Publications

Shock Physics codes in use at many Department of Energy (DOE) and Department of Defense (DoD) laboratories can be divided into two classes; Lagrangian Codes (where the computational mesh is (attached' to the materials) and Eulerian Codes (where the computational mesh is (fixed' in space and die materials flow through the mesh). These two classes of codes exhibit different advantages and disadvantages. Lagrangian codes are good at keeping material interfaces well defined, but suffer when the materials undergo extreme distortion which leads to severe reductions in the time steps. Eulerian codes are better able to handle severe material distortion (since the mesh is fixed the time steps are not as severely reduced), but these codes do not keep track of material interfaces very well. So in an Eulerian code the developers must design algorithms to track or reconstruct accurate interfaces between materials as the calculation progresses. However, there are classes of calculations where an interface is not desired between some materials, for instance between materials that are intimately mixed (dusty air or multiphase materials). In these cases a material interface reconstruction scheme is needed that will keep this mixture separated from other materials in the calculation, but will maintain the mixture attributes. This paper will describe the Sandia National Laboratories Eulerian Shock Physics Code known as CTH, and the specialized isotropic material interface reconstruction scheme designed to keep mixed material groups together while keeping different groups separated during the remap step.

A new capability for modeling thin-shell structures within the coupled Euler-Lagrange code, Zapotec, is under development. The new algorithm creates an artificial material interface for the Eulerian portion of the problem by expanding a Lagrangian shell element such that it has an effective thickness that spans one or more Eulerian cells. The algorithm implementation is discussed along with several examples involving blast loading on plates.

The specific problem to be addressed in this work is the secondary combustion that arises from shock-induced mixing in volumetric explosives. It has been recognized that the effects of combustion due to secondary mixing can greatly alter the expansion of gases and dispersal of high-energy explosive. Furthermore, this enhanced effect may be a tailored feature for the new energetic material systems. One approach for studying this problem is based on the use of Large Eddy Simulation (LES) techniques. In this approach, the large turbulent length scales of motion are simulated directly while the small scales of turbulent motion are explicitly treated using a subgrid scale (SGS) model. The focus of this effort is to develop a SGS model for combustion that is applicable to shock-induced combustion events using probability density function (PDF) approaches. A simplified presumed PDF combustion model is formulated and implemented in the CTH shock physics code. Two classes of problems are studied using this model. The first is an isolated piece of reactive material burning with the surrounding air. The second problem is the dispersal of highly reactive material due to a shock driven explosion event. The results from these studies show the importance of incorporating a secondary combustion modeling capability and the utility of using a PDF-based description to simulate these events.