Improving the wind energy harvesting performance with double upstream fractal bluff bodies

Fossil energy sources are not renewable and the technology to harness wind energy has gained considerable interest. This work proposes a wake galloping energy harvester with upstream fractal structures to promote the efficiency of wind energy harvesting. The dynamic response and energy harvesting performance of a conventional galloping energy harvester (GEH), a vortex-induced vibration energy harvester (VIVEH), a traditional wake galloping energy harvester with single or double upstream cuboids (WGEH-SC, WGEH-DC), and a wake galloping energy harvester with single or double fractal upstream structures (WGEH-SF, WGEH-DF) are evaluated numerically and experimentally. At a wind speed of 5.0 m/s, WGEH-DF increases the maximum root mean square (RMS) voltage from 19.36 V to 39.25 V, indicating an improvement of 102.7 % compared to the VIVEH. Meanwhile, the effects of the positions and windward angles of the upstream bluff bodies are discussed, and the WGEH-DF reaches its maximum average RMS voltage at an angle of 75° and x = 2 cm. It is found that when two fractal bluff bodies are placed upstream, the pressure difference increases on both sides of the downstream bluff body and the structural vibration becomes more violent. By comparing the pressure behind the two cuboids and two fractal bluff bodies, it is demonstrated that the negative pressure behind the fractal bluff bodies is increased. The flow field analysis further explains the aerodynamic mechanism that the fractal bluff bodies placed upstream improve energy harvesting performance.