Grid independence study of low speed catamaran operate in shallow water

Abstract

Selecting the number of grids for ship resistance simulation through Computational Fluid Dynamics (CFD) software is one of the important studies. Suitable number of grids is required to ensure the accuracy of CFD result and minimize the computational time. This paper is proposed to investigate a mesh independency study for the CFD setup used for simulating resistance of low-speed catamaran operating in shallow water. In this study, resistance of Catamaran Delft 372 model was simulated using Flow 3D software. The software uses the Reynold's Averaged Navier-Stokes (RANS) equation and a structured hexahedra mesh. The grid independence study was conducted for shallow water case to analyse the influence of seabed to number of grids needed for simulation. To optimise the grid number required, the resistance of Delft 372 simulated by different grid number were plotted against the number of grids used in simulation. This comparison was conducted with similar ship speed between experiment and simulation. Accuracy of ship resistance estimated by Flow-3D can be improved by increasing the number of grids. However, the time required to complete one simulation increases with the number of grids. Based on the grid independence study conducted, the optimise number of grids is obtained from the graph resistance versus grid number. To optimise grid number is the point where the resistance starts to become unchanged when the grid number increases. After completing grid independence study, the resistance of Delft 372 between Froude number 0.2 and 0.4 were estimated. The resistance of Delft 372 model obtained from Flow 3D was validated using published experimental data in deep water and shallow water case. Based on comparison, the number of grids required for shallow water is higher as compared to deep water case due to the viscous effect. As compared to experimental result, CFD results calculated by Renormalized Group (RNG) model are more aligned to experiment result when simulate the resistance of Delft 372 model in shallow water condition.