Numerical simulations of dynamic stall phenomena around an airfoil oscillat
ing in a coupled mode, in which the pitching and heaving oscillations have
some phase difference, have been performed with a Navier-Stokes code. The p
ropulsive efficiency and the thrust have been calculated for various combin
ations of the phase difference and the reduced frequency for two different
amplitude ratios. The effects of the dynamic stall phenomena on the behavio
rs of the propulsive efficiency and thrust are discussed in detail by exami
nation 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, fo
r which there appears no appreciable flow separation in spite of large-ampl
itude oscillations. For phase angles and reduced frequency other than this
best condition, efficiency is rapidly degraded by the occurrence of the lar
ge-scale leading-edge separation.