Abstract

We study the Boltzmann equation with external forces, not necessarily deriving from a potential, in the incompressible Navier–Stokes perturbative regime. On the torus, we establish Cauchy theories that are independent of the Knudsen number in Sobolev spaces. The existence is proved around a time-dependent Maxwellian that behaves like the global equilibrium both as time grows and as the Knudsen number decreases. We combine hypocoercive properties of linearized Boltzmann operators with linearization around a time-dependent Maxwellian that catches the fluctuations of the characteristics trajectories due to the presence of the force. This uniform theory is sufficiently robust to derive the incompressible Navier–Stokes–Fourier system with an external force from the Boltzmann equation. Neither smallness nor a time-decaying assumption is required for the external force, nor a gradient form, and we deal with general hard potential and cut-off Boltzmann kernels. As a by-product, the latest general theories for unit Knudsen number when the force is sufficiently small and decays in time are recovered.

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