Planar imaging measurements to study the effect of spanwise structure of upstream turbulent boundary layer on shock induced separation

The relationship between the upstream boundary layer and the unsteadiness of the separated flow in a Mach 2 compression ramp interaction is investigated by performing wide-field PIV and PLS measurements in streamwise-spanwise planes. Presence of spanwise strips of elongated regions of uniform momentum is detected in the upstream boundary layer. These long coherent structures exhibit strong similarities to those that have been found in incompressible boundary layers, which suggests an underlying similarity between the incompressible and supersonic regimes. At a wall normal-location of y/δ = 0.2, the upstream envelope of the separation region is found to oscillate between x/δ = -3 and -1 (where x/δ = 0 is the ramp corner). The instantaneous separation line (in the spanwise direction) is found to respond to the elongated regions of uniform momentum. The separation region exhibits an undulated envelope which conforms to the lowand high-speed regions in the upstream boundary layer. The low frequency unsteadiness of the separation region/shock foot observed in numerous previous studies can be reconciled by a turbulent mechanism that includes these elongated regions of uniform momentum.