Experimental study of the influence of wavy wall on hypersonic boundary layer instability on sharp cone at 6° angle of attack

Proper design of the wavy wall has been validated as an effective means to delay hypersonic boundary layer transition at 0° angle of attack. However, practical hypersonic aircraft often operate at an angle of attack to achieve a high lift-to-drag ratio. The mechanism by which a wavy wall affects hypersonic boundary layer transition at large angles of attack remains unclear. To address this, transition experiments have been conducted on a 7° half angle sharp cone with wavy and smooth surfaces at 6° angle of attack in the Φ 0.25 m Mach 6 Ludwieg tube tunnel of Huazhong University of Science and Technology (HUST). The development of instability waves along the streamwise direction at different azimuthal angles and the infrared transition front were analyzed using PCB high-frequency pressure sensors and high-speed infrared thermography. The coexistence of low-frequency instability waves (20 ～ 70 kHz), high-frequency instability waves (150 ～ 500 kHz), and stationary crossflow waves was observed. Power spectral density (PSD) analysis from PCB measurements shows that the wavy wall promotes the boundary layer transition at azimuthal angles of 0°, 45°, 90°, and 135°, with the most pronounced promotion observed at 0° azimuth where the transition front shifts upstream by approximately 24.1%. Further amplitude calculations, wavelet analysis, and bicoherence analysis results reveal that the amplitude development of high-frequency instability waves on the wavy wall is faster than that on the smooth surface, reaching saturation amplitude earlier, with faster development of nonlinear interaction. Infrared imaging results indicate that the overall transition front of the wavy-wall cone is more forward, and the high-temperature streaks caused by stationary crossflow vortices are more obvious. It is preliminarily inferred that the low-frequency instability waves measured in the experiment are traveling crossflow waves, and the high-frequency instability waves are secondary instability of crossflow vortices. The wavy wall promotes the development of the high-frequency secondary instability modes, ultimately leading to earlier boundary layer transition.