Influence of stiffness ratio on vortex-induced vibration of cylinder with low aspect ratio

Abstract:
Concern over the Vortex-induced Motions (VIM) acting on offshore structures, with special focus on monocolumn and spar platforms, mooring systems have crucial importance on system movements; the system has thus been transformed into a concept study herein. A floating and rigid circular cylinder with low aspect ratio (L/D=2) was used in the experiments carried out to investigate the influence of stiffness ratio (kx=ky) on Vortex- Induced Vibration (VIV). The cylinder was mounted in an elastic base composed of four springs with differences in in-line and transverse stiffness, defining: kx=ky =0.3, 0.5, 1.0, 2.0 and 3.0. The Reynolds number analysed belongs to a range between 0:2 104 and 2 104. Some good qualitative and quantitative agreements are found for in-line amplitudes, and higher kx=ky systems demonstrate significant oscillation for low relative velocities. This variation can be seen and justified when the XYplane trajectories were plotted. When kx=ky is defined as 2 and 3, the traditional VIV 8-shape is illustrated for reduced velocities between 3 and 6. In contrast, the other stiffness systems do not show significant movements and, consequently, a negligible XY shape. Roll and pitch degrees of freedom have shown the motions coupled with the transverse and the in-line motions respectively. Moreover, the yaw motion did not present considerable angles. kx=ky =2 has presented the highest lift force coefficients, without a great difference from the other aspects ratios, though. The drag force coefficient showed constant values for kx=ky = 2 and 3, the smallest results were observed for the system kx=ky =3.


Reference:
GAMBARINE, Dennis M.; MINIOLI, Luiz E.B.; GONÇALVES, Rodolfo T.; KOGISHI, Andre Mitsuo; FUJARRA, André L.C. Influence of stiffness ratio on vortex-induced vibration of cylinder with low aspect ratio. In: INTERNATIONAL CONFERENCE ON OCEAN, OFFSHORE AND ARCTIC ENGINEERING, 37., 2018, Madrid. Proceedings… 9 p.

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