Atmospheric re-entry of end-of-life satellites and numerical modelling of heat diffusion within anisotropic materials.

Today, the control of the end of life of a satellite is essential in order to limit the pollution of earth orbits on the one hand, and to reduce the risks of material and/or human damage linked to their uncontrolled re-entry into the atmosphere on the other hand. It is thus imperative to preserve some energy to deorbit the satellite at the end of its life and place it on an appropriate atmospheric re-entry trajectory. During the re-entry, the flow and the wall of the object are in strong interaction: the parietal heating linked to the transformation of kinetic energy into heat, leads to the degradation of the material by pyrolysis, oxidation, erosion and even sublimation. The surface of the satellite is then altered, which modifies its aerodynamic characteristics in return. The degradation must be sufficient on the trajectory to burn completely and thus destroy the satellite before it reaches the ground. ONERA is developing numerical tools to simulate the interaction between aerothermo-dynamics (hypersonic flow with chemical kinetic effects) and the thermochemical response of the material. In particular, the MoDeTheC code allows to model the heat and mass transfers in a porous anisotropic composite material, as well as its volumetric or surface degradation under the effect of external aggressions. An important component of this code is the modeling of heat diffusion. The objective of the work described in this thesis is to revisit the diffusive scheme already implemented in MoDeTheC, namely the scheme proposed by N. Leterrier in 2003, and replace it with a more advanced diffusive scheme proposed by P. Jacq in his 2015 thesis work. An in-depth study of the two schemes was conducted by implementing both of them on a code called DEEP-DIVE, which served as a testbed to analyze the numerical characteristics of the two diffusive methods and understand their peculiarities. In a second part of the thesis work, using the MoDeTheC code, some simulations were carried out to study the atmospheric re-entry of a sphero-conical body. The main focus of this second part of the work was to understand the influence of anisotropy on the physics of atmospheric re-entry by comparing the results obtained in the isotropic case with those obtained by modifying the thermal conductivity matrix in such a way as to introduce an anistropic component in the material.