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.

中文翻译：

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含超音速冲击的射流辐射噪声的方位角分解

提出了具有理想马赫数为1.14、1.38和1.50的未加热超音速欠膨胀射流的声学测量结果。在超音速喷射噪声的三个分量中，重点是宽带冲击相关噪声（BBSAN）分量。通过使用波包框架对BBSAN进行建模，一个可穿越的麦克风环被用来将声压分解为方位傅里叶模式。与下游辐射的噪声不同，BBSAN由1-3模式的方位角控制，该模式比轴对称分量强大约3-4 dB / St。至关重要的是，连续模式对BBSAN的相对贡献对观察者角度和射流工作条件很敏感。对于此处介绍的条件，必须使用四个方位模来将总BBSAN信号重构到1 dB / St精度以内。但是，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的喷气噪声模型所作的预测进行比较时，需要仔细考虑BBSAN的方位角含量。