| Bubble is a common phenomenon in nature,which has significant applications in many fields,such as underwater explosion,cavitation of hydraulic machinery,untrasound lithtripsy,ultrasonic cleaning,drag reduction by microbubbles in ships,seabed resource exploration,supercavity of underwater vehicle and so on.The applications of bubble in those fields can not be separated from the target structure,so making good use of load characteristics of bubble near structure and damage mechanism to structure can improve the damage power of weapon,extend the life of hydraulic machinery,enhance the capacity of marine development and improve medical conditions.Although bubble dynamics have been studied in detail and in depth by native and foreign scholars,the research on load characteristics of bubble near structure and overall damage mechanism of structure under bubble collapse is still not enough owing to the complexity of bubble motion,where many underlying mechanisms remain to be explored.So,aiming at the existing problems,the load characteristics of bubble near structure and the overall damage mechanism of structure under bubble collapse are studied by means of numerical simulation and experimental supplement in this thesis,to provide feasible suggestions and basic technical support for the applications of bubble in various fields.The application background of bubble in various fields is described firstly in this thesis,and the damage of underwater explosion bubble to structure and the erosion of cavitation bubble to structure are highlighted.Then,the research process and the latest development of bubble dynamics in experiment,theory and numerical simulation are elaborated,and the existing shortcomings and difficulties are summarized.First,the study on pulsating bubble by discretization method in computational domain is still immature.Second,the multiple reflection characteristics of shock wave between different structural surface and bubble surface and the influence of reflected pressure wave on bubble motion are not well underatood.Third,the study on the evolution mechanism of bubble in toroidal stage and jet load near structure is insufficient.Fourth,the systematic research on the relationship between characteristic parameters of underwater explosion bubble jet and initial parameters near structure is deficient.Fifth,the damage mechanism and critical damage condition of underwater explosion bubble to hull girder in plastic stage are not clear yet.Aiming at these problems and difficulties,the research work needed in this thesis is put forward.Aiming at the limitations of current numerical methods and the characteristics of bubble at different stages,some corresponding computational models are proposed.For the initial stage of bubble,considering the compressibility of fluid,the finite volume method is used to discretize the conservative non-linear compressible Euler equation.Considering the fluid-solid coupling and cavitation effects,the ghost fluid method is used to deal with the strong discontinuous Riemann problem at the interface,and the front tracking method is used to deal with the gas-liquid-solid interface,to establish the strong discontinuous computational model for initial shock wave induced by bubble together.For bubble pulsation and jet stage,because the velocity of flow field induced by bubble is much less than the sound velocity in most time,the influence of fluid compressibility on the motion characteristics of bubble is neglected.The incompressible Navier-Stokes equation and the gas-liquid interface are discretized and tracked by the finite volume mthod and the front tracking method,respectively,to establish the computational model for bubble pulsation and jet load.One-dimensional shock tube,two-dimensional underwater explosion bubble and cavitation examples are used to verify the effectiveness of the strong discontinuous computational model for initial shock wave induced by bubble.Then,aiming at the problem that the propagation characteristics of initial shock wave induce by bubble near different structural surfaces are not clear,on the basis of this calculation model,taking into account the effects of rigidity,elasticity and plasticity of structure,The propagation,reflection,transmission and induced cavitation of shock wave initially released by bubble at different distances between bubble and rigid plate,back-water elastic plate,back-air elastic plate,double break elastic plates,shallow-water elastic plate,single curved elastic plates and double curved elastic plates are studied,and the influence of reflected pressure wave on bubble motion and dynamic response of elastic-plastic structure are also analyzed.It is found that the pressure waves reflected back and forth between structure and bubble will form a ladder-like ripple with different types of wave alternating with each other.The peak pressure and unloading velocity of shock wave acting on rigid plate and elastic plate are obviously different,and the degree of cavitation induced easily also varies significantly with distance parameter and structural curvature.It is also found that the multiple reflection of pressure wave on structural surface and bubble has little effect on the initial expansion of bubble,so it will not have much effect on the later collapse of bubble.Rayleigh-Plesset equation and spark-generated bubble experiment are used to validate the computational model for bubble pulsation and jet load.It is proved that the finite volume method combined with the front tracking method has high accuracy in simulating bubble motion.Then,in view of the insufficient research on the evolution mechanism of bubble in toroidal stage and the characteristics of jet load,based on this model,the effects of structural surface distance parameter,buoyancy parameter,free surface distance parameters and Reynolds number on bubble collapse motion and jet load near structure are studied systematically,including bubble collapse deformation,water layer thickness,jet velocity,radial jet,splashing jet,toroidal bubble split,jet formation condition,jet load,etc,where the mechanical mechanisms are also analyzed.At the same time,on the basis of previous studies,the critical condition for non-penetration of bubble jet is summarized.Aiming at the lack of systematic research on the velocity,height,width and load of underwater explosion bubble jet.The boundary element method is simple,efficient and accurate in simulating the motion of underwater explosion bubble before jet penetrates,so the jet formation process are systematically studied by using boundary element method.After summarizing the rules,three mathematical models are established based on the least square method to characterize the relationship between jet velocity,jet width and jet height and charge weight,structural surface depth and dimensionless distance parameter.Then,on the basis of water hammer theory and mathematical models of jet velocity,jet height and jet width,a mathematical model for water jet load is established,which can provide guidance for engineering to predict the underwater explosion bubble jet load near structural surface.Aiming at the problem that the damage mechanism and critical damage condition of underwater explosion bubble collapse to hull girder are not clear yet,on the one hand,the interaction of small explosion bubble with small and middle-scale hull girders is studied experimentally.At the same time,the difference of total strength characteristics of small and middle-scale hull girders at different detonation distance is studied by using coupled Euler-Lagrange algorithm(CEL),where the damage mechanism under bubble collapse is analyzed.On the other hand,based on the engineering practice,the total strength characteristics of real-scale hull girder at different charge weight and detonation distance are systematically studied.The critical condition for the total strength failure of real-scale hull girder is obtained by summarizing and condensing,which provides a reference for the study of underwater explosion bubble damage to ships in the engineering field. |
PDF Full Text Request