Vortices in turbulent mixing layers

The importance of coherent vortices in the evolution of the mixing layer that forms between two fluid streams of different velocities has long been recognized. In transitional (i.e. preturbulent) mixing layers, there are well-known vortex structures (spanwise ''rollers'' and streamwise ''ribs''), the dynamics and stability of which are largely understood and are discussed. Similar vortex structures have been observed in a variety of experiments in fully turbulent mixing layers, though they are not observed in all circumstances. The conditions necessary for these transitional structures to exist in the turbulent how appears to be the continued occurrence of organized ''pairings'' of the spanwise rollers. This and the impact the structures have when they do appear are explored.