Vortex- and wake- induced vibrations in an array of cylinders

Abstract

Flow-induced vibration (FIV) is an important phenomenon, by which the flow around bluff bodies creates forces that excite vibration. When marine risers are designed in a tandem arrangement, two aspects of FIV including Vortex Induced Vibration (VIV) and Wake-Induced Vibration (WIV) are very important, resulting in strong vibrations and fatigue damage. In this thesis, the simultaneous effects of VIV and WIV are studied for the case of circular cylinders by means of Computational Fluid Dynamics (CFD) software ANSYS Fluent. An Arbitrary Lagrangian-Eulerian (ALE) formulation is applied on a deformable mesh needed for modelling a free vibrating cylinder. The response dynamics and wake interactions are addressed. Major aspects considered in the thesis include: the Reynolds number (Re), the mass-damping parameter, the degrees of freedom of a single cylinder and of a downstream cylinder and the combined effect of VIV and WIV.

The current predictions focus on sub-critical Re flow so that turbulence models are applied using two-dimensional Reynolds Averaged Navier-Stokes (RANS) equations. Force coefficients are analysed based on pressure distribution and Strouhal number. The VIV and WIV response is analysed by considering oscillating amplitude, frequencies and motion trajectories.

The work concentrates on FIV vibration in three main cases: a single circular cylinder, and a downstream cylinder in tandem and staggered arrangements. The cylinder was elastically mounted on a mass-spring-damper system, with 1 degree of freedom (dof), 1+1dof, 2 dof or 4 dof. The study results showed the cylinder’s vibration is strongly affected by the mass-damping ratio and reduced velocity. The coupling between inline and crossflow vibrations could increase the amplitude of motion dramatically compared to crossflow vibration only. The amplitude of vibration changes the wake pattern as well as the trajectory of the cylinder. The vibration in the inline direction on the downstream cylinder in the wake of the upstream one is remarkably high compared to the crossflow direction.

With a 2 dof system, the simultaneous effects of VIV and WIV give rise to vibration in each direction with two natural frequencies. WIV can be observed in the low-frequency response of the cylinder, which is considerably larger than the high-frequency VIV response. The combination of these two components can result in vibrations of the cylinder with higher amplitudes compared to any single form of excitation. The trajectories of the cylinder with a 4 dof system are very chaotic.

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ORCID for Pandeli Temarel: orcid.org/0000-0003-2921-1242

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