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dsmc.html
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<HTML>
<CENTER><A HREF = "main.html">Return to Steve Plimpton's home page</A>
</CENTER>
<HR>
<H3>Direct Simulation Monte Carlo - low-density reacting flows via particles
</H3>
<P>Note: The text below describes an older 2d DSMC code called ICARUS we
developed at Sandia in the 1990s. A newer 3d code called SPARTA, has
its own <A HREF = "https://sparta.github.io">webpage</A>. Links to SPARTA papers are also listed
here:
</P>
<P><B>Molecular-Level Simulations of Turbulence and Its Decay</B>,
M. A. Gallis, N. P. Bitter, T. P. Koehler, J. R. Torczynski,
S. J. Plimpton, G. Papadakis, Phys Rev Letters, 118, 064501 (2017).
(<A HREF = "abstracts/prl17.html">abstract</A>)
</P>
<P><B>Direct simulation Monte Carlo investigation of the Rayleigh-Taylor
instability</B>, M. A. Gallis, T. P. Koehler, J. R. Torczynski, and
S. J. Plimpton, Phys Rev Fluids, 1, 043403 (2016).
(<A HREF = "abstracts/prf16.html">abstract</A>)
</P>
<P><B>Direct simulation Monte Carlo investigation of the Richtmyer-Meshkov
instability</B>, M. A. Gallis, T. P. Koehler, J. R. Torczynski, and
S. J. Plimpton, Physics of Fluids, 27, 084105 (2015).
(<A HREF = "abstracts/pof15.html">abstract</A>)
</P>
<P><B>Direct Simulation Monte Carlo: The Quest for Speed</B>, M. A. Gallis,
J. R. Torczynski, S. J. Plimpton, D. J. Rader, T. Koehler, 29th Intl
Symposium on Rarefied Gas Dynamics in Xi'an, China, AIP Conf Proc,
1628, 27 (2014). (<A HREF = "abstracts/rgd14.html">abstract</A>)
</P>
<HR>
<HR>
<P>Direct Simulation Monte Carlo (DSMC) is a particle-based technique for
simulating low-density fluid flows -- those with a ratio of mean-free
path to geometry scale (the Knudsen number) of order 0.1 or
greater. The basic idea is simple. Particles of various chemical
species stream through a simulation domain bounded by inflow, outflow,
or body surfaces. In a single timestep, each particle moves
independently (without inter-particle collisions) to a new
position. The move may include boundary interactions such as bouncing
off a surface. At the end of the timestep an artificial grid is
superposed on the simulation domain and used to locate particles that
are near each other. Particles within a grid cell collide with each
other and perform chemistry reactions via Monte Carlo (random)
rules. Similarly, particles in grid cells that border a boundary
surface can undergo surface chemistry interactions during the particle
move. Statistics are gathered and a new timestep begins. Over the
course of a many-timestep simulation, statistics accumulated within
cells and at surfaces are used to compute various thermodynamic
quantities of interest for all chemical species, such as flow
temperature, density, and velocity.
</P>
<P>A Sandia DSMC code, called ICARUS, grids a 2-d domain with a
collection of adjoining body-fitted grids (see the 1st figure
below). Axi-symmetric problems can be simulated in 3-d by rotating the
grid around an axis of symmetry and applying 3-d boundary
conditions. ICARUS also allows for particle and cell weighting factors
which enable trace species to be accurately tracked with minimal
computational effort.
</P>
<P>ICARUS is parallelized by assigning a subset of grid cells and the
particles in those cells to each processor. This can raise
load-balancing issues as particle densities vary in space and
time. Our strategies for achieving reasonable load- balance and good
parallel performance are detailed in the papers below. On big problems
(tens of thousands of grid cells, millions of particles) ICARUS
typically runs efficiently on hundreds to thousands of processors. It
has been a workhorse code at Sandia on a series of parallel machines,
from the nCUBE 2 to the Intel Paragon to the Intel Tflops.
</P>
<P>Here is a snapshot of particle density (on a log scale) around a nose
cone as it re-enters the earth's atmosphere. The particle-tracking
grid is shown in the lower half.
</P>
<CENTER><IMG SRC = "images/dsmc_nosecone.gif">
</CENTER>
<P>This is the temperature profile of a gas escaping from an orifice
nozzle into vacuum with stream lines in black. The white region at the
upper left of the figure is the nozzle; the inlet orifice is at the
far left; the y-axis is a line of axi-symmetry.
</P>
<CENTER><IMG SRC = "images/dsmc_orifice.gif">
</CENTER>
<P>This is a calculation of flow around a wake shield that was deployed
on the space shuttle. The idea was to see if very low densities could
be achieved in the wake of the shield as it swept through the upper
atmosphere. These calculations of particle density showed a 6
order-of-magnitude reduction in density behind the shield, from the
reference ambient density.
</P>
<CENTER><IMG SRC = "images/dsmc_shield.gif">
</CENTER>
<P>A more down-to-earth setting where low-density flows are important is
in chemical vapor deposition (CVD) reactors for semiconductor
processing. This is the pressure profile from a simulation of flow
across a silicon wafer. Gas is injected at the top of the figure thru
small orifices, flows across the surface of the wafer, and is pumped
out at the lower right. Again, the left vertical axis is a line of
axi-symmetry.
</P>
<CENTER><IMG SRC = "images/dsmc_shower.gif">
</CENTER>
<P>Collaborator on this project:
</P>
<UL><LI> Tim Bartel, Sandia
</UL>
<HR>
<P>These papers provide some details of our parallelization technique and
illustrate the application of DSMC to a few problems of interest to
Sandia.
</P>
<P><B>Parallel Particle Simulations of Low-Density Fluid Flows</B>,
S. J. Plimpton and T. J. Bartel, in Proc of High Performance
Computing 1994, San Diego, CA, April 1994, p 31. (<A HREF = "abstracts/hpc94.html">abstract</A>)
</P>
<P><B>Direct Monte Carlo Simulation of Ionized Rarefied Flows on Large MIMD
Parallel Supercomputers</B>, T. J. Bartel, S. J. Plimpton, C. R. Justiz,
in Proc of 18th International Symposium on Rarefied Gas Dynamics,
Vancouver, Canada, July 1992, published by AIAA, A94-30156, p
155-165. (<A HREF = "abstracts/rgd92.html">abstract</A>)
</P>
<P><B>Monte Carlo Particle Simulation of Low-Density Fluid Flow on MIMD
Supercomputers</B>, S. J. Plimpton and T. J. Bartel, in Proc of Scalable
High Performance Computing Conference, Williamsburg, VA, April 1992, p
212, and in Computing Systems in Engineering, 3, 333-336
(1992). (<A HREF = "abstracts/shpcc92.html">abstract</A>)
</P>
</HTML>