Fluid-Structure Interaction and Underwater Hydrostatic Implosion of Thin-Walled Metallic Cylinders in Semi-Confined Conditions

Abstract

This study presents a comprehensive numerical investigation of the dynamic behavior and fluid-structure interactions (FSI) of metallic cylinders undergoing hydrostatic collapse in semi-confined fluid environments using a structured Arbitrary Eulerian-Lagrangian (ALE) formulation in LS-DYNA. The numerical model reproduces the experimentally measured collapse pressure of 3.69 MPa and predicts the first water hammer peak with a 1.01% error, demonstrating high predictive fidelity. Following validation, the effects of material type (aluminum and titanium), cylinder slenderness ratio (L/D), and confinement diameter on collapse behavior, pressure evolution, and fluid motion are examined. Titanium cylinders exhibited sharper collapses, higher water hammer pressures exceeding 70 MPa, and greater kinetic and strain energy accumulation than aluminum due to their higher stiffness and yield strength. Lower L/D ratios produced more abrupt collapses, whereas higher L/D ratios promoted more gradual, axisymmetric deformation. Larger confinement diameters intensified jet formation and increased fluid velocities. The simulations provide mechanistic insight into the coupling between structural deformation and surrounding fluid, showing that geometry, material stiffness, and confinement govern collapse-induced energy transfer. Full-field FSI analysis captures key phenomena, including radial jetting, peak fluid velocities, and internal cavitation, that are not evident from pressure-time histories alone. These findings provide quantitative guidance for the design and safety assessment of subsea pressure housings, marine pipelines, and other underwater structures subjected to extreme hydrostatic loading.