ON THE MECHANISM OF UNSTEADY SHOCK OSCILLATION IN SHOCK-WAVE TURBULENT BOUNDARY-LAYER INTERACTIONS

An experimental investigation into the mechanism of shock wave oscilla tion in compression ramp-generated shock wave/turbulent boundary layer interactions is presented. Particular emphasis is focused upon docume nting the respective roles played by both burst-sweep events in the tu rbulent boundary layer immediately upstream of the interaction and the downstream separated shear layer upon unsteady shock front motion. Un like the majority of compression ramp experiments which involve bulk s eparation and large-scale shock motion, consideration is given here to comparatively ''weak'' interactions in which the streamwise spatial e xcursion of the shock front is always less than one boundary layer thi ckness. In this manner any shock motion due to upstream burst-sweep ev ents should be more apparent in relation to that oscillation associate d with the separated region. A discrete Hilbert transform-based condit ional sampling technique is used to obtain wall pressure measurements conditioned to burst-sweep events. The conditional sampling technique forms the basis by which the instantaneous shock motion is conditioned to the occurrence of upstream bursting. The relationship between the separation bubble and shock motion is also explored in detail. The res ults of the experiments indicate that the separation bubble represents a first-order effect on shock oscillation. Although it is demonstrate d theoretically that the burst-sweep cycle can also give rise to unste ady shock motion of much lower amplitude, the experiments clearly demo nstrate that there is no discernible statistical relationship between burst events and spanwise coherent shock front motion.