摘要:

The model problem of a gas confined between stationary heated plates is analyzed numerically to quantify slip effects for gas densities ranging from near‐continuum to highly rarefied conditions. Calculations are performed with the direct simulation Monte Carlo technique using two gas–surface interaction models and with a finite‐difference Navier–Stokes method using slip boundary conditions. Comparisons are made with experimental density profiles and with previous discrete ordinate calculations using a model form of the Boltzmann equation. To the resolution of the experimental data, both direct simulation Monte Carlo and the Navier–Stokes equations with slip provide very good agreement for essentially all cases considered, while the discrete ordinate results are less accurate. The calculated pressure and temperature profiles show slightly larger differences. The details of the gas–surface interaction model have a relatively small effect on calculated density profiles. The direct simulation Monte Carlo and no‐slip Navier–Stokes results are found to agree quantitatively for all properties at very small Knudsen number [O(10−3)]. Detailed direct simulation Monte Carlo quantities near the plates are used to quantify discrepancies in the continuum slip models and suggest possible improvements.

ISSN:0899-8213    

DOI:10.1063/1.858804

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The stability of the flow over a flat plate to finite amplitude disturbances is studied using a model by Milinazzo and Saffman [J. Fluid Mech.160, 281 (1985)]. The traveling wave solutions which bifurcate from the Orr–Sommerfeld neutral stability curve of the Blasius profile are calculated numerically. This curve is the intersection of the zero energy plane with a neutral stability surface in energy, Reynolds number, and wave‐number space. Although the bifurcation is supercritical at the ‘‘nose’’ of this curve, it is shown that it is subcritical elsewhere and that the minimum Reynolds number for the existence of traveling waves is less than the critical Reynolds number of the Orr–Sommerfeld neutral stability curve. This result suggests that finite amplitude disturbances can initiate transition.

ISSN:0899-8213    

DOI:10.1063/1.858805

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Fully developed mixed convection flow in a loosely curved annulus is examined. Analytic solutions in power series form, where the radius ratio is properly taken into account, are derived. It is found that the power series solution is in good agreement with the numerical solution. It is also illustrated that the dimensionless numbers based on hydraulic radius are adequate to characterize the inclination of the secondary flow cells.

ISSN:0899-8213    

DOI:10.1063/1.858806

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In the laminar vortex street behind a circular cylinder, only few distinct shedding modes for a large range of aspect ratios and various end conditions are observed. Thenth mode is characterized by a Strouhal number Stnand a shedding angle, &thgr;n, both of which vary continuously with the Reynolds number. The experimentally observed discontinuities in the Strouhal–Reynolds number relationship are transitions between these shedding modes.

ISSN:0899-8213    

DOI:10.1063/1.858807

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Oscillating‐grid induced turbulence in confined geometries (tanks) is commonly used in the study of turbulence with zero‐mean shear. It is demonstrated that the mean secondary circulation generated during such experiments can be reduced by selecting conditions that lessen the Reynolds‐stress gradients within the fluid. A simple power law for the spatial decay of turbulent velocity fluctuations is realized only in the absence of such mean circulation.

ISSN:0899-8213    

DOI:10.1063/1.858808

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Moffatt has argued [J. Fluid. Mech.218, 509 (1990)] that it should be possible to control the motion of a metallic sphere immersed in a nonconducting fluid using the electromagnetic force and couple generated by means of a traveling and/or rotating magnetic field applied externally. Such a system has been realized experimentally: one or more aluminum spheres are placed in a vessel containing fluid (air, water, or silicone oil), the whole being placed in an inductor which provides an upward‐traveling magnetic field. The spheres move in response to the induced electromagnetic forces, the motion being influenced by gravity, viscous drag, vessel boundary reaction, and collisions. A range of possible behaviors, stable, unstable, and chaotic, are identified and discussed. The term ‘‘electromagnetic billiards’’ seems appropriate to describe this phenomenon.

ISSN:0899-8213    

DOI:10.1063/1.858809

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

There has been some question recently as to whether the far‐wake structure of a bluff body is dependent on, or ‘‘connected’’ with, the precise details of the near‐wake structure. Indeed, it has previously been assumed that there is no direct connection in scale or frequency. In the present work, observations are shown of a honeycomb‐like 3‐D pattern in the far wake, which is caused by an interaction between the decaying oblique shedding waves from upstream and large‐scale 2‐D waves in the far wake. A clear ‘‘connection’’ between the 3‐D scales of the near and far wakes is thereby demonstrated. The symmetry and spanwise wavelength of Cimbalaetal.’s [J. Fluid Mech.190, 265 (1988)] 3‐D pattern are precisely consistent with such wave interactions. In the presence of parallel shedding, the lack of a honeycomb pattern shows that the pattern is clearly dependent on oblique vortex shedding. It also follows that for oblique shedding, the far‐wake 2‐D waves cannot possibly be the result of vortex pairing.

ISSN:0899-8213    

DOI:10.1063/1.858810

出版商:AIP    

年代:1993

数据来源: AIP

ISSN:0899-8213    

DOI:10.1063/1.858899

出版商:AIP    

年代:1993

数据来源: AIP