Multiscale simulation of rarefied mixture gas flows with absorbing walls

The efficient simulation of rarefied gas-mixture flows with large mass ratios presents a significant challenge. Traditional methods, such as the direct simulation Monte Carlo and the discrete velocity method, often suffer from computational inefficiency. This study addresses these challenges by employing the general synthetic iterative scheme (GSIS). Through two-dimensional pressure-driven flow simulations, we demonstrate that GSIS exhibits fast convergence and asymptotic-preserving properties, enabling the rapid attainment of accurate solutions even on coarse spatial grids. The efficiency of GSIS is also maintained in three-dimensional complex geometries, such as the Divertor Tokamak Test facility. Using GSIS, we find that the absorbing boundary intensifies the flow separation between the two gases, an effect that becomes even more pronounced with an increase in the mass ratio. The lighter gas is more susceptible to the influence of the absorbing boundary, causing significant differences in the flow rates of the two gases through the boundary. The numerical tools and flow analysis presented here are valuable for the design and optimization of flow separation devices.