Research On Fluid-solid Coupling Between Hull And Free Surface Based On An Improved SPH Method

In the field of naval architecture and ocean engineering,the slamming and sinking of hulls is a kind of complex fluid-solid coupling problem,which involves physical phenomena such as large deformation and fragmentation of fluid free surface,transient slamming load and rigid motion,threating the safety of equipment and personnel and the vitality of hulls.The strong nonlinear fluid physical phenomena existing in the above fluid-solid coupling problems bring some challenges to the traditional mesh-based numerical methods.Meanwhile,the simulation requirements for practical problems in three-dimensional engineering also put forward higher requirements for numerical stability and computational efficiency.In view of the above physical problems and numerical difficulties,based on an improved Smooth Particle Hydynamics(SPH)model,a robust numerical model is established by independent programming to simulate the above processes.The purpose of this paper is to reveal the structural dynamic response law and fluid flow mechanism behind the relevant physical phenomena,so as to provide reference for ship structure design and marine rescue activities.At the same time,it provides technical support for China's independent industrial software development.Firstly,this paper summarizes the research status of ship hull slamming and sinking.It is found that,in recent years,scholars at home and abroad have studied the above problems by theoretical,experimental and numerical methods,and some progress has been made where the hull motion response and local load variation have been well analyzed.However,there are also some blind spots and deficiencies: the research on the water entry problems of some special structures,such as plates and ground effect wing(WIG)ships,is still insufficient.In the numerical study of the sinking of ship hulls,no scholar has systematically studied the water flooding of damaged ship hulls in wave environment and so on.In view of these shortcomings,combined with the characteristics of the SPH method and the application progress in recent years,the corresponding numerical innovation scheme and numerical research content are put forward,which lays the foundation for the following work.From the point of view of engineering application,the basic theory,the boundary conditions and some common accuracy improvement schemes of weakly compressible SPH(WCSPH)method are discussed systematically.Aiming at the SPH numerical simulation of the ship hull flooding process,on the basis of the original solid wall boundary conditions of multi-layer virtual particles,a new type of identifiable open boundary condition is established and added to intelligently distinguish the fluid particles inside and outside the hull structure of arbitrary shape,which solves the problem of non-physical interaction of fluid particles inside and outside the boundary in the simulation process.Aiming at the SPH numerical simulation process of three-dimensional large-scale engineering problems,using the idea of grid method for reference,an effective pre-and post-processing scheme of particle-based method is established,which improves the visualization of SPH calculation results,and the GPU parallel acceleration scheme is introduced to effectively improve the computational efficiency of the SPH model.Aiming at the problem of multiphase fluid-solid coupling with high density ratio,on the basis of the original SPH model,combined with the particle intelligent criterion of multiphase interface,a robust spatial displacement correction algorithm is added to solve the local spatial inhomogeneity in the process of multiphase SPH simulation.On the premise of keeping the multiphase interface clear and stable,the accuracy of SPH numerical calculation is improved.On this basis,the water entry problems of the wedge and the plate under multiphase conditions are numerically simulated,and the effects of air on the water entry process of the different types of structures are summarized.Considering the slamming of flared bow section,using the improved SPH algorithm in this paper,firstly,the effects of different entry velocity and angle on the slamming load and free surface evolution are studied,and the mechanism of free surface separation is revealed by flow field analysis.On this basis,the water entry process of a three-dimensional real-scale ship structure is numerically studied,and the variation rules of slamming load and motion response of the hull are analyzed.Considering the complex motion response of the WIG ship during ditching,the accuracy of the SPH numerical model and the the six-degree-of-freedom rigid body motion model established in the presented paper are fully verified by using the relevant theory and the existing experimental results.Then,the dtiching of a three-dimensional real-scale WIG ship is numerically simulated,and the reason and mechanism of taking off again after entering the water of the WIG ship are analyzed based on the attitude change of the ship during taxiing and the local load variation.Besides,the influence of water entry speed on the ditching process is studied.Considering the problem of water flooding of damaged ship hulls,firstly,the relevant experiments are carried out,and the present SPH model is adopted to restore the flooding process for comparison.On this basis,this chapter uses the SPH numerical model to study the flooding and sinking process of the damaged hull model under different opening radiuses,different opening positions and different cabin constructions,focusing on the analysis of hull motion response characteristics and internal influent flow law,grasping the final sinking state of hull and revealing the influence of internal influent flow on hull sinking process,so as to provide some reference for practical engineering problems and rescue.Considering the dynamic response of the damaged ship hull in waves,the numerical wave-making system is established in the SPH model,and the numerical verification is carried out according to the relevant experimental work.On this basis,the effects of different wave frequencies and the relative position of waves and holes on the sinking process of the hull are studied respectively.The trajectory of the ship during the sinking process is extracted,and its motion response law is analyzed,so as to provide some reference for ship structure design and practical engineering rescue.