Coherent structures in an airfoil boundary layer and wake at low Reynolds numbers

Abstract

Boundary layer and turbulent wake development for a NACA 0025 airfoil at low Reynolds numbers was studied experimentally. Wind tunnel experiments were carried out for a range of Reynolds numbers and three angles of attack. Laminar boundary layer separation occurs on the upper surface of the airfoil for all Reynolds numbers and angles of attack examined. Two flow regimes are investigated (i) boundary layer separation without reattachment and (ii) separation bubble formation. The results suggest that coherent structures form in the separated flow region and the wake of the airfoil for both flow regimes. The formation of the roll-up vortices in the separated shear layer is linked to inviscid spatial growth of disturbances and is attributed to the Kelvin-Helmholtz instability. Linear stability theory can be employed to adequately describe the salient characteristics of such vortices and the initial stage of the separated shear layer transition. The development of the roll-up vortices leads to boundary layer transition, and the vortices break down during the transition process. Vortex shedding also occurs in the airfoil wake and vortices form in the near-wake region. It is shown that the boundary layer behavior has a profound effect on the identified coherent structures, and each of the two flow regimes is associated with distinctly different vortex shedding characteristics.