▪ Abstract  The principles of and procedures for implementing direct simulation Monte Carlo (DSMC) are described. Guidelines to inherent and external errors common in DSMC applications are provided. Three applications of DSMC to transitional and nonequilibrium flows are considered: rarefied atmospheric flows, growth of thin films, and microsystems. Selected new, potentially important advances in DSMC capabilities are described: Lagrangian DSMC, optimization on parallel computers, and hybrid algorithms for computations in mixed flow regimes. Finally, the limitations of current computer technology for using DSMC to compute low-speed, high–Knudsen-number flows are outlined as future challenges.

The effect of varying the Knudsen number Kn in microchannel ows was simulated using the direct simulation Monte Carlo method (DSMC) combined with the monotonic Lagrangian grid (MLG). The DSMC -MLG, a method that provides automatic grid re nement according to number density, has been optimized for massively parallel computation and provides a fast, highly resolved description of the ow. New out ow boundary conditions, consistent with the DSMC -MLG algorithm, were developed to allow the user to specify the out ow pressures. The effect of varying Kn was examined for three different values of Kn (0.07, 0.14, and 0.19) for a high-speed in ow by varying the channel height. A Navier-Stokes computation was also performed to show continuum regime ow. The computations provide contours of pressure, temperature, and Mach number to show complex interactions among oblique shocks and boundary layers, and how these change with the Kn. Temperature jumps and slip velocities as functions of position along the wall are compared for all cases. The computations show that the velocity slip is approximately constant behind the shock, while the temperature jump is reduced. Nomenclaturestreamwise velocity, cm /s V = velocity x, y = Cartesian coordinate = mean free path Subscripts f = uid th = thermal x, y = Cartesian coordinate = freestream

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