Vortex shedding of multi-column floating structures is complex due to the wake interaction between front and rear columns. The vortex shed from upstream columns will impinge upon the downstream columns and change their pressure distributions on the surface, which sequentially affects the dynamic response of vortex-induced motions (VIM). This paper tries to reveal the mechanisms of the vortex shedding, wake interference, and their impacts on the VIM of a paired-column semi-submersible by means of computational fluid dynamics (CFD). In the present work, a scaled model (1:54) of paired-column semi-submersible (PC-Semi) is studied. The CFD solver used in this paper is an in-house CFD code naoe-FOAM-SJTU, which is developed on top of the OpenFOAM framework. Turbulent flows around the geometry are modeled by delayed detached-eddy simulation (DDES). Meanwhile, the motions of the model are constrained in the horizontal plane and obtained by solving six-degrees-of-freedom motions equations. Numerical simulations at different current headings and reduced velocities are performed. The overall motion responses of the structures are evaluated. Vortex shedding process and wake impingement on downstream columns are also discussed. These preliminary results show how the vortex shedding process and wake impingement influence the VIM characteristics of a multi-column floating structures.
Vortex shedding is a common physical phenomenon on flow past bluff body. It is a consequence of boundary layer separation, which is caused by the reduction of velocity in the boundary layer, combined with a positive pressure gradient. The vortex shedding will generate periodic pressure fluctuation on alternate sides of the bluff body. For long and thin cylindrical structures, the pressure fluctuation should lead to vortex-induced vibrations (VIV). In ocean engineering, the vertical columnar shaped floating structures will suffer similar excitations, which is called vortex-induced motions (VIM). VIM is very complicated due to the involvement of flow separation, rigid body motion, mooring stiffness and other physical properties of the system. Understanding the physical principle of VIM is vital to engineers to avoid mooring line fatigue failure.
