逆压梯度附着与分离边界层的不稳定机制及间歇性分布
Instability Mechanisms and Intermittency Distribution in Adverse Pressure Gradient Attached and Separated Boundary Layers
摘要：
本文对逆压梯度影响下的一个附着和两个分离边界层流动（包括小分离和大分离）进行了直接数值模拟。已经提供了三种情况下的过渡前边界层的统一图像，该图像揭示了"混合模式"不稳定性，包括来自不稳定波和流向条纹的贡献。对转捩速度信号的时频分析表明，随着雷诺数的减小，转捩速度信号的时频特征从"斑点"性质不断演变（表现出明显的湍流斑点），以实现"非斑点"行为（涉及湍流波动在时间上的更"均匀"分布）对于大分离情况，涵盖了整个过渡情景。过渡区内的不透明度因子的变化被认为是比较以及与Narasimha普遍不透明度分布。我们发现，虽然大分离的速度的时间变化是非斑点（或更“均匀”），展向变化的速度是斑点，表现出明确的集群高波数波动分离准层流区域。因此，所有病例均在过渡区表现出斑点，但表现形式不同。利用涡壁相互作用和展向涡结构不稳定性的思想，我们给出了附着和分离情况下转捩情景的物理动画。我们发现，集中击穿表现出的所有三种情况下，过渡开始和现场击穿过程是广泛一致的普遍的不稳定性分布的基本假设。
英文摘要：
Direct numerical simulation has been carried out on one attached and two separated boundary layer flows (involving small and large separation) under the influence of an adverse pressure gradient. A unified picture of the pre-transitional boundary layer for the three cases has been provided that reveals a "mixed-mode" instability, involving contribution from instability waves and streamwise streaks. A time-frequency analysis of the transitional velocity signals has been performed which shows that as the Reynolds number decreases, the character of the time traces evolves continuously from a "spotty" behaviour (exhibiting distinct turbulent spots) for the attached case to a "non-spotty" behaviour (involving more "uniform" distribution of turbulent fluctuations in time) for the large separation case, encompassing the entire spectrum of transition scenarios. The variation of the intermittency factor within the transition zone is seen to compare well with the Narasimha universal intermittency distribution. We find that although the time variation of velocity for large separation is non-spotty (or more "uniform"), the spanwise variation of velocity is spotty, showing a clear clustering of high-wavenumber fluctuations separated by quasi-laminar regions. Thus, all the cases exhibit spottiness in the transition zone with different manifestations. We present a physical cartoon for the transition scenarios for the attached and separated cases, using the ideas of vortex-wall interaction and instability of spanwise vortical structures. We find that concentrated breakdown is exhibited by all the three cases near the transition onset and the spot breakdown processes are broadly consistent with the postulates underlying the universal intermittency distribution.
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