The Bhatnagar–Gross–Krook kinetic model of the Boltzmann equation is solved for the steady cylindrical Poiseuille flow fed by a constant gravity field. The solution is obtained as a perturbation expansion in powers of the field (through fourth order) and for a general class of repulsive potentials. The results, which are hardly sensitive to the interaction potential, suggest that the expansion is only asymptotic. A critical comparison with the profiles predicted by the Navier–Stokes equations shows that the latter fail over distances comparable to the mean free path. In particular, while the Navier–Stokes description predicts a monotonically decreasing temperature as one moves apart from the cylinder axis, the kinetic theory description shows that the temperature has a local minimum at the axis and reaches a maximum value at a distance of the order of the mean free path. Within that distance, the radial heat flows from the colder to the hotter points, in contrast to what is expected from the Fourier law. Furthermore, a longitudinal component of the heat flux exists in the absence of gradients along the longitudinal direction. Non-Newtonian effects, such as a non-uniform hydrostatic pressure and normal stress differences, are also present.
