On the Validity of Macroscopic Models

The validity of macroscopic models in the limit of large mean-free path is examined by solving a one-dimensional model equation, where the term v⊇Vf 1 is retained in the kinetic equation. A standard thirteen-moments approximation yields accurate results in all collisionality regimes if the fluid velocity is sufficiently small. In contrast, the Chapman―Enskog scheme is only accurate in the collision-dominated regime