Computational analysis of bubble–structure interactions in near-field underwater explosion

Abstract

The response of underwater structures to a near-field explosion is coupled with the dynamics of the explosion bubble and the surrounding water. This multiphase fluid–structure interaction process is investigated in the paper using a two-dimensional model problem that features the yielding and collapse of a thin-walled aluminum cylinder. A recently developed computational framework that couples a finite volume compressible fluid dynamics solver with a finite element structural dynamics solver is employed. The fluid–structure and liquid–gas interfaces are tracked using embedded boundary and level set methods. The conservation of mass and momentum across the interfaces is enforced by solving one-dimensional bimaterial Riemann problems. The initial pressure inside the explosion bubble is varied by two orders of magnitude in different test cases. Three different modes of collapse are discovered, including an horizontal collapse (i.e. with one lobe extending towards the explosive charge) that appears counterintuitive, yet has been observed in previous laboratory experiments. Because of the transition of modes, the time it takes for the structure to reach self-contact does not decrease monotonically as the explosion magnitude increases. The fluid pressure and velocity fields, the bubble dynamics, and the transient structural deformation are visualized to elucidate the cause of each collapse mode and the mode transitions. The result suggests that, in addition to the incident shock wave, the second pressure pulse resulting from the contraction of the explosion bubble also has significant effect on the structure’s collapse. The phase difference between the structural vibration and the bubble’s expansion and contraction influences the structure’s mode of collapse. Furthermore, the transient structural deformation has clear effect on the bubble dynamics, leading to a two-way interaction. A counter-jet that points away from the structural surface is observed. Compared to the liquid jets produced by bubbles collapsing near a rigid wall, this counter-jet is in the opposite direction.