摘要:

A turbulent boundary layer subjected to multiple, additional strain rates, namely convex curvature coupled with zero, favorable, and adverse streamwise pressure gradients (ZPG, FPG, and APG) was investigated experimentally. The Reynolds stresses were suppressed, with respect to flat plate values, due primarily to the effects of strong convex curvature (&dgr;0/R≊0.10). Combined with curvature, the FPG reduced the strength of the wake component, resulted in a greater suppression of the fluctuating velocity components, and a reduction of the primary Reynolds shear stress relative to the ZPG curved case, whereas the APG counteracted the stabilizing curvature effect. The additional influence of streamwise pressure gradients on the normal stresses is most apparent in the outer part of the boundary layer (y+≥100). However, the primary Reynolds shear stress is impacted by the additional strain rates throughout most of the boundary layer, especially for the strong pressure gradients. Using theuv2quadrant burst detection method, including grouping of ejections, the normalized mean burst period in the convex boundary layer was found to increase in the presence of FPG and to decrease for APG. The time between bursts increased locally (by as much as 75%) in regions of locally strong acceleration. As the pressure gradient was relaxed to zero downstream, the bursting frequency in the case with stronger initial acceleration remained suppressed compared to that with milder initial acceleration, indicating a strain‐rate history dependence. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869089

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Modeled dissipation rate transport equations are often derived by invoking various hypotheses to close correlations in the corresponding exact equations. D. C. Leslie [ModernDevelopmentsintheTheoryofTurbulence(Oxford University, Oxford, 1972)] suggested that these models might be derived instead from Kraichnan’s [J. Fluid Mech.47(1971)] wavenumber space integrals for inertial range transport power. This suggestion is applied to the destruction terms in the dissipation rate equations for incompressible turbulence, buoyant turbulence, rotating incompressible turbulence, and rotating buoyant turbulence. Model constants likeC&Vegr;2are expressed as integrals; convergence of these integrals implies the absence of Reynolds number dependence in the corresponding destruction term. The dependence ofC&Vegr;2on rotation rate emerges naturally; sensitization of the modeled dissipation rate equation to rotation is not required. A buoyancy related effect which is absent in the exact transport equation for temperature variance dissipation, but which sometimes improves computational predictions, also arises naturally. The time scale in the modeled transport equation depends on whether Bolgiano or Kolmogorov inertial range scaling applies. A simple extension of these methods leads to a preliminary dissipation rate equation for rotating buoyant turbulence. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869090

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A Cartesian, two‐dimensional enclosure containing an isothermal rarefied binary gas mixture is studied as a limiting‐case model for actual crystal growth experiments conducted in reduced gravity environments. By employing a microscopic  approach related to the Boltzmann equation, it is demonstrated that in the presence of appreciable partialconcentrationgradients a steady‐state flow pattern develops,drivenby kinetic boundary layers adjacent to solid boundaries. In contrast, a macroscopic analysis based on the continuum transport equations and the classical no‐slip boundary condition would predict no flow whatsoever. For the case of equal mass species, the velocity scales involved are shown to increase with the disparity in accommodation coefficients, in agreement with expectations based on one‐dimensional, linearized Knudsen sublayer theory, while quantitative comparison between simulations and the latter theory reveals significant confinement effects. Simulation of concentration creep in binary mixtures composed of disparate mass species requires an alternative computational procedure, motivated by surface recombination/dissociation reactions. For this case, flow fields and creep coefficient values for a range of mass ratios are also reported. It is concluded that future continuum‐level modelling efforts should more fully exploit the detailed information now available from relevant microscopic simulations. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869094

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In the present study channel flow with the lower wall mounted with streamwise riblets is considered. The laminar flow state is computed and its stability is investigated by superimposing infinitesimal perturbations, which leads to a hydrodynamic stability problem in a complex geometry. Linear stability characteristics for riblets with several scalloped cross sections are computed. The computations show that at theprimaryinstabilitylevel the laminar flow in the presence of riblets is more unstable than channel flow with smooth walls. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869091

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A linear stability analysis is performed for steady source‐vortex and sink‐vortex flows for viscous and inviscid incompressible fluids. The analysis is based on a method recently developed by the authors [Phys. Fluids7, 2345 (1995)], which utilizes the irrotational nature of the basic flow and takes vorticity as a perturbation. It is shown that source‐vortex flows are always unstable. Sink‐vortex flows are found stable except for low flow‐rate flows of viscous fluid. The potential vortex is unstable for three‐dimensional perturbations, but stable for two‐dimensional perturbations if the fluid is inviscid. For inviscid fluid the linear stability of doublet and higher‐order singularities for plane perturbations is also studied. The general integral of the linearized vorticity equations is found and used for stability analysis. Only doublet flow is found stable. A mathematical criterion for stability of certain types of steady two‐dimensional flows of inviscid incompressible fluid is formulated. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869092

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We address the question of using a lattice gas method to study flows of complex fluids, such as non‐Newtonian or miscible fluids. A Bhatnagar, Gross, and Krook lattice technique provides a tool to simulate the flow of one fluid and the diffusion of a tracer in that fluid. We extend the technique to flows in which the viscosity is space and time dependent. This approach is suitable for non‐Newtonian fluids (shear dependent viscosity) and miscible fluids (concentration dependent viscosity). The modified scheme is tested on physical flow situations, analytically tractable for the sake of comparison. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869093

出版商:AIP    

年代:1996

数据来源: AIP

ISSN:1070-6631    

DOI:10.1063/1.869098

出版商:AIP    

年代:1996

数据来源: AIP