Vortex Dynamics of Free-to-Roll Slender and Nonslender Delta Wings

Experiments were conducted on various free-to-roll delta wings with sweep angles in the range of 40-70 deg in a wind tunnel. For low sweep angles, nonzero trim angles are observed until the wing stalls after which the trim angle becomes zero. The magnitude of the roll trim angle decreases with increasing sweep angle and almost disappears for the most slender wing. In a certain range of sweep angles, and just before the stall, large amplitude self-induced roll oscillations with nonzero mean were observed for thin wings with sharp leading edges for which separation point is fixed. Two different regions of angle of attack with free-to-roll oscillations were found for certain sweep angles. Velocity measurements suggest that the oscillations are initiated by the increasing proximity of the shear layers as the angle of attack is increased. Possible mechanisms of the self-induced roll oscillations based upon the phase-averaged velocity measurements have been proposed, incorporating varying vortex size and strength as well as the movement of the shear-layer reattachment locations. Hysteresis and time lag effects are significant even though typical dimensionless frequencies are very low.