Active control of turbulent separated flows over slanted surfaces

The experimental investigations in the present paper deal with the excitation of fundamental instability mechanisms in separated free shear layers on a bluff body and downstream of a diffuser by means of periodic forcing in order to reduce the expansion of flow separation. The experiments focus on a unique approach to separation control using fundamental frequencies for local forcing in two different shear layer configurations (inner and outer diffusers). Each separation process is characterized by the periodic occurrence of large spanwise vortex structures. These vortices scale with the difference in height between the ramp ends. The excitation of these large scale vortex structures by periodic forcing intensifies the momentum transfer between the separation region and the outer flow, resulting in a substantial reduction of the reattachment length. For the inner and outer diffuser configurations, a universal value for the optimum forcing frequency was established.