Absolute/convective instabilities and the convective Mach number in a compressible mixing layer

In this paper two aspects of the stability of a compressible mixing layer are considered: absolute/convective instability and the convective Mach number. It is shown that, for Mach numbers less than unity, the compressible mixing layer is convectively unstable unless there is an appreciable amount of backflow. A rigorous derivation of a convective Mach number based on linear stability theory for the flow of a multispecies gas in a mixing layer is also presented. In particular, the definition is based on the free‐stream Mach number in the laboratory frame and is independent of the speed of the large‐scale structures and the speed of the most unstable wave. The result is compared with the heuristic definitions of others and to selected experimental results.