This week we released an educational version of the KB suite, adding to the diversity of free software for scientists available on our resource site. KB is an integrated package of data and simulation programs for shock hydrodynamics and detonations. It could be a useful tool in departments of physics, mechanical engineering and civil engineering. The KB programs were initially developed for Los Alamos National Laboratory. A group involved in diagnostics of underground tests and stockpile-stewardship experiments wanted quick utilities to run on PCs. Their goal was to make extensive what-if calculations and then to refine the results with the large hydrodynamic codes available at the laboratory. In this post, I’ll describe some of the resources created during the project.
KB tables
The widely-distributed, unclassified Sesame tables from LANL are an excellent source of equation-of-state information at high density, temperature and pressure. When I began the project, the data for 140 materials were combined in a single binary file with a somewhat idiosyncratic format. The numbers could be accessed only with old-style FORTRAN routines supplied with the library. My first activity was to write programs to disassemble the Sesame library and to reconstruct in a more usable form. The resulting KB tables are in text format and therefore accessible through any computer language. There is an individual file for each material, so it is easy to add to the library without special maintenance routines. Finally, all quantities have been converted to SI units.
Shock hydrodynamic parameters
For some simulations (e.g., detonations), it is sufficient to use the shock equation-of-state. Data are available for a broad range of materials. The shock velocity us is related to the material velocity up through an equation of the type
us = C0 + S1*up + S2*up^2.
I wrote a program to fit the function to the raw data listed in S.P.Marsh (ed.), LASL Shock Hugoniot Data (University of California Press, Berkeley, 1980) and created a spreadsheet with (C0,S1,S2) data for over 400 materials.
One-dimensional hydrodynamic code
KB1 is a 1D simulation code for planar, cylindrical or spherical geometries. It is a descendant of a much more complex code described in the reference Finite-element simulation code for high-power magnetohydrodynamics. The previouscode could handle magnetic acceleration with self-consistent magnetic diffusion and temperature-dependent conductivity. KB1 uses the element-based approach described in the reference. It handles the full range of material types including explosives. Shock processes can be initiated by collisions, prescribed pressure waveforms or detonations.
Two-dimensional hydrodynamic code
KB2, the centerpiece of the suite, is a 2D code that can handle planar and cylindrical geometries. It was built from the conformable triangular mesh technology developed for our TriComp programs. The element-based Lagrangian approach has drawbacks and advantages. The main limitation follows from the requirement of logical continuity of the mesh. All objects (i.e., regions of different material properties) must be contiguous in the initial state. There can be no disconnected objects or holes. The implication is that the code is useful for assemblies like shaped charges but not suitable for bullets striking a wall or milk drops. On the other hand, the approach has decided advantages:
- Because elements preserve a unique material identity, it is straightforward to apply sophisticated material models and to represent complex histories.
- The compression and expansion of expansion of elements gives automatic zone refinement (i.e., the density of elements increases in critical regions).
- The approach is ideal for strong compressions and explosions. The final state of the assembly may be hundreds of times smaller or larger than the initial state.
Constraining the physics
A temptation in writing computer-simulation programs is to add adjustable parameters to represent processes where the physics is fuzzy or even unknown. This practice is largely responsible for the dubious reputation of simulations, particularly of shocks and detonations. An adjustable code can replicate any experiment but has no predictive power. In developing the KB programs, I avoided user parameters as much as possible. The only instances are the artificial viscosity for hydrodynamics (required for numerical stability at shock fronts) and the element detonation pressure for explosives. I believe that disagreement between a code and an experiment is a valuable piece of information. It shows that something is going on that you hadn’t planned on, and that you’d better look for it.
State of KB
I had planned to retire KB, but I recently got a request from Vilem Petr at the Colorado School of Mines. Although the application is fairly specialized, I decided it would be a shame to loose some of the unque resources. For the educational release, I tuned up the program interfaces, corrected several bugs and created a snappier-looking icon. The biggest effort was to combine individual manuals, reports and benchmark tests into a unified instruction manual. You can download the new manual at http://www.fieldp.com/manuals/kb.pdf.