ASC eNews—December 2007: Micron-Scale Atomistic Studies of Shock Ejecta Production Using BlueGene/L

A publication of the Office of Advanced Simulation & Computing, NA-114, NNSA Defense Programs

December 2007

Micron-Scale Atomistic Studies of Shock Ejecta Production Using BlueGene/L

Los Alamos researchers recently completed a series of simulations of shock ejecta production in copper. This fragmentation and atomization process is difficult to study experimentally, and various theories have been proposed. Atomistic-level simulations will contribute to the development of physics-based models as part of the LANL “Science@Scale” effort (see Sept. 2007 ASCeNews article). In particular, the dependence of ejecta production and transport on shock pressure (e.g., below and above the shock melting transition, to study material strength effects) and background gas (with either a vacuum as in previous simulations or an inert gas atmosphere) have been studied.

The simulation used the entire BlueGene/L machine at Livermore, which was expanded in the fall of 2007 to 212,992 CPUs. The longest simulation ran continuously for 88 hours (400,000 timesteps, or one nanosecond of simulated time) with nearly 800 million atoms generating 101 checkpoint dumps (4.18 TB of data) and 3024 images (14.5 GB). The nearly 20 million CPU-hours of this simulation alone are equivalent to more than 2 CPU-millennia, setting a new standard for HPC stability.

These simulations utilized a quasi-2D geometry, 5.7 microns in length and a 2.23-micron periodic cross-section (but only 1.5 nm in the third direction, also periodic, to preserve 3D equation-of-state and transport properties). Copper atoms were described by an embedded atom method (EAM) potential. The free surface opposite the impact plane is initially machined with a profile matching that measured in recent tin ejecta experiments at LANL,* with approximately 1:40 length scaling so that the ~1 micron experimental amplitude is ~25 nm. Earlier molecular dynamics simulations with a single machining groove demonstrated jet formation, with subsequent necking instabilities leading to jet breakup and droplet formation at later times.

Time sequences from two such simulations are shown in the figures, with the initial surface roughness and shock pressure equal in both cases. Color represents the local density in each case.

*M. B. Zellner et al., “Effects of shock-breakout pressure on ejection of micron-scale material from shocked tin surfaces,” J. Appl. Phys. 102, 013522 (2007).

Figure 1. For shock ejection into a vacuum, we observe jet formation from small-scale features at early times: Series of 8 snapshots, one every 10 ps, for shock ejection from a roughened copper surface. These jets, surrounded by a cloud of atomic ejecta, expand and subsequently merge or break up over time.

Figure 2. At later times: snapshot 300 ps after shock encounters the free surface, the longer-wavelength machining defects produce a Richtmyer-Meshkov instability, with three “bubbles” of vacuum pushing into the copper surface, and three large “spikes” of copper protruding out, with smaller-scale jets superimposed throughout. The bubbles and spikes are very asymmetric, as one would expect for Atwood number A=1.

Figure 3. A background gas (A < 1) inhibits jetting and leads to a more symmetric Richtmyer-Meshkov instability, as seen in the second simulation: Series of 9 snapshots, one every 50 ps, for Richtmyer-Meshkov instability development as a shock wave is transmitted from copper to a dense (0.5 g/cc) gas. In the third and fourth frames, one can clearly see the transmitted gas shock (still nonplanar), as well as a complex interaction of rarefaction fans from the roughened Cu/gas interface.