Separated Shear Layer Transition at Low Reynolds Numbers: Experiments and Stability Analysis

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-degrees angles of attack. With laminar boundary layer separation occurring on the upper surface of the airfoil for all cases examined, 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 centred at some fundamental frequency. The initial growth of disturbances centred at the fundamental frequency is followed by the growth of a sub-harmonic component, eventually leading to flow transition. The growing disturbances also cause shear layer roll-up and the formation of roll-up vortices. Inviscid stability calculations and experimental results show good agreement, implying that separated shear layer transition on an airfoil at low Reynolds numbers is essentially inviscid in nature. However, the results suggest that the proximity of the airfoil surface to the separated shear layer may cause viscous effects to influence the transition process to some extent at low angles of attack.