Investigation of Incipient Dynamic Stall at High Reynolds Numbers

Abstract

Abstract : In the first part of this study, surface shear-stress measurements were obtained on a NACA 0012 airfoil model, undergoing a pitch-up motion from 0 deg to 43 deg angle of attack at a constant rate using an array of surface-mounted hot-film sensors. Dominant features in these data and in the standard deviations computed from these data were examined and related to events in the development and evolution of the unsteady separation over the suction surface. Results were compared with well-known features of the dynamic stall process seen in the surface-pressure distributions. Trends in the behavior of these features are presented for a range of non-dimensional pitch rates and chord Reynolds numbers. Significant changes were seen in the behavior of these features at high Reynolds numbers. The results suggest that these changes are due to transition in the shear layer at high pitch rates and quasi-steady behavior at low pitch rates. In the second pan of this study, large amplitude sinusoidal motions were investigated for a wide range of Reynolds numbers and reduced frequencies. A combination of unsteady pressure and shear-stress data at the surface of the airfoil provided detailed information about the development and evolution of the flowfield. In particular, the formation of the dynamic stall vortex (DSV) during the upstroke of the motion profile was examined in detail as well as the reattachment process during the downstroke of the motion profile. Significant changes in behavior were seen with changing Reynolds number, reduced frequency, and amplitude of oscillation. The mean angle did not affect the development of the DSV except at the highest reduced frequency (k=0.4). Amplitude of oscillation did not affect the development of the reattachment process.