Effects of Mass Ratio and Restoring Condition on the Flow-Induced Vibration of Floating Cylinders with Low Aspect Ratio

Abstract The phenomenon of flow-induced vibration (FIV) generally acts on a floating body, being decisively affected by the relationship between its structural mass and the mass of the displaced fluid, which in turn is intrinsically affected by the nature of the mooring system. In this sense, solutions in stable equilibrium on the free surface are possible according to floating bodies restored by moorings, for which the mass ratio (ratio of the vibrating mass to the displaced mass in the fluid) is approximately unitary (m^* \approx 1), or for those maintained by vertically tensioned tendons that determine considerably lower mass ratios (m^* < 1). This study seeks to understand the fundamentals of the FIV responses due to different values of the mass ratio in floating cylinders with external diameter of 75mm and aspect ratio L/D=6.8. The experiments were carried out in a circulating water channel considering 12 reduced velocities for values of the mass ratio m^*=0.30, 0.60, 0.70, 0.80, 0.85, 0.90, 1.00. The floating cylinder was connected to the bottom of the channel using a tensioned tendon. Data acquisition was performed using a motion capture system that recorded the six degrees of freedom of the floating cylinder. The FIV responses provide valuable information on dynamic behaviour with respect to the structure mass and the mooring system, assessed by analysing trajectories, displacements, inclination, rotation, and oscillation frequencies. The results have shown how the significant amplitudes increase as the reduced velocity increases, similarly to cylinders with high aspect ratio. As the tendon allows the cylinder to rotate, the yaw rotation is significant for all the cases tested and contributes to amplitudes in the cross-flow direction higher than 1.5D. Knowing that floating offshore wind turbines (FOWT) are a trend for deep water generation, custom designed according to each site, this work brings important subsidies that can contribute to the selection of mooring systems, particularly with respect to the effects of the FIV phenomenon.