Vortex-induced vibrations in an active pitching flapping foil power generator with two degrees of freedom

The energy harvesting characteristics of actively pitching flapping foils under a two-degrees-of-freedom (2DOF) system were investigated through numerical simulations. At a Reynolds number of 1100, the effects of the pitching amplitude, reduced frequency, and structural parameters on the energy harvesting performance were compared with the traditional one-degree-of-freedom (1DOF) case. The optimal pitching amplitude (85 degrees), reduced frequency (0.16), and structural parameters ( b(x)*=0.5, k(x)*=0.7) of the streamwise vibrating flapping foil were determined. The additional velocity generated by streamwise vibrations increased the optimal reduced frequency and pitching amplitude over the traditional case. Streamwise vibrations accelerate the wake propulsion, and the wake vortevx spacing is about 0.8 times the chord length larger than that of the traditional case. Furthermore, the 2DOF case allows the vortex-shedding process of the flapping foil to participate in wake propulsion. The trajectory of the streamwise vibrating flapping foil was observed to be a figure "8" shape. The "8" shape gradually regularizes with an increased streamwise damping coefficient. There is an ideal parameter combination at the optimal reduced frequency that allows the flapping foil to reach the most unstable motion mode. The energy harvesting efficiency of the flapping foil can be increased by up to 25% due solely to vortex-induced vibrations of the 2DOF.