The supersonic mode in hypersonic boundary layers has been shown to be associated with an unstable Mack's second mode synchronizing with the slow acoustic spectrum, causing the disturbance to travel upstream supersonically relative to the mean flow outside the boundary layer. The flow conditions leading to the supersonic mode have not been thoroughly and systematically investigated. As a result, it is unknown whether or not the supersonic mode can become the dominant boundary layer instability over the traditional second mode. This work uses thermochemical nonequilibrium Linear Stability Theory (LST) to obtain a more complete investigation of the supersonic mode using both nonequilibrium and perfect gas models. The mean flow is obtained from Direct Numerical Simulation (DNS) assuming both thermochemical nonequilibrium and frozen flow models. The purpose is to analyze the real gas effect on the supersonic mode at conditions typical of experimental conditions. The simulation is Mach 5 flow over a 1 mm nose radius axisym-metric cone 1 meter in length. The LST results indicate that vibrational nonequilibrium effects are stabilizing to the second and supersonic modes. The nonequilibrium effects in the mean flow, however, are not as critical. Overall, the LST results demonstrate the importance of accounting for nonequilibrium effects when studying the supersonic mode.
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