surface instabilities at ablating interfaces

Ablating surfaces are often used in hypersonic applications as a thermal protection system. Numerical simulations of the interaction between the hypersonic flow and the ablating surface have a tendency to develop high wavenumber surface instabilities. In this work, the nature of the instabilities is investigated through numerical simulation of both a simplified model and the fully nonlinear coupled ablation system. In order to understand the nature of the instability, a simplified model of the linear inviscid flow over an isothermal (linear) porous ablator subject to small surface perturbations is studied. In the simplified model, the surface recession rate is proportional to the absolute value of the gas velocity at the surface. This is a representation of the well-known B' approach to surface ablation. Despite the fluid and porous material dynamics both having been linearized, the presence of the absolute value in the interface model still introduces a nonlinearity. Studying this semi-analytical system numerically shows the system to be unstable with amplification in the highest wavenumbers that are numerically represented.Moving to the full ablation system with nonlinear coupled physics, the supersonic flow over an ablating flat plate with a blunted leading edge is simulated. The results show the same type of behavior observed in the simplified model, where high wavenumber structures start developing along the surface. It is then demonstrated, that in both problems considered, the use of a low-pass filter can successfully mitigate the growth of the high wavenumbers.