Large-scale structures in a compressible mixing layer over a cavity

An experimental study was made of a flow 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 How (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 flow visualization experiments were carried out in the self-similar portion of the turbulent mixing layer at a nominal, convective Mach number of 1.1. The flow visualization technique was based on Rayleigh scattering from nanometer-scale contaminant particles present in the freestream flow. The interface marked by the vaporization of the particles revealed the large-scale organized turbulence structures in the mixing layer. Quantitative measures of the length scale, orientation, and speed of organized structures were derived from the flow 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 probe surveys, demonstrate that the flow over the cavity is essentially equivalent to a standard planar mixing layer flow, and thus forms a well-characterized initial condition for the reattachment flow downstream. In combination with our previous study, the present results add insight into cavity flow unsteadiness for the case where the driving mechansim is related to broad-band turbulent fluctuations, rather than discrete acoustic resonances.