摘要:

The Lagrangian and Eulerian frequency spectrum in isotropic turbulence is considered without a mean flow, concentrating on its second moment, the mean‐square acceleration. The pressure and viscous contributions are reviewed and the scaling properties of the advective term ⟨[(v⋅∇)v]2⟩ are examined. Random sweeping from this term is shown to be dominant at large Reynolds numbers if the fluctuation spectrum of the kinetic energyv2(x) scales ask−5/3. If on the other hand it satisfies the same Kolmogorov scaling as the pressure going ask−7/3, then the recent renormalization group prediction of no sweeping is recovered. This question is subject to direct experimental resolution. The experiments of Van Atta and Wyngaard [J. Fluid Mech.72, 673 (1975)] strongly indicate that the spectrum ofv2goes ask−5/3at high Reynolds numbers, thereby supporting the sweeping hypothesis.

ISSN:0899-8213    

DOI:10.1063/1.857684

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A quadratic nonlinear generalization of the linear Rotta model for the slow pressure‐strain correlation of turbulence is developed for high Reynolds number flows. The model is shown to satisfy realizability and to give rise to no stable nonzero equilibrium solutions for the anisotropy tensor in the case of vanishing mean velocity gradients. In order for any model to predict a return to isotropy for all relaxational flows, it is necessary to ensure that there is no nonzero stable fixed point that attracts realizable initial conditions. Both the phase space dynamics and the temporal behavior of the model are examined and compared against experimental data for the return to isotropy problem. It is demonstrated that the quadratic model successfully captures the experimental trends which clearly exhibit nonlinear behavior. Comparisons are also made with the predictions of the linear Rotta model, the quasilinear Lumley model, and the nonlinear model of Shih, Mansour, and Moin. The simple quadratic model proposed in this study does better than the Rotta model as anticipated, and also compares quite favorably with the other more complicated nonlinear models.

ISSN:0899-8213    

DOI:10.1063/1.857694

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The three‐dimensional (3‐D) time‐dependent compressible Navier–Stokes equations are numerically solved by a Fourier–Chebyshev collocation method to study the stability of supersonic flows over a flat plate. Several direct simulations carried out in this study suggest the existence of a secondary instability that might provide a route to transition. The interaction of the modes involved in the secondary instability is possibly amenable to a Floquet‐type analysis. Pertinent differences between this instability and the analogous incompressible K‐type secondary instability are pointed out. Some preliminary results of a 2‐D direct simulation of the nonlinear evolution of a second mode perturbation wave are also discussed.

ISSN:0899-8213    

DOI:10.1063/1.857695

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Following a recent theory suggesting that the local average rate of energy dissipation &egr;rfollows statistics predicted by the gamma distribution rather than the lognormal distribution, the gamma distribution is further examined here as a model for the local temperature dissipation rate &khgr;r. Comparisons of measured moments ⟨&khgr;mr⟩ with those predicted by both the gamma and lognormal distributions clearly favor the gamma model for predicting the power‐law exponents ofr/L. Based on gamma models for both &egr;rand &khgr;r, the bivariate gamma distribution is then examined as a possible model for the joint distribution of dissipation rates &egr;rand &khgr;r. In particular, the bivariate gamma model and the bivariate lognormal model are both compared with the inertial‐range behavior of measured temperature structure functions up to order 12. Some additional comparisons of these models are also made with data involving mixed moments of the velocity and temperature structure functions.

ISSN:0899-8213    

DOI:10.1063/1.857685

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A derivation of the convective‐diffusion equation for transport of a scalar quantity, e.g., surfactant, along a deforming interface is outlined. The direct contribution of interface deformation, giving rise to concentration variations as a result of local changes in interfacial area, is shown explicitly in a simple manner.

ISSN:0899-8213    

DOI:10.1063/1.857686

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Prior studies of Stokes flow over cavities have been limited to simple shear and simple stagnation point flows. In the present work, it is shown that the presence of a transverse pressure gradient has a dramatic effect on the character of the flow, eliminating the separated flow region in many cases. Consequences of this phenomenon for mass transfer are discussed.

ISSN:0899-8213    

DOI:10.1063/1.857687

出版商:AIP    

年代:1990

数据来源: AIP