In order to improve the energy extraction capability of oscillating hydrofoils, a dual Fowler flap structure is adopted as a device to increase lift. According to the motion of the oscillating hydrofoil, the double Fowler flaps retract and swing to increase the chord length and curvature of the entire hydrofoil. We proposed and established the structure and motion equation of Fowler flaps, and studied the influence of Fowler flaps on the overall performance of hydrofoils under fixed Reynolds number conditions. The numerical results indicate that compared to the NACA0018 hydrofoil without flaps, the application of Fowler flaps increase overall wing camber. By influencing the flow structure and pressure distribution around the trailing edge of the hydrofoil, Fowler flaps help generate lift, resulting in higher power coefficient and energy capture efficiency. The structure of the Fowler flaps keeps the entire hydrofoil in the streamlined state of traditional hydrofoils, with a smaller range of fluctuations in lift and drag.

The structure of the hydrofoil plays a crucial role in determining its hydrodynamic coefficients. Drawing inspiration from the swept-back morphology of fish caudal fins, this study developed two novel oscillating hydrofoil configurations: a leading-edge swept-back design and a trailing-edge swept-forward design. The corresponding mathematical formulations and parameter definitions are provided. Utilizing computational fluid dynamics techniques, the study investigates the influence of sweep angle and pitch axis position on the hydrodynamic performance of swept hydrofoils. The findings demonstrate that increasing the sweep angle can enhance the lift-to-drag ratio of the hydrofoil but also leads to an increase in the pitching moment power coefficient, with a more substantial impact on the latter. The increased negative work resulting from larger pitching moments further reduces the average power output. With the pitch axis oriented toward the trailing edge, the moment coefficients initially decline and subsequently increase. Accordingly, the average power output also exhibits a trend of first increasing and then decreasing. Therefore, the pitch axis should be positioned near the mid-chord of the hydrofoil. A larger sweep angle is preferable in complex flow environments, while a smaller sweep angle is more suitable when higher energy harvesting efficiency is desired.

Oscillating hydrofoils play a crucial role in tidal energy conversion systems, with their structural design exerting a profound influence on the hydrodynamic efficiency and overall performance of these devices. Inspired by the morphology of fish caudal fins, this study designs four biomimetic hydrofoils with diverse structural configurations to investigate their hydrodynamic performance: leading edge swept-back, trailing edge swept-forward, swept-back, and swept-forward. Computational fluid dynamics simulations are utilized to systematically analyze the impacts of sweep angles, pitch axis locations, heave amplitudes, and pitch angles on the hydrodynamic performance of these hydrofoil configurations. The results indicate that increasing the sweep angle significantly improves the lift-to-drag ratio of the hydrofoil, with the drag coefficient of the trailing-edge swept-forward configuration reduced by 12% as the sweep angle increases from 5° to 35°. Power output can be enhanced by adjusting the pitch axis location toward the mid-chord. Increasing the heave amplitude improves the lift-to-drag ratio but reduces the average power. At a pitch amplitude of 75°, the growth rate of the lift coefficient slows down, while that of the drag coefficient remains unchanged. Meanwhile, the average power coefficient reaches its maximum. This study contributes to the understanding of efficient hydrofoil design and highlights the importance of incorporating coupled motion parameters in computational models to better capture hydrofoil dynamics in future research.

In order to improve the energy extraction capability of oscillating hydrofoils, a dual-Fowler-flap structure is adopted as a device to increase lift. According to the motion of the oscillating hydrofoil, the double Fowler flaps retract and swing to increase the chord length and curvature of the entire hydrofoil. This article investigates the geometric parameter characteristics of an oscillating hydrofoil with Fowler flaps. Under the condition of a fixed Reynolds number Re = 2 × 10⁶, the effects of Fowler motion F and slot value S on the overall performance of the hydrofoil are studied. The numerical results show that the Fowler flap structure can increase the camber and chord length of the integral hydrofoil, and the movement of the Fowler flap is combined with the motion of the oscillating hydrofoil to increase the lift coefficient of the hydrofoil, thus increasing the energy collection efficiency of the oscillating hydrofoil—the maximum increase is 50%. By affecting the flow structure and pressure distribution around the trailing edge of the hydrofoil, the Fowler flap helps to generate lift, resulting in a higher power coefficient. U (overlap amount) = 0% is a dividing point, and the S (gap amount) value at this position has the greatest influence on the average power coefficient. The structure of the Fowler flaps maintains the streamlined state of the entire hydrofoil, and when S = 0 and F = 200, the lift and drag fluctuations of the oscillating hydrofoil are minimal. This is very beneficial for the stable operation of the hydrofoil.