Experimental and numerical investigation of tandem-arranged semi-active hydrofoils propulsion for wave glider

Bio-inspired flapping hydrofoils have garnered attention for their potential applications in propulsion or energy extraction. This research delves into both experimental and numerical investigations of six tandem semi-active hydrofoils developed for wave gliders. We examined the effects of the hydrofoil spacing (with gap G = 0.5c, 0.8c, and 1.0c) and stiffness on propulsion. Findings indicate that compared to a standalone hydrofoil, the tandem configuration with smaller gaps achieves a notable rise in both thrust and efficiency. The average thrust coefficient for tandem hydrofoils surpasses than that of an isolated hydrofoil counterpart by over 100 %. Meanwhile, the mean power coefficient experiences a rise of up to 35 %, leading to enhanced propulsion efficiency. This boost is prominent at mid and low frequencies. However, at higher oscillation frequency, propulsion coefficients align with those of an isolated hydrofoil, with even diminished efficiency. Notably, the thrust generated by different hydrofoils in the tandem setup strongly correlates with the amplitude of their passive pitch angles, and the passive pitch's peak times display a series of undulations. The benefits of tandem configurations with reduced gaps emphasize the potential for optimization in hydrofoil design for wave gliders.