Study On The Motion And Loads Characteristics Of Underwater Viscous Bubbles (Cavitations)

Kinetics and dynamics of bubbles and cavitations in fluids is of great significance in various fields, like fluid mechanics, environment engineering, medicine and ocean engineering. Take naval and ocean engineering for example, propeller cavitations, underwater explosion bubble and micro-bubbles in drag reduction technology are several famous phenomena and applications. The interaction between bubble (cavitation) and structure as well as the loads characteristics of bubble (cavitation) on the structure is also one of hot topics in engineering. Along with more applications, the researches on the bubble and cavitation dynamics are getting deeper, meanwhile, more challenges and problems appear, such as the strong non-linear bubble dynamic behaviors and jet loads in contact explosion, and the slide and detachment of micro-bubbles attached on the moving body.Take potential applications of bubbles and cavitations in naval and ocean engineering as background, this thesis summarizes the key mechanic problems, and study the bubble and cavitation dynamics with viscous effects both using experimental and numerical simulations. Different environmental flow field and various boundaries are studied to explore the bubble nonlinear dynamic behaviors and the loads characteristics.To start with, the developments on bubble and cavitation dynamics are reviewed from theoretical analytics, experimental technologies and numerical simulations. In theoretical analytics, the motion equation and collapse time are given for the spherical and circular bubble with viscous effects. In experimental technologies, both high-seed video recording and load measurement technologies are summed up. In numerical simulations, bubble dynamics near various boundaries and in different background pressure field is summarized, In detailed, the bubble near a rigid wall, attached on a moving body and near a free surface and in a gravitational field, a tip vortex field and a shockwave field are reviewed respectively. Further more, the effects of compressibility, surface tension and viscosity are also concluded. Based on the review, the weaknesses in the recent researches are presented.Based on incompressible Navier-Stokes equation, the bubble dynamic equations are modified considering viscous effect by using the boundary layer theory. On one hand, the normal additional stress is considered on the bubble surface; on the other hand, viscous modified pressured is introduced to replace the tangential additional stress, based on viscous dissipation energy equivalence principle. Both normal stress and tangential stress are kept continuous, and the weak viscous effect in the boundary layer is included. As a result, axisymmetric and3D boundary element models of the bubble with viscous effect have been erected. The numerical results are compared with spherical RP equation solution and analytical results, good agreements have been achieved and the models in this thesis are validated. On this basis, the results with and without viscous effect are contrasted. The influence of other factors like surface tension and gravity are also studied.For the cavitation in a tip vortex flow field after a moving vessel, the whole simulation can be divided into two stages:quasi-spherical and non-spherical stages, based on whether the cavitation captured by the vortex core or not. For quasi-spherical stage, RP equation and the momentum theorem are combined to solve the oscillation and migration of the cavitation; for non-spherical stage, the boundary element method (BEM) can be adopted to simulate the collapse and breakup of the cavitation. The output of first stage is taken as the input of the second stage, and the whole process can be simulated. The numerical results are compared with previous experimental data and good agreement has been obtained. On this basis, the effects of various factors like viscosity, lift force and slip velocity are considered. Further more, cavitaion cluster with given number and arrangement is studied.For the bubble breakup in a narrow flow field, on one hand, by using the spark generated bubble equipment and high speed camera system, the experiments are done to observe and record the oscillation and breakup of the bubble; on the other hand,3D bubble breakup model and rules are erected based on the previous axisymmetric breakup model, and the dynamic behaviors of the bubble are simulated. Numerical results and experimental data are compared and they validate each other well. The symmetric and asymmetric breakup of the bubble near two rigid walls and the circular breakup of the bubble in a narrow tube are studied respectively. The physical quantities like oscillation period and sub-bubble jet are investigated, and the influences of dimensionless parameters like distance and length parameters are also researched.For the bubble very close to a rigid wall, an improved algorithm with boundary-integral equation has been developed which predicts the whole bubble evolution. In numerical simulation,’water layer removal’technique is introduced, to avoid the distortion of bubble grids super close to the wall when λ≤0.5. When the jet impacting the rigid wall, another technique named’contact jet cutting’is adopted to obtain the impact pressure on the structure. On the other hand, the experiment is performed to record the deformation of the bubble near a super close wall with high-speed photography. The numerical results are compared with experimental data, and the good agreement achieved validates the feasibility of the improved model and the computation code. Besides, the numerical results of the improved algorithm are also compared with those of image method, whose agreements are also quite favorable. Some case studies are undertaken in which the physical quantities like jet and period are simulated. The effects of dimensionless distance between the bubble and the wall on the bubble dynamics are also investigated. Further more, reduced scaled experiments of the ship and box-beam model subjected the contact sparked bubble are done, and the dynamic responses of the complex elastic-plastic structures on the contact explosion loads are recorded and analyzed.For the deformable and sliding bubble attached on a moving body, the points of intersection between the bubble and body are treated specially in the numerical procedure, which guarantees the intersection point satisfy the bubble and rigid wall boundary conditions simultaneously. The indirect BEM is used to calculate the velocity in the flow field and the auxiliary function method is adopted to calculate the pressure on the body surface. The convergence study has been undertaken to assess the developed numerical method and the computation code. Some case studies are undertaken in which the interactions between the bubble, body and the incoming flow field are simulated. The effects of various physical parameters on the interactions are also investigated.For the bubble in an impinging shockwave, the modified BEM can be adopted to simulate its dynamic response, with the assumption that the early response is mainly inertia controlled and the compressibility of fluid surrounding the bubble are neglected. The contribution of the shockwave pressure is included in the Bernoulli equation. The interaction between the bubble and regular shockwave, typical impinging shockwave or reflected shockwave is simulated respectively. The calculated results of the three-dimensional model are compared with previous experimental data and those of other numerical methods. It is found that they coincide well, which shows that the algorithm and3D model of this thesis are effective and feasible. Through changing the parameters of shockwave and initial bubble state, corresponding dynamic characteristics of the bubble are studied, especially for the reflected shockwave in a near-ship field explosion.