Periodic solutions and frequency lock-in of vortex-induced vibration energy harvesters with nonlinear stiffness

The phenomenon of amplitude jump usually means the presence of multi-solution ranges and unstable periodic solutions, which has not been fully explored in the field of vortex-induced vibration energy harvesting. In this study, semi-analytical periodic solutions of vortex-induced vibration energy harvesters (VIVEHs) are obtained using the incremental harmonic balance method, and their stabilities are determined via the Floquet theory. It is found that the VIVEH with linear stiffness and those with three-order nonlinear stiffness have four saddle-node bifurcation points and two multi-solution ranges, which leads to the occurrence of amplitude jumps. The hardening effect can shift frequency lock-in regions to higher wind speeds, thus increasing the aerodynamic forces, and realizing wide wind speed bandwidths and high voltage outputs. The wind tunnel experiments verify the accuracy of theoretical calculations, the occurrence of amplitude jump phenomena, and the existence of multi-solution ranges. The VIVEH with nonlinear stiffness experimentally reached a maximum voltage of 9.87 V, and a frequency lock-in region of 2.27-5.36 m/s.