Hydrodynamic analysis of three oscillating hydrofoils with wing-in-ground effect on power extraction performance

Currently, oscillating-foil power extraction turbines have been developed as a new competitive way compared with rotor blade designs. In the present study, oscillating hydrofoils with a heaving amplitude of one chord at the Reynolds number of Re = 500,000 are employed in a fully prescribed motion to investigate the wing-in-ground (WIG) effect on the power extraction performance. To prescribe the heaving and pitching motions, the harmonic sinusoidal function is actively imposed. An unsteady Reynolds Averaged Navier-Stokes solver based on an open-source CFD code OpenFOAM is used to solve this problem. The fields of the pressure and velocity around a foil are analyzed when the foil approaching and departing from the ground. According to the distribution of pressure on the upper and lower surfaces of the foil, it is found that the efficiency of the power extraction increases mainly due to the increment of the positive pressure on the lower surface. The maximal improvement of the power-extraction efficiency is 7.36% when the minimum gap is 0.25 chord length. A parametric study with different gaps is conducted to predict the optimal power-extraction region. The optimal power-extraction zone shifts to the higher frequency as the gap decreases. The maximal efficiency of the power extraction reaches 41.67%. The wing-in-ground effect turbine (WIGT) by multi-hydrofoils is also simulated and it has a satisfying consequence with the increment of the power-extraction efficiency of 13.37% of the middle hydrofoil which experiences twice the WIG effect.