Nonequilibrium Hypersonic Aerothermodynamics Using the Direct Simulation Monte Carlo and Navier-Stokes Models.

Abstract

This dissertation presents a detailed, computational study quantifying the effects of nonequilibrium on the surface properties of a hypersonic vehicle by comparing Navier-Stokes-based Computational Fluid Dynamics (CFD) and direct simulation Monte Carlo (DSMC) simulation results for the flow about a cylinder and a wedge. Physical submodels contained in both computational methods are ensured to be as equivalent as possible. Translational nonequilibrium effects are isolated by considering a monatomic gas, argon. Thermal nonequilibrium effects are included by considering a diatomic gas, nitrogen. Several different flow regimes are considered, from the continuum into the transitional (freestream Knudsen numbers are 0.002, 0.01, 0.05 and 0.25), with Mach numbers of 10 and 25. Effects on surface properties (total drag and peak heat transfer rate) are quantified at each flow condition. Flow field properties are also compared. Continuum breakdown parameter values are compared with other flow and surface properties.

The effectiveness of several types of CFD slip boundary conditions is evaluated, and the velocity slip and temperature jump (including vibrational temperature jump) values are compared with those extracted from DSMC simulation results. The slip conditions of Gokcen (AIAA Paper 1989-0461) most accurately predict surface properties, while the slip conditions of Lockerby et al. (AIAA J. 43(6) (June 2005), 1391-1393) agree best with DSMC slip values.

For flows of argon and nitrogen about a cylinder, CFD total drag predictions remain within 6% of DSMC predictions, and heat flux agreement is 8% or better. For flows about a wedge, total drag differences range between 2% and 34%, mostly due to friction force differences. Peak heating differences are between 70% and 100%; DSMC predicts a much higher temperature near the leading edge than CFD.

Flow property differences near the wall surface are shown to be concentrated primarily in the Knudsen layer. Validation of the CFD code, as well as the effect of various levels of surface accommodation, are shown by considering a nitrogen flow over a flat plate and comparing the simulation results with experimental data.