Thermochemical nonequilibrium modeling

in state-resolved high fidelity method

Motivation

Shock-wave angle and shape is one of the important factors to design a re-entry vehicle and hypersonic aircraft because the shock-shape changes the center of aerodynamic force of the hypersonic vehicles. In compressible flow, this shock-shape is mostly affected by the specific heat ratio of real gas, and this specific heat ratio is determined from the internal energy population of gas species. Such internal energy populations of gas species are mainly occurred by the energy transitions and chemical reactions due to electron and heavy-particle collision and radiative transitions in the nonequilibrium states. The other important factor in designing a hypersonic vehicle is the heat transfer on the vehicle surface by the aerodynamic heating. Such aerodynamic heating is mostly caused by the non-Boltzmann radiation and the nonequilibrium chemical reactions. The nonequilibrium populations of the internal energies are closed related to these non-Boltzmann radiation and nonequilibrium chemical reactions behind a strong shock wave. Hence, the improvement of thermochemical nonequilibrium modeling in predicting the nonequilibrium population of the internal energies is needed in the accurate aerodynamic analysis of the hypersonic vehicles.

In previous hypersonic researches, the aerothermodynamic analyses were carried out by low-fidelity and empirical thermochemical models based on the experimental data from hypersonic facilities. In 1970’s to 1990’s, Apollo projects and space shuttle program led these hypersonic thermophysics researches, and the outcomes was enough to perform the United States space program. In the recent years, the remaining hypersonic work involves the space explorations, hypersonic commercial vehicles, and unsolved hypersonic problems. However, these remaining hypersonic works are somewhat different to the previous Apollo and space shuttle programs. In the space exploration, we need to analyze the aerodynamic characteristics of high-speed return capsule into Earth and the outer planets. In these re-entry cases, the previous thermochemical nonequilibrium model has limitations in estimating the shock-distance and measured nonequilibrium temperatures. In hypersonic commercial vehicles, the accuracy of thermochemical nonequilibrium model must be improved for the safety of passengers. In the last, the unsolved hypersonic problems are mainly related to the uncertainties of the previous thermochemical nonequilibrium models. In resolving such arising and future difficulties, the improvement of thermochemical nonequilibrium models is needed. In carrying out these new projects, the aerospace engineers need to understand the atomic and molecular structures from quantum mechanical view, need to improve the collisional and radiative transition cross sections by quasi- and semi-classical and quantum mechanical calculations, and need to derive the high-fidelity thermochemical nonequilibrium models from the microscopic calculations.

Research subjects

The research of our group is related to such improvement of thermochemical nonequilibrium models. In carrying out the thermochemical modeling work, we need five steps. 1) ab-initio electronic structure calculations, 2) designing a potential energy surface in hyper-dimensional space by the results of the ab-initio quantum calculation, 3) obtaining the complete sets of the state-to-state transition cross sections from trajectory calculations by quasi-/semi-classical or quantum mechanical methods, 4) master equation analysis in observing the thermochemical nonequilibrium phenomena, and 5) constructing the hydrodynamic equations and applications in continuum and rarefied flows. In performing each step, there exist a lot of difficulties coming from the complication of the molecular physics and the limitation of computational resources. The target species of present research is H2, He, N2 and O2. H2 and He are the main atmospheric gas of the outer planets and N2 and O2 are the main gas of Earth.

Acknowledgement

Funding from Air Force Office of Scientific Research Grant FA-9550-11-1-0309 and FA-9550-12-1-0483

Publications