3D Dynamic Behavior Of Underwater Explosion Bubble

Underwater explosion can imperil the vitality of the warship vary badly with complicated loads from the shock wave, the pulsating bubble generated by the detonation, the after flow induced by the bubble pulsation and the jet formed during the bubble collapse. Usually, the shock wave load causes local damage on the ship structure with high-frequency characteristics while the pulsating pressure and the after flow cause total damage with low-frequency characteristics. The pulsating pressure and after flow induced by the bubble can jeopardy the longitudinal strength of the ship and make it rupture in the mid-cross section. Meanwhile, the high-speed jet formed during the bubble collapse can damage the structure locally. Therefore, the damage from the underwater explosion bubble upon the structure can not be ignored. However, in the past, more attention has been paid to the underwater explosion shock wave instead of the complicated loads induced by the bubble motion. In the recent years, prototype explosion testing and model investigation all have indicated that the underwater explosion bubble can damage the structure very badly. Therefore, more and more research centers and researchers have focused on the interaction between the bubble and the free surface or the underwater structure. For instance, China Ship Scientific Research Center and the Institute of High Performance Computing in Singapore and other institutes are all studying the underwater explosion bubble dynamics. Although a lot of experiments and numerical simulations are carried out, the mechanics for the interaction between the bubble and boundary or structures hasn't been revealed.Therefore, the author summarized the research progress about the bubble dynamics from the theoretical analysis, experimental investigation and the numerical simulation in the thesis. It is found out that there are still a lot of issues needing to be investigated such as the rebound bubble after the jet, the influence of the characteristic parameters on the bubble motion and the control of the jet's motion direction. Besides, the particular phenomenon that the bubbles interact with each other when more than one bubbles exist hasn't been probed into well. There are rare literatures on the damage upon the structure from the after flow, the pulsating pressure and the jet, neither the literatures on the interaction between the bubble and the elastic-plastic structure with a free surface. On all these above issues, the underwater explosion bubble dynamics and its damage mechanics on the underwater are investigated by combining the boundary element method and the finite element method.The flow is assumed to be ideal. Axisymmetrical and three-dimensional bubble models are built based on the potential flow theory respectively, including the linear elements and the high-order curve elements. The boundary method is used to calculate the deformation and the location of the bubble and other boundaries. The efficiency and precision of different computing methods are compared and a highly accurate numerical method to simulate the bubble dynamics is presented. The numerical simulation for the bubble dynamics can be separated into two phases, before the jet (singly connected bubble) and after the jet (doubly connected bubble or the toroidal bubble). The methods for simulating the singly connected bubble coincide with each other approximately while there are disputations in the methods for simulating the toroidal bubble. The author established the axisymmetrical bubble with the vortex sheet and the vortex ring and established the three-dimension bubble with vortex ring respectively based on the achievements of the former researchers. The advantages and disadvantages of different models and their applicability are discussed. The three-dimensional toroidal bubble model is deduced and established and the key issue is analyzed.During the simulation, the numerical smoothing and the elastic mesh technique are introduced to avoid the numerical divergence caused by the mesh distortion. Finally, the author developed a whole visual program to simulate the dynamics of the axisymmetrical bubble and the 3-D bubble or any other bubble dynamics. The stability of the numerical method is validated by analyzing different models, element types, meshes and time steps; the numerical model and the computing method are validated by comparing the numerical results with the R-P model and the experimental data. Based on the numerical model and computing method, the bubble dynamics in gravity is investigated and the velocity of the after flow induced by the bubble motion along with time is obtained. It can be included that jet is formed in the bubble collapse phase and the direction of the after flow reverses with the expansion and collapse of the bubble. For universal use, the buoyancy parameterδ, strength parameterε, distance parameterγfand other parameters are introduced to study the bubble period, the jet and the rebound when parameters alters. From the study, it can be found out that the jet region is narrow if the charge is small but the speed of the jet can be as high as hundreds of meters per sec. so that it can cause local damage on the ship structure. However, the jet is slow when the charge is big but the jet region is very wide so that it can cause total damage on the structure. These features of the underwater explosion bubble can be used to guide the attack modes of the future weapons.Based on the characteristic parametersγf,δandε, the bubble dynamics near the free surface is studied including the generation of the spike phenomenon and the relation between the spike and the characteristic parameters. By changing the characteristic parameters, an upward or downward jet can be formed under the combined action of the Bjerkness force of the free surface and the buoyancy, which means that the motion direction of the jet is controllable and it is instructive for exploring new weapons. Furthermore, the intense repelling effect among multiple bubbles especially the out-of-phase bubbles and the natural phenomenon that the bubble will be pulled into the inside of the spike are also revealed, which is of much academic meaning for the research on the interaction between the bubble and the spike. It is found out during the investigation that the Blake criterion based on the Kelvin's impulse theory will fail under some conditions.The Blake criterion has a scope of application, and the reason for its failure is explained after further investigation.The bubble dynamics near the wall is very complex. The application scope of the Blake criterion is discussed in this thesis and the interaction between the bubble and the wall is studied. Study indicates that the Blake criterion is suitable for the bubble jet far from the wall but it fails when the jet is near the wall because of the simplification and assumption of Blake criterion. Furthermore, a bias jet is observed to be formed when the bubble is near an oblique wall. The angle and width of the jet are both related to the characteristic parametersγf,δandε. If the bubble is near a plane wall, an upright jet will be formed. When the Bjerknes force is in the same direction as the buoyancy, the jet advances towards the wall vertically; when the Bjerknes force is in the opposite direction as the buoyancy, the jet's direction should be decided by the criterion. These particular natural phenomena provide powerful support for the future research on the interaction between bubbles and boundaries.The elastic-plasticity being taken into account, the method to calculate the interaction among the bubble, the elastic-plastic structure and the free surface is presented by combing the boundary element method and the finite element method and a whole three-dimensional program is developed with the error between the numerical result and the experimental result within 10%. Through the calculation, the bubble is observed to be asymmetric; the attenuation and variation of the after flow are obtained also. With the program, the damage upon a plane plate, a cylinder and surface warship structures from the after flow, pulsating pressure and jet with a free surface or not are analyzed. Numerical results show that the after flow and pulsating pressure induced by the bubble causes total damage on the structure while the high-speed jet formed during the bubble collapse causes local damage. The investigation in this thesis is meaningful for the anti-shock and protective design of the warship and provides reference for the future research on the mechanics of the interaction between the underwater explosion bubble and the structure.