Load Characteristics And Shock Response Of Structrues Subjected To Near-Field Underwater Explosion Based On RKDG Method

Strong discontinuous shock wave,bubble pulsation and high-speed bubble jet can be generated by near-field underwater explosion,which may cause damage in warship.Hence,it is of great significance to study load characteristics and shock response of the warship subjected to near-field underwater explosion.The aforementioned loads of near-field underwater explosion with high-pressure and high-speed are more complex than that of mid/far-field underwater explosion.After the warship is impacted by these loads,it may be caused strongly nonlinear and unrecoverable damage.Although experimental research of real ship is the most effective and direct method to investigate underwater explosions,it has some advantages: higher risk to human operators,non-repeatability and limited information acquisition.Therefore,reduced-scale model is usually used in the experimental researches.Besides,with the development of computer technology,numerical method is generally combined with the experimental research.Hence,hybrid algorithms based on the Discontinuous Galerkin method(RKDG)and the Boundary Element Method(BEM)/ the Finite Element Method(FEM)are applied to solve non-linear Euler equations for strong compressible fluid.Subsequently,a fluid-structure interaction(FSI)model of near-field underwater explosion is developed.In order to verify the accuracy and the effectiveness of the mentioned hybrid algorithm,the numerical results are compared with experimental data and analytic solution.Finally,load characteristics including shock wave,bubble pulsing as well as bubble jet are investigated;the dynamic responses of structures under above different loads are studied.Researches of near-field underwater explosion loads and the shock response of structures are briefly summarized from three aspects-experimental method,theoretical analysis and numerical method.Then the development of these methods is reviewed and their shortcomings are pointed out.After that,the application of numerical methods,i.e.RKDG,Level-Set(LS),Ghost-Fluid(GF),in solving FSI problem is introduced.These numerical methods have advantages of dealing with strongly non-linear problems,which lays the foundation for further researches of near-field underwater explosion in this paper.The fluid is assumed to be strongly compressible since the Mach number is high in the process of near-field underwater explosion.A hybrid algorithm-RKDG-is proposed.The RKDG method is used to solve non-linear Euler equations and catch the strongly discontinuous shock wave in the fluid.The LS method is adopted to keep track of the moving interface.The GF method is an excellent tool for treating the two-fluid flows for underwater explosion and has no additional cost associated with geometry and physics complexities.Hence,it is employed to deal with the two individual fluid flows together with RKDG and LS methods.A numerical RKDG model with high resolution and high accuracy is eventually established in simulation of near-field underwater explosion.The presented hybrid algorithm is applied to solve transient FSI problems of near-field underwater explosion shock wave and shock response of structures under this load.Besides,a cavitation model is introduced in the RKDG model in order to study the formation and the collapse of the cavitation.Subsequently,the numerical FSI method considering the cavitation is used to investigate load characteristics and shock response of structures subjected to near-field underwater explosion.The shock wave propagation,the cavitation characteristics and structural dynamic response near free field and boundary are discussed.A combined algorithm based on the above RKDG method and the BEM is presented in dealing with column charge underwater explosion which takes the non-spherical detonation in consideration.This combined algorithm fully utilizing the advantages of the RKDG method and the BEM is adopted to simulate the entire process of near-field underwater explosion ranging from the detonation of column charge to the formation and the collapse of a bubble.Firstly,underwater explosions models of free-field and near rigid wall are built.Then the bubble dynamics during its jet impact,collapse impact and rebounding phases are investigated.Besides,two bubble-generating methods are used in the current experiments-small-charge underwater explosion and underwater discharges.After that,the experimental data is compared with numerical results in order to validate the presented combined algorithm.Finally,the effects of different parameters on jet impact,collapse impact as well as bubble splitting and rebounding is discussed;the movement law of the bubble near rigid wall is studied.Based on the above researches about load characteristics,a coupled BEM-FEM algorithm is proposed to realize the numerical simulation of load characteristics and dynamics of an elastic-plastic or a complex structure.BEM-FEM models of a plate,a box girder structure and a ship structure subjected to near-field underwater explosion are developed.The numerical results are compared the experimental ones with reduced-scale model.Subsequently,dynamic response of structures under the impact of shock wave,bubble pulsing as well as bubble jet is analysed.The results show that these loads may result in different shock responses: pressures induced by the shock wave and the bubble jet during its collapse phase lead to a local response while a global response is caused by the bubble pulsing.