Compressibility Effects on Dynamic Stall of Oscillating Airfoils

Abstract

A review of experimental results from an ongoing study of the effects of compressibility on dynamic stall of an oscillating NACA 0012 airfoil is presented. The study shows that compressibility effects become significant at a free stream Mach number of 0.3. Dynamic stall is accelerated above this Mach number but increasing unsteadiness delays onset of stall even under compressible flow conditions. Interferometric images of the flow show that process of dynamic stall occurs rapidly over a small angle of attack range. For certain flow conditions, multiple shocks form in the flow near the airfoil leading edge. The delay of stall has been shown to be due to delayed development combined with modification of the adverse pressure gradient in the flow. Transition has been shown to significantly modify the observed flow behavior, and thus is a very important factor to be considered, especially since it occurs near the vortex formation location. Proper modelling of its effects is critical in dynamic stall flow computations.