Effect of pivot location on the semi-active flapping hydrofoil propulsion for wave glider from wave energy extraction

This paper experimentally investigates the propulsion of semi-active hydrofoils from wave energy. For the case of NACA 0012 foil section, heave amplitude h0 = 1.0 chord, Reynolds number Re=2×104, a series of flapping experiments were carried out. We examine the effects of the pivot location xp = 0.09c, 0.16c, 0.25c, and 0.33c on the propulsion and 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 xp=0.33c, 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 xp≤0.25c is favored. Additionally, for the pivot location xp=0.09c with non-dimensional stiffness K∗=1.69, 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 xp close to 0.25c, a smaller torsion stiffness should be implemented to achieve better efficiency, and a bigger torsion stiffness for xp close to the leading edge. These results have significant references to the semi-active hydrofoil design of wave gliders.