Investigation of Effect of Twisted Surface on Suppression of Vortex-Induced Motion of a Square Column

Abstract

Suppression of vortex-induced motion (VIM) of multi-columns semisubmersibles is an important factor for the safety design and operating environment of floating platforms. Vortex generation around square columns is a key issue to cause VIM, and thus, an essential point for suppressing VIM is to desynchronize vortex shedding frequency and natural frequency of the system. In our work, we investigate the effect of a square column with the twisted surface on the suppression of VIM. The numerical tool we used in this work is an open source package, OpenFOAM, to solve transient flow pattern around a vibrating square column. The numerical validation of the present study is done by benchmarking with the reported experimental work at moderate Reynolds numbers (Re). In real sea state, the flow around floating structure is turbulent and is in high Re region. We implemented k-ω SST DES turbulence model in the present numerical model to solve complex turbulent flow around the square column with the twisted surface at high Re. By comparison with VIM of a square column, the VIM reduction of a twisted column is significant, which is up to 86% of VIM reduction. The vortex structure, flow separation points and vortex shedding frequency are varied in the spanwise direction of the twisted column, which causes the effect of vortex desynchronization on the VIM. The detailed flow pattern, 3D vortex structure (Q criterion) around the twisted column is demonstrated as well. A remarkable conclusion is made based on the present numerical findings.