Effects of Injection Gas Composition on the Stability of a High-Enthalpy Flow over a Blunt Cone

Vehicles traveling at hypersonic speeds encounter tremendous heat load near the stagnation region. To protect the underlying structure, transpiration cooling can be incorporated by employing gas injection, especially in the areas with high heat transfer rates, such as leading edges and nose regions. This study aims to understand how transpiration cooling applied from the stagnation region until 100 nose radii downstream of a blunt cone affect the stability characteristics of a high-enthalpy flow corresponding to T5 tunnel conditions. Air and CO$_2$ are considered as injection gases with different mass fractions and blowing rates. Although injected at the same rates, the larger molecular weight of CO$_2$ leads to a thicker boundary layer than for Air injection. A significant difference for the two injection gases is noticed between the gas properties in addition to the mean flow profiles along the boundary layer especially at larger blowing rates. A region of thermal-equilibrium flow is generated near the wall with CO$_2$ injection, which is not observed for Air even at larger injection rates. Injection with CO$_2$ is found to be more destabilizing as compared to air injected at a similar mass-flux. Increasing the injection rate leads to larger unstable regions in frequencies with higher disturbance amplitudes and shifts towards lower frequency content. In addition, as the injection velocity is enhanced, the neutral curve moves upstream towards the nose region of the cone. The transition location is found to vary linearly at lower blowing rates for both air and CO$_2$.