The Non-spherical Bubble Evolution Under Complex Boundaries And Dynamic Response Of A Floating Structure

The potential threat for a naval ship in a war can be divided into contact explosion and non-contact explosion,and a non-contact explosion includes Underwater Explosion(UNDEX)and Air Explosion(AIREX).For the same amount of explosive,UNDEX would cause much more serious damage to a ship compared with AIREX as the density of water is much heavier and its compressibility is much less than air.After the underwater explosion,a high-pressure shock wave and a gas bubble pulsation will emerge.The shock impact can be very high,its duration is very short compared with the bubble pulsations.As the difference of the damage mechanism to the structure nearby and the duration of time scale,they are usually investigated separately.The physical phenomena of an explosion detonation are complex and the response of the structure induced by explosion is determined by a series of factors,such as incident shock wave,secondary pressure pulses,gas bubble and so on.The experimental data about a ship test always involves the military secret and are difficult to obtain.The interaction between a non-spherical underwater explosion bubble with a complex and multi boundaries nearby are investigated.Literatures about underwater explosion bubble are collected and the fluid domain can be seen as incompressible,inviscid and irrotational flow according to the physical property.The three-dimensional boundary integral method is introduced to simulate the pulsation of an underwater explosion bubble based on potential flow theory.The singularity in this method is handled through analytical and numerical calculation,some numerical techniques such as elastic mesh method and element swap technique are implemented to address the associated numerical challenge and improve the distortion mesh.The vortex ring model is introduced to perform the transformation from a singly connected bubble to a toroidal bubble.Strong nonlinear interaction between multi-oscillation underwater explosion bubble and free surface is simulated.A direct approach is adopted to calculated solid angle on non-closed free surface.Various complex patterns of the free surface spike,the bubble centroid for different standoff distances,the buoyancy parameters and the strength parameters are obtained to reveal the nonlinear interaction between the bubble and the free surface."Plateau"shape,"plateau" with a hump and "crown" or "plateau" with water column shape are especially concerned during the rebounding non-spherical bubble phase.The amplitude of the second maximum bubble volume,the four typical patterns of the bubble jet and the free surface spike are examined in the context of the standoff distance.The large buoyancy is used to elevate the spray dome rather than the free surface spike.The interaction of an explosion bubble,a submerged or floating structure,and free surface waves are concerned.A three-dimensional fully nonlinear model has been developed based on the coupling of the Boundary Integral Method(BIM)for bubble dynamics and free surface waves and the finite element method for structure deformation.For a free-floating structure,the free surface causes not only a larger reduction in peak pressure for a rigid structure compared with a fixed body but also the modification of the bubble period and structural response.Three structure characteristics,including fixed,rigidly moving and flexible,are investigated separately,in the absence of a free surface,to determine their influence on bubble dynamics.Both the rigid motion and the deformation at the local structure appear in the simulation.Structural flexibility has an obvious influence on the reduction of pressure compared with the fixed or movable bodies.The interaction between a bubble with a surface ship based on the coupling of the boundary integral method,finite element method and DAA is investigated.Transient response of a surface ship model subjected to an explosion bubble is mainly studied.A hogging phenomenon can be seen at the early expansion stage,rigid-body motion and elastic deformation are investigated during the secondary bubble pressure period.Response mechanism of a three dimensional ship model subjected to an explosion bubble is also investigated,and the global and local responses of the ship model are performed.