Usefulness of inclined circular cylinders for designing ultra-wide bandwidth piezoelectric energy harvesters: Experiments and computational investigations

This study proposes to change the inclination angle (α) of a circular cylinder with respect to oncoming flow in order to broaden the effective wind speed bandwidth of the piezoelectric energy harvester. Comprehensive wind tunnel experiments are conducted to investigate the energy harvesting characteristics of the system under different inclination angles. The experimental results demonstrate that compared to the conventional vortex-induced vibration piezoelectric energy harvester (VIVPEH), an appropriate inclined cylinder can broaden the effective wind speed range for the VIVPEH. It is experimentally shown that the inclined cylinder at α = 60° can reduce the threshold wind speed and broaden the effective wind speed bandwidth by more than 229%. In addition, inclined circular cylinders at α = 25°, 30°, 35°, 40°, and 45° can produce torsional vibration and high voltage output under high wind speed. Through frequency analysis and computational fluid dynamics (CFD) simulations, the influence mechanism and causes of torsional vibration are explored in detail. The results show that the axial flow will prevent the free shear layer from falling off and cause the force imbalance at the top and bottom of the inclined cylinder, resulting in a torsional vibration and a series of other phenomena. The appearance of the torsional vibration rapidly reduces the frequency of the conventional transverse vortex-induced vibration (VIV). The superposition of the transverse bending vibration and the torsional vibration within a certain wind speed range results in a high voltage output.