Enskog theory deviations

Fig. 4. Deviation of velocity autocorrelation function V(5) from Enskog theory prediction at packing fractions (a) 17 = 0.0741 and (b) 0.148. Dashed curves are computer molecular dynamics results.

Using the theory developed by Chapman-Enskog (see Ref. 14), a hierarchy of continuum fluid mechanics formulations may be derived from the Boltzmann equation as perturbations to the Maxwellian velocity distribution function. The first three equation sets are well known (1) the Euler equations, in which the velocity distribution is exactly the Maxwellian form (2) the Navier-Stokes equations, which represent a small deviation from Maxwellian and rely on linear expressions for viscosity and thermal conductivity and (3) the Burnett equations, which include second order derivatives for viscosity and thermal conductivity. 

The viscous stresses only come into play for systems containing significant velocity gradients within the fluid. Nevertheless, very large gradients are not required as the Navier-Stokes equations can be derived from the Chapman-Enskog perturbation theory. On the other hand, for the non-equilibrium boundary layer and shock wave systems, i.e., systems which deviates considerably from equilibrium, higher order expansions are apparently needed [28]. Actually, at least for shock waves the validity of the Maxwell-Boltzmann equation becomes questionable. 

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