Piv-Based Examination of Deep Stall on an Oscillating Airfoil

A number of studies suggest that the deep stall flow fields produced by a pure-pitch and a pure-plunge oscillating airfoil are equivalent, when effective angles of attack are matched. This assumption is examined using particle image velocimetry (PIV). Sinusoidal oscillations of a pure-plunge and pure-pitch airfoil with matched reduced frequency k = 3.93 and with kinematically equivalent amplitudes of effective angle of attack are comparatively examined using results of PIV in a free surface water tunnel at AFRL/RB, Wright Patterson AFB. Experiments were conducted at Re = 10,000 and Re = 40,000, based on free stream velocity and airfoil chord, in order to observe the legitimacy of assuming Reynolds number insensitivity on the respective flow fields. Comparisons are made to computational flow field results collected in a separate, but coordinated, CFD effort. Results for both the pure-plunge case and the pure-pitch case confirm the Reynolds number insensitivity for the high frequency motions researched here. The resulting flow fields for pure-plunge case and the pure-pitch case were vastly different. Experimental results for the pure-plunge case closely resembled those achieved by computation. However, the pure-pitch case experimental and computation results were dissimilar even after considering possible wall effects of the water tunnel. The flow field disagreement between the two motions is not surprising considering trailing edge kinematic dissimilarities.