Enhancement of low-speed piezoelectric wind energy harvesting by bluff body shapes: Spindle-like and butterfly-like cross-sections

Fluid-structure interaction can be utilized to harvest the low-speed wind energy for sustaining the low-power sensors for structural health monitoring. To enhance the low-speed wind energy harvesting, this study proposes the novel spindle-like and butterfly-like bluff bodies by coupling both the vortex-induced vibration (VIV) and galloping phenomena. Comprehensive wind tunnel experiments are conducted to investigate the advantages of the two bluff bodies in terms of the bluff body cross-sections and installment directions. The experimental results demonstrate that for both the spindle-like and butterfly-like bluff bodies, the vertical installment and small width ratio are beneficial to the performance in a broad range of wind speeds. Compared to a conventional galloping-based energy harvester, owing to the coupling between the VIV and galloping, the vertical spindle-like bluff body with the smallest width ratio can reduce the threshold wind speed of activating the energy harvesting function by over 13%, and improve the maximum voltage output by over 160%. Finally, taking the spindle-like bluff body as an example, the computational fluid dynamics (CFD) studies are conducted by using XFlow software to interpret the physical insight of performance enhancement. The CFD results show that the vertical installment direction and a small width ratio play an important role. The two designs can lead to a stronger aerodynamic force due to the fast vortex shedding, which improves the energy conversion efficiency from the flow-induced vibrations.