Maxwellian momentum distribution function

Appl3ung the Enskog perturbation method we intend to describe the prop>-erties of gases which are only slightly different from equilibrium. Only under these conditions will the flux vectors be about linear in the derivatives so that the formal deflnitions of the transport coefficients apply. In this limit the distribution function is still nearly Maxwellian, and the Boltzmann equation can be solved by a perturbation method. The resulting solutions are then used to obtain expressions for the heat and momentum fluxes and for the corresponding transport coefficients. 

As might be expected, the model leads to a great simplification over the calculations required for molecules with a continuous potential energy function, as it enables the analysis to be confined to binary collisions and permits the definition of a collision frequency. Because there is no molecular interaction between collisions, the velocity distributions of two colliding molecules may be assumed to be re-established by the time a second collision occurs between them. Thus a Maxwellian distribution around the local mass velocity may be postulated for the calculation of the mean frequency of collision and the average momentum and energy transported per collision in the nonuniform state of the liquid. 

---