Bluff body structures exposed to ocean current can undergo vortex-induced motion (VIM) for certain geometric and physical conditions. Recently, the study of VIM has been gaining attention for many engineering applications, in particular offshore structures such as buoys, FPSOs, semi-submersibles, Spars and TLPs. The present work is a part of a systematic effort to investigate the VIM response of multi-columns floating platform. In real sea condition, floating platforms are in high Reynolds numbers region and flow patterns around structures are turbulent in nature. For the purpose of investigating and simulating accurately the nonlinear dynamic processes of vortex shedding, transport and wake interactions with the bluff body, the fundamental study of VIM around a square column at moderate Reynolds numbers (1500 ≤ Re ≤ 14000) is firstly investigated. In the present work, the transient flow pattern around a free vibrating square cylinder at moderate Reynolds numbers is numerically investigated by an open source CFD toolbox, OpenFOAM. Good consistency and agreement are found between the present numerical findings and that of experiments. The cylinder, with a blockage area of 4.2%, is mounted on an elastic support for free vibration in the transverse direction. Hybrid RANS-LES turbulence models are considered here for accurate prediction of massively separated turbulent wake flow while maintaining the reasonable computational cost. Three hybrid turbulence models, the DDES (Delayed Detached Eddy Simulation, the k-ω SST-DES (Detached Eddy Simulation), and the k–ω SST-SAS (Scale Adaptive Simulation), are studied and their results are compared with the reported experimental measurements. It is shown that the result of simulation with the k–ω SST-SAS model is closer to the reported literature than the other two and therefore, subsequently adopted for all the simulations of our study in this paper. The scaling effect of cylinder length in the spanwise direction is also studied with the objective to reduce the computational cost. From the comparison with the recent experimental measurements, the discrepancy between the present simulations of reducing cylinder length and the experiment increases only when Re ≥ 4000. This might stem from the increase in wavelength of some vortex shedding modes and turbulence intensity variation in the spanwise direction near the cylinder as Re ≥ 4000. The detailed flow patterns, 3D vortex structures (based on Q-criterion) and vortex-shedding modes are presented in this work as well.

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