Evolution and convection of large-scale structures in supersonic reattaching sheer flows

Double-pulsed Mie scattering studies were performed to characterize the evo lution of large-scale structures embedded within a planar supersonic base f low. Images were obtained at several streamwise stations along the shear la yers, at reattachment, and in the near-wake regions. From these time-correl ated images, the evolution characteristics of the large-scale structures we re examined over a range of nondimensional time delays. as defined by local integral length and velocity scales. The double-pulsed images indicated th at for short time delays (i.e., less than the representative eddy rollover time), the structures exhibited a simple translation in the streamwise dire ction. As the time delay was increased, rotation and elongation of the stru ctures were observed in addition to the translation feature. Time delays th at appreciably exceeded the local eddy rollover time generally resulted in a dramatic loss of structure identity. No eddy interactions, such as pairin g, were observed at any of the imaging locations. Images obtained near reat tachment provided evidence of shocklets moving in concert with the local ed dies. In the initial portions of the shear layers, the mean convection velo city was measured to be significantly higher than the isentropic estimate, which is consistent with the results of previous convection velocity studie s using mixing layers composed of supersonic/subsonic freestream combinatio ns. The eddies decelerate through the recompression and reattachment region s, presumably due to the influence of the adverse pressure gradient. Downst ream of reattachment, the large-scale structures accelerate as the wake dev elops. (C) 1999 American Institute of Physics. [S1070-6631(99)02308-9].