Outstanding issues in the area of compressible mixing

Issues that arose from the experimental investigation of the turbulent compressible shear layer in a variable-gas, variable-Mach number facility are explored. In that investigation, the growth rates were correlated with the convective Mach number Mc, i.e., the Mach number in the frame of reference of the large-scale structure. Initially, Mc was obtained theoretically, using arguments which give it approximately the same value on the two sides of the layer. The growth rate, normalized by its incompressible value, versus Mc falls on a universal curve that decreases with Mc I Implications of that curve, as well as possible intuitive scenarios for the growth-reduction mechanism, are discussed. Recent double-exposure photos of the flow reveal that, at high compressibility, Mc has substantially different values on the two sides of the layer. Reasons for this difference between experiment and theory are suggested. The effect of the walls and the possible three-dimensionality of the flow are identified as open questions whose resolution is very important