Numerical analysis of WIV phenomenon with two in-series cylinders: WIV suppression and energy harvesting

This numerical study aims to analyze the hydrodynamic characteristics of in-series structures with two cross -sections, as well as their power characteristics under different spacing (alpha) and radius (phi) ratios respectively. The results show that as the spacing ratio increases, the in-series circular/square-cylinder system displays VIV, whereas the in-series square-cylinder system can effectively suppress vibration within the interval of alpha = 5.3 - 5.7, 6 <= Ur <= 8 when phi = 1. It is worth noting that as the radius ratio increases, the vibration of the downstream square cylinder transits from VIV to galloping. Subsequently, the short-time Fourier transform and wavelet transform are employed to perform time-frequency analysis of different cases. In terms of energy harvesting, the in-series circular-cylinder system shows its maximum power of 48.83 mW at Ur = 9 when alpha = 6, phi = 1. Addi-tionally, the power of the in-series square-cylinder system at phi = 2 is higher than that in the other two cases (phi = 1 and 3), and in these three cases, the power at alpha = 6 is much greater than that of the case alpha = 8 and 4 regardless of the radius ratio. As to conversion efficiency, the case alpha = 6 and phi = 2 demonstrates the optimal energy harvesting performance. Thus, flexible control of the radius ratio and spacing ratio in a special range can not only suppress vibration but also enhance energy harvesting efficiency.