A New (DSMC) Collision Model for Polyatomic Molecules

We present a collision model for polyatomic gases which is intermediate between those that use a simple classical representation of the molecule (Melville 1972, Pullin 1974, Pullin and Harvey 1976) and the statistical echange schemes of Borgnakke and Larsen (1975) and Pullin (1978). The energy exchange in collision is calculated from a simplified representation of a collision in a multi-dimensional space. Any number of degrees of freedom may be included, but we present it here for three translational and two internal degrees of freedom. Unlike in the statistical exchange schemes of Borgnakke and Larsen (1975) and Pullin (1978), the post-collision state is not selected directly from an equilibrium distribution, but arises from a pseudo-collision calculation with a random choice of pre-collision impact parameters. The scheme can be used as part of a two-class exchange scheme, as in the standard Borgnakke-Larsen model. In order to satisfy detail balance, the model requires that collision pairs be chosen with a Maxwell collision probability - equal propability for all collision pairs. This appeared to be a serious limitation of the new model, if it meant that only a linear viscosity law could be used in the DSMC calculations. However, since the development of 'collision-rate DSMC' (Macrossan, M. N. J. Comput. Physics, v173, p600, 2001) this restriction no longer applies. Maxwell collision probability can be used, while any viscosity can be any specified function of temperature