MEASUREMENT OF UNSTEADY BOUNDARY-LAYER DEVELOPED ON AN OSCILLATING AIRFOIL USING MULTIPLE HOT-FILM SENSORS

The spatial-temporal progressions of the leading-edge stagnation, sepa ration and reattachment points, and the state of the unsteady boundary layer developed on the upper surface of a 6 in. chord NACA 0012 airfo il model, oscillated sinusoidally within and beyond the static-stall a ngle, were measured using 140 closely-spaced, multiple hot-film sensor s (MHFS). The MHFS measurements show that (i) the laminar separation p oint and transition were delayed with increasing a and the reattachmen t and relaminarization were promoted with decreasing alpha, relative t o the static case, (ii) the pitchup motion helped to keep the boundary layer attached to higher angles of attack over that could be obtained statically, (iii) the dynamic stall process was initiated by the turb ulent flow separation in the leading-edge region as well as by the ons et of flow reversal in the trailing-edge region, and (iv) the dynamic stall process was found not to originate with the busting of a laminar separation bubble, but with a breakdown of the turbulent boundary lay er. The MHFS measurements also show that the flow unsteadiness caused by airfoil motion as well as by the flow disturbances can be detected simultaneously and nonintrusively. The MHFS characterizations of the u nsteady boundary layers are useful in the study of unsteady separated flowfields generated by rapidly maneuvering aircraft, helicopter rotor blades, and wing energy machines.