Experiments on Natural Transition in Separation Bubbles

An experimental work on transitional separation bubbles is presented. The bubbles are generated on a flat plate by imposing, at some distance from the plate's leading edge, a region with a constant inviscid flow deceleration. Wall pressure, pitot-static probes, hot-wire, PIV, and LDV anemometry measurements are conducted to characterize the flow evolution. The different regions of the separation bubbles are studied and scaling laws are proposed to quantify the laminar shear layer length, its angle with the wall, the characteristic most amplified frequency, and the amplification rate during the linear stages of the transitional phenomena. The reattachment of the flow is found to be related to the evolution of the large scale structure after the shear layer rolls up. Downstream from reattachment, the velocity profile at the turbulent boundary layer seems to scale to a self-similar form.