Bubble Evolution And Air-water Interface Jet Induced By Under-water Directional Shock Load

The phenomenon of bubble evolution and liquid-gas interface jetting induced by underwater shock load widely occurs in the fields of marine engineering and medicine.To discover the laws and mechanisms of the phenomenon has both academic and engi-neering values.In this study,by means of high-speed photography together with theo-retical analysis and numerical simulation,we investigate the processes and dynamics of interfacial flow featured with bubbles and jets caused by directional underwater shock.Firstly,the evolution of liquid in a straight tube with a free surface subsequent to an underwater electrical explosion is investigated.The free surface is approximately flat but with a certain degree of curvature near the tube wall due to wetting effect.The study systematically reveals the shapes and developing features of explosion bubble and surface jet under varied conditions of explosion energy and liquid column scales.The electrical explosion produces a high temperature and high pressure bubble in the beginning.The bubble expands followed by a contraction,which drives the upper liq-uid column to rise and drop sequentially.With the decrease of tube width,the bubble evolution approaches one dimensional.Jet rises from the free surface in the process of bubble expansion.Two typical jet shapes are discovered.For small tube width the jet appears conical as a result of the relatively significant surface curvature,for large tube width the jet appears annular because the free surface is relatively flat,and a transitional type of jet possesses both features.The flow images obtained by high-speed camera are processed to extract data of the jet velocity and bubble size varying with the test conditions.Both the jet velocity and the maximum bubble height increase with the initial explosion energy.The jet ve-locity decreases with the tube diameter as well as the initial explosion depth,while the maximum bubble height is barely influenced by the explosion depth.The characteristic time of bubble evolution decreases with the tube diameter but increases with the initial explosion depth.A quasi-one-dimensional theoretical model was established to ana-lyze the evolution of the bubble and the jet velocity.The model describes the bubble dynamics from one dimensional flow to free spherical flow within the same theoretical frame.By comparing the maximum bubble sizes from the theoretical model and the experiment,we find that the effective energy proportion that contributes to the move-ment of surrounding liquid declines as the tube diameter decreases.By comparing the characteristic liquid column velocity obtained by the theoretical model and the experi-mental jet velocity,we find the velocity ratio(of experimental jet velocity to theoretical velocity)increases with the decrease of tube diameter,corresponding to the stronger kinematic focusing effect of in a thinner tube(larger free surface curvature).Secondly,the jet phenomenon caused by an underwater explosion acting on a liquid surface with a pit in a straight tube is investigated.Experimentally,the pit is generated by the impact of a free falling liquid drop on a free surface,and by arranging a synchro-nized explosion,the interaction of explosion with the quasi-static pit is realized.The investigation concerns the influence of explosion energy and pit size on the jet shape and velocity.It is found that the interaction induces a slim and smooth central jet from the pit bottom,and an annular satellite jet near the pipe wall.The central jet moves much faster than the satellite jet as well as the jet in the case of no surface pit with the same explosion energy.An examination on the flow details by numerical simulation indicates that central jet undergoes two acceleration processes-the first is the overall acceleration of the liquid surface,and the second is the local acceleration of the pit area which is highly related to the small high pressure spot beneath the pit.The second accel-eration also corresponds to the reverse of curvature of the pit bottom and the formation of upward cusp.Inertial force becomes dominant after the two accelerations,in which the jet velocity declines and turns stable.Thirdly,the response of an immersed bubble in liquid near a free surface(analoging cavities in material)under an impact,as well as the probably caused surface jet,is investigated.A projectile launched by a spring hitting the liquid tank from bottom provides the impact for experiment.The immersed bubble is found to collapse from the direction of impact and an inner jet develops from the lower bubble interface.The velocity of inner jet remains constant for different bubble depth.The inner jet penetrates the bubble and enters the liquid on top of the bubble.As the bubble is close enough to the free surface and the impact is strong enough,the inner jet may arrive the free surface before it is dissipated,and thereby causes a surface jet.The shape of the surface jet varies from irregular and asymmetrical to regular and symmetrical when the depth of bubble decreases.The bubble oscillates periodically after the impact.The amplitude of oscillation increases with the bubble depth,and the period of the oscillation increases with bubble size.Finally,though the numerical simulation we examined the flow details and dynamics of the surface jet.