Numerical study on the energy extraction characteristics of a flapping foil with movable lateral flaps

This study focuses on how to improve the energy extraction performance of flapping foils by using movable lateral flaps. Three flaps are designed firstly, including fixed lateral flaps (FLFs), movable lateral flaps on the pressure side (PMLFs) and movable lateral flaps on the suction side (SMLFs). The energy extraction characteristics of foils with different flaps at different reduced frequency f* are investigated and compared based on numerical simulations. The results show that PMLFs have a more positive effect on the energy extraction of the foil than other flaps. Then the effect of the geometrical parameters of PMLFs including opening amplitude beta(0), lateral flap length l(f) and position d(f) on the energy extraction efficiency eta of flapping foils is studied, respectively. When using the suitable PMLFs, a larger lift is obtained by improving the pressure distribution and velocity circulation around the foil, which increases eta. It is found that beta(0) has an optimal value, and a failure point exists when the sum of l(f) and d(f) is 0.9c. The best design scheme of PMLFs is at beta(0) = 30 degrees, l(f) = 0.2c and d(f) = 0.8c, which help the flapping foil obtain the largest eta = 48.0% at f* = 0.18, with an improvement of 21.7% than that of the conventional foil.