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

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.