Influence of resonance on the performance of semi-active flapping propulsor

ABSTRACT A semi-active flapping foil, which is a heaving foil attached to a torsional spring, efficiently generates thrust. In this report, the propulsive characteristics of such foils are studied using a boundary element method. The numerical investigation is performed to clarify the effects of foil inertia and resonance. It is found that the flapping propulsor is efficient over a wide range of operating conditions. The peak efficiencies occur approximately at an identical advance number and also correspond to the same pitch amplitude in spite of different inertia. The present study also supports that the immersed resonance is relevant to the optimal performance. On the other hand, the propulsive characteristics of all foils are identical regardless of foil inertia when structurally resonated. An evidence has been found that the foil can also perform as an energy harvester in a certain range of operating condition.