Ablation of carbon-based materials: Investigation of roughness set-up from heterogeneous reactions

Ablation of carbon-based materials is a key issue in atmospheric reentry; it displays a strong coupling between mass, momentum and heat transfers, the importance of which relies on the surface roughness. A new possible physical cause for roughness set-up is investigated, based on the coupling between diffusive transfer in the surrounding fluid on one hand, and heterogeneous reaction or sublimation on the other. Considering mass transfer in a 2D, isothermal, vertical-flux approximation, the surface is proved to be able to acquire, among others, a stable stationary morphology made of circle arcs connected by symmetrical singular points. Such a morphology has indeed been observed in the case of graphite ablation, and the computed roughness length scale, arising from the diffusion-to-reaction ratio, is compatible with observed data. A similar model based on the presence of a thermal gradient yields similar results, but with a larger length scale, also compatible with other observations.