Surface Pressure Study of an Airfoil Undergoing Combined Pitch and Low-Amplitude Plunge Motions

This paper describes the experimental study of surface pressure over a supercritical airfoil which was oscillated in pure pitching, pure plunging and combined pitch-plunge motions at the Reynolds number of 8.76*105. While the surface pressure distribution is of significant importance in stability and performance of an airfoil, not sufficient information is available on the pressure distribution in dynamic stall. The experiments were conducted in a closed-loop wind tunnel utilizing pressure transducers array. The motions were designed to maintain constant reduced frequency, Strouhal number and phase difference. Three different regions were assumed to represent the pressure distribution over the airfoil. The results showed that LEV formed on the upper surface manifested different behavior. In the attached flow region the LEV grew and shrunk over the upper surface but in the light stall region the LEV spilled on the airfoil while a small partial LEV remained at the leading edge. In the deep stall region the LEV spilled entirely and the flow was fully separated. The formation of Laminar Separation Bubbles and suction peaks were also reported in low angles of attack. Besides, the pitching moment Damping Factor was studied to determine the level of airfoil stall flutter stability. For lower amplitudes of pitching motion the airfoil seemed to be stable except where deep stall occurred. However for high amplitudes the airfoil had a tendency to enter the stall flutter. Nevertheless, forcing the airfoil to undergo a combined motion improved the stability condition in all cases.