Comments on nonlinear viscosity and Grad's moment method.
It is shown that the steady unidirectional flow with vanishing heat flux considered by B. C. Eu [Phys. Rev. E 65, 031202 (2002)], and earlier by Uribe and García-Colín [Phys. Rev. E 60, 4052 (1999)], is inconsistent with the exact conservation laws of mass, momentum, and energy. The inconsistency does not lie in the assumed symmetry properties of the flow but in the stationarity assumption. The unsteady problem is considered and its solution from the Boltzmann equation for Maxwell molecules is given.
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Andrés
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