Large-Scale Structures in a Compressible Mixing Layer over a Cavity

Abstract

An experimental study was made of a e ow in which a turbulent boundary layer separates at a backward-facing step, forms a free shear layer over a cavity, and reattaches on a ramp downstream. Accurate characterization of the mixing layer turbulence is important given the strong link between large-scale organized structures and intense unsteadiness at reattachment found in our previous study of this e ow (Poggie, J., and Smits, A. J., “ Shock Unsteadiness in a Reattaching Shear Layer,”Journal of Fluid Mechanics , Vol. 429, 2001, pp. 155‐ 185). To this end, detailed e ow visualization experiments were carried out in theself-similar portion of the turbulent mixing layer at a nominal convective Mach numberof1.1. Thee ow visualization technique was based on Rayleigh scattering from nanometer-scale contaminant particles present in the freestream e ow. The interface marked by the vaporization of the particlesrevealed thelarge-scaleorganized turbulence structures in themixing layer. Quantitativemeasures of the length scale, orientation, and speed of organized structures were derived from the e ow visualization data, and were found to agree well with conventional point-probe measurements. As has been found in other studies of planar mixing layers, the measured convection velocity varied moderately along the transverse direction, and the corresponding convective Mach number differed from the prediction of the isentropic model. The present results, along with previously published probesurveys, demonstratethat thee owoverthecavity isessentially equivalent to a standardplanarmixinglayere ow,and thusformsawell-characterizedinitialconditionforthereattachmente ow downstream. In combination with our previous study, the present results add insight into cavity e ow unsteadiness for the case where the driving mechansim is related to broad-band turbulent e uctuations, rather than discrete acoustic resonances.