Study of an SSC-A09 Airfoil in Compressible Dynamic Stall with Freestream Mach Oscillations

Particle image velocimetry (PIV) data was acquired over the upper surface of an SSC-A09 airfoil for the first thirty percent of chord. The test conditions included sinusoidal variation of pitch angle and Mach number, both in isolation and in tandem, in an unsteady transonic wind tunnel. The unique aspect of this study is experimental modelling of dynamic stall with oscillations of freestream velocity, along with the requisite pitch oscillations. Freestream velocity fluctuations are seldom studied in dynamic stall experimentation, but have been shown to be of first order importance to the dynamic stall characteristics. Furthermore, the time-varying freestream velocity of this work is in the compressible regime, with a mean freestream Mach number of 0.4 and fluctuation amplitude of 0.05. Other experimental conditions such as Reynolds number (1.8M to 3.7M based on chord) and reduced frequency (up to 0.05) were selected to match typical rotorcraft flight conditions. This work seeks to illuminate the physics governing formation and shedding of the dynamic stall vortex under these varying test conditions. Preliminary findings focus on the shear layer development and position, and unsteady compressibility effects that affect shedding of the dynamic stall vortex.