Vortex-Induced Motion of Floating Structures: CFD Sensitivity Considerations of Turbulence Model and Mesh Refinement

Abstract

Vortex-Induced-Motion (VIM) is an important issue in offshore engineering as it impacts the integrity of the mooring system for floating structures such as oil platforms and wind turbine platforms. Understanding and predicting VIM is a challenging task because of the inherent complexity of vortex structure shedding and fluid-structure interaction (FSI) in high Reynolds number flows. Computational Fluid Dynamics (CFD) is one of the key tools in VIM studies and optimization of the offshore systems design. We report a CFD sensitivity study with focus on turbulence model, mesh refinement, and time-step selection. Experimental measurements in a tow-tank facility are used to validate the CFD results. Three types of tank tests are modeled numerically: current drag, oscillating free decay, and VIM. The effect of turbulence model is evaluated by comparing Delayed Detached Eddy Simulation (DDES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) models. The influence of mesh refinement and time step is investigated by using the grid convergence index (GCI). For present geometry and flow conditions (Re∼105), the DDES turbulence model demonstrates better agreement with experimental measurement in model scale VIM compared to the URANS model. In addition, DDES simulation captures the vortex structure more realistically, as evidenced by Q-criteria and turbulent eddy viscosity distribution. Finally, we show how the mesh refinement and time step selection affect simulation accuracy. Two viscous-flow commercial solvers are tested: the finite-volume solver ANSYS-Fluent™, and the finite-element solver Altair AcuSolve™. The results of this CFD Sensitivity study provide useful guidelines for CFD simulation of FSI and VIM problems for offshore engineering applications.

A review of floating semisubmersible hull systems: Column stabilized unit

“…From this assessment, we can therefore speculate that the inclusion of the centralized pontoon section plays a significant role in the drag distribution around the hull of a typical PC-Semi. This unique behaviour has been investigated using CFD method in Kim et al (2015) where it was stated that the pontoon section was observed to be responsible for one-third of the entire drag around this hull. The maximum value for hull drag coefficient was recorded between 1.1 and 1.2, at a reduced velocity of 5.57.…”

Wave induced stress profile on a paired column semisubmersible hull formation for column reinforcement

“…A typical PC-Semi is made of eight rectangular columns, (four inner columns and four outer columns), a topside, pontoon, inner-outer column connections, and inner column braces. The inner columns are slightly smaller than the outer ones, to reduce the oscillating amplitude of the shed vortexes during high current velocities [11]. Figure 1 shows the geometry adopted in this study, which was extracted from [12].…”

CFD Simulation of Vortex Induced Motion of a Caisson during Installation Process

“…Brandtsegg [7] investigated a transient transcritical flow around of a submerged floating tunnel and examined the effect of vortex induced vibrations on a circular cylinder. Further detailed CFD studies have been carried out focusing on VIM modelling sensitivity, such as turbulence model, mesh refinement, Reynolds number and external damping [8][9][10].…”