Effects of Injection Gas Composition on High-Speed Boundary Layer Instability and Transition

High-speed vehicles experience severe aero-thermal loads, especially around the stagnation region and leading edges. To protect the underlying structure, transpiration cooling can be incorporated by employing gas injection which can dramatically reduce the heat transfer to the vehicle surface. Transpiration cooling first reduces the wall temperature through convection when the coolant passes through the porous media and secondly, it also forms a protective gas film reducing the heat transfer from the hot outside gas. Our work aims at understanding how transpiration cooling applied only in the nose region affects the stability characteristics of a Mach 9.81 flow around a blunt cone as compared to downstream injection. We will consider different injection gases and analyze how changing the composition of the gas mixture affects the flow transition to turbulence. Gas injection alters the boundary layer profiles by changing the peak temperature in addition to affecting the boundary layer thickness. We observe that the N-factors are not affected significantly if the injection of air is limited only in the nose region. However, if the injection is applied further downstream too, it can stabilize or severely destabilize the flow depending on the injection rates and types of gases used. In the case of air injection, we observe that increasing the blowing rates shifts the unstable flow region to a lower frequency range due to an increase in the boundary layer thickness. We also notice the appearance of the supersonic mode, in addition to the dominant second mode in the flow field at certain blowing rates.