  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.
