Investigating the coupled effect of different aspect ratios and leeward protrusion lengths on vortex-induced vibration (VIV)-galloping energy harvesting: Modelling and experimental validation

Abstract

Flow-induced vibration energy harvesting is one of the promising approaches for realizing self-powered wireless sensor networks. Incorporation of metasurfaces or surface protrusions on the bluff bodies is an emerging way of enhancing energy harvesting performance. This paper experimentally and numerically investigates the coupled effect of implementing different aspect ratios and leeward protrusion lengths on the performance of vortex-induced vibration (VIV)-galloping energy harvesters. A total of sixteen protruded bluff bodies with four different aspect ratios and four protrusion lengths are proposed for comparative studies. A comprehensive mathematical model is established based on the extended Hamilton’s principle and Tamura’s aerodynamic force model. Both the wind tunnel experiments and numerical simulations manifest a different development trend for the bluff bodies with aspect ratios greater or lower than the unity. For most of the bluff bodies, increasing the protrusion length can lead to an increase in both the VIV-galloping cut-in speed and output power at higher wind speeds. Wind tunnel computational fluid dynamics (CFD) simulations are also performed to investigate the underlying aerodynamic reasons for the observed phenomena. Based on the classic Den Hartog’s criterion and Hu’s findings, it can be found that there is a consistency among the wind tunnel CFD simulation results and the experimental and numerical findings. Furthermore, the wind speed ratio of quasi-steady galloping to Kármán-vortex resonance is selected as an index to evaluate the interaction extent of VIV and galloping oscillations. It is revealed that strongly conjoint VIV-galloping oscillations can be more easily observed when the ratio approaches unity, while no interaction can be detected when the ratio is higher than a value between 11.91 to 23.27 in this study. Further, impedance matching tests and forward wind sweep experiments show that the maximum output power measured among all the studied cases is 0.992 mW, which occurs at the test speed of 5.1 m/s and an optimal load resistance of 200 kΩ, for the bluff body with an aspect ratio of 3:2 and a leeward protrusion length of 10 mm. This outperforms the optimal case of our previous study by 31.04%. Moreover, using the optimization theory, this bluff body is proved to be the global optimal solution of this study, which can well balance the cut-in speed and output power for the applications in the natural gentle breeze.