Dynamic reactions of a free-pitching foil to the reverse Kármán vortices

In order to understand the energy saving mechanism of fish schooling, the dynamic reactions of a free-pitching downstream foil to the oncoming reverse Kármán vortices shed from a flapping leading foil are investigated numerically. When the position of the hindfoil is optimized in a staggered arrangement, a significant augmentation of the thrust efficiency for the whole two-foil system is obtained because of the suction effect of a vortex pair formed at the leading edge of the hindfoil and the guide-plate effect of the hindfoil on the mean-flow jet originated at the trailing edge of the forefoil.