Skin-friction measurements in hypersonic transitional boundary layer (HTBL) flows on a flared cone at Mach 6 at zero angle of attack are obtained using oil-film interferometry (OFI). In addition, the amplitude evolution of instability waves is investigated using fast-response pressure sensors and Rayleigh-scattering flow visualization (RSFV). The connection between the skin-friction coefficient and near-wall dynamics during the whole transition process is illustrative and the underlying mechanism of the generation of the wall skin friction is revealed. There exist two peaks of skin-friction rise along the streamwise direction of the model. The first one is at the transitional region (denoted as FS) which is caused by the second-mode instability. The second one is at the region where the transition is completed (denoted as FT). The drastic growth of skin friction near the end of transition is mainly attributed to the rapid boundary-layer breakdown into small scales. It is found that the strong shear in the near-wall region in the quiet zone where dominant hairpin-shaped vortices evolved from the second modes can also cause a local skin-friction rise (denoted as FQ). Further investigations show that in the region between FS and FQ, there is a slight increase in the skin friction, which is associated with the growing low-frequency waves. The skin friction coefficient value decreases slowly after FT but does not reach the expected turbulent theoretical value by the end of the test domain, indicating that the development towards the completely turbulent boundary layer appears to be a gradual process.

中文翻译：

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在高超声速过渡边界层流中使用油膜干涉测量表面摩擦


使用油膜干涉测量法 (OFI) 获得零攻角、马赫数 6 的扩口锥体上高超音速过渡边界层 (HTBL) 流的表面摩擦测量。此外，还使用快速响应压力传感器和瑞利散射流可视化 (RSFV) 研究了不稳定波的振幅演化。整个转变过程中蒙皮摩擦系数与近壁动力学之间的联系是说明性的，并且揭示了壁面蒙皮摩擦产生的潜在机制。沿着模型的流向存在两个表面摩擦力上升峰值。第一个是在过渡区域（表示为 FS），这是由第二模式不稳定性引起的。第二个是过渡完成的区域（记为 FT）。接近转变结束时表面摩擦的急剧增长主要归因于边界层快速分解成小尺度。研究发现，由第二模态演化而来的主导发夹形涡的静区近壁区域的强剪切也会引起局部蒙皮摩擦力的增加（记为FQ）。进一步的研究表明，在 FS 和 FQ 之间的区域，皮肤摩擦力略有增加，这与低频波的增长有关。 FT后表面摩擦系数值缓慢下降，但在测试域结束时并未达到预期的湍流理论值，表明向完全湍流边界层的发展似乎是一个渐进的过程。