Comparison of the energy extraction performance of flapping foils using movable flaps with different modes

This study aims to compare the common features and the differences in energy extraction performance of flapping foils using movable flaps with different modes. These modes includes movable lateral flaps on the pressure side (PMLFs), the movable trailing edge flap (MTF) and the movable Gurney flap (MGF). Under the condition of different reduced frequency f∗ and flap relative angle α_LF, the energy extraction coefficients of foils using movable flaps with different modes are calculated and compared based on numerical simulations. The results show that, all the three types of flaps could improve the energy extraction efficiency η by increasing the curvature of the pressure side of the foil. However, MTF deflects the suction side of the foil thereby separating the fluid on the lower surface of the foil near the flap deflection point, which accelerates the development of the leading edge vortex at f∗ = 0.10 and produces continuous vortex shedding near the trailing edge at f∗ = 0.18. The movement of the MGF towards the suction side of the foil blocks the flow on the suction side of the foil and significantly increases the pitching power loss at f∗ = 0.18. Only the PMLF shows an excellent improvement of η over the full frequency range.