Characteristics of transition to turbulence over a Mach 6 flared cone

We study turbulence production at the end of a second-mode-induced transition in a Mach 6 boundary layer based on both experiments and numerical simulations. By using ultra-fast visualization and particle image velocimetry, we succeed in capturing a soliton-like wave packet that plays a determining role in turbulence production. We reproduce the experiment by direct numerical simulation, revealing that this wave packet arises from the vortical first Mack mode (the counterpart of Tollmien–Schlichting waves in low-speed boundary layers), rather than the rapidly growing dilatational second Mack mode, which triggers a Λ-vortex and consequent turbulent structure. These results reveal the existence of a transition mechanism in hypersonic boundary layers similar to that in low-speed boundary layers.