Detailed Analysis of Thermochemistry Modeling for Hypersonic Air Flows

Abstract

Hypersonic vehicles operate at speeds in excess of Mach 5, producing extreme temperatures and nonequilibrium flow phenomena. A principle challenge associated with designing hypersonic platforms lies in the modeling of the nonequilibrium environment in which they will operate. The overall goal of this thesis is to perform a detailed investigation into the thermochemistry modeling of hypersonic air flows and is broken into two main sections.

The first section provides an assessment of different assumptions for thermochemistry modeling on hypersonic air flows. A computational analysis is used to study flows over a double-cone using three different thermochemical approaches: nonequilibrium flow, equilibrium flow, and frozen flow for air at several different freestream conditions. The thermochemical model effects on the flow field and surface properties are specific areas of interest. The resulting aerothermodynamic loads are compared to experiments performed in the CUBRC LENS-I and LENS-XX facilities and indicates that thermochemistry modeling plays an important role in determining surface properties. The results indicate that the specific thermochemistry model used to describe hypersonic flow over a double-cone plays an important role in determining surface properties for both CUBRC facilities, especially at high enthalpies. A comparison of Park and Modified Marrone-Treanor thermochemistry models is also made and conclude that both models produce similar surface properties, a result of the freestream density, and fail to reproduce experimental results. Careful analysis concludes that consistent over and under prediction of pressure drag and heat load indicates there is some unknown fundamental difference between the actual experiments and the simulations, thus limiting the usefulness of these double-cone experiments for validation of thermochemistry models.

The second section focuses on understanding the uncertainties between computational simulations and experiments by conducting a sensitivity analysis on the thermochemical kinetics of hypersonic flow over a cylinder. A computational analysis is used to model Mach 5 and Mach 7 flows over a cylinder, where freestream properties are representative of experiments to be conducted in the Hypervelocity Expansion Tube at the California Institute of Technology. The sensitivity analysis is conducted using the polynomial chaos expansion method and Sobol indices are used to determine which thermochemical nonequilibrium phenomena most affect various quantities of interest. The results show that the O2-N2 and O2-O reactions dominate surface pressure, surface heat transfer, drag, heating rate, and rotational temperature, while the first two Zeldovich reactions dominate the surface number density of NO. The O2-O2 reaction was found to be less important than other reactions. Surface pressure and drag are also shown to be relatively insensitive overall. The results also indicate flow separation and recirculation near the trailing edge of the cylinder. These findings will help diagnostic developments to lower discrepancies between computation and experiments, and indicate that the Hypervelocity Expansion Tube experiments should be successful for the future evaluation of thermochemistry modeling.