Requirement for hydrostatic initialization in LS-DYNA/USA finite element models

Abstract

The LS-DYNA/USA (Underwater Shock Analysis) coupled finite element codes are being investigated as a tool lot predicting the local response of compliant plate structures subjected to far-field underwater explosion. It had previously been observed in LS-DYNA/USA models that extraneous pressure build-ups emanating from the DAA (doubly asymptotic approximation) boundaries may occur in the surrounding fluid region of the model, which inevitably lead to erroneous modelling of fluid-structure interaction and inaccurate structural responses. These instabilities typically result in divergence of the solution and eventually premature termination of the simulation. After a comprehensive investigation, it was found that the instabilities did not arise if the finite element model was hydrostatically initialized before conducting the LS-DYNA/USA simulation. The purpose of this study is to investigate the need for achieving hydrostatic equilibrium prior to the modelling of the shock wave propagation through the fluid-structure media. The method for achieving static equilibrium with the current version of the LS-DYNA/USA software is presented. The example simulations presented show that the hydrostatic initialization procedure is effective it, removing instabilities occurring at the DAA-fluid boundary, associated with the USA ambient hydrostatic pressure condition.