Improvement of energy extraction performance for flapping foils by using retractable leading edge flaps

This study aims to improve the energy extraction efficiency of flapping foils by using the retractable leading edge flaps (RLEFs). The effects of three parameters, including the designed rotation amplitude γ₀, flap tail position d_lef, and additional rotation angle γ_add of RLEFs are studied at different reduced frequency f^*. The usage of the RLEFs increases the effective contact area without increasing the sweep distance, improves the pressure difference near the leading edge, and delays the developments of the leading edge vortices on the flapping foil, resulting in the improvement of the energy extraction efficiency η at f^* = 0.08-0.16. The optimal parameters are γ₀ = 21°, d_lef = 0.25c, and γ_add = 0°, which helps the foil obtains a relative increase in 29.0% at f^* = 0.08. A combined flap scheme using both RLEFs and the movable lateral flaps on the pressure side of the flapping foil is proposed. This scheme gets the highest η at f^* = 0.08-0.14 among all the foils, and its maximum η is 44.9% at f^* = 0.14, with a relative increase in 16.3%. Its maximum enhancement of η occurs at f^* = 0.08, with a value of 33.6%, which is 41.3% higher than that of the conventional foil. It also forms a relatively stable high-efficiency region (η > 41.5%) at f^* = 0.10-0.22.