Research On Fluid Structure Interaction Of Underwater Explosion Based On Doubly Asymptotic Approximation

Fluid-structure interaction widely exists in nature and engineering field.In ship and ocean engineering field,scholars have focused on the fluid-structure interaction of underwater explosion for a long time.Underwater explosion is very complex,so it has great academic significance to investigate the fluid-structure interaction of underwater explosion.In addition,under underwater explosion loading,the ship structure exhibits large plastic deformation or rupture,or even breakoff.The investigation of the fluid-structure interaction of underwater explosion can help us to understand the damage mechanism of ship structure to underwater explosion,guide us to design the ship structure.And it has great realistic meaning for increasing the ability of shock resistance and the vitality of the ship structure.Therefore,in this paper the research on the fluid-structure interaction of underwater explosion is carried on based on the doubly asymptotic approximation method,including the cavitation effect of shock wave,the influence of internal fluid volume to dynamic response of structure subjected to shock wave,the dynamic characteristic of bubble and the interaction between bubble and structure.Furthermore,the investigation of dynamic response of stiffened cylindrical shell structure subjected to underwater explosion and hydrostatic pressure is conducted experimentally to verify the numerical methods established in this paper.The basic phenomenon of the fluid-structure interaction of underwater explosion is expounded as the process of underwater explosion firstly.Then,the researches and developments of the fluid-structure interaction of underwater explosion are reviewed.Based on the review,some weaknesses are presented.In this paper,fluid motion is assumed small,inviscid,irrotational and compressible,based on the linear wave equation and retarded potential equation,doubly asymptotic approximations for external fluid volume and internal fluid volume are obtained.And indirect doubly asymptotic approximation is modified based on doubly asymptotic approximation for external fluid volume and fluid finite element.On basis of indirect doubly asymptotic approximation,a fluid-structure interaction numerical model is established to analyze the cavitation effect of underwater explosion.In the implementation,doubly asymptotic approximation other than curved wave approximation is used to simulate non-reflecting boundary.Based on the numerical model,the cavitation effects near free surface,plane plate and spherical shell are investigated.The occurrence mechanisms of cavitation and reflected rarefaction wave are analyzed.The influence parameters of cavitation effect are analyzed systematically.And the difference between doubly asymptotic approximation non-reflecting boundary and curved wave approximation non-reflecting boundary is investigated.It's found out that air impedance far less than water impedance is the main occurrence reason of cavitation effect near free surface,which determines the reflected wave is rarefaction wave.The velocity of plane plate causes the cavitation effect near plane plate,the cavitation effect near plane plate occurs when the velocity of plane plate reaches a certain value.In addition,as the depth of water deeper,the influence of cavitation effect on structure dynamic response less,that is,the cavitation effect is more obvious in shallow water than deep water.Based on doubly asymptotic approximations for external fluid volume and internal fluid volume,a fluid-structure interaction numerical model is established to analyze the influence of internal fluid volume on structure dynamic response.The dynamic responses of spherical shell,cylindrical shell and stiffened cylindrical shell with internal fluid volume are carried out with the numerical model.Then,the influence of parameters of shock wave on structure dynamic response with internal fluid volume is investigated.And the influence of internal fluid volume on structure dynamic response is analyzed.As the research shows,the existence of internal fluid volume makes the velocity amplitude and oscillation frequency decrease.A modified compressible bubble numerical model is established that accounts for compressibility of external liquid and wave effect of internal gas,based on doubly asymptotic approximations for external fluid volume and internal fluid volume.And the volume acceleration model is applied to determine initial velocity of bubble.The difference among incompressible model,compressible model and modified compressible model is investigated.The energy loss mechanism of bubble system is analyzed.The influence of initial bubble velocity on bubble dynamic is carried on.Based on the established bubble model,the bubble dynamics in gravity and shock wave field are investigated.According to the result,the established model is very suitable to simulate the underwater explosion bubble dynamic.The compressibilities of external liquid and internal gas are the main energy loss mechanism.During the simulation of bubble dynamic,the initial velocity of bubble should take into account.The interaction between a bubble and a cylinder with different diameters is investigated experimentally via a spark generator.And the influence of parameters on the interaction between bubble and cylinder is analyzed.Then,a fluid-structure interaction numerical model is established to study the interaction between bubble and cylindrical shell.The spherical equivalent method is proposed to simulate the bubble dynamic after jet pierces the bubble surface.And the damage of bubble to cylindrical shell with different initial distance is investigated,based on the fluid-structure interaction numerical model.It's found that the behavior of a bubble near a rigid cylinder is distinct from that near a rigid plate.The spherical equivalent method is reasonable to investigate the interaction between bubble and structure.And as the initial distance decrease,the damage of bubble to cylindrical shell is more obvious.The dynamic response of stiffened cylindrical shells subjected to underwater explosion and hydrostatic pressure is investigated experimentally through underwater explosion experiment.The strain,acceleration and pressure of stiffened cylindrical shells have obtained.Then the influence of hydrostatic pressure on the strain and acceleration of stiffened cylindrical shells in far field explosion,and the influence of hydrostatic pressure on the plastic deformation of stiffened cylindrical shells in close field explosion are analyzed.As the results show,in far field explosion,the hydrostatic pressure has little influence on the strain.But,in close field explosion,exist of hydrostatic pressure makes the strain and deformation increase,and deformation domain expand.