Wavepackets and trapped acoustic modes in a turbulent jet: coherent structure eduction and global stability
Wavepackets and trapped acoustic modes in a turbulent jet: coherent structure eduction and global stability
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DOI：
10.1017/jfm.2017.407
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发表时间：
2017-07
影响因子：
3.7
通讯作者：
O. Schmidt;A. Towne;T. Colonius;A. Cavalieri;P. Jordan;G. Brès
中科院分区：
文献类型：
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作者：
O. Schmidt;A. Towne;T. Colonius;A. Cavalieri;P. Jordan;G. Brès
通过对马赫数为0.9、雷诺数为10 ^{6}$的湍流射流的大涡模拟，得到了射流的相干特征。除了众所周知的Kelvin-Helmholtz不稳定性的剪切层，一类新的被困的声波被确定的潜在的核心。基于湍流平均流进行了全局线性稳定性分析。被捕获的声波在整体谱中形成离散特征值的分支，并且相应的整体模式精确地匹配导出的结构。离散的捕获声学模式发生在由其径向和轴向顺序确定的层次结构中。局部色散关系构造的全球模式，并发现同意有利的经验色散关系推导出的模拟数据。直接和伴随模式之间的产品，然后被用来隔离被困波。在某些条件下，共振形式的拍频之间发生捕获声波的正和负群速度。这种共振解释了为什么被困模式在模拟中被突出地观察到，并且在以前的实验研究中作为音调。在过去，这些音调被归因于外部因素。在这里，我们表明，它们是一个内在的特点，高亚音速射流，可以明确地确定了全球线性稳定性分析。
Coherent features of a turbulent Mach 0.9, Reynolds number $10^{6}$ jet are educed from a high-fidelity large eddy simulation. Besides the well-known Kelvin–Helmholtz instabilities of the shear layer, a new class of trapped acoustic waves is identified in the potential core. A global linear stability analysis based on the turbulent mean flow is conducted. The trapped acoustic waves form branches of discrete eigenvalues in the global spectrum, and the corresponding global modes accurately match the educed structures. Discrete trapped acoustic modes occur in a hierarchy determined by their radial and axial order. A local dispersion relation is constructed from the global modes and found to agree favourably with an empirical dispersion relation educed from the simulation data. The product between direct and adjoint modes is then used to isolate the trapped waves. Under certain conditions, resonance in the form of a beating occurs between trapped acoustic waves of positive and negative group velocities. This resonance explains why the trapped modes are prominently observed in the simulation and as tones in previous experimental studies. In the past, these tones were attributed to external factors. Here, we show that they are an intrinsic feature of high-subsonic jets that can be unambiguously identified by a global linear stability analysis.