On the transient process of flow reattachment by external excitation

The transient reattachment process of a turbulent flow to an inclined flat surface was investigated experimentally. The process was initiated by periodic excitation that was introduced in a step-wise fashion at the hinge of the flap where the flow normally separated. Reattachment of the separated flow is essentially a constant-rate process whose slope depends upon the frequency and amplitude of the excitation and the flap deflection. Exception to this is an adverse surge in the flap aerodynamic loading immediately following the onset of the excitation. The duration of the surge scales with the flight time of the first induced vortex over the entire flap and it is unrelated to the nature of the oscillations. Overall reattachment time attains a minimum at reduced frequencies around F⁺=1.5. This optimal frequency is independent of the amplitude. The entire process is dominated by large spanwise vortices that were initiated by the excitation and amplified by the shear layer. It was demonstrated that as the coherent structures grow they contribute to net transport of fluid directed away from the surface that causes favorable pressure difference across the reattaching flow.