摘要:

Inappropriate initial conditions discovered in a recently reported model calculation of the thermal mixing layer are corrected. Good agreement with measurement is restored by a reoptimization of model coefficients.

ISSN:0031-9171    

DOI:10.1063/1.865020

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A modified version of the drag theory of permeability for a granular porous medium is presented. The technique employs the Brinkman equation to interpolate between the microscopic (Stokes) equation and the macroscopic (Darcy) equation, but differs from previous treatments in that the medium is built up iteratively beginning with the smallest grains. A discussion of the geometry corresponding to this construction is presented. The result for the permeability takes the form of a nonlinear differential equation which may be integrated numerically for a given choice of porosity and grain size distribution. Numerical results for the case of spherical grains with a log‐normal distribution of sizes are presented, and compared to the predictions of the Kozeny equation for the same geometry. It is found that for narrow distributions the present method gives a somewhat higher permeability than the Kozeny equation, but that for wide distributions it gives a lower permeability. It is shown that in the limits of narrow or wide distribution the differential equation for the permeability may be solved explicitly, and it is suggested that in the latter case the result may be exact.

ISSN:0031-9171    

DOI:10.1063/1.865021

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A compound liquid drop system is comprised of three immiscible concentric fluids: a core fluid of density &rgr;(i)surrounded by a shell of density &rgr;(s)which is embedded in a medium of density &rgr;(0). In this analysis, the fluids are incompressible and inviscid. The effect of rotation upon the modes of oscillation of a compound drop is investigated. Rotation rate is considered as a small perturbation of the normal modes of the compound drop, thus introducing the effects of a Coriolis force and a centrifugal distortion.

ISSN:0031-9171    

DOI:10.1063/1.865022

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The region of buoyant fluid resulting from the initiation of heating of an infinite fluid is called the starting plume. Here, velocity field measurements of this flow pattern are presented for the first time. The measurements were carried out by processing digitized tracer particle path photographs. Similarity of the velocity field of the starting plume as it rises was found to exist in the limited range of heat injection rates investigated. The vorticity computed from the velocity field was diffuse, there being no evidence of a distinctive core. Additional flow visualization experiments show the spiral structure in the starting plume cap.

ISSN:0031-9171    

DOI:10.1063/1.865023

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The Wilton ripples phenomenon with a background current is examined in detail. The case of most interest occurs when the current is near the threshold value such that direct resonance occurs for the Wilton ripples. Interaction between the fundamental mode and its second harmonic occurs on a much faster time scale. Bifurcation diagrams and stability analysis are presented.

ISSN:0031-9171    

DOI:10.1063/1.865024

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

This study considers a large‐scale flow maintained by a small‐scale periodic force field. If the field is essentially anisotropic, the system of small‐scale eddies thereby generated turns out to be unstable to long‐wave disturbances, i.e., the effective viscosity of the corresponding large‐scale flow is negative. If the applied field is sufficiently isotropic, the long‐wave instability disappears so that the effective viscosity is positive.

ISSN:0031-9171    

DOI:10.1063/1.865025

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Velocity and wall shear stress measurements were made along the centerline of turbulent spots generated in a laminar boundary layer on a flat plate. A momentum integral calculation method based on two‐dimensional boundary layer theory and the assumption of potential flow above the spot was developed by transforming the unsteady momentum equation into nondimensional similarity coordinates. The centerline wall shear stress, calculated using ensemble‐averaged data for the velocity field, shows fairly good agreement with the measured values.

ISSN:0031-9171    

DOI:10.1063/1.865026

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The joint probability density function of the temperature &thgr; and its instantaneous dissipation &khgr; is important for the development of turbulent reacting flow models. Measurements are presented, in the nearly self‐preserving region of a turbulent plane jet, of the joint statistics of &thgr; and an approximation &khgr;1to &khgr;. Properties of the three, separately obtained, dissipation components are first compared to justify the choice of &khgr;1. The quantity chosen is an appropriately weighted linear combination of the squared values of the temporal derivative, shown to be an adequate substitution for the derivative in the streamwise direction, and the derivative, in either the lateral or spanwise directions, of temperature. The correlation between &thgr; and &khgr;1is weak, receiving contributions primarily at small frequencies. The assumption of independence between &thgr; and &khgr;1, allowing the joint probability density function of these two quantities to be written as a product of the individual probability density functions, improves with distance from the jet centerline.

ISSN:0031-9171    

DOI:10.1063/1.865027

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The evolution of weak double layers in ion‐acoustic turbulence in one and two‐dimensional particle simulations is examined. Weak double layers (e&Jgr;≲Te) evolve in simulations when a subthermal electron drift is imposed on a long or nonperiodic system withTe/Ti≫1. Their growth rate increases with the electron drift, and they decay because of ion trapping. They do not form in weakly magnetized or unmagnetized two‐dimensional (2‐D) systems unless a nonuniformity is introduced in the initial or boundary conditions. When the plasma is strongly magnetized (&ohgr;ce>&ohgr;pe), they emerge from 2‐D ion‐acoustic turbulence as coherent structures localized transversely to the magnetic field.

ISSN:0031-9171    

DOI:10.1063/1.865028

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In this study, properties of small‐amplitude parallel and oblique whistler mode waves are investigated for a wide range of plasma parameters by numerically solving the full electromagnetic Vlasov dispersion equation. To investigate the cold‐plasma and electrostatic approximations for the whistler mode, the results are compared with results obtained using these descriptions. For large wavelengths (kc/&ohgr;i≲100), the cold‐plasma description is often accurate, while for short wavelengths (kc/&ohgr;i≳100) and sufficiently oblique propagation, the electrostatic description is often accurate. The study demonstrates that in a Vlasov plasma the whistler mode near resonance has a group velocity more nearly parallel to the magnetic field than that predicted by cold‐plasma theory.

ISSN:0031-9171    

DOI:10.1063/1.865029

出版商:AIP    

年代:1985

数据来源: AIP