Research Of Numerical Computation Methods On Water Entry Hydrodynamics Of Structure

In the field of navy architecture and marine engineering as well as navy equipment,water entry problem is a common fluid-structure interaction phenomenon and have caught wide attention from experts and scholars.Studies of the impact of solid objects onto horizontal liquid surfaces have led to a detailed understanding of the forces that act on the object in the very early stages of impact and on the shape and size of an air cavity formed adjacent to the object.This information is helpful for predictions of the motion of objects that breach the surface as they pass into the liquid.For involving a series of complex processes such as moving structure boundary,multiply splashing of free surface and dramatic change of liquid-gas two phase media,which bring huge difficulties and challenges to numerical simulation of water entry process.In this paper,two methods were used for research of simple structures on water entry characteristics.Based on the discrete lattice Boltzmann method(LBM),the first method conducts numerical analysis on motion characteristics and surface pressure of wedge while entering water.The second method is based on coupled Euler-Lagrange method,which simulates the moving characteristics,as well as vacuolar morphology of sphere.Lattice Boltzmann method is one of those mesh-free computational methods which perform well on problems related to remarkable deformation of free fluid surface.Therefore,based on advanced fluid mechanical software Xflow using lattice Boltzmann method water entry of two-dimensional vertical wedge is simulated.The free surface flow model in software Xflow is used to calculate its surface pressure where the water contacts with the wedge.Through the simulation pressure of the whole flow field is detected.Comparing with the experimental results,the reliability of the numerical method is verified.In detail,lattice Boltzmann method in application of water entry simulation of wedge is available in solving surface pressure during impact.However,it is found that lattice Boltzmann method could not simulate morphology accompanied with the wedge which appears later after impact.In order to simulate the morphology phenomenon,coupled Euler-Lagrange(CEL)method is used via finite element software ABAQUS,for numerical simulation of sphere entering water,compared with corresponding experimental data from MIT.Water entry simulation of the sphere with different densities is conducted to obtain their hydrodynamic properties such as motion character,morphology features.The numerical results are highly agreed with the experimental values.To summary,application of lattice Boltzmann method and coupled Euler-Lagrange method in water entry hydrodynamics is achieved,and their advantages and disadvantages are compared.It is simple and efficient using software Xflow based on lattice Boltzmann method.But this method cannot take into consideration elasticity of structure.While coupled Euler-Lagrange method could deal with well structure hydro-elasticity,it is strong in modeling morphology in water entry.However the cost for using coupled Euler-Lagrange method turns out to be very high.Based on above studies,in order to study the influence of relevant numerical parameters used in CEL method to the sphere moving feature,by changing the distributions and sizes of grid in ABAQUS computation,length of time steps,elastic modulus of the structure,Poisson's ratio and friction coefficient of contact surface,these characteristic parameters are examined for their effects to ball into water.Finally,as with LBM method dealing with water slamming on structure,related numerical parameters are considered for their impact on the calculation.Respectively,by changing water viscosity,surface tension of water,these parameters are examined for their effects on displacement and surface pressure of wedge into water.