The characteristics and evolution of large-scale structures in compressible mixing layers

Abstract

Single- and double-pulsed visualizations were employed to study the characteristics and evolution of large-scale structures in compressible mixing layers with convective Mach numbers (Mc) of 0.51 and 0.86. Instantaneous images and spatial correlations based on large ensembles of images show that large-scale structures which span the entire mixing layer thickness are a more dominant feature at Mc=0.51 than at Mc=0.86. Double-pulsed images in the Mc=0.51 developing region show the structures’ formation to proceed as the roll-up of a wavy mixing region, in agreement with the Kelvin–Helmholtz picture of structure formation derived from studies of incompressible mixing layers. However, little indication of roll-up was present for Mc=0.86. In the Mc=0.51 fully developed region, the large-scale structures evolve similarly to those of incompressible mixing layers, even undergoing pairing processes. Similar structure evolution processes were not encountered in Mc=0.86; instead, it was more difficult to even track large-scale motions between the initial and delayed images. Convective velocities derived from space-time correlations of ensembles of double-pulsed images are in good agreement with the theoretical convective velocity at the center of the mixing layer, but are higher than the theoretical value toward the high-speed side of the mixing layer and lower toward the low-speed side.