**Direct Simulation Monte Carlo on petaflop supercomputers and beyond**

S. J. Plimpton, S. G. Moore, A. Borner, A. K. Stagg 1 , T. P. Koehler, J. R. Torczynski, M. A. Gallis, Physics of Fluids, 31, 086101 (2019).

The gold-standard definition of the Direct Simulation Monte Carlo
(DSMC) method is given in the 1994 book by Bird ** Molecular Gas
Dynamics and the Direct Simulation of Gas Flows (Clarendon Press,
Oxford, UK, 1994)**, which refined his pioneering earlier papers in
which he first formulated the method. In the intervening 25 years,
DSMC has become the method of choice for modeling rarefied gas
dynamics in a variety of scenarios. The chief barrier to applying DSMC
to more dense or even continuum flows is its computational expense
compared to continuum computational fluid dynamics methods. The
dramatic (nearly billion-fold) increase in speed of the largest
supercomputers over the last 30 years has thus been a key enabling
factor in using DSMC to model a richer variety of flows, due to the
method's inherent parallelism. We have developed the open-source
SPARTA DSMC code with the goal of running DSMC efficiently on the
largest machines, both current and future. It is largely an
implementation of Bird's 1994 formulation. Here, we describe
algorithms used in SPARTA to enable DSMC to operate in parallel at the
scale of many billions of particles or grid cells, or with billions of
surface elements. We give a few examples of the kinds of fundamental
physics questions and engineering applications that DSMC can address
at these scales.

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