Force and motion measurements of a passively oscillating hydrofoil

Abstract

Energy extraction using flapping foils is a novel concept in the field of renewable energy, especially when the system's locomotion is fully passive and solely dependent on fluidic forces. In order to investigate this concept, a water-tunnel test rig was designed and fabricated in such a way that a hydrofoil is able to periodically heave and pitch under hydrodynamic forces. Energy extraction performance was investigated systematically through simultaneous measurements of the hydrofoil’s two degree-of-freedom (DoF) motions and hydrodynamic forces/torques, at two flow velocities (corresponding to the Reynolds Number (Re) ≈ 0.9 × 105 and 1.1 × 105). This study is focused on the effect of maximum pitching angle of the hydrofoil on the performance of the system. Hydrodynamic forces, extracted power and efficiencies were measured and calculated to evaluate the system performance at three maximum pitching angles. It was observed that increasing the free-stream velocity increased the energy extraction efficiency and power output of the system. Also, a peak appears in average power coefficient and efficiency when the maximum pitching angle is at the intermediate value.