Title: A comment on `An improved macroscale model for gas slip flow in porous media',
Abstract: In a recent paper by Lasseux, Valdés-Parada and Porter (J.~Fluid~Mech.
\textbf{805} (2016) 118-146), it is found that the apparent gas permeability of
the porous media is a nonlinear function of the Knudsen number. However, this
result is highly questionable, because the adopted Navier-Stokes equations and
the first-order velocity-slip boundary condition are first-order (in terms of
the Knudsen number) approximations of the Boltzmann equation and the kinetic
boundary condition for rarefied gas flows. Our numerical simulations based on
the Bhatnagar-Gross-Krook kinetic equation and regularized 20-moment equations
prove that the Navier-Stokes equations with the first-order velocity-slip
boundary condition are only accurate at a very small Knudsen number limit,
where the apparent gas permeability is a linear function of the Knudsen number.