Study On Water Entry Of Torpedo And Damage Characteristics Of Ship Structures Subjected To Its Near-field Underwater Explosion

Comparing with those of the far-field underwater explosion,the characteristics of the loading and the damage of structures caused by the underwater near-field explosions of torpedo are more complicated when the torpedo is launched from its platform above the water or in the air and enters the water safely.The underwater structures are subject to the damage by the shock wave loading,the shrapnel and the impact loading of the bubble jet.Nonlinear structural failure modes include large deformation and large orifices,etc.The dynamic characteristics of the bubbles and the characteristics of water jet of underwater explosion near crevasse of the structure vary significantly when compared with those of intact structures.Because of the fully nonlinearity of torpedo water entry,the complexity of the underwater near-field explosion load and the resulted destructive structures,the researches on the water entry of torpedo and the fluid-structure interaction of near-field underwater explosion are essential to the improvement of ship design and vitality.On such basis,present thesis elaborate the features of hydrodynamic characteristics of water entry of torpedo and three types of loadings(shockwave,shrapnel and bubble jet);study on the bubble jet dynamics near complex boundaries with crevasses are carried out experimentally at the same time;the damage mechanism of ship structures subjected to underwater explosion of torpedo and anti-shock performance of integrated ship equipment are investigated.The researches begin with the characteristics of torpedo,the basic phenomenon and classification of underwater explosion.The phenomenon and features of water entry of torpedo and the fluid-structure interaction of underwater explosions and damaged structures were demonstrated.The states of art are reviewed in terms of theoretical study,numerical simulation and experiments of hydrodynamic characteristics of water entry,underwater near-field explosions loadings,near-field underwater explosion damageability,anti-shock ship equipment integration and bubble interaction with complex boundaries.The deficiencies of theoretical,experimental and numerical studies are discussed,upon which the work of present studies are determined.The numerical model on the loading and damageability of near-field underwater explosions was established based on the SPH and the coupled Euler-Lagrangian method subsequently.Based on the potential flow theory and the boundary element method,the impact of torpedo water entry are studied.The stretched coordinate system and the auxiliary function are introduced to simulate the full-nonlinear torpedo water entry numerically.The free surface elevation and the pressure distribution over the torpedo head were analyzed when vertical and oblique water entries are concerned.SPH method was introduced to study the shockwave loads and structural damage due to near-field or contact underwater explosion.Numerical model is established for the damage on side liquid tank caused by contact explosion of torpedo.The peak pressure and impulse distributions of underwater explosion in free field and near seabed are studied respectively;the shockwave reflection and truncation effect of wall and submarine are analyzed.On this basis,the generation and propagation law of underwater explosion shock wave in the structure are studied;the design criteria of the tank width and outer plate thickness are given.This paper summarizes the damage law of the shock wave to the tank structure in the far field,near field and contact explosion conditions.For the damage and protection of warship structure subjected to high-velocity shrapnel and jet loads,the SPH method is introduced to simulate the high velocity shrapnel of different incident angles and the high-velocity jet loads impacting on the broadside liquid cabin.Firstly,the attenuation law of the velocity of shrapnel and the shock wave pressure are studied.Considering various width of the cabin,the thickness of the outer plate and incidence angle of the shrapnel,as a reference of the design of liquid cabin,are provided.The generation and propagation of the shock waves and the influence of different jet shapes and velocity on warship structure is observed and discussed.For the problem of pulsation characteristics and jet characteristics of underwater explosion bubble near complex boundary conditions,experimental study was carried out on the dynamics of underwater explosion bubbles subject to the effect of free surface and complex boundaries with crevasses using spark bubbles and high speed photography.The characteristics of the underwater explosion bubbles near the crevasse of the single-layer wall are firstly investigated to get the law of the influence of the various crevasse sizes,distances from the explosive source,explosion angles and depths from the free surface on the period of bubble pulsation,the jet flow velocity and the deformation of the free surface.The coupling effect among the explosion bubble,the free surface and the damaged structure with crevasse are investigated as well.On the basis of this and to simulate the flow field of the cabin after the torpedo attack,three typical conditions of full tank,partial filled and dry tanks are selected to investigate different flow field in the presents of free surface.The characteristics of the bubble fluctuation in the vicinity of the double-layer wall and the single-layer wall are found and the variation law of the jet in the different tank state is also observed.The damage characteristics of multi-layer structure of large ships subjected to the underwater near field explosion of torpedo are investigated.The jet loading is calculated with the water-hammer model.The shock wave loading is obtained with the semi-empirical Geers & Hunter model.The jet and shock wave loadings are then imposed to near-field structures of large ships to check the strength of structure;failure pattern of different panels are analysed;the characteristics of damage of objective ships due to underwater near-field torpedo explosion are investigated;assessment on safety zones for equipment and crew is obtained.Finally,the analysis of the integrated impact on ship equipment is provided.