title: Extension of the aerothermodynamics modelling in direct simulation Monte Carlo
Creators: name: Civrais, Clément Henri Bernard
Publisher: name: University of Glasgow
Year: 2024

description: The modelling of aerothermodynamic flows is extremely challenging as it combines both aerodynamics and thermodynamics disciplines. Aerothermodynamic applications typically involve high-speed and high-temperature gas flows, giving rise to many processes to the extent of large variation in the transport properties of the gas. This thesis concentrates on expanding the current state of the art of aerothermodynamic modelling for a moderate to high degree of rarefaction. Over the years, the direct simulation monte carlo method (DSMC) has emerged as the standard method for simulating rarefied gas flows due to its ability to model complex non equilibrium effects in the flows such as non-equilibrium in the internal degrees of freedom of molecules or non-equilibrium chemical processes in great detail. However, the traditional approaches for modelling the vibrational excitation of molecules, the chemical reactions and the interaction between internal modes in the DSMC method present some limitations.

The first research axis focuses on the vibrational modelling of molecular species with an anharmonic oscillator model. This has been motivated by the fact that high-fidelity calculations or state-of-the-art radiation solvers compute the vibrational excitation with an anharmonic oscillator model, whereas the standard approach in the DSMC method relies on a harmonic oscillator model. Therefore, the first objective of this thesis is to quantify the difference between the standard approach in the DSMC method and an anharmonic oscillator mode for the reproduction of the thermodynamic properties. The quantification analysis is then extended to the context of an Earth’s planetary reentry of a cylindrical body at an altitude of about 75 km. 

The second research axis focuses on the extension of the quantum kinetics (QK) chemistry models, in which vibrational excitation is modelled with an anharmonic oscillator model. This custom version of the QK models has been constructed by a generalisation of the reaction rates to incorporate the modelling of the vibrational excitation with an anharmonic oscillator model. These formulations are extensively investigated for the most representative dissociation reactions occurring in an Earth’s atmospheric reentry under thermal equilibrium and nonequilibrium conditions. The new formulations are compared against an extensive compilation of well-established theoretical chemistry models, experimental measurements, and high-fidelity calculations. Through this comprehensive study, the limitations of the new formulations are identified, demonstrating an excessive utilisation of the relative translational energy and underutilisation of the vibrational energy to promote dissociation reactions. Based on these observations, an extension of these formulations of QK models is herein proposed incorporating tunable parameters to accurately reproduce the most representative experimental measurements and high-fidelity calculations in both thermal equilibrium and non-equilibrium conditions. These formulations are then applied for the reproduction of in-flight measurements of the surface heat flux experienced by the Space Shuttle Columbia during its second mission at an altitude of about 92.35 km.

The third research axis focuses on the development of a novel model for electronic excited states of molecular species in DSMC. The standard approach in the DSMC method is to treat separately each mode of a chemical species which prohibits any interaction between internal modes. However, aerothermodynamic processes involve the coupling between all internal modes which becomes particularly significant in the analysis of molecular radiation, where achieving the correct distribution of vibrational and electronic excitation energy is crucial. As a result, a novel model that assumes a coupling between the vibrational and electronic modes allowing each electronic excited state to excite its vibrational quantum levels is proposed. Considering the challenge of measuring experimentally chemical processes involving electronic excited states, the novel model is verified against an extensive compilation of theoretical studies. Additionally, the model is applied for a canonical hypersonic flow in Earth’s atmosphere past an infinite cylindrical body at an altitude of 85 km.