Experimental and numerical studies on the excitation loads and vortex structures of four circular section cylinders in a square configuration

A variety of concepts of deep draft semi-submersibles (DDS) have been developed in order to improve the vertical motions. However, the DDS may experience critical vortex-induced motions (VIMs) stemming from the fluctuating forces on the columns. Aiming to investigate the excitation loads and the fluid mechanism of VIM, this paper presents experimental and numerical studies of the three-dimensional (3-D) flow around four circular section cylinders in a square configuration at subcritical Reynolds numbers (Re). The experimental measurements were carried out by using the piezoelectric load cells in a wind tunnel, while the numerical simulations were conducted by employing the Large Eddy Simulation (LES) method. The effects of three spacing ratios (L/D) and four array attack angles () ranging from 0 degrees to 45 degrees in 15 degrees increments were investigated. The results show that the vortex structures are the key to understand the behaviours of the forces and the Strouhal number (St). Consequently, the downstream cylinders are normally subjected to smaller mean drags but more serious fluctuating forces than the upstream cylinders, due to the influence of unsteady wake vortices. Furthermore, at the present range of L/D ratios, the effect of spacing ratio on the excitation forces is not significant.