A Study On The Effect Of Complex Boundaries On Dynamic Behaviors Of Underwater Explosion Bubbles

In near-field underwater explosions aimed at surface vessels and submarines,the load caused by the underwater explosion bubble is one of the major cause of damage.Usually,the behaviors of underwater explosion bubbles are greatly affected by the boundary conditions of the fluid field in which it is generated,and the dynamic loads a bubble causes largely depends on its behavior.In order to analyze the potential damage of underwater explosions to warships,it is necessary to identify the behaviors of underwater explosion bubbles and associated bubble loads,especially with complex boundary conditions.In this paper,a systematic experimental study was carried out focusing on the various features of bubble behaviors and bubble loads corresponding to different boundary conditions.Behaviors of the bubble were captured using high-speed photography,bubble loads were measured using pressure gauges and responses of underwater structures were recorded with strain gauges.Three bubble-generating methods were applied in the current experiments,including low-voltage underwater discharges with both atmospheric pressure and reduced pressure,and small-charge underwater explosions.Bubble were generated with different buoyancy effect,based on which the bubble behaviors were divided into two categories,i.e.bubble behaviors with insignificant buoyancy effect and with significant buoyance effect.Except for different buoyance,experiments were carried out where bubbles interacts with a variety of simple and complex boundary conditions,namely free field,simple boundary(free surface or rigid plane boundary),combined simple boundaries,rigid curved boundary(i.e.ship bilge),incomplete boundaries(i.e.curved/plane rigid boundaries with circular openings),elastic-plastic boundaries(i.e.steel panel)and the special boundary condition of another bubble oscillating synchronically.First,the effect of buoyancy on bubble behaviors were studied since buoyancy is always present in all cases.Experiments were carried out in a pressure tank in which the air pressure above water was controlled to obtain different buoyancy parameters.The radius,migration,oscillating period,jet velocity,etc.,were recorded and analyzed against the buoyancy parameter.Subsequently,bubble behaviors and its loads on nearby boundaries were studied with bubble oscillating near a simple boundary with buoyancy.Three scenarios were carried out with buoyancy and Bjerknes effect being opposite to each other,in the same direction and perpendicular to each other.Based on a number of experiments,the criterion for the alternation of the direction of bubble collapse and jetting is presented in terms of non-dimensional boundary distance and buoyancy parameter.Bubble behaviors with simple boundary and insignificant buoyancy were also studied,discovering new features such as the "gas torus" and the duo jetting behavior,and giving a systematic study to the morphology of water spikes.Subsequently,complex boundary conditions were implemented to observe the features of bubble behaviors and bubble loads.Three types of bubble morphology were given for bubble oscillating near the curved rigid surface of ship bilge.Further,bubble motion features near a bilge with a circular opening were studied with varying opening diameters and stand-off distances.Five types of bubble morphology and associated bubble loads were given or inferred.Motion features such as jetting away from rigid boundary was discovered and consequent bubble loads were analyzed.In the experiments with combined simple boundaries,bubble behaviors were captured and analyzed with bubble oscillating near a vertical wall and free surface,between a horizontal wall and free surface,and between two horizontal walls.For the second case,the distance between the bubble and the wall/free surface were adjusted to study the change of bubble shapes,periods and free surface spike motions;for the third case,special bubble behaviors such as annular jetting and splitting were studied as well as the bubble loads caused.In the experiments with elastic-plastic boundaries,a steel panel in the form of double bottom were used to interact with underwater explosion bubbles.Pressure gauges were used for measuring bubble loads in fluid field and strain gauges were deployed in the corner,at the edge and at the center of the girds of the panel.Comparisons were made across the strains at different places on the same grillage,as well as at the same place on grillages subjected to different underwater explosion conditions.Results showed that the strains of the grillages caused by bubble loads are sometimes comparable or even larger than that caused by underwater shockwaves.For a bubble interacting with other bubbles,the latter can be regarded as a special kind of boundary condition to the former.In this paper,the dynamics of 2-4 evenly spaced,synchronically oscillating bubbles were studied.Cases with both insignificant and significant buoyancy effects were included.Motion features of bubbles oscillating independently and bubble coalescence were analyzed and compared to the motion features of bubbles oscillating near rigid plane walls.New features such as splitting of the middle bubble in a 3-bubble system and the counter-directed jets after coalescence were found.