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Ablative heat-shield materials are a crucial component of atmospheric entries. Comprehensive experiments in their operative conditions, coupled with numerical work, are fundamental to advance the knowledge of their response for thermal protection system design. Most numerical models describe oxidation and sublimation, but do not treat nitridation of the carbon surface, which may impact surface recession and heat flux. We present experiments of graphite ablation in nitrogen plasmas, produced by the VKI Plasmatron facility, aiming at the estimation of nitridation reaction probabilities of carbon in a relevant temperature range relevant for heat-shield operation, and determination of CN species densities in the boundary layer for comparison and validation of the numerical model. The gas-phase radiative signature was observed using a high resolution spectrometer, combined with a two-dimensional ICCD array. From the rebuilding of local emission intensities, obtained through Abel inversion, local mole fractions of the CN molecule were estimated by comparison to a spectroscopic model. Nitridation reaction probabilities were extracted from the tests using a model including an ablative boundary condition. The computed stagnation-line CN species densities agree very well with those obtained by the experimental approach. An estimation of the translational-rotational and vibrational-electronic temperatures Trot and Tvib of the CN violet system yielded strong evidence that Trot and Tvib are far from thermal equilibrium at low surface temperatures (low Plasmatron power), but seemed to equilibrate at higher surface temperatures. Our investigation is yet inconclusive on the origin of the high deviation of estimated rotational and vibrational temperatures, but the gathered data suggest that non-dissociated nitrogen in the boundary layer might play a significant role in the vibrational excitation of the CN molecule.

The work is focused on numerical simulations of roughness induced transition for hypersonic flow on a flat plate wall mounted roughness element. Numerical simulations are compared to experimental results in order to resemble the physics highlighted in the tests. In particular, stress has been placed on the detection of the vortices in the wake behind the roughness element and on the onset of transition.

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