Vortex Dynamics of Delta Wings Undergoing Self-Excited Roll Oscillations

Previous explanations of the flow mechanisms causing self-excited roll oscillations of non-slender delta wings were found to be inadequate following observations of a wing of 50° sweep with sharp leading edge. Experimental work was performed on a series of wings with sharp leading-edge and varying sweep angle (40° to 70°) on a free-to-roll sting and the results are presented here. The largest oscillations were found for wings around the slender/non-slender boundary, and two different regions of angle of attack with free-to-roll oscillations were found for the more slender wings tested. PIV measurements confirmed that the mechanism for the oscillations of the sharp leading edged wings in their first regions of oscillations were the same. A possible mechanism based upon the PIV measurements has been proposed for the motion incorporating changing windward vortex size and movement of the leeward shear layer reattachment point. For the second region of oscillations at higher angle of attack a different mechanism is thought to be taking place. PIV data suggest that the leeward shear layer is stalled at the maximum roll angle, but reattached flow is present at the minimum roll angle, thus these oscillations appear to be out of phase with the first region of oscillations.