Effect of airfoil shape on flow control by small-amplitude oscillations

The forces and flow fields associated with small-amplitude oscillations of a NACA 0012 airfoil and flat plate are compared for zero and a post-stall angle of attack of fifteen degrees at a Reynolds number of 10,000. For zero degrees angle of attack at high Strouhal numbers the NACA airfoil experiences stable deflected jets, whereas the flat plate experiences deflected jets that are prone to periodic oscillation in direction resulting in oscillation of the lift coefficient with a period on the order of 100 cycles. At fifteen degrees angle of attack the flat plate is shown to produce a comparable increase in lift up to a Strouhal number of unity but after this the lift performance deteriorates. This is due to the Leading Edge Vortices (LEVs) convecting further from the upper surface. At higher plunge velocities a new mode of leading-edge vortex behavior is observed, for the NACA airfoil the leading-edge vortex is formed during the downward motion and then remains near the leading-edge and therefore loses its coherency through impingement with the upward moving airfoil. For the flat plate the upper surface LEV pairs with the lower surface LEV to form a dipole that self-advects normal to the free stream and is rapidly destroyed.