Incomplete molecular chaos within dense-fluid shock waves

Large-scale molecular dynamics simulations of an $M_s=4.3$ shock in dense argon ($\rho =532\mathrm{kg}∕{\mathrm{m}}^3$, $T=300\mathrm{K}$) and an $M_s=3.6$ shock in dense nitrogen ($\rho =371\mathrm{kg}∕{\mathrm{m}}^3$, $T=300\mathrm{K}$) have been performed. The two-point molecular velocity correlation function is calculated within slices in plane with the shock wave. Long-range ($\sim$ten molecular radii) positive correlations (correlation coefficient $0.05$) are observed for the shock-normal velocity component, the in-plane velocity component, and the rotation rates. No correlations are found upstream or downstream of the shock, indicating that this is a nonequilibrium effect. These correlations violate one of the assumptions underlying the Boltzmann equation.