Comment on "Model kinetic equation for low-density granular flow"

Hydrodynamic equations for a system of inelastically colliding granules were systematically derived on the basis of the Boltzmann equation by the Chapman-Enskog method. [A. I. Goldshtein, V. N. Poturaev, and I. A. Shulyak Fluid Dyn. (USSR) 25, 305 (1990); A. Goldshtein and M. Shapiro, J. Fluid Mech. 282, 75 (1995)]. We feel that this problem has recently been incorrectly treated by J. J. Boy, F. Moreno, and James W. Dufty [Phys. Rev. E 54, 445 (1996)] using the Bhatnagar-Gross-Krook approximation of the kinetic equation. In this Comment, the inconsistency of their approach is analyzed, and possible adequate methods for stability analyses of the homogeneous state of inelastic granules are delineated.