Rotation‐translation collision model for DSMC with restricted energy exchange

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AbstractIn the standard implementation of Borgnakke–Larsen (BL) rotation and translation energy exchange in DSMC not every rotational mode of each molecule is active in every collision. In some collisions, the rotation mode of one molecule is “frozen” (cannot be changed by the collision); in other collisions the rotation mode of both molecules is frozen. Different relaxation rates are obtained by varying the proportion of frozen‐rotation collisions. In 1974 Larsen and Borgnakke proposed a “restricted exchange” version of BL in which a portion of the energy of each mode is frozen in each collision; the method seemed physically plausible but did not satisfy the detailed balance condition. In 1978 Pullin proposed a general restricted exchange of BL exchange which did satisfy the detailed balance condition; it was more CPU intensive than the original BL scheme. It was seen as “laborius” and “cumbersome” and it seemed to be incompatible with vibration energy‐exchange models. Here a simplified version, a special case, of Pullin's restricted exchange scheme is described. The new method is only 12%–17% more CPU‐intensive than standard BL methods and can be used with the standard DSMC quantum vibration exchange model. It is shown that the elimination of frozen‐rotation collisions changes the high energy distribution of vibration energy in nonequilibrium conditions which may slightly decrease the nonequilibrium dissociation rate when restricted exchange is used with the quantum‐kinetic method for dissociation reactions.