Compressibility Effects on Aerodynamic Damping During Dynamic Stall Events

Experimental results of attached, light, and deep dynamic stall regimes of an oscillating airfoil are presented at freestreamMach numbers ranging from 0.2 to 0.6, and Reynolds numbers up to 3.5x106. High bandwidth pressure transducers in the airfoil are used to capture the dynamic surface pressures and spatially-integrated airloads and moments. The time-resolved pitch-moments are used to determine the time-resolved aerodynamic coefficient within the pitching cycle. This involved the use of discrete Hilbert transforms of the pitch-moment time series. This time-resolved aerodynamic damping analysis offers new insight into the mechanism of transient stall flutter in rotorcraft operations. The experiments include a comprehensive investigation of independent parameters including Mach number, reduced frequency, and stall regime that is controlled by the maximum angle of attack. In addition, the effect of leading edge turbulent "trips" on the transient and cycle-average aerodynamic damping are investigated. It is demonstrated that the cycle-integrated aerodynamic damping coefficient masks much of the physics underlying the destabilizing effect of the dynamic stall process. In particular, conditions that exhibit positive cycle-integrated aerodynamic damping, often include time intervals of large negative aerodynamic damping during the pitching cycle. These negativelydamped portions of the pitch cycle could account for high-frequency stall flutter divergence from the rotor control input in helicopter applications.