Control of flow separation by dynamic excitation of the free shear layer

Abstract

Reattachment of a fully separated flow at a NACA 0012 airfoil at angles of incidence above the critical angle could be achieved by periodic suction and blowing through a narrow slot close to the leading edge of the airfoil. A small cavity below the slot was connected to a horn-driver providing periodic pressure fluctuations to drive the flow through the slot. The present research confirms the results of previous studies [1] that the mechanism for such a control consists of stimulation of momentum exchange across the separated free shear layer. Three optimum frequency bands were found where the periodic excitation was very effective (i.e. where low sound pressure levels were required to obtain an attached flow). The first frequency band was about the natural instability frequency of the separated free shear layer. The second frequency band was centered about the subharmonic of the natural shear layer instability frequency. It was assumed that excitation within these two frequency bands promotes the development of streamwise vortices in the free shear layer which promote entrainment of the outer flow. The third frequency range was below about 1/10 of the fundamental free shear layer instability. Large-scale vortical structures are believed to be generated by excitation within this frequency band.