This Letter explores the potential effect of implementing different surface protrusions on galloping energy harvesters. Three types of protruded bluff bodies with rectangular, triangular, and elliptical metasurfaces are proposed, and four kinds of surface treatments are deployed to vary their protruded shape. Wind tunnel experiments reveal that adding the protrusions can obviously change the mode of oscillations, and only the backward protrusions can enhance the galloping response. Both the experiments and simulations show that elliptical surface protrusions have the greatest potential to enhance the galloping energy harvesting performance. Specifically, with a backward protruded length of 15 mm, the maximum output power in the experiments is measured to be 0.757 mW, which occurs at 5.1 m/s, and an optimal load resistance of 300 kΩ. In this case, the energy harvester outperforms its counterpart carrying a simple square prism by 157.48%.

This Letter proposes a rotational energy harvester with hybrid piezoelectric and electromagnetic mechanisms and dynamic bistability. It consists of a piezoelectric stack with a force magnification frame and two connected springs, and two electromagnetic coils with inner impact stoppers. On the one hand, the hybrid energy transduction is applied in the gravity-based rotational energy harvester to enhance the energy output. On the other hand, the dynamic bistability by utilizing the centrifugal force is proposed to improve low-frequency performances. Simulation and experiments demonstrate that the proposed harvester starts to exhibit the bistability as the rotational frequency is increased from zero with subsequent interwell, chaotic, and intrawell motions, among which the interwell motions can increase the peak power from piezoelectric and electromagnetic parts by 24.99% and 57.41%, respectively. Specifically, the maximum total output power in experiments is measured to be 2.98 mW at 7.5 Hz and the spring stiffness of 4200 N/m. Moreover, the total power and frequency bandwidth are both higher but broader/narrower with a higher spring stiffness or impact distance, respectively.