Unsteady Lift and Moment of a Periodically Plunging Airfoil

To simulate the effects of gusts and maneuvers, lift, moment, and flow measurements are presented for a periodically plunging airfoil at a Reynolds number of 20,000 over a wide range of reduced frequency (k≤1.1), amplitude (A/c≤0.5), and mean angle of attack (0–20 deg). For this parameter range, the maximum lift in the cycle is determined by the circulatory lift, regardless of whether a leading-edge vortex (LEV) is formed or not, whereas the maximum nose-down moment is determined by the competition between the added mass and the arrival of the LEV near the trailing edge. The LEV generally increases the mean lift and decreases the mean moment. This increase, which is not predicted well by the reduced-order models, correlates with the maximum effective angle of attack of the motion, rather than the Strouhal number. In contrast, the amplitude and phase of lift are predicted well by the Theodorsen theory (Theodorsen, T., “General Theory of Aerodynamic Instability and the Mechanics of Flutter,” NACA TR 496, 1949), whether LEVs are present or not. Hence, LEVs have a minimal effect on the fluctuating lift. However, the amplitude of the pitching moment is not predicted well by the Theodorsen theory, if LEVs are present. The competition between the added mass and the LEV causes the nonmonotonic variation of the moment amplitude as a function of reduced frequency.