Propulsive efficiency of oscillating foils

The dynamics of the vortex structures generated by a foil in steady forward motion, plus a combination of harmonic heaving and pitching oscillations, is determined by means of the numerical solution of the vorticity equation. The force and the torque acting on the foil are also computed. The investigation extends a previous study of the phenomenon (J. Fluid Mech. 410 (2000) 323-341), where only heaving oscillations were considered. The pitching motion turns out to be the only means to produce thrust when the forward motion of the foil vanishes, i.e., for foils operating in the hovering mode. Moreover pitching oscillations added to the heaving motion of the foil produce much larger values of the thrust and better efficiencies. Results are obtained for many combinations of the parameters characterizing the phenomenon and the investigation allows us to identify the range of the parameters providing high propulsive efficiency. The numerical results agree well with previous experimental measurements and, in addition, allow access to the velocity and vorticity fields as functions of space and time, which in turn allows us to identify the underlying thrust production mechanisms more easily. (C) 2003 Elsevier SAS. All rights reserved.