A hybrid continuum/particle approach for modeling subsonic, rarefied gas flows

A hybrid approach that combines a continuum approach solving the Navier-Stokes equations and a particle method called the information preservation (IP) method is implemented to simulate subsonic, rarefied gas flows accurately and efficiently. The coupling between the continuum and particle approaches relies on a continuum/particle interface, which is investigated in detail. The IP method preserves information at the macroscopic level that allows the continuum approach to directly use the information from the particle region. In order to correctly generate particles from the continuum region, two strategies are proposed. One strategy adopts a Marshak-type condition, which requires many particles in a cell to control the flux fluctuation due to the microscopic motion of particles. In the second strategy, reservoir cells and buffer cells are used, and this is the approach adopted in our general hybrid scheme. The location of the interface is determined using a continuum breakdown parameter. Studies show that the continuum breakdown parameter suggested by Garcia et al. can be used to determine the location of the interface, but the implementation of the interface also affects the location of the interface. Simulation of a flow over a flat plate shows the performance of the hybrid approach and reveals the effects of the cutoff value to a continuum breakdown parameter. This approach is also applied to study the aerodynamics of a micro-scale airfoil, which is very difficult for a single method. The hybrid approach generally spends less computational time than the IP method for rarefied gas flows, and the numerical performance of the hybrid approach depends on the number ratio of continuum cells to particle cells.