摘要:

Regions in a high‐symmetry flow [J. Phys. Soc. Jpn.54, 2132 (1985); Phys. Fluids6, 2757 (1994)] where the pressure Hessian term remains diagonal for all times are identified. It is found that vorticity also vanishes in such regions. The components of the pressure Hessian tensor are equal only at the origin, toward which the vortices approach. For the case of isotropic pressure Hessian with no vorticity, and in the absence of viscosity, the resulting equations for the evolution of the eigenvalues and theQinvariant of the strain rate tensor are integrated numerically and analytically respectively. It is found that all the eigenvalues and theQinvariant diverge to infinity in finite time. In a recent simulation of a high‐symmetry flow [Phys. Fluids6, 2757 (1994)], it is found that the two (positive) eigenvalues of the strain rate tensor become equal as all the eigenvalues and theQinvariant show trends of divergence and as the vortices approach to the origin. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868563

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The dispersion of tracer particles in zero‐mean shear flows with no‐slip boundaries is found to be diffusive when the particle trajectories are chaotic and ergodically explore the flow domain. This diffusive spreading occurs even when the molecular diffusivity vanishes. The long‐time behavior for zero‐mean shear flows thus differs quantitatively from non‐zero mean shear flows. A model problem is formulated and the moment equations solved using techniques from the theory of random walks. Dispersion of ‘‘perfect’’ tracers in oscillatory Rayleigh–Be´nard convection is also found to behave diffusively. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868564

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experimental observations of various flows have led to the conclusion of the existence of streamwise vortices involved in the destabilization process of these flows. In the plane Couette flow, a linear shear flow, such structures have never been observed, because of the linear stability of the flow. The flow was slightly modified by introducing a wire in its central plane, parallel to the spanwise direction. A destabilization then occurs. It generates streamwise structures periodically spaced in the spanwise direction. These structures have been identified as pairs of counter‐rotating streamwise vortices. This Letter characterizes the dependence on the Reynolds number of the behavior of these vortices and how their destabilization leads to turbulence. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868565

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Batch sedimentation experiments were conducted with suspensions of noncolloidal spherical particles. Using nuclear magnetic resonance imaging (NMRI), the time evolution of the volume fraction versus height profile was measured for initial suspension volume fractions, &fgr;i, ranging from 0.08 to 0.44. NMRI clearly delineates the clear fluid layer at the top of the suspension, below which there is a transition to a region having the initial mean particle concentration. The hindered settling function determined from these data corresponds well with previous results. The spreading of the interface in excess of that expected from the combined effects of polydispersity and self‐sharpening was analyzed as a diffusion process. The measured values of the self‐induced hydrodynamic diffusivity agreed with those reported previously, and they decreased sharply for &fgr;i≳0.15. The concentration profile was also measured through the fan region into the sediment, where the volume fraction is near maximum packing, and determined the fan thickness as a function of time. The fan thickness is found to increase as the initial suspension volume fraction is increased. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868566

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The primary instability of axisymmetric steady thermocapillary flow in a cylindrical liquid bridge with non‐deformable free surface is calculated by a mixed Chebychev‐finite difference method. For unit aspect ratio the most dangerous mode has an azimuthal wavenumberm=2. The physical instability mechanisms are studied by analyzing the linear energy balance of the neutral mode. If the Prandtl number is small (Pr≪1), the bifurcation is stationary. The associated neutral mode is amplified in the shear layer close to the free surface. For large Prandtl number (Pr=4), the basic state becomes linearly unstable to a pair of hydrothermal waves propagating nearly azimuthally. Both mechanisms are compared with those previously proposed in the literature. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868567

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The aim of this paper is the derivation of the Ginzburg–Landau equation [as introduced by A. C. Newell and J. A. Whitehead, J. Fluid Mech.38, 279 (1969)] from the hydrodynamic equations for an infinite Hele–Shaw cell. The dimensional analysis and the asymptotic study allow one to distinguish two nonlinear formulations, each one depends on the order of magnitude of the Prandtl number. The first formulation corresponds to the case Pr=O(1) or Pr≫1, whereas the second corresponds to the case Pr=O(&egr;*2), where &egr;*≪1 denotes the aspect ratio of the cell. Here a weakly nonlinear analysis is performed for the two formulations. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868568

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Convection in a rotating annulus with no‐slip sidewalls, stress‐free ends, radial gravity, and sideways heating is considered. The transition from fully three‐dimensional convection cells to Taylor columns with increasing rotation rate is studied and its dependence on the annulus parameters is established. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868569

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The motion of an axisymmetric column of Navier–Stokes fluid with a free surface is considered. Due to surface tension, the thickness of the fluid neck goes to zero in finite time. After the singularity, the fluid consists of two halves, which constitute a unique continuation of the Navier–Stokes equation through the singular point. The asymptotic solutions of the Navier–Stokes equation are calculated, both before and after the singularity. The solutions have scaling form, characterized by universal exponents as well as universal scaling functions, which are computed without adjustable parameters. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868570

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Numerical solutions for the steady flow of an arbitrarily oriented incompressible capillary‐viscous jet in the presence of gravity are presented. A combination of the finite element method and an elliptic mesh generation technique are used to compute solutions over a wide range of the four relevant physical parameters: Reynolds number, Stokes number, capillary number, and angle of the jet nozzle with respect to gravity. It is shown that both gravity and surface‐tension forces can be instrumental in the mechanism of jet turning. In addition, it is found that multiple solutions are possible when the jet nozzle is directed close to vertically upward. The linear stability of the steady solutions to two‐dimensional disturbances is briefly considered. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868571

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A boundary element method (BEM) has been developed to solve for the nonlinear evolution of a liquid jet acting under the influence of both surface tension and the aerodynamic interactions with the surrounding atmosphere. For longer waves, aerodynamic effects are shown to cause a ‘‘swelling’’ of the liquid surface in the trough region. The model predicts the presence of satellite drops in the first wind‐induced regime, and predicts the evolution of a ‘‘spiked’’ surface at the periphery of the jet for conditions consistent with the second wind‐induced regime. The effects of the disturbance wave number, the liquid Weber number, and the density ratio between the liquid jet and the surrounding gas on the breakup of the jet have been examined. Transition points between various flow regimes have also been identified. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868572

出版商:AIP    

年代:1995

数据来源: AIP