Effect of key parameters on receptivity of high-speed cone boundary layer to freestream slow acoustic waves

Predicting the boundary-layer transition process is a hot research issue in aerodynamics as well as an engineering demand for aircraft design. The e^N method based on linear stability theory is widely regarded as the most scientific method for transition prediction, which has been applied in engineering practices. However, this method has theoretical shortcomings as it traditionally ignores the receptivity of the boundary layer to external disturbances, i.e., the process of exciting boundary-layer unstable waves by external disturbances. The influence factors of receptivity are numerous, but at present, their influence laws are relatively less studied and not systematically understood yet. In this study, the high-speed cone boundary layer is taken as the object to investigate the influence of key parameters such as nose bluntness, wall temperature, unit Reynolds number, and frequency on the receptivity of boundary layers to freestream slow acoustic waves. The used methods include numerical simulations of receptivity and linear stability theory. It is found that, overall, the receptivity coefficient decreases as the nose bluntness increases, and increases as the wall temperature decreases, but is almost unaffected by the unit Reynolds number. For freestream disturbances with different frequencies, the unstable mode excited in the boundary layer under adiabatic or heated wall temperature conditions include the first and second modes, among which the receptivity coefficient of the first mode is 1-2 orders of magnitude higher than that of the second mode. In addition, the receptivity coefficient decreases with increasing frequency in the first-mode excited frequency range, while it is mainly affected by the nose bluntness in the second-mode excited frequency range. The receptivity coefficient decreases first then increases with the frequency for small bluntness, decreases monotonically with the frequency for medium bluntness, and increases first then decreases for large bluntness.