Numerical Simulation Of Motion And Wave Load Responses Of Moderate Or High Speed Ships Based On 3D Time Domain Methods

With the increasing of ship dimensions and forward speeds,it is of great importance to develop an accurate and stable hydrodynamic simulation tools,so that it can be applied to predict the motion response and wave loads of the vessels with moderate or high speeds.In recent years,due to the fast development of the high performance computers,the 3D time domain boundary element methods have been tried to accomplish this work.However,the existing time domain methods still have some problems in stabilities and accuracy.The aim of this thesis is to improve the accuracy as well as the stability of the current 3D time domain methods,i.e.the 3D free surface Green function method,the 3D Rankine panel method,the 3D Rankine-Green mixed source method,and investigate the properties as well as the applicabilities of each method.So that they can be used in the hydrodynamic predictions of moderate or high speed vessels.The major contributions of this work are as follows:The theories related to the ship hull mathematical expressions and boundary panel generations are analyzed.The B-spline curves and B-spline surface are used to express the ship wetted surface mathematically,and do the generation of the boundary element panels.Besides,according to the waterline geometries,the panels on the rectangular free surface are generated by the finite interpolation scheme,and the panels distributed on the Oval type free surface are generated by solving the Poisson equation.To control the free surface mesh density distributions,two additional parameters are introduced.The method based on the time domain free surface Green function is studied and improved.The boundary integral equations of the disturbed velocity potentials are established in the earth-fixed coordinate system,which can take into account the influence of the steady wave potential automatically.To avoid the instabilities of the finite difference scheme,the acceleration potentials are introduced,and the related boundary conditions are derived.Besides,an interpolation scheme based on the nine nodes finite element shape functions are developed to improve the computational efficiency of the time domain free surface Green functions.In order to accelerate the convergence speed,the eight nodes higher order panels are applied to discrete the boundary integral equations.The comparisons of the numerical results with the analytical and model test values demonstrate that this method can be applied in the hydrodynamic prediction of the ships without flare.The method based on the time domain Rankine source is studied and improved.The initial and boundary conditions in the reference coordinate system are derived.To improve the numerical stabilities,the rectangular pattern free surface is chosen,and the bi-directional derivatives are adopted to compute the partial derivatives of the velocity potentials on the free surface.The infinite frequency added mass is introduced,which decouples the hydrodynamic force from the body accelerations.Besides,the three nodes filter scheme is adopted to filter out the generated spurious waves.Considering the computational efficiency and suitable for the complex body surface,the eight nodes higher order panels are applied to solve the boundary integral equations.The comparisons of the numerical results with the analytical and model test values demonstrate that this method can be applied in the hydrodynamic prediction of the ships with or without flare.Then the time domain Rankine-Green mixed source method is studied and improved.The total fluid domain is decomposed into an interior sub-domain and exterior sub-domain by an additional control surface.Then the Rankine source is applied in the interior sub-domain,so that the algorithm can be used to simulate the ships with large flare.The free surface Green function is used in the exterior sub-domain,so that for the same control surface the free surface Green function only need to be computed once.To keep the continuousness of the velocity potentials at the intersection points of different surfaces,the eight nodes higher order panels are adopted to discrete the integral equations.Besides,the integral form free surface condition is adopted to improve the time marching stabilities.Instead of using the difference scheme,the B-spline functions are introduced to compute the first order and second order partial derivatives of the velocity potentials on the free surface,so that the developed algorithm can be used to simulate the ships with complex geometries on the bow and sten.The comparisons of the numerical results with the analytical or the model test values demonstrate that this method can be applied in the hydrodynamic predictions of the ships with or without flare.Finally,the properties as well as the applicabilities of each method,i.e.the 3D free surface Green function method,the 3D Rankine panel method,the 3D Rankine-Green mixed source method,are analyzed.Then the time domain Rankine-Green mixed source method is chosen as the basic theory,and used to develop the computer code for the motion and wave load predictions of ships with moderate or high speeds.The applicabilities of the developed computer code are further verified by the simulations of DTMB5512’s motion responses with different forward speeds.Then the developed computer code is applied to do the motion and section load predictions of the first design plan of a high speed displacement vessel,which provides a reference for the final choice of the design plan.