Frequency response of separated flows on a plunging finite wing with spoilers

The effectiveness of lift and bending moment reduction on a plunging wing was investigated in an experimental study to understand the effects of unsteady wing motion. The frequency response of the lift/moment reduction was studied for finite-span mini-spoilers placed on a finite wing. Single, multiple, and spanwise periodic mini-spoilers located at various spanwise locations all exhibit a decaying frequency response of the effectiveness of lift/moment reduction with increasing frequency and amplitude of the oscillations. This happens for pre-stall angles of attack as the mini-spoilers induce shear layer separation, roll-up of vorticity, and recirculation regions, generating additional lift. For the post-stall angles of attack, stronger leading-edge vortices are already shed from the clean wing at high frequencies, and the spoilers are not able to influence the development of the vortices, resulting in a decaying frequency response. The cut-off frequency for all spoiler configurations is similar and scales with the reduced frequency at pre-stall angles of attack, whereas the Strouhal number based on the peak-to-peak amplitude of the plunge oscillations becomes the scaling parameter at post-stall angle of attack. The spoilers exhibit similar cut-off frequency to their two-dimensional counterparts. The finite-spoilers delay or displace the shear layer away from the wing surface, similar to the two-dimensional mechanisms.