Active and passive control of reattachment on various low-sweep wings

The use of wing oscillations as an active flow control method and wing flexibility as a passive flow control method was studied for various low-sweep wings in wind and water tunnel experiments. Spectral features of separated flows over the swept wings, their relation to the observed optimum frequencies, the effect of wing sweep angle, and the effects of symmetric and antisymmetric perturbations were investigated. Highly three-dimensional separated flows exhibit dominant peaks in the spectra of velocity fluctuations in the range of St = 1 to 5 in the post-stall region. These dominant peaks are in the same range as the optimum frequencies observed for oscillating or flexible wings. For A = 50°, 40°, and 30° simple delta and cropped delta wings, an optimum frequency around St = 1 was identified for which reattachment or vortex re-formation can be controlled best. The results also suggest that the lower limit of sweep angle below which the beneficial effect of wing sweep diminishes is around A = 20°. Symmetric perturbations also promote reattachment and vortex re-formation. For active flow control purposes, both symmetric and anti-symmetric excitations are effective. However, passive control for a flexible wing is possible only in the anti-symmetric mode.