Physical models for nonequilibrium plasma flow simulations at high speed re-entry conditions

The project focuses on diatomic gases with vibrational and electronic mode disequilibrium, environments commonly encountered in the shock layer of spacecrafts while re-entering into earth atmosphere. The known-how gained during former aerospace missions allows for the key role of Computational Fluid Dynamics (CFD) simulations in the development of hypersonic applications to be highlighted in strong interaction with ground testing. The relevance of these simulations is linked to aerothermochemistry features such as high-temperature gas effects in hypersonic flows. For instance, the design of the heat shield used to protect spacecraft is based on the estimation of the heat fluxes to the vehicle surface by means of experimental and numerical resources. CFD predictions of these quantities strongly rely upon the accuracy of the model used to describe the flow. During the entry of a spacecraft into a planetary atmosphere, the translational energy of the fluid particles rises through the shock. A high number of collisions is then needed to equilibrate the internal energy modes (electronic for atoms; rotational, vibrational, and electronic for molecules) with the translational one. Hence, these modes turn out to be in nonequilibrium at the convective time scale. In addition, particles dissociate, recombine, and ionize in the shock-layer, the flow is found to be in chemical nonequilibrium. The prediction of the heat fluxes strongly depends on the completeness and accuracy of the physical model used to describe thermo-chemical nonequilibrium phenomena. There is thus a critical need to develop an accurate model for the Lunar and Martian return missions. In this manucript, we present a finite rate chemistry mechanism to determine the species concentration. The rotational mode is considered in equilibrium with the translational mode. Vibrational energy and free electron kinetic energy equations deal with thermal nonequilibrium. Radiative heating can approach and possibly exceed the level of convective heating that results from the frictional flow of the atmosphere over the thermal protection material. This situation occurs during Earth's reentries at given the large amount of radiators produced in the shock layer at hypervelocity ($ v > 10$ km/s). Radiation modeling involves the determination of population distributions over the internal energy levels and of the radiative contribution of each of these levels. In flight conditions, the electronic energy level populations are expected to depart from equilibrium. We will resort here to a hybrid collisional-radiative/Boltzmann model. The model adopted, combines an electronic collisional-radiative model to determine the population of the electronic energy levels by solving a system of rate equations and Boltzmann distributions for the rotational and vibrational energy level populations. One-dimensional shock-tube and nozzle flow solvers have been developed in order to validate the models. The models have been assessed by means of comparison between the computed results and data obtained by hypervelocity demonstrators and shock-tube experiments representative of flight conditions. The detailed CR model employed for the analysis of nonequilibrium flows in compressing as well as expanding situations has been used as a baseline to create a new reduced kinetic mechanisms for air flow conditions typical of reentry applications. The number of electronic levels of atoms and molecules considered in the model has been reduced by grouping similar energy levels. This allowed us to extend the use of the CR model for 2 and 3D computational fluid dynamic simulations. The final part of the thesis is devoted to the implementation of the simplified model in multidimensional (2D and 3D) solver for re-entry application, which allows to simulate the flowfield surrounding a space vehicle while reentering into earth atmosphere.