Improved Transient Fluid-Structure Interaction Model Of Underwater Explosion And Its Applications

The warship subjected to underwater explosion(UNDEX)loads is a complex physical problem.The shock wave,bubble and cavitation reloading caused by UNDEX could threaten the survivability of naval ships and submarines seriously.The physical process involves a series of key problems,such as capturing the shock,transitions between liquid and gaseous phases,large deformation of gas-liquid interface,nonlinear transient fluid-structure interaction(FSI),plastic deformation of structure and so on.On the basis of acoustic finite element method and local discontinuous Galerkin(LDG)method.a transient FSI nuemrical model is established in this paper,which can calculate the shock wave and cavitation loading efficiently and accurately.The model can improve the traditional coupled acoustic-structrual model and overcome the calculation difficulties,distortion of shock front and loss of pressure peak.Based on the potential fluid theory and explicit finite element method,the fully coupled model of the interaction between explosion bubble and elastic-plastic structure is established,which can reveal the mechanism about damage characteristics ofstructure subjected to UNDEX and study the coupled characteristics between bubble and structure.Firstly,the classifications and basic phenomenon of UNDEX are introduced.And the domestic and overseas researching progresses about UNDEX loads and interaction between the explosion bubble and structure are investigated.In order to improve the precision,stabilities and efficiency of the current numerical model,the novel numerical model are required to be established to solve the nonlinear transient FSI effects.The present model is aimed to extend to calculate three dimensional marine structural response subjected to underwater explosion and apply to the related ocean engineering field.In Chapter 2,the basic theory of acoustic finite element method is expounded.On the basis of local discontinuous Galerkin(LDG)method,a strongly discontinuous axisymmetric numerical model about acoustic pressure is established.The model can capture the strong discontinuity caused by the shock wave in fluid field.The pressure cutoff model is employed to deal with the cavitation effect due to the reflection of the shock wave.The model can calculate the dynamic pressure in the fluid field and capture the high-resolution shock wave.In comparison to solve Euler equations by DG method,the LDG model can not only ensure the resulting accuracy,but also improve the calculation efficiency.The numerical model is verified by comparing with the analytical solution of the cavitation effect near the structure in one dimension.As for the cavitation characteristics of inclined wall near free surface,the two dimensional LDG far-field UNDEX loads model is established in Chapter 3.The influence of inclination angle on the explosion loads and cavitation characteristics is studied.Due to the effects of the complex fluid field boundary,the reflection law of far-field explosion load is analyzed.Under the combined action of wall and free surface,the pressure wave reflected by wall makes the whole bulk cavitation zone split into smaller zone suddenly.The cavitation region becomes the discontinuous shape.The cavitation phenomenon near the third-order Stokes wave surface is studied,and the influence of wave surface on the cavitation characteristics is analyzed.As for FSI effects under the action of shock wave and cavitation loads,the FSI algorithm of LDG model is proposed in Chapter 4.The three dimensional LDG FSI model with self-programmed codes is established.The FSI model couples the LDG method with explicit finite element method(EFEM)method to study the FSI effects of elastic structure subjected to UNDEX loads.And the axisymmetric LDG FSI model is adopted to study the rigid spherical shell subjected to UNDEX loads.The influence of the depth of spherical shell on the FSI effects is studied.Besides,the impact dynamic response of the floating body caused by UNDEX loads is simulated.The distribution of pressure on the wet surface is discussed.The growth process of cavitation zone near the structure is studied.In order to improve the current loosely coupled model,the novel fully coupled BEM-FEM model is proposed to simulate the interaction between explosion bubble and elastic-plastic structure with self-programmed codes in Chapter 5.The BEM is adopted to simulate the bubble dynamics,and the explicit finite element method(EFEM)is employed to solve the nonlinear transient response of structure.Based on the acceleration potential theory,the mutual dependence between the hydrodynamic load and the structural motion is decoupled.Through coupling the equations for the state variables of the fluid and structure at the fluid-structure interface,a set of coupled linear algebra equations is established.By solving the boundary integral equations,the explosion bubble loads are predicted directly and accurately,which can improve numerical precision and stabilities.The fully coupled model has expounded the theory about the interaction between bubble and elastic-plastic structure.The numerical results correspond well with the experimental data,in terms of bubble shapes and structural strain response.Compared with the traditional modal superposition method the present model has an obvious advantage over calculation efficiency.As for the damage characteristics of structure subjected to explosion bubble loads,the fully coupled model is used to study the coupled characteristics of interaction between bubble and typical ship structure in Chapter 6.Initially,the influence of stand-off distance and plate thickness on the interaction between bubble and plate fully submerged in water is studied.With different parameters,the bubble dynamic behavior,peak pressure and vibration frequency and amplitude of the plate are discussed,which can provide a reference for the anti-shock researches of naval ships.Besides,the influence of stand-off distance and structural stiffness on the interaction between bubble and spherical shell and stiffened plate is studied.And the influences of the internal fluid on the dynamic response of the spherical shell are discussed.This paper reveals the mechanism of the bubble dynamics near elastic-plastic boundaries.