Abstract
The velocity field over a moderately low aspect ratio retreating rotor blade is investigated using particle image velocimetry as the two-bladed teetering rotor with cyclic pitch operates in a low-speed wind tunnel. The phase-averaged velocity field shows that the stall vortex circulation is comparable to the section dynamic lift, confirming it as a dynamic stall vortex (DSV). Strong tip vortex effects suppress stall at outboard locations. The DSV on the rotating blade after liftoff is elongated and remains in close proximity to the blade surface in contrast to classical two-dimensional DSV on non-rotating blades. The phase-averaged DSV is roughly 40 % weaker and spatially diffused than those observed in individual instantaneous velocity fields. Spanwise variations of the vortex are also discussed. Cycle-to-cycle variations of the instantaneous velocity fields suggest a radial-flow-induced stabilization in the strength, but not in the spatial location of the DSV.
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