Effects of flexibility on energy extraction performance of an oscillating hydrofoil under a semi-activated mode

The aim of this paper is to numerically investigate the energy extraction performance of a flexible hydrofoil under the semi-activated mode. The effects of flexure amplitude (alpha), pitching amplitude (theta(0)) and reduced frequency (f*) on power extraction were studied and compared with rigid hydrofoils. The evolution of effective angle of attack, vortex and pressure field are examined. The results show that averaged power coefficient ((C-P) over bar) and efficiency (eta) of the flexible hydrofoil are better than that of rigid hydrofoils. The increase of alpha can obviously improve the zones of high eta at 0.15 < f*<0.25 and 50 degrees < theta(0) < 80 degrees. Especially, increasing a could obviously improve (C-P) over bar and eta at a low theta(0). Further, at a high theta(0), the factors causing decline of power extraction are the separation of leading-edge vortex at low frequency and the shifting of pressure center caused by the decrease of pitching angular rate at high frequency. Compared with the rigid hydrofoil, the flexible hydrofoil can increase the pressure difference between the upper and lower surfaces of hydrofoils, thus lift is improved, which is beneficial to the improvement of energy extraction performance. (C) 2021 Elsevier Ltd. All rights reserved.