Passive Post-stall Flow Control on Nonslender Delta Wings Using Flags

The lift force on two nonslender delta wings of sweep angles of Λ = 40° and 50°, with flags attached to the leading-edge, was investigated. Substantial lift enhancement in the post-stall regime of the clean wings and the stall delay were observed. With a flag, the flow field measurements showed the reattachment of the shear layer onto the wing surface and the re-formation of the leading-edge vortex, whereas the clean wing had completely stalled flow at the same angle of attack. Over a wide range of angle of attack, mass ratio, and flag length, the maximum lift enhancement was attained when the dimensionless frequency of flag oscillations was in an optimal range and the flag-tip velocity had sufficient amplitude. The optimal dimensionless frequency range corresponded to the natural shear layer instabilities of the clean wing. The excitation in this range substantially increased the coherency of the separated flow due to the flag oscillations. The main mechanism of the lift increase is the excitation of the shear layer, which exhibits a convective instability. This is very different from the lift enhancement on airfoils for which the flags locks-in to the wake instability, which is known to have an absolute instability at the post-stall angles of attack of the clean airfoil.