Numerical Analysis of the Diffusive Transport Phenomena in Hypersonic Flows

Abstract

One of the main focuses of hypersonic research is understanding the relevant physicochemical phenomena that characterize hypersonic flows. Shock-induced heating and strong thermochemical non-equilibrium are significant occurrences in high-enthalpy, high-speed flows. To accurately simulate such flows, one must ensure that the relevant effects are described in the physical model of the gas. In conventional CFD, we solve a set of governing equations, the Navier-Stokes equations, that include the terms related to viscous dissipation, heat transfer, and mass diffusion of multiple chemical species present in the flow. These diffusive processes are a continuum manifestation of transport processes at the molecular scale. According to kinetic theory, the Boltzmann equation fully describes the statistical behavior of dilute gas mixtures. A mathematical link between the Boltzmann and the Navier-Stokes equations provides a complete description of the transport phenomena with additional terms neglected in the conventional continuum flow representation. Thus, with this work, we study the effects of diffusion transport properties and chemical kinetics by simulating different hypersonic flows in the near-continuum regime. In particular, we compare the solutions obtained with US3D, a code routinely used for complex hypersonic computational fluid dynamics simulations, and MGDS, a code capable of large-scale 3D Direct Simulation Monte Carlo calculations. This work is part of a long-term effort to strike a balance between computational efficiency and accuracy in simulations and perform eventually coupled hybrid CFD-DSMC simulations of hypersonic flows.