Numerical Simulation On Vortex-induced Motions Of A Deep Sea Multi-column Floating Platform

With the development of the deep draft semi-submersible(DDS)platform,the vortex-induced motions(VIM)of DDS has become an important issue in offshore and ocean engineering.This paper presents numerical studies on the vortex-induced motions(VIM)of a deep draft semi-submersible platform with four square columns base on improved delayed detached-eddy simulation(IDDES).Moreover,flow past circular cylinders and square cylinders at high Reynolds numbers are also simulated by IDDES,including a circular cylinder,a square cylinder,two tandem circular cylinders and two tandem square cylinders.It is found that the effect of Reynolds number on the global quantities for square cylinders is not much in this range of Reynolds numbers,which is different for circular cylinders.There is no drag crisis phenomenon for flow past a square cylinder at 2.0x 10~3<Re<1.0×10~7.The Strouhal number is Reynolds-independent at Re>2.0× 10~3,and the Reynolds-independent is also observed for the mean drag coefficient and the RMS lift coefficient.Simulation on two tandem circular cylinders is performed at Reynolds numbers of 2.2 × 10~4 and 3.0 × 10~6 for five different spacing L to diameter D ratios:L/D=2.0,2.5,3.0,3.5 and 4.0.At the critical spacing(L_c/D)there is found a distinct step-like jump of mean drag coefficient and RMS lift coefficient at the subcritical Reynolds number of 2.2×10~4,and the mean drag coefficient of the downstream circular cylinder is negative for L/D<L_c/D.However,the mean drag coefficient and the RMS lift coefficient are seen to be slightly affected by spacing for Re=3.0×10~6,and the mean drag coefficient of the downstream circular cylinder is always positive.Flow past two tandem square cylinders is considered at Reynolds numbers of 1.6×10~4 and 1.0×10~6.The abrupt change in mean drag coefficient and RMS lift coefficient at the critical spacing is clearly seen on both upstream and downstream square cylinders for both Reynolds numbers.When L/D<L_c/D,the mean drag coefficient of the downstream cylinder is negative for both Reynolds numbers.It is found that the different lock-in regions in VIM of a semi-submersible are observed for 0°,22.5°and 45°current incidences.The lock-in behaviors occur at 6.0<V_r<8.0 for both 22.5°and 45°current incidences,while it occurs at 9.0<V_r<11.0 for 0°current incidence.However,yaw motions tend to monotonically increase with the growing reduced velocities for all current incidences.According to the results,there are no eight-shaped trajectories as those typically presented in Spar VIM for all three current incidence angles.The more rounded edge of column where four edges per column are rounded can lead to a significant increase in transverse and yaw motions.However,the characteristic amplitudes for transverse motions and yaw characteristic amplitudes change slightly with the growing corner radii of columns where only one edge per column is rounded.It is also found that the lower draft condition can attenuate the VIM of semi-submersible platforms.The present study can offer valuable reference for the design of the deep draft semi-submersible platform.It not only has the importance of engineering,but also has great academic value.