ON THE DYNAMICS SCALING OF FORCED UNSTEADY SEPARATED FLOWS

Abstract

The active control of separated flows in turbulent boundary layers was achieved experimentally using the periodic oscillations of a spoiler-like flap inside the separated zone. Results were obtained for three heights of the separation generator, using several freestream velocities and numerous forcing frequencies. The dynamical scaling analysis of reattachment control as a function of reduced frequency revealed the existence of at least two distinct mechanisms controlling flow reattachment. At low reduced frequencies (k < 0.04) , the primary mechanism corresponds to the momentum exchange induced by the modulation of the separated shear layer, and leads to the periodic shedding of the Separation bubble. This mechanism scales with the characteristic height of the separation region. For larger values of the reduced frequency parameter, the dominating mechanism is the formation and shedding of energetic vortices caused by the oscillation of the flap. This mechanism scales with the reduced frequency based an the flap height.