A mixed Boltzmann–BGK model for inert gas mixtures

We propose a mixed Boltzmann-BGK model for mixtures of monatomic gases, where some kinds of collisions are described by bi-species Boltzmann operators and the others by the binary BGK terms given in [Bobylev et al., Kinetic and Related Models 11 (2018)], that is the relaxation model for mixtures with the closest structure to the Boltzmann one. At first, we assume that collisions occurring within the same species (intra-species) are modelled by Boltzmann operators, while interactions between different constituents (inter-species) are described by BGK terms. This option allows us to rigorously derive hydrodynamic equations not only in the classical collision dominated regime, but also in situations with intra-species collisions playing the dominant role (as in mixtures with very disparate particle masses). Then, we present a general form of this mixed Boltzmann-BGK model, characterized by further parameters allowing us to select which binary interactions have to be described by Boltzmann integrals or by BGK operators. We prove that this model preserves conservations of global momentum and energy, positivity of all temperatures and the validity of Boltzmann H-theorem, allowing us to conclude that the unique admissible equilibrium state is the expected Maxwellian distribution with all species sharing a common mean velocity and a common temperature.