Wong Marcus H, Kirby Rhiannon, Jordan Peter, Edgington-Mitchell Daniel
Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC), Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC 3800, Australia.
Département Fluides, Thermique, Combustion, Institut Pprime, CNRS-Université de Poitiers-ENSMA, Poitiers, 86000, France.
J Acoust Soc Am. 2020 Oct;148(4):2015. doi: 10.1121/10.0002166.
Acoustic measurements of unheated supersonic underexpanded jets with ideally expanded Mach numbers of 1.14, 1.38, and 1.50 are presented. Of the three components of supersonic jet noise, the focus is on the broadband shock-associated noise (BBSAN) component. Motivated by the modelling of BBSAN using the wavepacket framework, a traversable microphone ring is used to decompose the acoustic pressure into azimuthal Fourier modes. Unlike noise radiated downstream, BBSAN is dominated by azimuthal modes 1-3, which are approximately 3-4 dB/St stronger than the axisymmetric component. Crucially, the relative contribution of successive modes to BBSAN is sensitive to the observer angle and jet operating condition. Four azimuthal modes are necessary to reconstruct the total BBSAN signal to within 1 dB/St accuracy for the conditions presented here. The analysis suggests, however, that the number of modes required to maintain this accuracy increases as the peak frequency shifts upward. The results demonstrate the need to carefully consider the azimuthal content of BBSAN when comparing acoustic measurements to predictions made by jet noise models built on instability theory.
本文给出了马赫数分别为1.14、1.38和1.50的未加热超声速欠膨胀射流的声学测量结果。在超声速射流噪声的三个组成部分中，重点关注宽带激波关联噪声（BBSAN）部分。受使用波包框架对BBSAN进行建模的启发，采用了一个可移动的传声器环将声压分解为方位角傅里叶模式。与下游辐射的噪声不同，BBSAN由方位角模式1 - 3主导，这些模式比轴对称分量大约强3 - 4 dB/St。至关重要的是，连续模式对BBSAN的相对贡献对观测角度和射流运行条件敏感。对于此处给出的条件，需要四个方位角模式才能将总BBSAN信号重构到1 dB/St的精度范围内。然而，分析表明，随着峰值频率向上移动，保持该精度所需的模式数量会增加。结果表明，在将声学测量结果与基于不稳定性理论建立的射流噪声模型的预测结果进行比较时，需要仔细考虑BBSAN的方位角内容。
