Abstract
Over the past half century, a variety of computational fluid dynamics (CFD) methods and the direct simulation Monte Carlo (DSMC) method have been widely and successfully applied to the simulation of gas flows for the continuum and rarefied regime, respectively. However, they both encounter difficulties when dealing with multiscale gas flows in modern engineering problems, where the whole system is on the macroscopic scale but the nonequilibrium effects play an important role. In this paper, we review two particle-based strategies developed for the simulation of multiscale nonequilibrium gas flows, i.e., DSMC-CFD hybrid methods and multiscale particle methods. The principles, advantages, disadvantages, and applications for each method are described. The latest progress in the modelling of multiscale gas flows including the unified multiscale particle method proposed by the authors is presented.
Highlights
1 Introduction While the traditional computational fluid dynamics (CFD) methods based on Navier-Stokes-Fourier (NSF) equations have been widely used for the simulation of gas flows, they encounter difficulties when applying to nonequilibrium gases, with too less molecular collisions in the flow timescales to ensure local thermodynamic equilibrium
(2019) 1:12 direct simulation Monte Carlo (DSMC) method [1, 2], which tracks a large number of representative molecules, assuming that molecular motions and inter-molecular collisions are uncoupled during small time intervals
5 Summary In order to solve multiscale gas flows encountered in engineering problems, it is desirable to develop multiscale modelling methods instead of using the traditional computational methods designed for one scale
Summary
While the traditional computational fluid dynamics (CFD) methods based on Navier-Stokes-Fourier (NSF) equations have been widely used for the simulation of gas flows, they encounter difficulties when applying to nonequilibrium gases, with too less molecular collisions in the flow timescales to ensure local thermodynamic equilibrium. On the contrary, when the gas flow is in the transition or free molecular regime, the NSF equations with linear constitutive models break down, and the Boltzmann equation based on the kinetic theory of gases is proved to be an appropriate counterpart instead It is well-known that solving the Boltzmann equation is cumbersome due to the complexity of the collision integral term. Spacecrafts reentry meets the full Knudsen number range continuously when descending, i.e. from free molecular flow in the outer atmosphere to the continuum regime close to the earth Another example is micro/nano-devices developed rapidly in the past three decades.
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