The propulsion of two flapping foils with tandem configuration and vortex interactions

The propulsive performance of two flapping foils with the tandem configuration has been analysed. The trajectories of the foils are prescribed with typical propulsive motions. The hind foil performs oscillatory motion in the wake of the fore foil. The local flow around the hind foil and the effective attack angle have been changed by the vortex street. The velocity potential theory and the boundary element method are introduced to study the interactions of the vortices and the foils. The propulsive performance of the tandem flapping foils is affected significantly when various longitudinal distances and phase differences are adopted. The typical vortex interaction modes are investigated in terms of global phase shift. The thrust coefficient and the propulsive efficiency of tandem NACA0012 foils at typical global phases are analysed. The optimal global phase shifts for the highest thrust and highest efficiency have been found.