Roughness Induced by Chemical Ablation in Carbon Based Ablators: Onset and Macro-Scale Handling

Roughness of heat-shield materials has been studied in the frame of aerodynamics and the laminar-turbulent transition through a wide range of experimental and numerical works. On the contrary, the dynamic link between chemical ablation and roughness has been poorly tackled. In the present work, the role of chemical reaction in the onset, development and stabilization of composite roughness is exposed. A second part is dedicated to the handling, at macro-scale, of an heterogeneous rough reactive surface. By design composites are chemically non-uniform. At surface, this property leads to a different behavior of the material components with respect to the oxidants brought by the surrounding flow. A minimal micro-scale model coupling diffusion and reaction is used to illustrate roughness evolution from onset to stabilization. The use of 3D numerical simulation feeds numerical experiments that are successfully compared to SEM observations. At TPS scale, the chemical reactions leading to ablative mass loss are seen as being purely surfacic. Indeed as classical roughness pattern is two or three magnitude scale smaller than the vehicle, a direct handling is not a suitable approach. In order to take into account the effect of roughness, an upscaling of the phenomenon occurring at micro-scale is proposed. This procedure uses a domain decomposition technique. The theoretical transformation of equations allows building, through closure relations, an explicit link between micro-scale reaction and macro-scale mass loss. In this study, the theoretically obtained relations are validated by comparing a direct numerical simulation and the homogenized problem