Nonlinear Wave Process Hierarchies and the Cyclic Development of Quasi-Ordered Structures in Turbulent Shear Flows

摘要

The nonlinear wave interpretation and description of the large scale coherent structures commonly observed in free as well as bounded turbulent shear flows has been discussed. The experimentally observed cyclic development of nonlinear two-dimensional vorticity concentrations in homogeneous, incompressible, free mixing layers has been linked to the growth/equilibration of finite amplitude, spatially amplifying instabilities and their cyclic regeneration under the action of intrinsic secondary instabilities evoked by flow nonlinearity. The mathematical modeling of such nonlinear behavior in terms of matched asymptotic expansion solutions of the Navier Stokes equations, being reported under separate cover, has been reviewed. The relevance of the physical viewpoint and the mathematical model to more general turbulent flows has been examined and supported by an analysis of selected, conditionally sampled, measurements in transitional and turbulent boundary layers. On that basis, a dominant role of specific nonlinear wave processes has been indicated, and the approach to their systematic mathematical modeling from first principles has been outlined. (Author)