Research On Updated Lagrangian Particle Hydrodynamics (ULPH) Method And Its Applications

In recent years,Computational Fluid Dynamics(CFD)has been widely applied to the research of ship and ocean engineering hydrodynamics.CFD methods are mainly divided into grid methods and meshfree methods.The grid method discretizes the computational domain into several non-overlapping grids,and the set of controlling equations of fluid dynamics is solved numerically on the grid,and the quality of the grid affects the accuracy and precision of the computational results.Therefore,the grid methods often face the difficulties of large grid deformation and interface tracking when dealing with large deformation of free surface flows,multiphase flow splitting and fusion,wave overturning and breaking,fluid-structure interaction,explosion impact,etc.To solve the inherent defects of grid methods,the node-based meshfree method has been developed rapidly in CFD field.In this paper,based on nonlocal theory and nonlocal differential operator,a new meshfree method,namely,the Updated Lagrangian Particle Hydrodynamics(ULPH)method,is improved and refined.Through a completely autonomous programming approach,a set of robust ULPH numerical models have been established,and the ULPH method has been applied to the research in the field of computational fluid dynamics,aiming to provide technical support for China’s autonomous industrial software development.In this paper,the research status of meshfree particle class method is reviewed.Through investigation,it is found that ULPH method,as a new meshless particle method,is not perfect in theoretical research,and its theoretical model and calculation method need further study.Besides,the practical application of ULPH method in engineering is still very weak.In order to perfect the theoretical system of ULPH method,based on the nonlocal theory and Taylor expansion principle,this paper derives the high-order nonlocal differential operator,and uses the high-order nonlocal differential operator to decouple the fluid control equation discretely,which improves the precision of ULPH method.Based on the linked list search method,an improved self-adaptive linked list search algorithm is proposed.The time integration adopts an improved predictive correction time stepping scheme,and the OpenMP parallel acceleration scheme is added to the self-compiled program,which improves the calculation efficiency of ULPH method,thus establishing a set of efficient and stable 2D and 3D hydrodynamic calculation programs.According to the ULPH numerical simulation of three-dimensional practical engineering problems,a set of effective pre-processing and post-processing schemes of meshless particle method is established by using the grid method processing ideas for reference,which improves the visualization level of calculation results of ULPH method.Considering the problem of multiphase flow with variable interfaces,this paper establishes a multiphase flow ULPH model that considers physical viscosity and surface tension.The introduction of the interface sharpness force at the multiphase interface prevents particles from penetrating at the interface to ensure the clarity and stablility of the interface.In order to eliminate the pressure oscillation in the flow field,the density reinitialization is used for continuity correction to obtain a stable pressure field.On this basis,a series of numerical examples are used to comprehensively verify the convergence,stability and accuracy of the multiphase flow ULPH model.In view of the complicated nonlinear multiphase flow problems in ship and ocean engineering,such as rising bubbles,bubble coalescence,interaction between bubbles and free surface,bubble and obstacles,etc.,the multiphase flow ULPH model is used to analyze the bubble dynamics during the bubble rising process.Numerical simulation analyzed the shape changes,force conditions,velocity changes,coalescence and splitting conditions of rising bubbles,which provided technical support for the research of complex gas-liquid two-phase flow in ship and ocean engineering.Aiming at the large deformation of the free liquid surface that is common in ships and ocean engineering,this paper establishes a single-phase flow ULPH model based on the basic theory of ULPH.The introduction of the density dissipation term in the continuum equation can effectively prevent the pressure oscillation in the flow field.Introducing an improved particle shifting correction technology to solve the uneven distribution of particles and ensure uniform distribution of particles throughout the calculation process.An improved free surface search algorithm is proposed in this paper,which can effectively detect the particles in the free area and calculate the normal direction of the particles.The shape tensor of the particles in the area near the free surface is optimized to solve the problem that the shape tensor of the particles in the area near the free surface becomes an ill-conditioned matrix due to the large deformation of the free surface.The single-phase flow ULPH model proposed in this paper is used for numerical simulation of droplet oscillation,tank sloshing,three-dimensional dam break and other issues,and compared with the experimental results and other numerical methods,verifying the accuracy and effectiveness of the single-phase flow ULPH method.For the problem of object water entry,the tension instability control technology was added on the basis of ULPH theoretical model,which solved the tension instability caused by negative pressure in the flow field.On the basis of reasonable calculation of the domain size,in order to eliminate the reflection of pressure wave when an object impacts into water,a sponge layer is added to the flow field near the solid wall to realize the non-reflection boundary condition.On this basis,numerical simulation studies were carried out on the water entry of cylinder,flat plate and ball with different angles.The water entry process,slamming load and evolution process of free surface of the object were analyzed,which laid a foundation for the ULPH simulation research on the water entry of complex structures.