Investigation of mechanisms of high lift for a flat-plate airfoil undergoing small-amplitude plunging oscillations

Two high-lift mechanisms, namely, convected leading-edge vortices and stable deflected jets, have previously been identified for an airfoil undergoing small-amplitude plunging oscillations. In this paper the effect of geometry is investigated through direct comparison of the forces and flowfields associated with small-amplitude plunging oscillations of a NACA 0012 airfoil and a flat plate for 0 deg and a poststall angle of attack of 15 deg with a Reynolds number of 10,000. For 0 deg at high Strouhal numbers, the NACA airfoil experiences stable deflected jets responsible for very large lift coefficients, 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. It is postulated that this jet switching is driven by the leading-edge vortex (LEV). At 15 deg 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 convecting further from the upper surface. At higher plunge velocities, a new mode of LEV behavior is observed. The upper-surface LEV pairs with the lower-surface LEV to form a dipole that convects against the freestream and is rapidly dissipated. This results in a highly separated time-averaged flow, and thus in low lift and high drag.