The deterministic description of the coherent structure of free shear layers

Abstract

The large coherent structures observed in turbulent mixing layers are tentatively identified with the unsteady flow which is generated in two dimensions by the same initial and boundary conditions. This flow is briefly described and means of calculating it are reviewed. Typical results of such calculations are illustrated. For large Reynolds numbers and the outer folds of the interfacial spirals, a boundary-layer-like treatment allows the calculation of the evolution of the diffusive attributes of the flow in terms of the rate of stretching of the material interface. This important quantity has been calculated as a Lagrangian function of time and initial location for a roll-up and pairing and for a moderate Reynolds number. It is suggested that interfacial stretching does not depend strongly on Reynolds number. The use of this convenient approach is illustrated in a simple example of a diffusion-limited reaction.