Interferometric Investigations of Compressible Dynamic Stall Over a Transiently Pitching Airfoil

Abstract

The dynamic stall flow field over a NACA 0012 airfoil pitching transiently from 0 - 60 degrees at a constant rate under compressible flow conditions has been studied using the real-time technique of point diffraction interferometrv. This investigation using nonintrusive diagnostics-provides a quantitative description of the overall flow field, including the finer details of dynamic stall vortex formation, growth and the concomitant changes in the pressure distribution. Analysis of several hundred interferograms obtained for a range of flow conditions shows that the peak leading edge suction pressure coefficient at stall is nearly constant for a given free stream Mach number at all nondimensional pitch rates. Also, this value is below that seen in steady flow at static stall for the same Mach number, indicating that dynamic effects significantly affect the separation behavior. Further, for a given Mach number, the dynamic stall vortex seems to form rapidly at nearly the same angle of attack for all pitch rates studied. As the vortex is shed, it induces an anti-clockwise trailing edge vortex, which grows in a manner similar to that of a starting vortex. The measured peak suction pressure coefficient drops as the free stream Mach number increases. For free stream Mach numbers above 0.4, small multiple shocks appear near the leading edge above the shear layer. The unsteady flow peak suction pressure coefficient at any angle has been experimentally demonstrated to lag that of the steady flow at the same angle of attack.