Keywords: underwater implosion mitigation, bubble collapse, fluid-structure interactions, wave equation, finite element modeling.

When an implosion occurs in high pressure environments such as deep underwater, the gas inside the bubble rapidly contracts. This triggers a dynamic cycle of expansions and contractions that produces pressure waves propagating in the radial directions. These pressure waves can damage nearby structures and possibly cause other pressure volumes to implode.

In this paper, the mitigating effect of structural deformation on the process of underwater implosion dynamics is investigated. For this purpose a simple spherical implosion with a spherical shell placed inside is considered. The focus is to examine the sensitivity of the implosion process to the variation of such parameters as the inner shell location relative to the bubble radius and the initial pressure inside the bubble. The benchmark for comparison is the peak acoustic water pressure at a specified distance from the initial bubble.

Underwater implosions involving deformable structures are an example of fluid-structure interaction problems. For the underwater implosion problem, the convection term in the equation for the momentum conservation is negligible. This assumption leads to the wave equation. The analytical model adopted in the present paper couples a finite element model for the structural subsystem with a finite element approximation of the wave equation for the fluid subsystem. The fluid and the structure are coupled by the gas in the bubble. For the analysis of air-filled pressurized structures, the problem necessitates the tracking of three interfaces. The interfaces are between the water and the air, between the outer gas and the outer wall of the structure, and between the inner wall of the structure and the gas inside the structure. The resulting set of equations is solved using a fourth order Runge-Kutta time marching scheme. For the fluid the pressure boundary condition is applied at the bubble surface and a displacement boundary condition is applied at the far end of the fluid domain. The fluid subsystem mesh is updated at each time step.

The results of numerical analyses demonstrate potential methods for tailoring structures to mitigate the pressure exerted on nearby objects during an implosion. To reduce the peak acoustic pressure in the water, one may minimize the collapse range of the bubble by placing interior wall structures as close as possible to the initial bubble radius, as long as the structural integrity of the inner wall can be maintained. This could be accomplished by using double wall structures with an air gap in between the two walls, or sandwich structures with porous core materials. Increasing the gas pressure inside the bubble also has the effect of decreasing the bubble collapse range, and thus decreasing the acoustic pressure peaks. Another tailoring method could be to use highly deformable interior structures, which are effective energy absorbers.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £50 +P&P)