14- and 17-moment systems for polytropic gases - Comparison regarding transport coefficients.

Moment approximations are used to reduce the complexity of kinetic equation by projecting onto physically relevant variables given by moments. In this paper, moment equations with 14 and 17 moments are derived starting from the continuous kinetic model for polytropic gases with an extended version of the collision kernel. For a fixed collisional cross section model characterized by a set of parameters, we compute explicit linearized production terms in case of 14 and 17-moment in order to get estimates for transport coefficients. We show that both moment systems provide the same estimates for shear and bulk viscosities while heat conductivity estimates are different that lead to two different estimates for the Prandtl number. We compare these two Prandtl number estimates and find their agreement for a specific choice parameters in the collision kernel. This can be seen as a possible way to determine the cross section model which exhibits stabilty when increasing the moment approximation. Even with this choice, the remaining cross section parameters provide flexibility in the range of possible values for Prandtl number. [ABSTRACT FROM AUTHOR]