For reactive burn models in hydrocodes, an equilibrium closure assumption is typically made between the unreacted and product equations of state. In the CTH [1] (not an acronym) hydrocode the assumption of density and temperature equilibrium is made by default, while other codes make a pressure and temperature equilibrium assumption. The main reason for this difference is the computational efficiency in making the density and temperature assumption over the pressure and temperature one. With fitting to data, both assumptions can accurately predict reactive flow response using the various models, but the model parameters from one code cannot necessarily be used directly in a different code with a different closure assumption. A new framework is intro-duced in CTH to allow this assumption to be changed independently for each reactive material. Comparisons of the response and computational cost of the History Variable Reactive Burn (HVRB) reactive flow model with the different equilibrium assumptions are presented.
A new capability for modeling graded density reactive flow materials in the shock physics hydrocode, CTH, is demonstrated here. Previously, materials could be inserted in CTH with graded material properties, but the sensitivity of the material was not adjusted based on these properties. Of particular interest are materials that are graded in density, sometimes due to pressing or other assembly operations. The sensitivity of explosives to both density and temperature has been well demonstrated in the literature, but to-date the material parameters for use in a simulation were fit to a single condition and applied to the entire material, or the material had to be inserted in sections and each section assigned a condition. The reactive flow model xHVRB has been extended to shift explosive sensitivity with initial density, so that sensitivity is also graded in the material. This capability is demonstrated for use in three examples. The first models detonation transfer in a graded density pellet of HNS, the second is a shaped charge with density gradients in the explosive, and the third is an explosively formed projectile.
The Extended History Variable Reactive Burn model (XHVRB), as proposed by Starkenburg, uses shock capturing rather than current pressure for calculating the pseudo-entropy that is used to model the reaction rate of detonating explosives. In addition to its extended capabilities for modeling explosive desensitization in multi-shock environments, XHVRB's shock capturing offers potential improvement for single shock modeling over the historically used workhorse model HVRB in CTH, an Eulerian shock physics code developed at Sandia National Labs. The detailed transition to detonation of PBX9501, as revealed by embedded gauge data, is compared to models with both HVRB and XHVRB. Improvements to the comparison of model to test data are shown with XHVRB, though not all of the details of the transition are captured by these single-rate models.
Explosive shock desensitization phenomena have been recognized for some time. It has been demonstrated that pressure-based reactive flow models do not adequately capture the basic nature of the explosive behavior. Historically, replacing the local pressure with a shock captured pressure has dramatically improved the numerical modeling approaches. A pseudo-entropy based formulation using the History Variable Reactive Burn model, as proposed by Starkenberg, was implemented into the Eulerian shock physics code CTH. Improvements in the shock capturing algorithm in the model were made that allow reproduction of single shock behavior consistent with published Pop-plot data. It is also demonstrated to capture a desensitization effect based on available literature data, and to qualitatively capture multi-dimensional desensitization behavior. This model shows promise for use in modeling and simulation problems that are relevant to the desensitization phenomena. Issues are identified with the current implementation and future work is proposed for improving and expanding model capabilities.