  A combination of molecular-dynamics simulations, theoretical predictions, and
previous experiments are used in a two-part study to determine the role of the
Knudsen layer in rapid granular flows. First, a robust criterion for the
identification of the thickness of the Knudsen layer is established: a rapid
deterioration in Navier-Stokes-order prediction of the heat flux is found to
occur in the Knudsen layer. For (experimental) systems in which heat flux
measurements are not easily obtained, a rule-of-thumb for estimating the
Knudsen layer thickness follows, namely that such effects are evident within
2.5 (local) mean free paths of a given boundary. Second, comparisons of
simulation and experimental data with Navier-Stokes order theory are used to
provide a measure as to when Knudsen layer effects become non-negligible.
Specifically, predictions that do not account for the presence of a Knudsen
layer appear reliable for Knudsen layers collectively composing up to 20% of
the domain, whereas deterioration of such predictions becomes apparent when the
domain is fully comprised of the Knudsen layer.
