1 Delay of Stall by Small Amplitude Airfoil Oscillations

Particle Image Velocimetry and force measurements were conducted for small amplitude plunging motion of a NACA0012 airfoil at a post-stall angle of attack. Time-averaged flow shows that the region of separation is significantly reduced compared to a stationary airfoil. At low Strouhal numbers phase-averaged flow shows the formation and convection of leading-edge vortices, followed by their destructive interaction with the trailing-edge vortices. This mode of vortex flow is modified substantially at a critical Strouhal number. In this new mode, the leading-edge vortex is generated during the downward motion of the airfoil, but dissipates rapidly during the upward motion of the airfoil. Hence the convection of the leading-edge vortices is not observed. The strong interaction between the airfoil and vortex appears to destroy the coherency of the vortex. The switch between modes occurs at lower Strouhal numbers with increasing plunge amplitude. The critical Strouhal number corresponds to a Strouhal number based on the peak-to-peak amplitude SrA = 0.25- 0.42. The point of drag to thrust switch correlates strongly with the point of mode-switch. Nomenclature a = amplitude of plunging motion Cd = time-averaged drag coefficient Cd0 = drag coefficient of stationary airfoil c = chord length f = frequency of oscillation h(t) = plunging displacement of the airfoil Re = Reynolds number, ρU∞c / µ SrA = Strouhal number based on peak-to-peak amplitude, 2fa/U∞ Src = Strouhal number based on chord length, fc/U∞ t = time T = plunge period U ∞ = free stream velocity V = magnitude of velocity vector x = streamwise direction y = cross-stream direction α = angle of attack µ = viscosity ρ = density ω = vorticity I