document: Numerical simulation of the gas flow through such non-trivial internal geometries is, however, extremely challenging. This is because conventional continuum fluid dynamics, which assumes that locally a gas is close to a state of thermodynamic equilibrium, becomes invalid or inaccurate as the smallest characteristic scale of the geometry (e.g. the channel height) approaches the mean distance between molecular collisions, λ [1]. An accurate and flexible modelling alternative for these cases is the direct simulation Monte Carlo method (DSMC) [2]. However, DSMC can be prohibitively expensive for internal-flow applications, which typically have a geometry of high-aspect ratio (i.e. are extremely long, relative to their cross-section). The high-aspect ratio creates a formidable multiscale problem: processes need to be resolved occurring over the smallest characteristic scale of the geometry (e.g. a channelʼs height), as well as over the largest characteristic scale of the geometry (e.g. the length of a long channel network), simultaneously.
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