Supersonic mode in a low-enthalpy hypersonic flow over a cone and wave packet interference

A computational fluid dynamics study is conducted in which acoustic-like waves are observed emanating from the boundary layer of a Mach 8 slender blunt cone with a relatively low freestream enthalpy and a warm wall. The acoustic-like wave emissions are qualitatively similar to those attributed to the supersonic mode. However, the supersonic mode responsible for such emissions is often found in high-enthalpy flows with highly cooled walls, making its appearance here unexpected. Linear stability analysis on the steady-state solution reveals an unstable mode S (Mack's second mode) with a subsonic phase velocity and a stable mode F whose mode F- branch takes on a supersonic phase velocity. It is thought that the stable supersonic mode F- is responsible for the acoustic-like wave emissions. Unsteady simulations are carried out using blowing-suction actuators at two different surface locations. The analysis of the temporal data and spectral data using Fourier decomposition reveals constructive/destructive interference occurring between a primary wave packet and a satellite wave packet in the vicinity of the acoustic-like wave emissions. The constructive/destructive interference between the wave packets also appears to have a damping effect on individual frequency growth in both unsteady simulations. Based on this study's results and analysis, it is concluded that a supersonic discrete mode is not limited to high-enthalpy, cold wall flows and that it does appear in low-enthalpy, warm-wall flows; however, the mode is stable.