Abstract: Based on the immersed boundary method, this study examines the energy harvesting performance of a flapping foil under the effects of the phase difference φp between its pitching motion and heaving motion as well as the parameter β of its nonharmonic pitching motion. Firstly, the effects of phase difference under different pitching amplitudes θ0 on energy harvesting performance are studied. We find that at θ0 = 30°, the foil cannot collect flow energy because its effective attack angle and heaving induced angle have the same trend of variations. As θ0 increases, the foil’s energy collection performance is gradually improved. At θ0 = 76.3°, the optimal phase difference of 247.5° is obtained, and the foil achieves a pure power output of 0.895 W/kW and an energy collection efficiency of 40.2%, which is 5.2% higher than the highest efficiency (35.0%) in previous studies. Then, we examine the effect of non-harmonic pitching motion on the energy harvesting performance at θ0 = 76.3° and φp = 247.5°. The results show that this motion can further improve energy collection efficiency; at β = 1.85, the maximum pure power output coefficient reaches 1.09. And at β = 1.15, the maximum energy harvesting efficiency reaches 45.6%, or an increase of 5.4% compared with the harmonic pitching motion; the pure power output coefficient is 1.05.

Key words: immersed boundary method, flapping foil, phase difference, non-harmonic pitching motion, energy harvesting performance