Compressible Dynamic Stall Mechanisms Due to Airfoil Pitching and Freestream Mach Oscillations

An experimental investigation was undertaken to simulate dynamic stall and study the effects of airfoil pitching and freestream Mach oscillation. This work was performed in the recently modified OSU 6" x 22" unsteady transonic wind tunnel. The flow oscillates at a rate up to 21Hz or a reduced frequency of 0.05, with a Mach number of 0.42 ± 0.5 and a mean Reynolds number from 1.7 to 2.8 million. The work presented in this paper investigates the effects of an unsteady freestream on the airfoil at static angles of attack near stall. Specifically, the impact of reduced frequency on lift and pitching moment was investigated. The impact of time-varying compressibility is juxtaposed with measurements acquired for a pitching airfoil in a steady freestream. Variations between phase-averaged and time-accurate compressibility effects are also highlighted using particle image velocimetry (PIV) and pressure-sensitive paint (PSP).