摘要:

Turbulence structures near the interface between two flowing fluids have been resolved by direct numerical simulation. As a first step the interface has been kept flat, corresponding closely to the recent gas‐liquid flow experiments of Rashidi and Banerjee [Phys. Fluids A2, 1827 (1990)], with the fluids coupled through continuity of velocity and shear stress boundary conditions. For density ratios between the fluids typical of air and water, the turbulence characteristics on the gas side are quite similar to that in wall regions. The liquid side shows larger velocity fluctuations close to the interface and ejections originate closer to the interface. The mean velocity distribution, turbulence intensities, Reynolds stress and various other statistical measures are significantly altered compared to those in the wall region of channel flows. Quasi‐streamwise vortices form in the areas between high and low shear stress on both sides of the interface. At any given instant, about a fifth of these appear to be coupled across the interface. Whether the others are, but the coupling is too weak for the detection technique used, or were coupled previously remains an open question. In any case, sweeps usually occur on the high shear stress side of these vortices and ejections on the low shear stress side. Significant coupling exists across the interface with over 60% of the Reynolds stress in the region close to the interface being associated with coupled events –the main coupling coming through gas ejection‐liquid ejection events over low shear stress regions, with a lesser but significant number of gas sweep‐liquid sweep events over high shear stress regions. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868937

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An analytic theory of the Richtmyer–Meshkov (RM) instability for the case of reflected rarefaction wave is presented. The exact solutions of the linearized equations of compressible fluid dynamics are obtained by the method used previously for the reflected shock wave case of the RM instability and for stability analysis of a ‘‘stand‐alone’’ rarefaction wave. The time histories of perturbations and asymptotic growth rates given by the analytic theory are shown to be in good agreement with earlier linear and nonlinear numerical results. Applicability of the prescriptions based on the impulsive model is discussed. The theory is applied to analyze stability of solutions of the Riemann problem, for the case of two rarefaction waves emerging after interaction. The RM instability is demonstrated to develop with fully symmetrical initial conditions of the unperturbed Riemann problem, identically zero density difference across the contact interface both before and after interaction, and zero normal acceleration of the interface. This confirms that the RM instability is not caused by the instant normal acceleration of the interface, and hence, is not a type of Rayleigh–Taylor instability. The RM instability is related to the growth of initial transverse velocity perturbations at the interface, which may be either present initially as in symmetrical Riemann problem, or be induced by a shock passing a corrugated interface. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868938

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

It is now well known that the wake transition regime for a circular cylinder involves two modes of secondary three‐dimensional instability (modes ‘‘A’’ and ‘‘B’’), depending on the regime of Reynolds number (Re). However, there exists a surprisingly large scatter in previous measurements of critical Re for the inception of the mode A instability (Re from 140 to 190) and in previous measurements of spanwise length scale. It is deduced in this work that the large variation in previous measurements concerning mode A are due to the presence of vortex dislocations. In the absence of such dislocations, we find an excellent agreement of the critical Re as well as spanwise wavelength of mode A with the linear secondary stability analysis of Henderson and Barkley [Phys. Fluids8, 1683 (1996)]. We further demonstrate that these large‐scale dislocations in wake transition are triggered at the sites of some of the vortex loops for mode A; they are an intrinsic feature of transition, independent of end conditions. These studies lead us to a new clarification of the possible flow states through wake transition, as follows. If one defines a Mode A* as (Mode A+Dislocations), then the route through transition appears to follow the scenario of wake modes: (2D→A→A*→B). ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868949

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Secondary instability of flow past a circular cylinder is examined using highly accurate numerical methods. The critical Reynolds number for this instability is found to beRec=188.5. The secondary instability leads to three‐dimensionality with a spanwise wavelength at onset of 4 cylinder diameters. Three‐dimensional simulations show that this bifurcation is weakly subcritical. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868939

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An experimental analysis of the scaling properties of the velocity increments correlation function is presented. It is found that the scaling behavior predicted by a shell model of turbulence, called the Gledzer‐Ohkitani‐Yamada (GOY) model, is in substantial agreement with present experimental results. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868950

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

H. Holway presented a proof, based on the Boltzmann equation, in which he shows that for the moment method there exists a critical Mach number beyond which no continuous shock solution is possible. In this paper, the results of Holway’s proof are reinterpreted. From this reinterpretation, it follows that no upper bound for a critical Mach number exists. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868947

出版商:AIP    

年代:1996

数据来源: AIP