Experimental Study On Underwater Explosion Bubble Loads And Damage On The Structure Nearby

In recent years, the shock wave and the bubble of underwater explosions and theirdamage effects to ship structures have become the forefront and heated issues in theresearches internationally. Fruitful results have been achieved on the far-field underwaterexplosions, shock wave propagations, bubble dynamics and structure responses both at homeand aboard. The structural load is more complicated in the near field underwater explosionswhere impacts are caused by not only shock pressure but also bubble pulsations, jets andcavitation on the structures, and hence can be much severer. As a result of the complexboundary conditions, the specificity of the loads and the strong non-linearity of the structuralresponses, the basic formulations and assumptions for far-field explosions become invalid innear-field cases, hence immense difficulties arise in the researches. So far, the loadcharacteristics and the mechanism of structural damages in near-field underwater explosionsare still far from being fully revealed.As the best research method for underwater explosions, the full-scale test, despite itshigh cost, has been carried out in small amounts in China and some developed countries.However, there is much constraint to the full-scale test for the near-field explosion in bothtechnical and financial aspects due to its great destructive power, and it has been rarelyconducted domestically. Therefore, model tests and principle experiments become favorableand particularly important for the studies on the loads and structural damages in near-fieldunderwater explosions. In this paper, the loads and structural damages induced by theunderwater explosion bubbles are studied with a combined method of principle experimentsand model tests, with respect to the engineering in ship safety.A classification of underwater explosions and analysis on the characteristics of thenear-field cases were presented first, with which the scope and content of this study wasidentified. The research progress on shock wave and bubble load as well as ship structureresponses was then reviewed in three aspects, namely the experiments, analytical analysis andnumerical simulations. The major defect, i.e. the neglecting of buoyancy effects, in existingmedium and low voltage spark-discharge bubble experiments, as well as the deficiencies inthe researches on the damage effects of near-field bubble jets was analyzed. The necessity ofexperimental studies on bubble loads and structural damages in near-field underwaterexplosions were identified. All of these laid the foundation of this study.Based on a review of classical theories and algorithms of bubble dynamics, a3-D bubblesimulation method is proposed. Since the buoyancy effect, which atmospheric experiments fail to capture, is dominating the contraction and upward migration of the bubble in actualunderwater explosions, the spark-discharge bubble experiments in this study are conducted insub-atmospheric pressure; in this way, the strong buoyancy effect in full scale tests can bereproduced under laboratory conditions by adjusting the air pressure above the water.With the pressure reduction and a high-speed photographing system, principleexperiments were first conducted for bubble pulsations in free fluid field with differentpressures. Physical quantities representing bubble behaviors, including bubble radius,pulsation period and migrations of bubble center, top and bottom, etc., were obtained.Variations of the period, the maximum radius and jet speed with the fluid pressure werestudied. The measurements and analysis were carried out not only within the first pulsationperiod of the bubble, as in most of the previous works, but also in all visible periods thereafter.On this basis, the interactions of two in-phase bubbles were investigated, where one of thebubbles is seen as a special boundary for the other. The coalescence, contraction, counter jetsand reverse jets of the bubble and the variations of these behaviors with the distanceparameter and the buoyancy parameter were examined. The principles were obtained for thecoalescence of two bubbles located horizontally or vertically, and could be utilized in thedesign of launch schemes for multiple missiles. Besides, the "water curtain"(or the "waterspike") and the bubble jet, both strongly affected by buoyancy, were categorized into differenttypes. Their formation mechanisms were analyzed; the combined effects of both the buoyancyparameter and the distance parameter on the form and height of the water spike, the jet speed,the migration of bubble center and the pulsation period were studied. The results werecompared with the "Blake principle" and could be of use in the development of anti-missilewater curtains.Further, experiments were conducted on bubble behaviors near complex boundary underbuoyancy effect with reduced pressure. The dynamics of the bubble close to a rigid wall wasstudied with the direction of the Bjerknes force being the same, the opposite andperpendicular to that of the buoyancy. Through analysis of the pulsation period and themigration of the bubble and the direction, angle and speed of the jet, a "bubble splitting zone"over a horizontal wall, together with a relationship between the angle of the jet and the featureparameters, were identified. In addition, bubble dynamics were discussed near a rigid cylinder,an elastic boundary and combined boundaries of rigid wall and free surface, which could bereferred to in underwater weapon designs and utilizations.With the pressure reduction, different levels of buoyancy effect were obtained and thusfull scale underwater explosion bubbles and their interactions with different boundaries were simulated. The study on the complex bubble behaviors including expansion, contraction, jetand splitting in free field, near free surface or near a rigid wall was combined with numericalsimulations with the3-D dynamic bubble model constructed in this paper. The numericalresults were validated by the reduced-pressure experiments and contribute to the3Dnumerical methods for underwater explosions. The pulsations after the first period and thebehaviors of the bubble generated very close to the boundary (distance parameter less than0.5)were unable to be simulated numerically but can be easily observed in the experiments; thislaid the foundation for the following analysis on the model tests utilizing the reduced-pressureexperiments.A full scale cabin model was tested in near-field underwater explosion experiments tostudy the plastic dynamic response of local ship structures under explosion loads withdifferent impact factors; the structural damage due to the bubble jet and the area affected bythe jet were obtained. The results of the model test were combined with that of the principleexperiments in the analysis of pressure, strain, displacement, etc. of the structure; on this basis,the pattern of the structural damage induced by the bubble load preceded by the impact of aninitial shock wave was obtained. The results could offer references for researches on thedamage mechanism of ship structures under near-field underwater explosion loads.