Coherent Structure Eduction and Convection in Supersonic Wake Shear Layers

Abstract

Coherent structures in blunt-base cylinder wakes at 0o and 10o angle-of-attack to an M=2.46 freestream flow have been studied using high-speed flow visualization. Conventional cross-correlation analysis has been performed to determine the convection velocity of the coherent structures. The coherent structures in the developing shear layers of both wakes convect significantly faster than the isentropic convection velocity, in accordance with the stream selection rule for compressible shear layers. The coherent structures decelerate in the axisymmetric wake during recompression and reattachment, but not in the lateral plane of the angle-of-attack wake, apparently due to differences in the freestream velocity distribution in these wakes. A conditional eduction technique has been developed to identify and average the coherent structures at several scales. This structure eduction procedure has been combined with the cross-correlation technique in a conditioned convection velocity measurement, which tracks the coherent structures explicitly. These data show that the convection velocity is highly dependent on the transverse position of the structures in the shear layer. Quantitative measurements of the structure evolution indicate that on average the structures evolve by dissipating, in that they no longer resemble the average coherent structure at the given imaging position.