The Aerodynamics of the Transition from Thrust Production to Dynamic Stall on a Pitching Airfoil

Many swimmers maneuver using a thrust producing propulsor, which is typically modelled as a pitching airfoil and has been heavily studied. Vortices are generated and produce the classical reverse von Karman vortex street and thus thrust at select Strouhal numbers. Other pitching foils, such as helicopter blades, experience similar pitching oscillations at similar reduced frequencies but instead experience a dynamic stall event. Typically in dynamic stall the vortices are shed from the leading edge. The clear difference between the two problems is the differing mean angle of attack. This presentation will experimentally examine how mean angle changes cause a transition from thrust production to dynamic stall behavior. A NACA 0012 airfoil was oscillated about ¼ chord at reduced frequencies of k =0.25 and a Reynolds number of 12,000. High speed particle image velocimetry (PIV) measurements were taken to analyze the formation location and trajectory of vortex structures formed on the wing. The transition between thrust generating flow structures and dynamic stall will also be discussed.