Separated-Shear-Layer Development on an Airfoil at Low Reynolds Numbers

Abstract

Flow transition in the separated shear layer on the upper surface of a NACA 0025 airfoil at low Reynolds numbers was investigated. The study involved wind-tunnel experiments and linear stability analysis. Detailed measurements were conducted for Reynolds numbers of 100,000 and 150,000 at 0-, 5- and 10-degree angles of attack. For all cases examined, laminar boundary-layer separation takes place on the upper surface of the airfoil. The separated shear layer fails to reattach to the airfoil surface for the lower Reynolds number, but reattachment occurs for the higher Reynolds number. Despite this difference in flow development, experimental results show that a similar transition mechanism is attendant for both Reynolds number flow regimes. Flow transition occurs due to the amplification of natural disturbances in the separated shear layer within a band of frequencies centered at some fundamental frequency. The initial growth of disturbances centered at the fundamental frequency is followed by the growth of a subharmonic component, eventually leading to flow transition. The growing disturbances also cause shear-layer roll-up and the formation of roll-up vortices. The results show that inviscid stability theory can be employed to adequately estimate such salient characteristics as the frequency of the most amplified disturbances and their propagation speed. This implies that the roll-up vortices can be attributed to inviscid instability. However, the results suggest that viscous and nonparallel effects need to be accounted for to effectively model the convective growth of the disturbances in the separated shear layer.