Turbulent shear layers and wakes

This paper considers the lectures on turbulent shear layers and wakes presented and discussed at the Marseille meeting in 1961 and provides our perspective on progress in understanding the mechanics of these flows since that time. The initial discussion is based on the understanding in 1961 gained from prior work. Particular emphasis is then placed on the subsequent experimental revelation of the large-scale vortical structure (coherent structure) found to be essential to understanding the mechanics of the turbulent shear layer. Critical insight into the mechanics that determines the growth rate (the shear stress), for example, is provided by the Biot–Savart relationship. Conclusions are drawn from the experiments and some unresolved questions posed. This is followed by a discussion of plane wakes. Four regions of the plane wake are identified and experimental results on the large-scale structure are discussed. Again emphasis is placed on the vorticity and the vorticity fluxes that contribute directly to the derivative of the principal Reynolds stress. Results from numerical calculations offer new insights into the mechanics, especially through the vorticity and vorticity fluxes that could not previously be measured. For this case too, conclusions are drawn and outstanding questions posed.