Effects of dynamic stall on propulsive efficiency and thrust of flapping airfoil

Numerical simulations of dynamic stall phenomena around an airfoil oscillating in a coupled mode, in which the pitching and heaving oscillations have some phase difference, have been performed with a Navier-Stokes code. The propulsive efficiency and the thrust have been calculated for various combinations of the phase difference and the reduced frequency for two different amplitude ratios. The effects of the dynamic stall phenomena on the behaviors of the propulsive efficiency and thrust are discussed in detail by examination of each flow pattern obtained. Highest efficiency has been observed for the case in which the pitching oscillation advances 90 deg ahead of the heaving oscillation and the reduced frequency is at some optimum value, for which there appears no appreciable flow separation in spite of large-amplitude oscillations. For phase angles and reduced frequency other than this best condition, efficiency is rapidly degraded by the occurrence of the large-scale leading-edge separation.