Hydrodynamic Limit of a Kinetic Gas Flow Past an Obstacle

Given an obstacle in R3 and a non-zero velocity with small amplitude at the infinity, we construct the unique steady Boltzmann solution flowing around such an obstacle with the prescribed velocity as |x| → ∞, which approaches the corresponding Navier–Stokes steady flow, as the mean-free path goes to zero. Furthermore, we establish the error estimate between the Boltzmann solution and its Navier–Stokes approximation. Our method consists of new L6 and L3 estimates in the unbounded exterior domain, as well as an iterative scheme preserving the positivity of the distribution function.