Study On Viscous/potential Method For Ship Hydrodynamics

The prediction of ship hydrodynamic performance is an important part in the process of ship design and production while numerical simulation of ship hydrodynamic has becoming a common method to assistant ship design, so it is vital to explore new numerical algorithms with better efficiency. The potential theory, which includes the using in wavemaking and Strip theory, has been widely used in prediction of ship resistance and seakeeping performance along with the development of computer. The Computational Fluid Dynamics(CFD) method can consider the viscous property of fluid and nonlinear effect of flow comprehensively while it takes much computer resources and lots of computation time and even to extend the period of design, the usage of this method then shall be limited in the ship design. We proposed a viscous/potential method for ship hydrodynamic based on velocity decomposition, which can take full advantage of both potential theory and CFD method. We also developed a viscous/potential program using Open FOAM software platform and the theory and the software has been validated.Firstly, we accord to the Helmholtz velocity decomposition theory, a modified Navier-Stokes(NS) equation is obtained by decomposing the velocity into the potential flow component and the remained one, and we give out the numerical method to solve the modified NS equation. Then, we applied the method into the case of flow around two-dimensional cylinder, and finished the program in Open FOAM with the potential part is given by analytical solution. By calculating each component of velocity and pressure and compared with the experiment result, our theory is validated and the calculation precision is guaranteed.We also generalized the theory to three-dimensional(3D) case, and simulated the flow around 3D submerged body. We adopted simple Green function method for the calculation of potential part and proposed a source dipole drifting theory to address this problem. We also finished the program for the flow around 3D submerged body with present method based on the platform of Open FOAM. The computation stability is also demonstrated in analyzing the details of flow field and comparison between resistance between theory and experiments.At last, a preliminary exploration on the prediction of ship hydrodynamic performance has been done with the case of flow around Wigely hull. The total resistance can be decomposed into wavemaking resistance and viscous resistance. The wavemaking resistance is calculated by potential method while the viscous resistance is obtained from the calculation of flow over double model by viscous/potential method. The potential part is solved by simple Green function method and then the modified NS equation is solved for the solution. The ship resistance obtained by present method is compared with the experiment result and they agreed with each other very well.The result shows that our theory is validated and provide a new way for the simulation of viscous flow with better efficiency.