Mechanism for frustum transition over blunt cones at hypersonic speeds
Mechanism for frustum transition over blunt cones at hypersonic speeds
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DOI：
10.1017/jfm.2020.261
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发表时间：
2020-05
影响因子：
3.7
通讯作者：
P. Paredes;Meelan Choudhari;Fei Li
中科院分区：
文献类型：
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作者：
P. Paredes;Meelan Choudhari;Fei Li
数值和实验研究表明，尖锥上高超音速边界层的层流-湍流转捩是通过平面Mack模不稳定性的模增长实现的。然而，由于马克模式增长在更高的鼻子钝值的强烈减少，所观察到的锥截头体上的转变的发病机制是目前未知的。线性非模态增长分析表明，对于中到大的头部钝度，在熵层内达到峰值的平面和斜向行进扰动都经历了可观的能量放大。然而，由于其边界层区域内的弱签名，通过非模态增长的旅行扰动过渡开始的路线仍然不清楚。非线性抛物化稳定性方程（NPSE）和直接数值模拟（DNS）被用来识别在AFRL马赫数6高雷诺数设备中测试的7度钝锥上转捩的潜在机制。具体地说，计算进行研究的非线性发展的一对倾斜的，非定常的非模态干扰的制度，适度钝头。NPSE和DNS预测之间表现出良好的一致性。结果表明，即使线性非模态扰动主要集中在边界层外，它们的非线性相互作用可以产生稳定的条纹，穿透和放大边界层内，最终诱导的转折点通过这些条纹的崩溃。结果表明，当一对倾斜的受控非模态扰动的初始幅值取自由流速度的0.15%左右时，可以在实验测得的位置处产生转捩。
Numerical and experimental studies have demonstrated laminar–turbulent transition in hypersonic boundary layers over sharp cones via the modal growth of planar Mack-mode instabilities. However, due to the strong reduction in Mack-mode growth at higher nose bluntness values, the mechanisms underlying the observed onset of transition over the cone frustum are currently unknown. Linear non-modal growth analysis has shown that both planar and oblique travelling disturbances that peak within the entropy layer experience appreciable energy amplification for moderate to large nose bluntness. However, due to their weak signature within the boundary-layer region, the route to transition onset via non-modal growth of travelling disturbances remains unclear. Nonlinear parabolized stability equations (NPSE) and direct numerical simulations (DNS) are used to identify a potential mechanism for transition over a 7-degree blunt cone that was tested in the AFRL Mach-6 high-Reynolds-number facility. Specifically, computations are conducted to study the nonlinear development of a pair of oblique, unsteady non-modal disturbances in the regime of moderately blunt nose tips. Excellent agreement was demonstrated between the NPSE and DNS predictions. Results reveal that, even though the linear non-modal disturbances are primarily concentrated outside the boundary layer, their nonlinear interaction can generate stationary streaks that penetrate and amplify within the boundary layer, eventually inducing the onset of transition via the breakdown of these streaks. The results indicate that a pair of oblique, controlled non-modal disturbances can produce transition at the location measured in the experiment when their initial amplitude is chosen to be approximately 0.15 % of the free-stream velocity.