Study On Dynamics Of Underwater Explosion Bubbles Near Structures

Underwater bubble dynamics is a widely concerned issue in many fields such asshipbuilding, marine science and medical science. Although a lot of research work has beendone, our knowledge on the various characteristics of bubbles is still insufficient due to theircomplexity and there are many unknown mechanics to be revealed and studied, especially inthe field of shipbuilding and ocean engineering. The damage to underwater structures, whichis caused by underwater explosion bubble load and usually severe, is in the first place to bestudied. Therefore, starting from basic phenomenon of bubble dynamics, a boundary integralequation considering fluid field compressibility is proposed based on wave equations, whichis able to simulate energy dissipation that cannot be captured with incompressible potentialflow theory. Thereafter the dynamics of bubbles under different boundary conditions isstudied both numerically and experimentally. Finally coupling effects between bubble andunderwater structures are studied by analyzing the dynamic response of a cylinder shell underthe load of near-field bubble jet with the DAA method.In the introduction, firstly the researches and developments of bubble experiments arereviewed with a discussion of the features of bubbles generated by different sources, theirmerits and drawbacks. Secondly, the spherical bubble theory based on analytical method andthe Boundary Element Method based on numerical method are summarized, including theirlatest progress. Subsequently, the progress in other methods of bubble dynamics issummarized and some unsolved problems are raised as a guide for the work in the future.Assume that the fluid movement is irrotational and the viscosity ignorable, incompressible flow, the mass conversation equation and the momentum conversation equationare simplified into the Laplace equation and the compressible Bernoulli equation, separately,and in incompressible flow they are simplified into the linear wave equation and theincompressible Bernoulli equation. These are the theoretical basis for subsequent chapters.Simplifications are made in both time and frequency domain for the construction of theboundary integral equation considering flow compressibility. Thus the complexity andinstability of directly solving the retarded potential equation is avoided and the energydissipation in bubble oscillation is captured, which cannot be simulated in incompressibleflow. Validity of this numerical method is proved by comparing the numerical resultsconsidering compressibility with the theoretical solution. When the sound speed c approachesinfinity, the numerical model of bubble dynamics in incompressible flow reduces into theboundary integral equation in incompressible flow. Besides the compressibility of the surrounding fluid, the transmission of heat and mass between bubble and fluid, the chemicalreaction and bubble breaking can also lead to energy loss during bubble pulsation. The finiteelement method basing on non structural mesh and boundary element method are combined tosimulate the temperature distribution inside bubble. If we do not discuss the reason of energyloss, a model is carried out by considering the energy loss accurs when bubble reaches itsminimum volume and numerical simulation is performed.Based on the model of bubble dynamics in compressible flow, the energy dissipation inthe oscillation of spherical bubble is calculated and the motion of non-spherical bubbles infree flow field is discussed. The jet speed turns out to be lower when compressibility is takeninto account, and the influence of the compressibility on the bubble shape is more significantif the jet occurs after the moment when the bubble reaches its minimum volume, and lesssignificant otherwise. Besides, the numerical model can be easily extended and applied to thedynamics of vertex ring bubble. Dynamic characteristics are simulated under differentboundary conditions and dimensionless parameters, and conclusions are drawn. But in thestudy of underwater explosion bubble dynamics, bubble pulsation in the first period is mainlyconsidered. In this process, the compressibility of the surrounding fluid does little effects onbubble pulsating period and maximum radius. Although bubble jet velocity reduces with theconsideration of fluid compressibility and the radiation of bubble pressure delays in time, itcan still be ignored in engineering. If bubble pulsation in acoustic field is considered, it isnecessary to take the effectness of fluid compressibility into account. Therefore the potentialflow theory in incompressible fluid is adopted in the following chapters to analyze bubbledynamics.The Gauss-Lobatto-Legendre (GLL) integration is applied to solve the boundary integralequation and the accuracy of the result is improved. This method is applied in the simulationof the bubble motion near different boundaries. By comparing the numerical results and theexperimental results, this method is validated and the scope of the application of the GLLintegration is therefore expanded.For the cases where underwater explosion happens near seabed or at oil-water interfaceand based on previous researches, a dynamic model of the bubble near different boundaries isestablished with incompressible potential flow theory, with the effect of buoyancy and surfacetension taken into account. The seabed or oil-water interface is treated as an interface of twoelastic fluids with different densities, with the dimensionless elastic factor κ and elasticmoduleE memput forward to describe the elasticity of the fluids and the interface,respectively. Computer codes were written and the bubble motions near different boundaries are systematically studied. The numerical model is validated by comparing its results to thatof experiments. The reason why mushroom-shape bubble is formed is discussed and bubblemotion after split is simulated as well. The above numerical model can be applied under manyconditions and the free field, free surface and rigid boundary are only some of them.Experiments are conducted subsequently. Bubbles are generated by200V DC EDMcircuit and the bubble motion features under different boundary conditions are captured with ahigh-speed camera. Specially studied are the motion features near free surface, rigid boundary,shipboard, seabed and that close to both shipboard and free surface. The coupling and fusioncharacteristics are discussed with some criterion proposed.The fusion of bubbles may occur when bubbles are generated at the same time and arevery close, or a cluster of bubbles interacts with each other. With the3D boundary elementmethod, the fusion of two underwater explosion bubbles into one is studied with the thicknessof the fluid between the two bubbles defined as the fusion criterion. The numerical resultscoincide well with the experiment results from Chahine. In addition, the influences of thedimensionless distance and depth of the charge on bubble fusion are also discussed.The oscillation load of underwater explosion will cause damage to structures nearby, andwhen the charge is ignited closely to underwater structures, the high-speed jet also causessevere local damage. First estimate the range within which the explosion bubble will generatea direct jet towards a submarine under typical amount and depth of the charge, then calculatethe jet impact with the water column impact model based on the DAA method. As for warship,the free surface effect is dealt with a image method, which is aimed at providing morereferences for the research of bubble dynamics.