Research on the mechanism of power extraction performance for flapping hydrofoils

Utilizing the UCFD (Unified Computational Fluid Dynamics) software, this paper describes simulation of the extraction of marine tidal energy using a flapping hydrofoil. Firstly, the validity of the numerical simulation was confirmed, where the time signal of heave force coefficient agreed well with the experimental result. Secondly, a parameter optimization study was conducted under different influence factors, results such as heaving force and work done by the heaving and pitching motions were analyzed and compared. When the quarter-period effective angle of attack is 34.4°, the heaving amplitude is 1.0c or the Strouhal number is 0.36, the power efficiency of the flapping foil was found maximized. Finally, by combining the vortex variation and pressure coefficient distribution on the hydrofoil surface at different moments, it was found that if there is a relatively large leading edge vortex at the bottom surface of the foil at 0.25 T, and also if the difference between the pressure coefficients for top and bottom surfaces is relative large, then a high power efficiency could be achieved. This result could be used as a necessary, but not precisely sufficient condition for evaluating the power extraction, since the pitching amplitude should also be considered for analysis.