SEPARATION AND INSTABILITIES IN THE VISCOUS-FLOW OVER AIRFOIL LEADINGEDGES

The two-dimensional, unsteady, leading-edge flow over stationary, pitc hing and oscillating airfoils is studied using the Navier-Stokes equat ions for flow past a parabola. Grid-converged solutions have been obta ined for the initial stages of unsteady separation for impulsively-sta rted unsteady flow and for pitch-up at constant rate. The results comp are favorably with corresponding full-airfoil computations. It is show n that small perturbations in the flow can generate eddies in the boun dary layer before how reversal occurs in the base flow. Oscillations a ppear in the skin friction which grow in amplitude and convect downstr eam with time. Above a certain amplitude, the skin Friction becomes ne gative locally, giving rise to small recirculating eddies. This type o f eddy creation prior to reversal in the base flow is in general agree ment with the theory of Rayleigh instabilities. The wavelength of thes e instabilities scales as Re-0.43 which is in reasonable agreement wit h the theoretical value of Re-1/2 for the Rayleigh instability. (C) 19 98 Elsevier Science Ltd. All rights reserved.