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Streak Instability and Bypass Transition

条纹不稳定和旁路过渡

基本信息

批准号：
EP/F045093/1
负责人：
Xuesong Wu
金额：
$ 45.01万
依托单位：
Imperial College London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2008
资助国家：
英国
起止时间：
2008 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FF045093%2F1
关键词：
Streak Instability Bypass Transition

项目摘要

Laminar-turbulent transition is a fundamental problem in fluid mechanics that has great implications for the prediction of drag, aerodynamic heating, and flow separation. For example, transition to turbulence can bypass flow separation in compressors, and in the low-pressure-turbine stage, 50% of the flow can be transitional. While the end-state may always be an equilibrium turbulent boundary layer, transition itself can be mediated by different mechanisms and each results in distinctly different transition locations and lengths. The literature generally divides transition scenarios into the natural, or orderly, route and the bypass mechanism. The former is characterized by viscous Tollmien-Schlichting (T-S) instability waves which can be observed experimentally under quiet testing conditions. In the presence of free-stream disturbances, which are loosely referred to as free-stream turbulence (FST), transition takes place more swiftly, thus bypassing the natural route, and hence the term bypass transition . In the bypass mechanism, elongated high-amplitude disturbances, known as boundary layer streaks, are observed in the pre-transitional region of the flow. Breakdown has been attributed to a secondary instability of these streaks, but the exact dependence has only been speculated and never satisfactorily quantified. The proposed research is first to accurately and efficiently compute the non-linear streaks by using a combined theoretical and numerical approach. We will also analyze the stability of these streaks, and our study will provide an estimate for the required threshold of free-stream disturbances for streak instability. It will also give a quantitative relation between the location where the streaks first become unstable and the intensity of the free-stream disturbances. Such information will be useful for the design of airfoils which operate in transitional flow regimes.

层流-湍流转捩是流体力学中的一个基本问题，对阻力、气动加热和流动分离的预测有重要意义。例如，过渡到湍流可以绕过压缩机中的流动分离，并且在低压涡轮机级中，50%的流动可以是过渡的。虽然最终状态可能总是一个平衡的湍流边界层，过渡本身可以通过不同的机制，并在明显不同的过渡位置和长度的结果。文献通常将过渡情景分为自然或有序路线和旁路机制。前者的特点是粘性Tollmien-Schlichting（T-S）不稳定波，可以在安静的测试条件下观察到实验。在存在自由流扰动的情况下，通常被称为自由流湍流（FST），转捩发生得更快，从而绕过了自然路线，因此称为旁路转捩。在旁路机制中，细长的高振幅扰动，被称为边界层条纹，在流动的前过渡区观察。击穿被归因于这些条纹的二次不稳定性，但确切的依赖性只是推测，从未令人满意地量化。该研究首次采用理论和数值相结合的方法准确有效地计算非线性条纹。我们还将分析这些条纹的稳定性，我们的研究将提供一个估计所需的阈值的自由流扰动条纹不稳定性。它还将给出条纹首先变得不稳定的位置与自由流扰动强度之间的定量关系。这些资料对于在过渡流态下工作的翼型的设计是有用的。