Plane and axisymmetric mixing layers

Christophe Bogey and Christophe Bailly

Velocity-gradient regions between two streams are found in numerous flows such as plane or axisymmetric jets. These simple flow configurations, usually referred to as shear layers or mixing layers, have been extensively investigated as reported in the review of Ho and Huerre (1984). These studies have allowed better understanding of the transition to turbulence occurring in initially laminar or transitional shear layers thanks notably to the development of instability theories (Michalke 1984) and to the observations of coherent structures. Among the experiments supporting the latter observations, the famous one conducted by Brown and Roshko (1974) shows clearly that large-scale coherent structures are intrinsic features of mixing layers even at high Reynolds numbers. These structures, through their interactions such as vortex mergings, have been recognized to be appreciably responsible for the spreading of the shear layers or for noise generation, as stated by Winant and Browand (1974).

From the preceding considerations, the large-eddy simulation (LES) approach appears especially well suited to mixing-layer computations because with LES all the scales larger than the grid size, and consequently a large part of the coherent structures, are calculated. The earlier large-eddy simulations of practical flows have thus often involved mixing layers. They have permitted testing of the LES methodology for simple transitional flows.