The temporal and spatial evolution of the second-mode instabilities at the final stage of transition in a hypersonic boundary layer is investigated. Experiments are conducted on a flared cone in a Mach 6 wind tunnel using time-resolved Rayleigh-scattering flow visualization, fast-response pressure sensors. The second mode lifts up away from the wall and evolves into a hairpin-shaped vortex at the final stage of transition, which can also be indicated by a rapid broadening of the disturbance amplitude spectra. Then, these hairpin vortices deform, mutually interact, and break down to smaller scales rapidly as the flow starts to become turbulent.

中文翻译：

---

高超声速边界层中第二种模式的演变

研究了高超声速边界层过渡最后阶段第二模式不稳定性的时空演变。使用时间分辨瑞利散射流可视化，快速响应压力传感器，在6马赫风洞的扩口圆锥上进行了实验。第二种模式从壁上抬起，并在过渡的最后阶段演变成发夹形涡旋，这也可以通过扰动幅度谱的快速展宽来表示。然后，随着气流开始变得湍流，这些发夹状涡流发生变形，相互相互作用并迅速分解成较小的比例。