Shock unsteadiness in a reattaching shear layer

The origin of shock unsteadiness in a Mach 2.9 turbulent reattaching shear layer was investigated experimentally using temporally resolved flow visual ization and measurements of wall pressure fluctuations. In this how, the se paration point of a turbulent boundary layer is essentially fixed at a back ward-facing step, and the reattachment point is free to move along a ramp. In order to examine the influence of disturbances originating in the incomi ng shear layer, artificial disturbances were introduced into the flow throu gh steady air injection in the vicinity of separation. The effect on the re attachment shock system was dramatic: the intensity of the pressure fluctua tions and the amplitude of the shock motion increased substantially, and po wer spectra of the pressure fluctuations showed a distinct shift to lower f requency. The spectra collapsed, onto a common curve in non-dimensional coo rdinates based on a length scale derived from two-point cross-correlations of the flow visualization data and a convection velocity derived from cross -correlations of the pressure measurements. The data were compared to a the ory developed by Plotkin (1975), which is based on perturbation of a shock by random fluctuations in the incoming turbulent flow. Plotkin's model mimi cs the manner in which relatively broad-band perturbations in the incoming turbulent flow lead to relatively low-frequency motion of the separation bu bble and its associated shock system. It is an excellent fit to separation shock motion, such as that generated in a blunt fin flow (briefly illustrat ed here). In the present shear layer flow, this low-frequency motion was de tectable in the spectra near reattachment, but contained considerably less energy relative to the shock motions caused by direct perturbations by the incoming turbulent structures. These results indicate that the shock motion in the reattaching shear layer is primarily caused by organized structures in the incoming turbulent how.