The Dynamic Response Of A Ship Hull Structure Subjected To A Spherical Bubble

The damage of warships when subjected to underwater explosion has always been a concerned topic for national defense, on which a great many researchers have done extensive studies for years. Analyzing the underwater explosion problems requires understanding many different areas, including the simulation of underwater explosion loads, explosion gas bubble behavior, fluid-structure interaction phenomena and structure dynamic response.A ship hull structure immersed in the vicinity of an underwater explosion is affected by two types of time-dependent loads:the transient shock wave and pulsating bubble. Compare with the shock wave, the bubble pulsation load in the underwater explosion can cause more severe damage on the warships. It is a serious threat to the vitality of the warship. At present, the dynamic response of the ship structure subjected to the underwater explosion bubble load is an international research focus. However, a lot of failure mechanism and essence on this problem remain open today, due to their difficulties.The present thesis is mainly to describe the dynamic responses of the ship hull structure subjected to a mid-field underwater bubble. Our focus is on the study of the hydroelastic responses, rigid-body motions, hydro-elastic-plastic responses and3-D structure dynamic responses of the ship hull. Our study aims to reveal the mechanism and characteristics of the ship hull’s dynamic responses to the underwater bubble pulsation loading.Firstly, based on the potential flow theory, we introduce a spherical underwater bubble model with the bubble migration, free surface effect and drag force taken into account. The motion characteristics of the bubble and important parameters’effects are analyzed. And the numerical results of the bubble loads are verified.Secondly, the effect of rigid-body motions on the whipping response of a ship hull subjected to a spherical underwater bubble is studied. The dynamic elastic response of a floating ship hull girder to an underwater bubble is normally composed of two parts: rigid-body motion and elastic deformation. However, the effects of rigid body motion have consistently been neglected in the current literature based on the assumption that they are small. Next, our focus is on the study of rigid-body motion effects on the hull girder’s elastic deformation, also known as the’whipping response’. A theory of interaction between a gas bubble and a hull girder is presented. The model of a ship hull girder floating on water subjected to the underwater bubble load is established. The rigid-body and elastic responses of the hull that are induced by the impulsive pressure of a bubble are calculated using the methods presented herein. Two different examples of real ships are given to demonstrate the effect of rigid-body motion on whipping responses. The hydroelastic response characteristics of the hull girder are analysed. And the resonance mechanism in hull girder’s whipping response to an underwater bubble is discussed in detail. The time histories of the bending moments and displacements of the two different hull girders are presented. The numerical results show that rigid-body motions reduce the amplitudes and vibration natural periods of the bending moments of the hull girder. These effects can be ignored for slender hulls, but must be taken into account for shorter/wider hulls so as not to underestimate the longitudinal strength.Then, the dynamic hydro-elastic-plastic response of a floating ship hull girder subjected to a spherical underwater bubble is studied. We divide the hull girder’s response into three phases:The first phase, the hull girder experiences elastic deformation and rigid-body motion. Up to the point when the bending moment somewhere in the hull girder reaches the critical bending moment, a plastic’hinge’forms at the failure point. The second phase, plastic deformation accumulates until the point when the plastic deformation rate becomes zero. And the third phase, the bending moment is smaller than the limiting moment, the hull girder is plastically deformed experiences elastic deformation and rigid-body motion as an elastic hull girder. We analyse each phase of the hull girder’s response separately and then obtain the whole motions of the hull girder. The theories and numerical methods of the responses in the three phases are derived and presented. The formation of the plastic hinge when the hull girder’s longitudinal bending moment exceeds its ultimate bending moment is investigated. Real-scale ship examples are given to discuss the features of the dynamic elastic and plastic response. The numerical results show that the bending moment’s peak value is not directly caused by the bubble pressure. It is induced by the hull girder’s whipping motion. And the plastic deformation of the hull girder features a sharp rise and a short duration.Furthermore, based on the doubly asymptotic approximation (DAA) theory, using the finite element method with the boundary element method, the transient dynamic responses of a3-D surface ship structure subjected to a spherical underwater bubble are studied. We develop a procedure which couples the finite element method with boundry element method to study this problem. The theories and numerical methods of the structure response equation, the fluid surface equation and fluid-structure interaction are derived and presented. The global and local responses of the ship model in vertical, transverse and longitudinal directions are performed. The acceleration, velocity and displacement time histories are presented. The mechanism and characteristics of both the global and local responses of the ship hull structure are discussed in detail.At last, based on the theories and numerical methods described in this study, we develop the software of "The calculation of dynamic responses of the ship structure subjected to the bubble load". The software is introduced in detail from the functional modules, embedded procedures and the operation flow.