Wall Distance Effects on Transition to Turbulence in Low-Reynolds-Number Separated Flows

Airfoils operating at Rec<50,000 suffer from laminar separation that degrades aerodynamic efficiency. Transition to turbulence improves performance but becomes difficult to achieve at low Reynolds numbers. Experiments at separation Reynolds numbers, Re<theta>S=24-46, with a rounded leading edge and a sharp back-step, were performed to distinguish between the effects of the wall proximity and Reynolds number on the shear layer transition to turbulence. When the shear layer is sufficiently far from the wall (HS>20), the transition distance, normalized by the model chord, is proportional to the Reynolds number at separation. The growth of instabilities are in good general agreement with both linear stability calculations based on the measured velocity profiles, and the predictions obtained using the computational tool XFOIL. For shape factors larger than 20, high growth rates occur, even for Re theta S as low as 5. For smaller shape factors, instabilities are greatly reduced or eliminated, and the wall stabilizing effect becomes more significant as the Re number decreases. Guidelines for the design of turbulator devices are discussed.