Numerical simulations of the Los Alamos gapstick experiment

The gapstick is a relatively new high explosive (HE) experiment that was inspired by the traditional gap test. The gapstick consists of a series of HE and inert pellets, which are increasingly longer, in a rate stick configuration. As the shock attenuates in the inert pellets it becomes more difficult to initiate successive HE pellets. Simulations of these experiments present numerous challenges. To start with, the material model for Polyvinylidene Fluoride (PVDF), used for the inert pellets, is not well characterized but is quite important. Properly modeling the initiation and detonation of the HE is also important and requires a reactive burn model with sufficient mesh resolution to capture the reaction scales. For this work, we employ the Scaled Uniform Reactive Burn (SURF) and Arrhenius Wescott-Stewart-Davis (AWSD) reactive burn models. These burn models are sensitive to mesh resolution and other code settings which are investigated (e.g. artificial viscosity in the Lagrangian-based staggered-grid hydrodynamics solver). Finally, running the full gapstick simulation at a reasonable mesh resolution is very computationally demanding and mesh strategies such as adaptive mesh refinement (AMR) will be explored. Despite these challenges, the gapstick is a very unique experiment that can be used for validation and potential calibration of reactive burn models.