Mixing Layer Vortices

Abstract

A mixing layer develops between two parallel, but different velocity, streams separated by a splitter plate upstream. The mixing layer is a flow of paramount importance for understanding the development of turbulence in external aerodynamics or combustion, as well as in atmospheric or oceanic flows. These so-called ``spatially-growing mixing layers'' are characterized by the formation of big spiral vortices resulting from a Kelvin-Helmholtz type instability. The instability is due to the inflectional nature of the upstream velocity profile; the vortex sheet initially created is linearly unstable and rolls up to form the coherent vortices. This roll-up was clearly shown experimentally at both low and high Reynolds numbers (Brown and Roshko 1974; Winant and Browand 1974). For instance, Figure 2.1.1, taken from Brown and Roshko (1974), displays very clearly the presence of these coherent vortices in a mixing layer at a very high Reynolds number. These big spiral vortices, shed in spatially-growing mixing layers, are of major importance for the transport of heat and momentum. (The transfer of momentum is, or course, directly related to the creation of drag). They are also essential in combustion and acoustics, and their manipulation is very important for turbulence control.