Acoustic resonance in the potential core of subsonic jets
Acoustic resonance in the potential core of subsonic jets
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DOI：
10.1017/jfm.2017.346
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发表时间：
2017-07
影响因子：
3.7
通讯作者：
A. Towne;A. Cavalieri;P. Jordan;T. Colonius;O. Schmidt;V. Jaunet;G. Brès
中科院分区：
文献类型：
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作者：
A. Towne;A. Cavalieri;P. Jordan;T. Colonius;O. Schmidt;V. Jaunet;G. Brès
本文的目的是描述和模拟在亚音速射流的潜在核心内观察到的波，并将它们与以前观察到的近喷嘴区域的音调相关联。波检测的数据从大涡模拟的马赫数0.9等温射流和建模使用平行和弱非平行线性模态分析的欧拉方程线性化的湍流平均流，以及简化模型的基础上的圆柱形涡面和声学模式的圆柱形软管道。除了开尔文-亥姆霍兹不稳定波，三种类型的波与负相速度被确定在潜在的核心：上游和下游传播的管道状的声学模式，经验的剪切层作为一个压力释放表面，因此径向局限于潜在的核心，和上游传播的声学模式，代表一个弱耦合之间的射流核心和自由流。缓慢的流向收缩的潜在核心强加了一个依赖于频率的结束条件的波，被建模为一个弱非平行近似波的转折点。这些转折点提供了一种机制，上游和下游行波可以通过反射和透射过程相互作用和交换能量。与由喷嘴提供的第二端部条件配对，这导致在有限频带中共振的可能性，该有限频带由复波数平面中的两个鞍点限定。预测的频率与在喷流外部检测到的观测到的音调非常匹配。涡面模型，然后使用系统地探讨马赫数和温度比的依赖性的现象。对于等温射流，该模型表明，共振可能发生在一个狭窄的马赫数范围内，
The purpose of this paper is to characterize and model waves that are observed within the potential core of subsonic jets and relate them to previously observed tones in the near-nozzle region. The waves are detected in data from a large-eddy simulation of a Mach 0.9 isothermal jet and modelled using parallel and weakly non-parallel linear modal analysis of the Euler equations linearized about the turbulent mean flow, as well as simplified models based on a cylindrical vortex sheet and the acoustic modes of a cylindrical soft duct. In addition to the Kelvin–Helmholtz instability waves, three types of waves with negative phase velocities are identified in the potential core: upstream- and downstream-propagating duct-like acoustic modes that experience the shear layer as a pressure-release surface and are therefore radially confined to the potential core, and upstream-propagating acoustic modes that represent a weak coupling between the jet core and the free stream. The slow streamwise contraction of the potential core imposes a frequency-dependent end condition on the waves that is modelled as the turning points of a weakly non-parallel approximation of the waves. These turning points provide a mechanism by which the upstream- and downstream-travelling waves can interact and exchange energy through reflection and transmission processes. Paired with a second end condition provided by the nozzle, this leads to the possibility of resonance in limited frequency bands that are bound by two saddle points in the complex wavenumber plane. The predicted frequencies closely match the observed tones detected outside of the jet. The vortex-sheet model is then used to systematically explore the Mach number and temperature ratio dependence of the phenomenon. For isothermal jets, the model suggests that resonance is likely to occur in a narrow range of Mach number, $0.82