Study on a Boltzmann/Navier-Stokes Hybrid Scheme for Simulating Multi-scale flows

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In this study, a novel Boltzmann/Navier-Stokes Hybrid scheme has been developed for simulating the multi-scale flows ranging from the continuum to free molecule flows. For the Boltzmann solver, instead of the direct simulation Monte Carlo method, a deterministic CFD method for a kinetic model Boltzmann equation is used. The same CFD method is also used for the Navier-Stokes equations. Solution adaptive domain decomposition strategy using local switching parameter based on the three gradient-length local Knudsen numbers for the density, velocity, and temperature is adopted for coupling the Boltzmann solver and the Navier-Stokes solver. At the interface between the Boltzmann solver and the Navier-Stokes solver, moments of the numerical fluxes of the Boltzmann equation are transferred to the Navier-Stokes solver from the Boltzmann solver, while the Chapman-Enskog distribution functions are transferred to the Boltzmann solver from the Navier-Stokes solver. Reliable solution can be obtained for both subsonic flows and supersonic flows with the hybrid scheme at lower computational costs compared to the direct simulation Monte Carlo method as well as the pure CFD Boltzmann method..