The modelling of large eddies in a two-dimensional shear layer

Large coherent eddies have been observed in turbulent shear layers and seem to play an important role in their growth, mixing and noise production. Winant & Browand (1974) have observed that the pairing of large eddies is central to the question of shear-layer development, and they model the pairing process with discrete line vortices. It is shown here that the growth of the layer and amalgamation of large eddies are not adequately treated by isolated-line-vortex models, which are proved to be essentially non-evolutionary. A more detailed model of the shear layer, itself consisting of vortex elements, is shown to provide the definition required to observe the evolution and coalescence of the large eddies. The observed development of this model shear layer is consistent with many features of experimentally observed flows.