FLOW STRUCTURE ON STALLED DELTA-WING SUBJECTED TO SMALL AMPLITUDE PITCHING OSCILLATIONS

The flow past a harmonically oscillating delta wing is investigated using high image-density particle image velocimetry. The overall objective is to alter the development of the leading-edge vortex and decrease the extent of stall on the wing. To this end, instantaneous velocity fields, contours of constant vorticity, and streamline patterns are obtained over an entire plane of the how at successive instants of the wing motion. The degree of repeatability of the flow patterns during successive cycles of the wing oscillation is highly dependent on the di mensionless frequency of oscillation; it is possible to generate flow patterns that are phase locked at every cycle, as well as every other cycle, of the wing motion. In each case, a well-defined leading-edge vortex appears, and the instantaneous extent of the stall region is decreased relative to that on a stationary wing at the same angle of attack. However, cycle-averaged patterns show only minor departures from the flow structure on the stationary wing. Ensemble-averaged patterns, obtained by phase referencing to the wing motion, reveal that small-scale vortical structures originating in the unstable leading-edge shear layer are averaged out; the overall vorticity levels of the entire flowfield are lowered substantially, even for the average of only a small number of instantaneous images.