Numerical Study On Added Resistance And Motions Based On Viscous Flow And Coupled Viscous Potential Method

A ship is affected by waves to generate motion response,which also changes the ship resistance.Because of added resistance,ships in waves must consume more fuel than in calm water and produce more carbon emissions,which is harmful for ships’ economy.Also,ship motions in waves,especially for resonance conditions,have been the focus of ship resistance and safety.It is particularly important to accurately calculate the ship motions and added resistance in waves,which is significant to the ship’s sea-keeping,safety and economy.For many years,the ship resistance and motions have been widely studied.The computational fluid dynamics(CFD)method has become an important method in the field of ship fluid mechanics.Traditional researches on ship motions mostly apply dynamic deformed grid technology and its calculation accuracy is limited by the motion amplitudes.There is little research using overset grid technology,and predicting ship motions with overset grid technology in large-scale viscous wave fields requires huge calculation assumptions.In the meantime,most traditional researches only consider the head waves.Serious dissipation is easy to appear in numerical simulations of short and oblique waves.Simulating accurate wave environment has high requirements for grids,solvers,and wave-maker and wave-elimination technologies.In this paper,the CFD method and overset grid technology are used to systematically study the KCS ships’ motions and added resistance.Firstly,the motions and added resistance in regular waves at different wavelengths are simulated numerically.A series of numerical verification is performed to ensure the accuracy of the numerical wave tank and overset grid.The results in different wavelengths are compared to validate the reliability in short-wave conditions.The heave and pitch are free or fixed to study the characteristics of diffraction resistance and radiation resistance.The Fourier analysis method is used to study the frequency distribution of motions and resistance.The results show that the wavelength has little effect on the diffraction resistance.When the wavelength is close to the ship length,the hull motion amplitude and added resistance reach the peak value.The violent motions causes strong radiation resistance.Sailing in this condition should be avoided.Further,the wave direction is extended to the oblique waves,and the motions and added resistance of KCS ships in regular waves with different wave directions are studied.That is significant for predicting the resistance and motions in the actual wave environment.In order to alleviate the huge computational cost caused by large-scale wave-making,and fully apply the high-efficiency advantage of high-order spectrum(HOS)method,the HOS method and the CFD method are combined to predict motions and added resistance in resonance conditions.This coupled model is used to simulate the whole wave field.The numerical results are compared with previous CFD data and experimental data to verify the accuracy of the coupled model.The coupled method can provide a reference for the resistance and motion problems of ships in waves.