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A plane, isothermal, chemically reacting mixing layer has been experimentally investigated to analyse the origin and the development of three-dimensional stream-wise vorticity. The results show that early in its evolution, the plane, free shear layer is composed of counter-rotating pairs of streamwise vortices superimposed upon the spanwise ones. This coherent, streamwise vortical structure was found to be the result of the unstable response of the layer to three-dimensional perturbations in the upstream conditions. Depending on the magnitude and location of the upstream disturbances, the location of the transition to three-dimensionality varied. However, the concentrated streamwise vorticity was always seen to form first on the braids between consecutive spanwise vortices and then to propagate into their cores. For the low and moderate Reynolds numbers of this study, it was found that the onset of the so-called ‘mixing transition’ does not necessarily coincide with that of the formation of concentrated streamwise vorticity. These vortices were observed to have a scale, as measured by the size of their cores, somewhat smaller than but comparable with that of the spanwise ones, thus contributing substantially to the entrainment process in the early stages of mixing-layer development.
Lasheras et al. (1986) studied this question.