Passive Poststall Flow Control on Nonslender Delta Wings Using Flags

Abstract

The lift force on two nonslender delta wings with sweep angles of Λ=40 and 50°, with flags attached to the leading edge, was investigated. Substantial lift enhancement in the poststall regime of the clean wings and the stall delay were observed. With a flag, the flowfield 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 angles of attack, mass ratios, and flag lengths, 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 convective instability. This is very different from the lift enhancement mechanism on airfoils where the flags lock-in to the wake instability, which is known to have an absolute instability at the poststall angles of attack of the clean airfoil.