Interaction of vortex shedding processes on flow over a deep-draft semi-submersible

A numerical study on the flow over a deep-draft semi-submersible (DDS) for both stationary and vortex-induced motions (VIM) was carried out using the computational fluid dynamics (CFD), with the aim to investigate the overall hydrodynamics of the structure. In order to study the fluid physics associated with VIM, a comprehensive numerical simulation was conducted to examine the characteristics of vortex formations, shedding processes and especially their interactions due to the multiple cylindrical columns. In addition to the vortex shedding characteristics, the drag and lift forces on each member of the overall structure were calculated. It is revealed that under 45 degree incidence, the transverse forces induced by the portside and starboard side columns are the dominant excitation forces responsible to VIM while the horizontal member - pontoons restraining VIM. In addition, the hysteresis phenomenon observed between the force and motion domains - the peak lift force occurs slightly earlier than the peak transverse motion is mainly due to the vortices shed from the upstream column move back to impinge on one of the side columns after impinging on the other side column and the symmetrical strong vortices which shed from the side columns.