Propulsive efficiency of oscillating foils

We determine the flow field generated by a two-dimensional foil in steady forward motion and a combination of harmonic heaving and pitching oscillations, by numerically solving momentum and continuity equations in the stream function-vorticity formulation. Attention is focused on moderate values of the Reynolds number and Mach numbers much smaller than one. The versatile tool which has been developed has allowed the achievements of results for a large set of values of the paramenters and the identification of those giving rise to optimal propulsive performances. Conditions for optimal production of thrust are: -operation at Strouhal number between 0.25 and 0.40; -large amplitude of heave motion-to-chord ratio (of order one); -large maximum angle of attack, between 15 and 25 degrees; -phase angle between heave and pitch (pitch leading heave) of about 75 degrees when the reference point for heave motion is at the one-third chord length from the leading edge.