On the largescale structure in turbulent free shear flows
Abstract
The existence of organized structures in turbulent shear flow has been the subject of recent observational discoveries in both the laboratory and in the atmosphere and ocean. The recent work on modeling such structures in a temporally developing, horizontally homogeneous turbulent free shear layer has been extended to the spatially developing mixing layer, there being no available rational transformation between the two nonlinear problems. The basis for the consideration is the kinetic energy development of the mean flow, largescale structure and finegrained turbulence with a conditional average, supplementing the usual time average, to separate the nonrandom from the random part of the fluctuations. The integrated form of the energy equations and the accompanying shape assumptions, is used to derive amplitude equations for the mean flow, characterized by the shear layer thickness, the nonrandom and random components of flow which are characterized by their respective energy densities. In general, the largescale structure augments the spreading of the shear layer and enhances the finegrained turbulence by taking energy from the mean flow and transferring it to the turbulence as it amplifies and subsequently decays. The maximal amplitude of the largescale structure is attained by the initially most amplified mode, however, the relative enhancement of the finegrained turbulence is achieved by both the magnitude of the largescale structure and its streamwise lifetime. Thus a greater enhancement of the turbulence is achievable by the lower frequency modes which have longer streamwise lifetimes. The largescale structure can also be controlled by increasing the initial level of turbulence, which would render its decay more rapidly.
 Publication:

NASA STI/Recon Technical Report N
 Pub Date:
 June 1977
 Bibcode:
 1977STIN...7733452L
 Keywords:

 Shear Layers;
 Turbulent Flow;
 Turbulent Mixing;
 Atmospheric Models;
 Kinetic Theory;
 Mixing Layers (Fluids);
 Ocean Models;
 Fluid Mechanics and Heat Transfer