Coherent Vortical Structures in the Separated Flow over a 3-D Hump*

The separated flow over a three-dimensional hump model in an approaching fully turbulent boundary layer is investigated as part of a program to generate a data set used in validating CFD simulations of 3-D separated flows. The flow regimes include a range of Mach numbers from 0.05 to 0.2 that correspond to Reynolds numbers based on the hump height of 8.3e4 to 3.3e5.  Single-camera and dual-camera-stereo PIV sampling several y-z planes downstream of the hump apex are used to quantify the characteristics of the flow separation and reattachment. Over the range of Mach numbers, the flow past the hump apex transitioned from being fully attached to fully separated.  When fully separated, the PIV measurements revealed the presence of lifted large-scale, streamwise-oriented symmetric vortical structures whose presence was initially identified with surface flow visualization. These vortical structures are believed to be a universal feature of all 3-D separated flows.  Their spatial evolution is characterized for the range of freestream Mach numbers. Of special interest is the role that these coherent vortical motions might have on flow reattachment.