A Comparative Study On Numerical Methods For Predicting Ship Added Resistance

With the energy efficiency design index EEDI enforced to limit the carbon dioxide emis-sions of ships,energy saving and low carbon have been focus on for the shipbuilding industry in recent years.Reducing installed power and speed is the fastest and most efficient way to satisfy energy efficiency requirements.Based on the required EEDI,The International Mari-time Organization has proposed the minimum propulsion power of ships to measure the ship's ability to maintain its maneuverability under adverse sea conditions.One of the problems in the certification of the minimum propulsion power is how to accurately calculate the wave resistance induced by waves.The CFD method theoretically reflects the real fluid flow be-cause there is no assumption limitation.Compared with the potential flow results,the CFD method can be used to predict the wave added resistance problem on strong nonlinear waves.In this paper,the STAR.CCM+viscous flow solver is used to predict wave added resistance.The wave added resistance is the average increase drag of the wave resistance relative to the calm water resistance.Firstly,the calm water resistance of the ship is studied.In this paper,the convergence analysis of the computational domain size,grid size and time step setting is performed for calm water resistance.In addition,since the viscosity dominates the ship drag problems in still water,the boundary layer setting i5 also studied.By adoptting the recom-mended numerical simulation strategy of calm water resistance,the numerical prediction re-sults are compared with the experimental data for ships with both different speeds and various types.The calm resistance prediction reaches the expected accuracy within 3%.Making waves through numerical wave tank is the basis of seakeeping problems.Firstly,the convergence analysis of the computational domain,grid and time step settings of pure wave numerieal simulation is carried out.Considering the ship with a forward speed,thus,wave plus current numerical simulations is also studied.Compared with stream function re-sults after self-convergence verification for pure waves and wave plus current conditions,the results show that CFD can bring damping effect on the process of wave-making.This problrm can be solved by increasing the input wave height.The overlapping grid method can be taken to solve wave interaction problems.Conver-gence analysis is performed by using overset mesh to obtain recommended numerical simula-tion strategy.According to KCS wave resistance,results adopting overset mesh method agree well with the experimental data compared with that without overset-grid method.However,For short wave conditions,the non-overlapping mesh method can be used to predict wave added resistance.Depending on the nonlinearity of the incident wave,the potential history results is compared with CFD.It is found that the potential and the viscous flow results agree well on linear and weakly nonlinear waves.However,as for the strong nonlinear waves,the discrepancy between them is apparent.Finally,the results of wave added resistance show that the potential method can accurately predict the linear and weakly nonlinear wave conditions.CFD can predict the ship wave added resistance in good agreement with experiments whatev-er the nonlinearity of the incident wave.