the standardturbulent viscosity, but that there are also other terms which are important for the system dynamics and which do not reduce to a simple turbulent viscous effect. In the compressible case, we show that the small scales of the turbulent flow create aturbulent pressureforce which resists compression. Anadiabatic indexis estimated for this pressure and is found to decrease with time scale, from a stronglyadiabaticbehavior down to asubisothermalbehavior. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869839

出版商:AIP    

年代:1998

数据来源: AIP