Computational Modeling of Dynamically Initiated Instabilities and Implosion of Underwater Cylindrical Structures in a Confined Environment

This paper details a numerical study of the dynamic stability of a cylindrical shell structure under combined hydrostatic and dynamic pressure loading within a tubular environment as compared to the traditional loading of hydrostatic pressure alone. Simulations are executed using a coupled Eulerian-Lagrangian scheme, within the dynamic system mechanics advanced simulation (DYSMAS) code, to explicitly model the (1) structural response of a single unstiffened cylindrical shell to dynamic pressure loading and (2) the fluid flow field within the surrounding environment due to the shock and the shell structural response. Simulations involve a non-pressure-compensated aluminum 6061-T6 cylindrical structure with a length-to-diameter ratio, L/D, equal to 9.6. This structure is 31.8 mm (1.25-in) in outer diameter and is concentrically and longitudinally centered within the outer tube, which has an inner diameter of 177.8 mm (7.00-in) and total internal length of 2.13 m (84-in). Simulations are run at four hydrostatic tank pressures, which are categorized by percentage of measured critical collapse pressure, Pc, of the shell structure: 66% Pc, 80% Pc, 85% Pc, and 90%Pc. For each case, the shell structure is subjected to shock loading created by the detonation of a commercial blasting cap at a given standoff to the structure within the confining tube. Simulated pressure histories are compared to experimental pressure data at gage locations. The simulations and corresponding experiments produce the same overall result for three of four cases (i.e., survive: 66%Pc or implode: 85%Pc and 90%Pc). For the 80%Pc case, the overall result differs between simulation and experiment in that the specimen in the experiment survives but the simulated cylinder implodes. However, the discrepancy between the overall experimental result and corresponding simulation is not deemed a failure for the 80%Pc case; instead, this signifies a transitional case for the dynamic stability of the shell structure (i.e., collapse is sensitive to small deviations from assumed conditions in this regime).