To mimic the passage through a region of reverse flow as observed on the blades of a helicopter rotor of vertical axis wind turbine, an experiment was designed including a NACA0012 harmonically oscillating in stream-wise direction in a constant free-stream at a fixed angle of attack. The surging parameters were chosen to ensure the occurrence of reverse flow on the downstream motion of the wing. To analyze the unsteady flow development during the full surging cycle, including the transition in and out of reverse flow conditions, time resolved particle image velocimetry and direct force measurements were conducted. Six flow development stages were observed within the full surging cycle which are successively characterized by bounded leading edge vortex formation, shear layer lift off, trailing edge vortex formation, alternate vortex shedding, shear layer stagnation, and vorticity wipe out. The transition timing between the stages was determined based on a POD analysis of the flow field.
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