Simulation of Second-Mode Instability in a Real-Gas Hypersonic Flow with Graphite Ablation

A new high-order shock-fitting method with thermochemical nonequilibrium and finite-rate chemistry boundary conditions for graphite ablation is presented. The method is suitable for direct numerical simulation of boundary-layer transition in a hypersonic real-gas flow with graphite ablation. The new method is validated by comparison with three computational datasets and one set of experimental data. Direct numerical simulations were run for a 7 deg half-angle blunt cone at Mach 15.99 to find how graphite ablation and thermochemical nonequilibrium affect boundary-layer receptivity and instability. The real-gas simulation is compared with ideal-gas simulations that set their wall temperature and wall blowing from the real-gas simulation. Weak planar fast-acoustic waves in the freestream are used to perturb the steady base flow. A 525 kHz second-mode wave was found to be significantly unstable for the real-gas simulation, whereas in the ideal-gas simulations, no significant flow instability was seen. For the specific flow conditions tested, it was found that real-gas effects significantly destabilize second-mode waves.