The unsteady boundary layer developed on an oscillating NACA 0012 airfoil model was investigated using multiple hot-film sensor arrays. The results show that (ⅰ) the spatial-temporal progression of the boundary-layer transition and separation points were measured both nonintrusively and simultaneously, (ⅱ) the transition (reattachment) was delayed (promoted) with increasing (decreasing) angle of attack and oscillation frequency, relative to the static case, and (ⅲ) the dynamic stall process was found not to originate with the bursting of a laminar separation bubble, but with a breakdown of the turbulent boundary layer. These measurements are useful in the study of unsteady separated flow fields generated by rapidly maneuvering aircraft, helicopter rotor blades, and wind energy machines.
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