The delay of turbulent boundary layer separation by oscillatory active control

The flow over a solid wedge from which a fully developed, turbulent boundary layer separates naturally, was investigated experimentally. The flow which separates at the geometric discontinuity turnes into a free mixing layer downstream of it. However the flow can be forced to reattach by the introduction of two-dimensional, harmonic, and small-amplitude perturbations at the apex of the wedge. The temporally-averaged characteristics of the reattached boundary layer are typical to flows at less severe pressure gradients, with the exception of the spanwise coherence near the solid surface which has been notably enhanced by the imposed perturbations. Phase-locked and ensemble-averaged results indicate that the subharmonic frequency dominates the flow at large distances from the apex at all forcing frequencies considered thus far. The preliminary results presented, indicate that this might be an effective way to delay separation of turbulent as well as laminar boundary layers.