摘要:

The variance in the sedimentation speed for a homogeneous suspension of solid spheres in a Stokes fluid is calculated for a particular choice of the distribution function of the spheres. In the infinite particle number limit, the variance is found to be infinite.

ISSN:0031-9171    

DOI:10.1063/1.865095

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A new mechanism that can destabilize kinetic ion‐cyclotron waves in the presence of a nonuniform electric field perpendicular to the uniform ambient magnetic field is given. In the absence of the electric field, the mode energy is positive, while in the presence of a uniform electric field the mode energy could be negative. However, when the electric field is nonuniform, it is possible for a finite region to be a negative wave energy surrounded by regions of positive wave energy. A nonlocal wave packet couples the two regions so that a flow of energy from the region of negative wave energy to the region of positive wave energy will cause the mode to brow. This gives rise to the instability.

ISSN:0031-9171    

DOI:10.1063/1.865096

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A gyrating electron beam supports negative energy modes near the harmonics of electron‐cyclotron frequency. An electromagnetic wave passing through such a beam parametrically up‐converts into high‐frequency electromagnetic modes separated from the pump frequency by the electron‐cyclotron harmonics. The growth rate for this process varies directly as the oscillatory velocity of beam electrons caused by the pump and as square root of the beam density. It has a maximum at values of scattering angle close to 180° and is also implicitly dependent on the beam velocity and the cyclotron frequency of electrons. The effect of a cold electron component is to reduce the growth rate.

ISSN:0031-9171    

DOI:10.1063/1.865097

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

It is shown, by explicit calculation, that the influence of a plane wall supporting the suspension on the sedimentation velocity is such that the convergence problems of this quantity encountered in an unbounded suspension do not occur—even in the limit of an infinitely distant wall.

ISSN:0031-9171    

DOI:10.1063/1.865098

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A KdV equation modified by viscosity is derived for weakly nonlinear long waves propagating in a channel of uniform but arbitrary cross section. The case of high Reynolds number is considered, and the method of matched asymptotic expansion is employed. The equation derived here is found to be similar to the corresponding equation for a two‐dimensional layer of liquid derived by previous authors. The only difference is that the dispersive, nonlinear, and viscous terms are multiplied by constants dependent on the cross section geometry of the channel.

ISSN:0031-9171    

DOI:10.1063/1.865044

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

An experimental study has been made of the hydrodynamic stability of viscous fluid flow contained between the differentially heated walls of a tall vertical annulus. Tests were conducted using different glycerol solutions spanning a range of Rayleigh numbers (104–106) and moderately high Prandtl numbers (15–150) for a height/gap ratio of 64 and a radius ratio equal to 0.62. Visualization studies show that the unstable flow consists of two separate progressive wave systems, one ascending the hot inner wall and the other descending the cold outer wall. The approximate Rayleigh number, phase speed, and wavelength at the onset of instability for each wave system were determined for several Prandtl numbers. The regular wave patterns observed near the stability boundary persist over only a very limited range of supercritical Rayleigh numbers above which flow dislocations set in. At higher Rayleigh numbers these random dislocations appear in increasing numbers and ultimately render the flow turbulent.

ISSN:0031-9171    

DOI:10.1063/1.865045

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A linear stability analysis for a Be´nard problem with two layers is considered. The equations are not self‐adjoint. The system can lose stability to time‐periodic disturbances. For example, it is shown numerically that when the viscosities and coefficients of cubical expansion of the fluids are different, a Hopf bifurcation can occur, resulting in a pair of traveling waves or a standing wave. This may have application in the modeling of convection in the Earth’s mantle.

ISSN:0031-9171    

DOI:10.1063/1.865046

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A single filament with a variable core size parameter is used to model how a vortex tube breaks down in the Euler equations. The first singularity is a self‐similar collapse which brings two antiparallel pieces of filament together at a point. This pairing then quickly encompasses a finite fraction of the initial data and the arclength begins to grow faster than exponential. A local model should exist which would allow one to understand the stage of rapid stretching in terms of simpler processes.

ISSN:0031-9171    

DOI:10.1063/1.865047

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Analytical solution of the equations of motion of a spherical particle suspended in a turbulent flow near a plane wall has been obtained. The equations include the lift force and wall effects on the drag force. The solution shows that the particle turbulent motion is affected by the wall presence in the following manner: (a) The wall augments the response of the particle to fluid turbulence. The ratio between the particle rms velocity fluctuation near the wall and that of an identical particle in an unbounded flow is always greater than unity. This ratio increases by increasing the particle density and diameter and decreasing the particle distance from the wall. (b) Wall effects in a direction normal to it are more pronounced than those in the parallel direction. This is attributed mainly to the lift force acting in the normal direction. (c) Effects of the drag force on particle intensity are confined close to the wall whereas the lift effects extend to larger distances.

ISSN:0031-9171    

DOI:10.1063/1.865048

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A simple, new method of measuring velocity and temperature simultaneously without imparting any disturbances to the fluid is developed. The key idea is to use a certain liquid that undergoes thermochromism as a test fluid, the temperature of which can be observed directly by its color change or measured quantitatively by a suitable optical method. If a laser Doppler velocimeter is used, the temperature at the measuring point, in addition to the velocity, can be determined simultaneously and instantaneously by detecting the otherwise discarded intensity of the transmitted light of the laser beams. This method was applied to measurements of distributions of velocity and temperature of a plume in a Hele–Shaw cell. Results were in fairly good agreement with similarity laws derived theoretically in this paper.

ISSN:0031-9171    

DOI:10.1063/1.865049

出版商:AIP    

年代:1985

数据来源: AIP