Large-scale molecular dynamics simulations of normal shock waves in dilute argon

Large-scale molecular dynamics (MD) simulations using the Lennard-Jones potential are performed to study the structure of normal shock waves in dilute argon. Nonperiodic boundary conditions in the flow direction are applied by coupling the MD domain with a two-dimensional finite-volume computational fluid dynamics (CFD) solver to correctly generate the inflow and outflow particle reservoirs. Detailed comparisons are made with direct simulation Monte Carlo (DSMC) solutions using the variable-hard-sphere (VHS) collision model. By performing realistic MD simulations of full shock waves, this article presents a more sensitive evaluation of the VHS model parameters (via temperature and velocity distribution functions) than is possible using available experimental density measurements. In the high temperature range (300–8000 K), where the Chapman–Enskog theory supports the VHS model assumptions, near-perfect agreement between MD and DSMC solutions is demonstrated and inverse shock thickness predictions reproduce experimental measurements. In the low temperature range (16–300 K), theory predicts and MD simulation confirms that the VHS collision model becomes less valid.