DYNAMIC IMPLOSION OF UNDERWATER CYLINDRICAL SHELLS : Experiments and Computations

The implosion of an underwater structure is a dynamic event caused by the ambient constant pressure environment. It produces a short duration pressure pulse that radiates outwards and can damage adjacent structures. This paper presents results from a combined experimental/numerical study that aims to understand the underlying physics and establish the parameters that govern the nature of such pressure pulses. Collapse experiments on small-scale metal shells were conducted in a custom testing facility under constant pressure conditions representative of those in deep waters. The dynamic collapse of the shells was monitored using high-speed photography and the pressure around the structure with dynamic pressure transducers. Synchronization of the high-speed images with the data acquisition allowed temporal and spatial resolution of the events and the pressure pulses. Results from two experiments on shells that buckled and collapses in modes 4 and 2 are reported. A computational framework developed for the solution of highly nonlinear fluid-structure interaction problems characterized by shocks, large deformations, and self-contact is outlined. It features an Eulerian embedded boundary method for Computational Fluid Dynamics capable of achieving secondorder spatial accuracy including at the fluid-structure interface; an explicit structural analyzer with nonlinear geometric, material, and contact capabilities; and a loosely-coupled implicitexplicit fluid-structure time-integrator with a second-order time-accuracy and excellent numerical stability properties. The numerical tool is used to simulate the two experiments and shown to reproduce with good accuracy both the large deformations of the structure as well as the compression waves that emanate from it. The results demonstrate that the pressure pulse generated is influenced by the mode of buckling as well as the associated localization of collapse.