The Migration of Underwater Explosion Bubbles Near Different Boundaries
Date Issued
2011
Date
2011
Author(s)
Hwangfu, Jing-Jia
Abstract
Damage of structures by impacts of nearby collapsing bubbles is an important problem in the military, engineering, and medicine. The migration direction of bubbles is closely related to damage. When a bubble migrates toward a boundary, serious damage can be induced. To neutralize and even utilize this damage, it is essential to understand the mechanism of bubble migration and to be able to predict migration direction. However, related studies are rare, which motivates the interesting of the author.
Research in cavitation has produced some results on the subject of bubble migration. Since these bubbles seem similar macroscopically, the author has assumed that these results can be applied to underwater explosion bubbles. To verify this assumption, experiments involving underwater explosion bubbles, produced by small charges, near four side-clamped air-backed metal plates and the free surface, were conducted. The behavior of spark/laser induced bubbles, which have been used in cavitation studies, were compared with underwater explosion bubbles. The author found that the behaviors of these two bubbles are similar. Experimental data were substituted into equations of the Kelvin impulse and charts of boundary inertia m* and boundary stiffness k*, which were developed to predict the migration of spark/laser bubbles. It was found that none of them are able to comprehensively predict the migration when boundary elasticity is considered. However, the Kelvin Impulse can predict the migration direction when the bubble is near the free surface. On the other hand, the author found that the phase between the boundary response and the bubble pulsation could help develop a predictive index. To obtain more information, a detailed investigation was undertaken using simulation methods.
The finite element code LSDYNA was adopted for the simulation. The author first simulates the scenario of the experiment. Consistent results were achieved by adjusting meshes and tuning command parameters. After the numerical model was verified, detailed exploration of different boundary thicknesses, materials, and stand-off distances was carried out. By observing the bubble behavior and boundary responses in simulation, the mechanism of bubble migration was analyzed. The author found that the ratio of the boundary vibration period to the bubble period, r, and the normalized bubble migration index, BRMIN/L, are linearly related: BRMIN/L=1.55r. From r and through this relation, we can estimate the migration index and thus find the migration direction of the bubble. The author has also found that the primary shock has no effect on the bubble motions. This indicates that the bubble migration relation mentioned above, can be applied to spark/laser and cavitation bubbles. These results will help to neutralize, or even utilize bubble damage in future studies.
Subjects
underwater explosion
cavitation bubble
bubble dynamics
bubble migration
bubble jetting
Kelvin impulse
LSDYNA
ALE
fluid-structure interaction
Type
thesis
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