Nonequilibrium corrections in the pressure tensor due to an energy flux

The form [Formula Presented] for the pressure tensor for a system submitted to an energy flux [Formula Presented] (U being the identity matrix, [Formula Presented] the specific energy, and [Formula Presented] the specific volume) widely used for anisotropic radiation and proposed to be more general by Domínguez and Jou [Phys. Rev. E 51, 158 (1995)] has been recently questioned by R. E. Nettleton [Phys. Rev. E 53, 1241 (1996)]. We provide a physical basis, in a completely different way, for this expression for anisotropic radiation and ultrarelativistic gases and we criticize some previous physical interpretations. We recall the necessity of an understanding of this kind of expression in a thermodynamic framework. © 1996 The American Physical Society.