Lift enhancement over flexible nonslender delta wings

Unsteady aerodynamics of flexible nonslender delta wings is investigated in an experimental study using various techniques. Dramatic fluid/structure interactions emerge with increasing wing flexibility and result in substantial lift enhancement in the poststall region. This recently discovered phenomenon appears to be a feature of nonslender wings. Self-excited antisymmetric vibrations of the wing promote reattachment of the shear layer, which results in the lift enhancement. These self-excited vibrations are not observed for a half-model. The Strouhal number of the dominant frequency of the structural vibration is on the order of unity for all nonslender wings, which also corresponds to the frequency of the shear-layer instabilities for the rigid wing. Velocity measurements demonstrate the striking difference between the flows over the flexible and rigid wings in the poststall region. The effect of flexibility is to promote the reattachment of the shear layer near or downstream of the apex, depending on the incidence. There are substantial effects on the vortical flow with increasing wing flexibility, which might lead to the axial flow developing within the reattached region. The time-averaged vorticity flux increases due to the oscillating leading edge, which leads to increased circulation.