In the present work, numerical simulations of VIV of flexible risers in different tandem arrangements are carried out by the in-house CFD solver viv-FOAM-SJTU. Firstly, the accuracy of the solver is verified. Then, vibration responses of two risers are presented. Numerical results show that the interference effect between two risers makes amplitude responses of the two risers different from the isolated riser, and the upstream riser is more likely to exhibit multi-modal vibration characteristic. Through the vorticity map at the middle section of two risers, the wake vortex reattachment phenomenon at the downstream riser is observed.
Vortex-induced vibration (VIV) is one of the main causes of fatigue damage of marine risers and has long been the focus of researchers. A lot of research has been carried out on VIV of an isolated riser, mainly focusing on inflow conditions and structural parameters of the riser. In the actual engineering environment, marine risers often appear as groups, which makes the VIV responses of multi-riser system different from the isolated riser and more complicated. However, the research on VIV of multi-riser is still less. For the sake of simplicity of research, a few researchers have carried out some research on VIV of risers in tandem and side-by-side arrangements. Sumner (2010) summarized the research on the VIV of two "infinite" circular cylinders that have appeared in the last 20 years.
Assi, Bearman and Meneghini (2010) investigated the mechanism of wake-induced vibration (WIV) of two tandem cylinders with an aspect ratio (ratio of length to diameter) of 13. Assi (2014) further investigated the influence of lateral separation on wake interference between two cylinders, and the evidence of wake-stiffness was observed in tandem arrangement. King and Johns (1976) conducted experiments on two tandem flexible cylinders. Experimental results showed that the upstream cylinder responsed like an isolated cylinder, while the downstream one performed a large amplitude in the mode transition region.
