A rigorous model reduction for the anisotropic-scattering transport process

In this letter, we propose a reduced-order model to bridge the particle transport mechanics and the macroscopic fluid dynamics in the highly scattered regime. A rigorous mathematical derivation and a concise physical interpretation are presented for an anisotropic-scattering transport process with arbitrary order of scattering kernel. The prediction of the theoretical model perfectly agrees with the numerical experiments. A clear picture of the diffusion physics is revealed for the neutral particle transport in the asymptotic optically thick regime.