Nonuniform motion of leading-edge vortex breakdown on ramp pitching delta wings

Vortex flow characteristics on ramp pitching delta wings of sweep angles from 59 to 70 deg were studied by flow visualization and LDA measurement. Experimental observations indicate that the movement of the breakdown point is strongly dependent on the sweep angles and the pitching rates. The moving behavior of the breakdown point can be characterized by the occurrence of two delays. The first delay is due to the fact that the flow initially takes a length of time comparable to C/U_∞ to respond to the pitching motion. The second delay exists only for specific sweep angles and occurs in the course of pitching-up motion. It is characterized by the phenomenon that the breakdown point is moving slowly or even standing still above the wing surface for a certain length of time. The chordwise location corresponding to this occurrence shifts upstream as the pitching rate increases. This delay diminishes as the sweep angle increases. The LDA data obtained further reveal that both delays are associated with the underdevelopment of the primary vortex.