Application Of The Improved Bubble Dynamics Model In The Shock Resistance Research Of Ship

Bubble dynamics is a classical topic of fluid dynamics.Because of the diversity of characteristics presented in different circumstances,it has many applications in multiple fields.Researchers paid great attention to it.As an import branch of the bubble dynamics,the underwater explosion bubble dynamics can provide references for the anti-shock researches of warships and the destruction mechanism of underwater weapons which is intensely studied in this thesis.Beginning with the application background and the basic phenomenon,several key issues of bubble dynamics are firstly discussed.Then the domestic and overseas researching progresses are summarized from the theoretical,experimental and numerical aspects.In spite of the long-term and in-depth researches promoted in bubble dynamics,there are many important questions haven't been answered satisfied for both the researching methods and the bubble dynamics itself.Therefore,aiming at several typical problems among them,the traditional bubble dynamics model based on boundary element method is improved both theoretically and numerically.And then study the bubble dynamics under special circumstances and the interaction characteristics with ships which are unsubstantial in the former researches.Under the assumption that the fluid around the bubble is incompressible and ideal,the basic equations of boundary element method are deduced to establish the bubble dynamics numerical model in different environments.To enhance the accuracy and the stability of traditional toroidal model,a new method to calculate the vortex ring induced potential is come up with together with some other improvements such as the new numerical surgical cut,vortex ring update and so on.Inspired by the Elastic Mesh Technology,the density potential is introduced to represent the local nodes density,according which the new method named Density Potential Method is established to resolve the discordant between the bubble motion and the mesh density change.Then,incorporated with the topology optimization procedure based on edge swapping,the new three-dimensional surface mesh optimization method is come up with.By comparison with other methods,it shows good validity and robustness.For the opened-boundary problems in boundary element method,the singularity of doubled-potential integration is solved with the equivalent theory between solid angle and vortex ring induced potential.Besides,the boundary integral equation is revised to consider the influence of undiscerned boundary in far field,which shows an obvious improvement in accuracy.Then,the'double-node' technique is adopted to treat the over constrain caused by the confluence of the different types of boundary conditions,such as the intersection between the free surface and the body surface.Under the potential flow assumption,the motion equations of coupling between the fluid and both the rigid and elastic body,which establishes the FSI numerical model considering the nonlinear free surface.The bubble dynamics in both free field and the vicinity of simple boundaries followed with great interest since it is the foundation of the study on the complex situations.With the improved numerical model,the bubble dynamics in free field and both vicinity to free surface and rigid boundary is simulated.The pulsation,jet penetration and rebound characteristics are compared with the experimental results for the validation of accuracy and stability.Especially the results of 3d toroidal bubble model match the results of axisymmetric model for the first time in the deformation details,which shows the new model develops a distinct improvement in accuracy and stability.The non-axisymmetric problem of bubble dynamics under the conjunct effects of vertical wall and the buoyancy is analyzed.We find that the inclined jet developed during the collapsing phase will be turn itself to the perpendicular to the wall.As the extension of the simple boundaries,the bubble dynamics near both air-backed and water-backed broken boundaries is studied in this paper.For the water-backed broken boundary,the mechanism of opposite-jets and the influence of distance and broken parameters are analyzed.We find that the wall and the hole dominates the bubble dynamics for a large and small distance parameter respectively.Meanwhile,for the cases with an extreme small distance parameter,the domain detaching method is adopted to solve the fusion between bubble surface and the wall.For the air-backed broken boundary,the motion of free surface in the central hole shows salient nonlinearity which differs from the simple free surface situation because of the discontinuity at the intersection between the free surface and the wall.For some specific cases,the free surface can develop multiple spikes at different stages.And the influence of boundary on the bubble can be regarded as the conjunction of the free surface and the wall.The interesting feature of air-backed broken boundary is that the hollow at the edge of the free surface can fuse with the bubble when the distance of the bubble is close enough.It will consume most energy of the bubble,which decreases the impact of the bubble load.These researches are academically significance for the anti-shock researches for the broken warships.For the bubble generated nearby the ship side,the coupling process between the bubble and the free surface is very complex because of the sudden change of the boundary condition at the intersection of the free surface and the ship body.Two typical ship sides are analyzed in this paper.We found that different nonlinearity arises at the intersection for the flare out wall and the falling-in wall respectively,which cause an impact on the wall by the free surface at the later stage.Because of the low-frequency characteristic of the bubble load,significant rigid and longitudinal bending motions can be excited which threat the stability and the overall strength of the ship.For the rigid motion of the ship,2d numerical model is established considering both the bubble load and the impact of the waves generated by the bursting of the bubble at the free surface.The influence of the case parameters on the rigid motion of the floating structure is summarized.And the damage region of the free surface respect to the charge is come up with.For the longitudinal bending motion,the 3d FSI model takes the nonlinear free surface,initial static moment and the wave into consideration.We analyze the influence of added mass matrix changes due to various factors on the bending motion.And we find that the wave affect the whipping response nonlinearly.The ship on the wave crest is more sensitive than that on the wave trough to the change of wave amplitude,which can provide references for the anti-shock researches of warships.