Effect of Pivot Location on the Propulsion and Energy Extraction of Semi-Active Flapping Hydrofoil for Wave Glider

This paper experimentally investigates the wave energy extraction for propulsion of semi-active hydrofoils used in wave glider. For the case of NACA 0012 foil section, heave amplitude [[EQUATION]] =1.0 chord, Reynolds number [[EQUATION]] , a series of flapping experiments were carried out. We examine the effects of the pivot location [[EQUATION]] = 0.09c, 0.16c, 0.25c, and 0.33c on the propulsion and energy extraction efficiency of the semi-active flapping hydrofoils. Combined with numerical simulation and Theodorsen's theory, some conclusions are given. The results demonstrate that for the pivot location [[EQUATION]] , the semi-active flapping hydrofoil might fall into a pitch unstable state at a low oscillating frequency, which leads to low efficiency(<20%). For the semi-active hydrofoil, a pivot location [[EQUATION]] is favored. Additionally, for the pivot location [[EQUATION]] with non-dimensional stiffness [[EQUATION]] , we find that an energy conversion efficiency of more than 40% is achievable at nearly all Strouhal numbers St=0.2~0.77. Meanwhile, for the pivot location [[EQUATION]] close to 0.25c, a smaller torsion stiffness should be implemented to achieve better efficiency, and a bigger torsion stiffness for [[EQUATION]] close to the leading edge. These results have significant references to the semi-active hydrofoil design of wave gliders