The effects of expansion strength on large-scale structures in compressible free shear layers

Planar visualizations of two compressible free shear layers were performed immediately downstream of centered expansions of differing strengths in ord er to assess the influence of expansion strength on the embedded large-scal e structures. The free shear layers studied here were formed through the se paration of an approach flow, either a Mach 2.0 stream or a Mach 2.5 stream , from a planar backstep. In addition to side-view and end-view visualizati ons, spatial correlations (computed from large image ensembles) and laser D oppler velocimetry surveys of the free shear layers were also examined to d iscern relationships between the structure dynamics and the underlying pre- and postexpansion velocity fields. The instantaneous images clearly illust rate that ellipsoidal, highly coherent structures were present in both shea r layers downstream of the expansion corner. The dissimilar expansion stren gths did not appear to produce qualitatively different structures in the sh ear layers; however, as compared to the weaker expansion, the stronger expa nsion did result in an increase in the growth rate of the large-scale struc tures, apparently from an augmentation of the <u(')v(')> partial derivative U/partial derivativey production term in the TKE equation. Furthermore, qua ntitative measurements of the mean structure geometry, as determined from t he spatial correlation fields, revealed that a stronger expansion strength resulted in a larger aspect ratio of the mean structures (i.e., the structu res were stretched preferentially in the streamwise and transverse directio ns as compared to the spanwise direction during the expansion process). Qua drant decompositions of the instantaneous velocity fluctuations within the approach boundary layers and within the free shear layers indicated a defin ite increase in structure organization across the expansion region, which i s in contrast with studies of expanded supersonic boundary layers without s eparation. The instantaneous image data, spatial correlations, and velocity decompositions uniformly suggest that the separation process itself, and n ot the expansion strength, is the primary influence on initial eddy structu re in the postexpansion free shear layer. (C) 2001 American Institute of Ph ysics.