| The near-field flow structure of the tip vortex generated by a NACA 0015 wing oscillating though the attached-flow, light-stall and deep-stall regimes was investigated at Rec = 1.86 x 105. Particular emphasis was placed on the effects of oscillation frequency and mean incidence upon the spatial and temporal evolution of the unsteady vortex structure. Phase-locked, ensemble-averaged cross-flow and axial velocity fields, vorticity distributions, and turbulence structures over a full cycle of oscillation were compared to static wing-tip vortex results, and the dynamic effects upon the vortex strength, size, trajectory and associated induced drag were examined. Through the attached-flow and light-stall oscillations, most vortex properties were qualitatively similar to the static cases, though a small degree of hysteresis between the pitch-up and pitch-down phases of motion was observed. The radial distributions of circulation within the inner region of the vortex were self-similar, and showed only small variations from the static case. When the wing was oscillated through the deep-stall regime, a dramatic decrease in tip vortex strength and concentration was observed at the end of the upstroke, as a result of the growth of the leading-edge vortex and subsequent catastrophic flow separation. The use of passive spoilers and active flaps to control the strength and trajectory of the tip vortex was also investigated. |
