Flowfield measurements of reverse flow on a high advance ratio rotor

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Lind, A.H.
Trollinger, L.N.
Manar, F.H.
Chopra, I.
Jones, A.R.
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A 1:7 m-diameter Mach-scaled slowed rotor was tested at advance ratios up to _ = 0:9 and three shaft tilt angles _s = ??4_; 0_; 4_. Two-component time-resolved particle image velocimetry was used to characterize the flow field around a blade element in the reverse flow region, nominally positioned at = 270_ and y=R = 0:4. Four dominant flow structures were observed: the reverse flow starting vortex, the blunt trailing edge wake sheet, the reverse flow dynamic stall vortex, and the tip vortex. As advance ratio increases, the duration of reduced time that the blade element spends in the reverse flow region also increases. This affects the strength, trajectory, and predicted vortex-induced pitching moment of the reverse flow dynamic stall vortex. Shaft tilt angle also has a strong effect on the evolution of the reverse flow dynamic stall vortex with forward shaft tilt resulting in dramatically increased strength and size. The results of this characterization and sensitivity study are aimed at informing the development of unsteady reverse flow models for use in comprehensive rotorcraft codes.