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Microscale gas flows are the subject of increasingly active research due to the rapid advances in micro-electro-mechanical systems (MEMS). The rarefied phenomena in MEMS gas flows make molecular-based simulations desirable. However, it is necessary to reduce the statistical scatter in particle methods for microscale gas flows. In this paper, the development of an information preservation (IP) method is described. The IP method reduces the statistical scatter by preserving macroscopic information of the flow in the particles and the computational cells simulated in the direct simulation Monte Carlo (DSMC) method. The preserved macroscopic information of particles is updated during collisions and is then modified to include the pressure field effects excluded in the collisions. An additional energy transfer model is proposed to describe the energy flux across an interface, and a collision model is used to redistribute the information after both particle–particle and particle–surface collisions. To validate the IP method, four different flows are simulated and the solutions are compared against DSMC results. The results from the IP method generally agree very well with the DSMC results for steady flows and low frequency unsteady flows ranging from the near-continuum regime to the free-molecular regime. [Copyright &y& Elsevier]

Fluid dynamic equations are valid in their respective modeling scales, such as the particle mean free path scale of the Boltzmann equation and the hydrodynamic scale of the Navier–Stokes (NS) equations. With a variation of the modeling scales, theoretically there should have a continuous spectrum of fluid dynamic equations. Even though the Boltzmann equation is claimed to be valid in all scales, many Boltzmann solvers, including direct simulation Monte Carlo method, require the cell resolution to the order of particle mean free path scale. Therefore, they are still single scale methods. In order to study multiscale flow evolution efficiently, the dynamics in the computational fluid has to be changed with the scales. A direct modeling of flow physics with a changeable scale may become an appropriate approach. The unified gas-kinetic scheme (UGKS) is a direct modeling method in the mesh size scale, and its underlying flow physics depends on the resolution of the cell size relative to the particle mean free path. The cell size of UGKS is not limited by the particle mean free path. With the variation of the ratio between the numerical cell size and local particle mean free path, the UGKS recovers the flow dynamics from the particle transport and collision in the kinetic scale to the wave propagation in the hydrodynamic scale. The previous UGKS is mostly constructed from the evolution solution of kinetic model equations. Even though the UGKS is very accurate and effective in the low transition and continuum flow regimes with the time step being much larger than the particle mean free time, it still has space to develop more accurate flow solver in the region, where the time step is comparable with the local particle mean free time. In such a scale, there is dynamic difference from the full Boltzmann collision term and the model equations. This work is about the further development of the UGKS with the implementation of the full Boltzmann collision term in the region where it is needed. The central ingredient of the UGKS is the coupled treatment of particle transport and collision in the flux evaluation across a cell interface, where a continuous flow dynamics from kinetic to hydrodynamic scales is modeled. The newly developed UGKS has the asymptotic preserving (AP) property of recovering the NS solutions in the continuum flow regime, and the full Boltzmann solution in the rarefied regime. In the mostly unexplored transition regime, the UGKS itself provides a valuable tool for the non-equilibrium flow study. The mathematical properties of the scheme, such as stability, accuracy, and the asymptotic preserving, will be analyzed in this paper as well. [ABSTRACT FROM AUTHOR]