摘要:

Conditional averages of the velocity field, subgrid‐scale (SGS) stresses and SGS dissipation are calculated using the velocity fields obtained from the DNS of plane channel flow. The detection criteria isolate the coherent turbulent structures that contribute most strongly to the energy transfer between the large, resolved scales and the subgrid, unresolved, ones. Separate averages are computed for forward and backward scatter. The interscale energy transfer is found to be strongly correlated with the presence of the turbulent structures typical of wall‐bounded flows: quasi‐streamwise and hairpin vortices, sweeps and ejections. In the buffer layer, strong SGS dissipation is observed near lifted shear layers; the forward scatter is associated with ejections, the backscatter with sweeps. Both backward and forward scatter occur in close proximity to longitudinal vortices that form a very shallow angle to the wall. Further away from the solid boundary, in the logarithmic region and beyond, both forward and backward energy transfer are associated prevalently with ejections. Eddy viscosity models do not predict the three‐dimensional structure of these events adequately, while scale‐similar models reproduce the correlation between the large‐scale coherent structures and the SGS events more accurately. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868829

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of density gradients on two‐dimensional (2‐D) supersonic wall modes (acoustic modes) of a 2‐D confined compressible shear layer are investigated using linear analysis. Due to the inadequacies of the hyperbolic tangent velocity profile, the boundary layer basic flow profiles are used. First, a test case is taken with the same parameters as in a previous analysis by Tam and Hu [J. Fluid Mech.203, 51 (1989)], who used a hyperbolic tangent profile. For the boundary layer profiles, three generalized inflection points are found giving rise to three modes. The first two show similar properties to those found by Tam and Hu, whereas the third is a new mode that can have a higher growth rate than the others. As the density ratio is increased above that of the test case, the smallest of the three neutral phase speeds tends toward the speed of the lower‐velocity stream, and the other two eventually coalesce and then disappear. These two effects lead to a linear resonance between some of the modes that increases the cutoff frequency and growth rate of the lowest mode. In fact, growth rates of two to four times the test case were found as the density ratio was increased to 7. A similar trend (linear resonance) is observed as the density ratio is decreased from the test case, but the growth rate is only slightly changed from the test case. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868830

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of density gradients on three‐dimensional (3‐D) supersonic acoustic modes and 3‐D subsonic modes in a compressible confined shear layer were investigated using linear analysis. The compressible Rayleigh equation with the solution of the steady compressible boundary layer equations as its basic profiles was solved for two different density ratios, 1.398 and 3.0, and several spanwise wave numbers. For a density ratio of 1.398, the 2‐DC01mode had the highest growth rate for a fixed aspect ratioB/H=2. At the higher‐density ratio, the 3‐D mode had a slightly higher maximum growth rate than the two‐dimensional (2‐D) mode, and the maximum growth rate was at a location where the phase speed was supersonic with respect to the fast stream. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868831

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The coupling between combustion processes and gasdynamic effects in reactive mixtures with nonuniform temperature or concentration distributions may lead to the formation of detonations, producing very large‐pressure waves. This type of large‐pressure wave generation is one of the main mechanisms responsible for the knocking phenomenon in internal combustion engines and for the transition from deflagation to detonation in accidental explosions. In this paper, both theoretical analysis and one‐dimensional numerical simulation are carried out to determine the critical conditions for the large overpressure generations by a nonuniform hot pocket. The analysis based on the square wave model shows that due to the curvature effect, a self‐sustained quasi‐CJ spherical detonation may not exist when the radius of the detonation front is smaller than a critical radius. It is found that the critical conditions for the self‐sustained propagation of detonations control the formation of detonations: it occurs only when the position of the formation determined by Zeldovich’s condition is larger than this critical value. The order of magnitude of the so‐determined critical size of the hot pocket for successful initiation is much larger than the critical size estimated from Zeldovich’s condition. This point is confirmed by our direct numerical simulations. The magnitude of the critical size is also in good agreement with the experimental results. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868832

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The interpretation of bulk viscosity as a relaxation parameter for describing nonequilibrium polyatomic gases is challenged both qualitatively and quantitatively. Its presumed proportionality to a relaxation time is an artificial concept restricted to very rapid equilibration of translational and internal modes compared with some characteristic time of the system. Application of a generalized methodology to small‐signal sound waves of intermediate and high frequencies demonstrates this artificiality and shows that bulk viscosity can at best be used as a nonphysical adjustable parameter to force fit a single aspect of relaxation. Sound absorption can be force fitted in this way, but not the dispersion of the sound speed. Accordingly, recent applications of bulk viscosity in solving such problems as heat transfer in hypersonic boundary layers may be seriously deficient. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868833

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A rarefied gas flow through a thin slit caused by a small pressure difference is studied on the basis of the kinetic equation. The Bhatnagar–Gross–Krook model equation is solved numerically by the optimized discrete velocity method. The gas–surface interaction is assumed diffuse‐specular. It is shown that the dependence of the mass flow rate on the gas–surface interaction law is negligibly small at any Knudsen number. Analytical formulas for the flow rate embracing the whole range of the Knudsen number are obtained. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868834

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The relativistic Navier–Stokes equations following from the first‐order theory of Eckart are derived. Exact analytic solutions, which are the generalizations of the well‐known nonrelativistic solutions for Couette flow, are found. In particular, it is shown that even in the case when the dissipative coefficients are constant the temperature distribution influences the velocity distribution. The relativistic generalization of the classical Busemann theorem is found for the case of dissipative coefficients varying with temperature. For almost every type of these flows the velocity and temperature profiles are flattened as compared with respective nonrelativistic profiles. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868883

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The critical path theory of Katz and Thompson [Phys. Rev. B34, 8179 (1986)] is extended to estimate the permeability of strongly disordered macroscopic porous media. The theory is based on percolation concepts and for lognormal distributions predicts an asymptotic exponential dependence on the standard deviation. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868835

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We model the incompressible, inviscid flow in a hydraulic jump in a closed horizontal tube. Downstream of the jump the flow fills the cross‐section of the tube, and there the pressure is hydrostatic plus an additional pressure caused by the confines of the tube. We calculate the rate of energy loss by the jump and use this to account for the dramatic damping seen when an enclosed tube, partly‐filled with water, swings freely like a pendulum. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868836

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Linear stability analysis, which includes the effects of nonparallelism and body curvature, is used to calculate theNfactors of stationary and traveling disturbances for the same experimental conditions measured by Poll for flow over a long swept cylinder. The correlatingNfactors for traveling disturbances have less scatter than the correlatingNfactors of stationary disturbances. The mean value of the correlatingNfactor for traveling disturbances is 14.9 and 7.4 for stationary disturbances. Therefore, for an initial amplitude of a stationary disturbance that is 100 times larger than that of a traveling disturbance, the amplitude of the traveling disturbance is 18 times larger than the amplitude of a stationary disturbance at the location of transition onset. The calculations indicate that transition is due to traveling disturbances; this result is in agreement with the experimental observations. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868837

出版商:AIP    

年代:1996

数据来源: AIP