Energy harvesting of unequal-height cylindrical FIV considering wind direction

To improve the performance of flow-induced vibration (FIV) energy harvesters, we propose the use of unequalheight tandem cylinders. Specifically, we design cylinder models with height ratios (h/H = 0.2, 0.4, 0.6, 0.8, 1.0) and conduct wind tunnel tests under different wind directions and spacing ratios. Our approach enhances the understanding of how wind direction impacts the vibration and energy harvesting performance of unequalheight tandem cylinders. Through wind tunnel experiments, we identify nine distinct vibration modes, which are further analyzed using wavelet transforms for precise frequency identification. Notably, at the spacing ratio l = 1.2, we observe a strong coupling effect between the upstream and downstream cylinders. For shorter cylinders, significant vibration amplitudes in vibration modes IV and VII remain stable despite changes in wind direction, a phenomenon we term the "short cylinder instability effect". This effect is consistent across all wind conditions tested. Through theoretical analysis, the electromechanical prediction model of coupled vortexinduced vibration and wake-induced galloping is established, which is in good agreement with the experimental results. In addition, at h/H = 0.4, vibration is suppressed in the upwind configuration, while in the downwind configuration, the maximum voltage generated at l = 2.5 is 9.602 V, which is 204 % higher than that of equal-height cylinders. To better understand the effects of wind direction and height variations, we also perform computational fluid dynamics (CFD) simulations. These simulations reveal how vortex shedding at a microscopic level contributes to the shielding effects caused by changes in wind direction and cylinder height. The CFD simulations further reveal how wind direction and cylinder height influence vortex shedding and shielding effects.