Local conservation laws and entropy inequality for kinetic models with delocalized collision integrals

This paper presents a common setting for the collision integrals St appearing in the kinetic theory of dense gases. It includes the collision integrals of the Enskog equation, of (a variant of) the Povzner equation, and of a model for soft sphere collisions proposed by Cercignani (Comm. Pure Appl. Math. 36 (1983) 479–494). All these collision integrals are “delocalized”, in the sense that they involve products of the distribution functions of gas molecules evaluated at positions whose distance is of the order of the molecular radius. Our first main result is to express these collision integrals as the divergence in v of some mass current, where v is the velocity variable, while viSt and |v|2St are expressed as the phase space divergence (i.e. divergence in both position and velocity) of appropriate momentum and energy currents. This extends to the case of dense gases an earlier result by Villani (Math. Model. Numer. Anal. M2AN 33 (1999) 209–227) in the case of the classical Boltzmann equation (where the collision integral involves products of the distribution function of gas molecules evaluated at different velocities, but at the same position. Applications of this conservative formulation of delocalized collision integrals include the possibility of obtaining the local conservation laws of momentum and energy starting from this kinetic theory of denses gases. Similarly a local variant of the Boltzmann H Theorem, involving some kind of free energy instead of Boltzmann’s H function, can be obtained in the form of an expression for the entropy production in terms of the phase space divergence of some phase space current, and of a nonpositive term.