  The gas dynamics under external force field is essentially associated with
multiple scale nature due to the large variations of density and local Knudsen
number. Single scale fluid dynamic equations, such as the Boltzmann and
Navier-Stokes equations, are valid in their respective modeling scales, and it
is challenging for the modeling and computation of a multiple scale problem
across different regimes and capture the corresponding non-equilibrium flow
physics. Based on the direct modeling of conservation laws in the discretized
space, a well-balanced unified gas-kinetic scheme (UGKS) for multiscale flow
transport under external force field has been developed and is used in the
current study of non-equilibrium gaseous flow under external force field. With
the variation of modeling scale, i.e., the cell size and time step, the UGKS is
able to recover cross-scale flow physics from particle transport to
hydrodynamic wave propagation. Theoretical analysis based on the kinetic model
equation is presented to conceptually illustrate the effects of external force
on the non-equilibrium heat transport.
