Free Surface Effect in the Current Load Calculation Based on CFD

The free surface resolution in current load calculations increases costs significantly when using computational fluid dynamics (CFD), even though the small Froude number of a dynamic positioning (DP) vessel suggests minimal wave effects. This study examined the impact of including and excluding the water-air interface in CFD simulations to identify a more efficient method for load computation. For each current condition, this study analyzed two virtual domains: one modeled the free surface using the Volume of Fluid method, while the other treated the free surface as a symmetry plane based on the double body assumption. The Reynolds-averaged Navier-Stokes (RANS) algorithm with the Shear Stress Transport k-ω model was used, and the results were validated against the empirical formula and experimental data. These findings showed that the free surface had a negligible effect on the resulting hydrodynamic force at the targeted low Froude numbers. Despite this, implementing free-surface modeling costs a hundred times more than the simulations using the steady RANS approach. The substantial increase was mainly due to the low convergence rate of the unsteady solver for a low-propagating flow. The study enhanced the credibility of the steady RANS solver without free-surface resolution in numerically calculating the current load on a DP vessel.