| Underwater explosion has a wide range of applications in both military and civil fields,such as the impact damage on the warships subject to underwater weapons,blasting removal of underwater buildings and ice breaking for the polar channel.It consists the shock wave phase and bubble phase,and there are great scale ratios in both time and space.It is difficult to balance the accuracy and efficiency to analyze with the traditional methods.Thus,the shock wave and bubble motion phases are usually analyzed separately with two different methodologies often compromised with reduced precision.Against this background,a complete set of analysis methods and numerical models is developed to analyze the whole process of underwater explosion under one framework,which provides technical support to fields related explosion loads.Firstly,it is crucial to choose the suitable governing equation corresponding to the characteristics of underwater explosion.Based on Eulerian finite-element method and Volume of Fluid Method(VOF)method,governing equation of the multiphase flow in underwater explosion is numerically discretized and solved.Significant improvements are made to the numerical model.These include:a new non-reflecting boundary condition is proposed applicable to both the shock wave and bubble motion phases;and the Monotonic Upstream Scheme for Conservation Laws(MUSCL)scheme is modified in axsymmetric problem to maintain the conservativity.The work in this part provides the basic method for the residual studies.An axisymmetric underwater explosion numerical model based on the above method is established and the results are compared with the results from an underwater discharge bubble with great buoyancy to validate the present model.The results show good agreement such that the present method can be used in the following study.Secondly,the multi-cyle pulsation motion and the energy dissipation are analyzed based on the free-field underwater explosion model.Comparison between the shock pressure results of the present continuous simulation and the experiment shows that the present method is able to simulate the shock wave and bubble motion accurately at the same time.The asymmetry of the near-field pressure load is observed which is significantly affected by the buoyancy.Additional comparison is made to validate the multi-cycle pulsation simulation.Results from both the simulation and experiment show that bubble fragmentation also contributes to the energy dissipation.It is also shown that the initial discontinuity is significant to the energy dissipation.Based on the linearized theory of the energy conservation of the bubble system,a new non-dimensional parameter Ma is modified from the Mach number to represent the energy dissipation due to wave effects.It is found that the dissipated energy is related linearly to Ma,which can be used to predict the energy dissipation of a general case.As for explosion detonated in the shallow water with the nonlinear interaction,it is more complex when the bubble breaks and re-closes at the free surface.An axisymmetric bubble dynamics model based on the Eulerian finite element method with the interface tracked by the volume of fluid(VOF)method is established to numerically investigate the breaking and reclosure of a bubble near a free surface.An experimental validation of the numerical model reveals its good accuracy.The motion,breaking,and re-closure of small and large bubbles at a free surface are simulated.Complex phenomena are observed in the simulation after the break of the bubble.Under the effects of inward gas flow and the pressure difference between its inside and outside,a broken bubble finally recloses.The convergence and impact of a spike wall generate an upward water spray higher than that from an intact bubble.Furthermore,the critical depth at which the bubble period reaches its peak has a negative relationship with the weight of the explosive charge used to generate the bubble.As for the underwater explosion at the bottom of the sediment seabed,it is simplified as the movement of the underwater explosion bubbles at the interface of two kinds of dense fluid,which can be analyzed by an axisymmetric numerical model established based on the axisymmetric Eulerian finite-element method.It is found that the interface overflow,whirling and the ring-shaped jet impact will occur during the expanding phase of the bubble,and the density interface tends to induce the bubble to develop a jet that drills into the heavier liquid during the bubble collapse phase.The non-dimensional period of oscillation increases with increasing density ratio of the two liquids,and an annular jet impacts,generating a pressure peak,when the density ratio exceeds 1.5.When the effect of gravity is considered,it is found that the annular impact is enhanced,while the downward jet is weakened.In addition,a significantly sized bubble is split from the main bubble by the impact of the annular jet.When the buoyancy parameter exceeds a threshold value,gravity comes to dominate the bubble motion,and neither an annular jet nor a downward jet will develop.If the buoyancy parameter is large enough,the bubble migrates upward and detaches from the liquid-liquid interface during the collapse phaseAs for the jet development and impact load of underwater explosion bubble on a horizontal solid wall,a numerical model is established in a cylindrical coordinate system and validated by comparing the results with a spark-generated bubble experiment.The influences of the wall location(upside or downside)and the stand-off distance from the wall are analyzed.The results show that the features of the jet impact load are much more complicated than those of the shock wave.Nearby a downside wall,the buoyancy and Bjerknes force compete to dominate the bubble motion with opposite influences.By contrast,they enhance the effect of each other to develop a liquid jet towards the upside wall.The pressure peak,impact range,and duration time nonlinearly depend on the combination of the case parameters and are not monotonic to one.Within a proper range of the parameter combination,the jet impact load can reach its maximum and be more destructive than the shock wave because of a comparable pressure peak and a much longer duration.Finally,for load characteristics of underwater explosion bubble nearby a vertical solid wall,which no longer satisfies the axisymmetric condition,a three-dimensional numerical model for bubble dynamics simulation is established.Considering the significant computational cost,the Block-based adaptive mesh refinement technology is introduced to both maintain the precision of the concern elements and to reduce the number of degrees of freedom.Meanwhile,dynamic load balancing technology is also introduced and an efficient parallel numerical model with high precision corresponding to the integrated complex multi-scale process of underwater explosion is established.The present model is validated by comparing the results using the conventional boundary element method,and a fast convergence is observed with the increase of the maximum refinement level.Further,several cases of the bubble collapse nearby a solid wall are numerically simulated,and the toroidal bubble motion and their pressure load on the solid wall are analyzed.Complex physics of the collapsing bubble is examined,such as the formation of the crescent-shape bubble,the air cushion effect during the jet penetration and the relationship between the jet impact pressure and the angle between the jet velocity and the opposite bubble surface.In summary,a multi-scale parallel calculation method based on Euler finite-element method is established in this thesis for the numerical simulation of underwater explosion consisting shock wave propagation and bubble motion.Based on the method and the corresponding models,the load characteristics of underwater explosion in different environments,such as in free field,nearby free surface,walls and liquid-liquid interface is analyzed,aimed at providing technical support for the engineering application related to underwater explosion loads. |
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