Hydrodynamic characteristics of wing-in-ground effect oscillating hydrofoil on power extraction performance

The energy contained in the tidal motion of the seas and oceans has the potential to be a significant source of renewable energy. The oscillating hydrofoil current-energy turbine has a good performance to extract energy from the coupling of its heaving and pitching motions. In the present study, the wing-in-ground (WIG) effect has been considered to improve the power-extraction performance of the oscillating hydrofoils. The overset grid in the commercial computational fluid dynamic (CFD) software STAR CCM+ is applied to study the flapping hydrofoil with dynamic WIG effect between two hydrofoils. The simulation results show that the WIG effect can greatly improve the power extraction performance of the flapping hydrofoil. The WIG effect is asymmetric over the course of the foil moving toward or leaving from the symmetry plane. The distance of the gap has a major influence on the hydrodynamic performances of the flapping hydrofoil. For a moderate gap, the positive pressure on the lower surface enhances as the hydrofoil departs from the symmetry plane and causes an improvement of lift and moment coefficients. As the gap decreases further, the increasing negative pressure between the leading edge and the symmetry plane plays an essential role improving the power extraction as the hydrofoil approaches the symmetry plane. Compared to the case without the WIG effect, the power-extraction efficiency has an increment of 16.34% in the present study.