摘要:

The convection arising in a cylindrical container, heated from the ends and designed for a measurement of Soret coefficient, is examined. A three‐dimensional method using a pseudostationary scheme with a finite differences technique is used. Attention is focused on a horizontal cylinder with an aspect ratio of 6, a Prandtl number of 0.6, and a Schmidt number of 60. The influence of convection on the separation and on the mass fraction profiles is examined for moderate Grashof numbers (0.01≤GrH≤10) and realistic Soret parameters (−0.75≤S≤1). A domain is found where the flow has no influence on the separation, corresponding to a ‘‘separation’’ regime. Extensions of the results to different Prandtl and Schmidt numbers and to larger aspect ratios are proposed.

ISSN:0031-9171    

DOI:10.1063/1.865834

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The vaporization of a spherically symmetric liquid droplet subject to a high‐intensity laser flux is investigated on the basis of a hydrodynamic description of the system composed of the vapor and ambient gas. In the limit of the convective vaporization, the boundary conditions at the fluid–gas interface are formulated by using the notion of a Knudsen layer in which translational equilibrium is established. This leads to approximate jump conditions at the interface. For homogeneous energy deposition, the hydrodynamic equations are solved numerically with the aid of the CON1D computer code (‘‘CON1D: A computer program for calculating spherically symmetric droplet combustion,’’ Los Alamos National Laboratory Report No. LA‐10269‐MS, December, 1984), based on the implict continuous–fluid Eulerian (ICE) [J. Comput. Phys.8, 197 (1971)] and arbitrary Lagrangian–Eulerian (ALE) [J. Comput. Phys.14, 1227 (1974)] numerical mehtods. The solutions exhibit the existence of two shock waves propagating in opposite directions with respect to the contact discontinuity surface that separates the ambient gas and vapor.

ISSN:0031-9171    

DOI:10.1063/1.865783

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The Kramers–Moyal expansion is used to derive an infinite hierarchy of Eulerian moment conservation equations from a random‐walk model with non‐Gaussian random forcing, thus generalizing the Langevin‐equation–Fokker–Planck analysis of van Dopetal. [H. van Dop, F. T. M. Nieuwstadt, and J. C. R. Hunt, Phys. Fluids28, 1639 (1985)]. By imposing the condition that an initially well‐mixed state should remain so, equations for random forcing moments of arbitrary order are derived in terms of the Eulerian velocity moments of the turbulence. This procedure makes explicit the equivalence of the different procedures used by van Dopetal. and Thomson [D. J. Thomson, Q. J. R. Meteorol. Soc.110, 1107 (1984)] to derive the first few forcing moments and extends their results. It is then shown that the random forcing approximation implies an infinite hierarchy of Eulerian closure assumptions, the first few of which were derived by van Dopetal. The analysis is extended to a class of rescaled random‐walk equations, and it is shown that the version developed by Wilsonetal. [J. D. Wilson, B. J. Legg, and D. J. Thomson, Boundary‐Layer Meteorol.27, 163 (1983)] is unique in that it alone is realizable for inhomogeneous Gaussian turbulence and then has Gaussian random forcing.

ISSN:0031-9171    

DOI:10.1063/1.865784

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The statistical‐mechanics theory of the passive scalar field convected by turbulence, developed in an earlier paper [Phys. Fluids28, 1299 (1985)], is extended to the case of a small molecular Prandtl number. The set of governing integral equations is solved by the equation‐error method. The resultant scalar‐variance spectrum for the inertial range isF(k)∼x−5/3/[1+1.21x1.67(1+0.353x2.32)], wherexis the wavenumber scaled by Corrsin’s dissipation wavenumber. This result reduces to the − (5)/(3) law in the inertial‐convective range. It also approximately reduces to the − (17)/(3) law in the inertial‐diffusive range, but the proportionality constant differs from Batchelor’s by a factor of 3.6.

ISSN:0031-9171    

DOI:10.1063/1.865785

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A reduction in skin friction drag is shown when gas is introduced into the liquid turbulent boundary layer of a submerged axisymmetric body. The 89 mm diameter, 632 mm long body has a cylindrical balance 273 mm long. Free stream speeds in the 305 mm diameter tunnel are as high as 17 m/sec, giving length Reynolds number of up to 10 million. In general, skin friction reduction is shown to increase with increasing free stream speed. At high speeds, helium injection is shown to be more effective at reducing skin friction than is air injection. Maximum skin friction reduction is near 80%—a value in good agreement with the maximum value observed in the flat plate work of Madavanetal. [Phys. Fluids27, 356 (1984)]. While maximum skin friction reduction was found at a free stream speed of 5 m/sec for the flat plate geometry, maximum skin friction reduction was at a free stream speed of 17 m/sec for the axisymmetric geometry.

ISSN:0031-9171    

DOI:10.1063/1.865786

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The mixing layer was studied at a Reynolds number (U1&dgr;*/&ngr;) of 270 and at six velocity ratios ranging from 1.0 to 0.46. Velocity measurements were carried out in the developing region of the mixing layer. Results show that naturally developing velocity fluctuations reach the maximum at a certain stream location in the developing region, then decay, and that the streamwise velocity fluctuation increases again in the further downstream region where the mean flow approaches the hyperbolic‐tangent shape. There is an indication that the fluctuating motion initiated in the wake region with its characteristic scale can pick up additional energy in the downstream hyperbolic‐tangent mean‐flow region, although stability theory predicts a most unstable length scale that is substantially larger.

ISSN:0031-9171    

DOI:10.1063/1.865787

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The closure method developed in earlier papers [J. Qian, Phys. Fluids26, 2098 (1983);27, 2412 (1984)] is applied to the study of the inverse energy cascade in two‐dimensional turbulence. The resultant inertial‐range energy spectrum isE(k)=2.58g0.244&egr;2/3k−5/3. Heregis a localization factor and &egr; is the rate of energy cascade. This result is compatible with the numerical experiments by Lilly [D. K. Lilly, Phys. Fluids Suppl. II12, 24 (1969)], Siggia and Aref [E. D. Siggia and H. Aref, Phys. Fluids24, 171 (1981)], and Frisch and Sulem [U. Frisch and P. L. Sulem, Phys. Fluids27, 1921 (1984)].

ISSN:0031-9171    

DOI:10.1063/1.865788

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The topology of a typical group of large‐scale coherent structures in the far‐field of a slightly heated cylinder wake is clearly delineated by using averages of the velocity vector components conditioned on the occurrence of spatially coherent temperature fronts. Velocities are determined relative to a frame of reference traveling with the approximate convection velocity of the structures. When the results for a number of positions across the wake are assembled a clear topology picture of a typical group of structures in the plane of the main shear is obtained. The contributions of the coherent and random motions of all structures to the momentum and heat transports have been determined. In all cases the contribution that results from the random motion is larger than that of the coherent motion. This difference is more pronounced for the momentum transport than for the lateral heat transport.

ISSN:0031-9171    

DOI:10.1063/1.865789

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The ionization of argon by strong normal shock waves is studied. The conservation equations are solved to yield the plasma behavior behind the shock wave front. Very good agreement is obtained between experimental findings and the present numerical results for the electron number density, plasma density, and degree of ionization, especially at the electron avalanche region of the relaxation zone. The high accuracy of the present numerical solutions in reproducing the electron avalanche is attributed to the use of accurate threshold collision cross sections for excitation of argon by electron collisions. To support this claim it is demonstrated that if different assumptions were used to describe the ionization process, then the computed results would be different only upstream of the electron avalanche region, i.e., it is shown that the proposed model for ionizing shock waves enables a highly accurate reproduction of the electron avalanche but is less accurate in predicting its exact location.

ISSN:0031-9171    

DOI:10.1063/1.865790

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

An idealized statistical theory of chaining in a dilute suspension of macroscopic magnetic particles in a rarefied gas is presented when an external homogeneous magnetic field is present. The primary colloidal particles are assumed to be spherical, of the same size, and to have saturated magnetic moments. The magnetochemical potentials and the association–dissociation equations are derived for chains consisting ofv≥1 magnetic grains, in dependence of the temperatureT, the densityNvof chains, and the homogeneous magnetic fieldB0. High field intensitiesB0are shown to shift the chain length distributionF=F(v) in favor of long chains,v≫1, whereas increasing temperaturesTmove the maximum of this statistical distribution to smaller chain lengths,v→1. The theory appears to be in qualitative agreement with oven experiments using an external magnetic field for the alignment and stabilization of the chains.

ISSN:0031-9171    

DOI:10.1063/1.865791

出版商:AIP    

年代:1986

数据来源: AIP