Wavepackets in the velocity field of turbulent jets
Wavepackets in the velocity field of turbulent jets
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DOI：
10.1017/jfm.2013.346
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发表时间：
2012-06
影响因子：
3.7
通讯作者：
A. Cavalieri;D. Rodríguez;P. Jordan;T. Colonius;Y. Gervais
中科院分区：
文献类型：
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作者：
A. Cavalieri;D. Rodríguez;P. Jordan;T. Colonius;Y. Gervais
摘要研究了具有湍流边界层的圆形喷嘴非强制高雷诺数亚音速射流的速度场。本研究的目的是减少这种流中的波包，并探讨它们与辐射声的关系。速度场测量使用热线风速计和立体，时间分辨PIV系统。该场可分解为频率和方位角傅里叶模式。用传声器环阵列同步测量低角声辐射。与以往的观测结果一致，速度场和声压场的方位角波数谱是明显的。初始混合层的速度谱在4 ～ 11的方位角波数$m$处出现峰值，且峰值与混合层的局部动量厚度成正比。另一方面，声压场主要是轴对称的，这表明速度场轴对称模式的相对声效率增加，理论上可以证明这一特性是由声源的径向致密性引起的。相关系数高达10，证实了这一点 %, between the axisymmetric modes of the velocity and acoustic pressure fields, these values being significantly higher than those reported for two-point flow–acoustic correlations in subsonic jets. The axisymmetric and first helical modes of the velocity field are then compared with solutions of linear parabolized stability equations (PSE) to ascertain if these modes correspond to linear wavepackets. For all but the lowest frequencies close agreement is obtained for the spatial amplification, up to the end of the potential core. The radial shapes of the linear PSE solutions also agree with the experimental results over the same region. The results suggests that, despite the broadband character of the turbulence, the evolution of Strouhal numbers $0. 3\leq St\leq 0. 9$ and azimuthal modes 0 and 1 can be modelled as linear wavepackets, and these are associated with the sound radiated to low polar angles.
Abstract We study the velocity fields of unforced, high Reynolds number, subsonic jets, issuing from round nozzles with turbulent boundary layers. The objective of the study is to educe wavepackets in such flows and to explore their relationship with the radiated sound. The velocity field is measured using a hot-wire anemometer and a stereoscopic, time-resolved PIV system. The field can be decomposed into frequency and azimuthal Fourier modes. The low-angle sound radiation is measured synchronously with a microphone ring array. Consistent with previous observations, the azimuthal wavenumber spectra of the velocity and acoustic pressure fields are distinct. The velocity spectrum of the initial mixing layer exhibits a peak at azimuthal wavenumbers $m$ ranging from 4 to 11, and the peak is found to scale with the local momentum thickness of the mixing layer. The acoustic pressure field is, on the other hand, predominantly axisymmetric, suggesting an increased relative acoustic efficiency of the axisymmetric mode of the velocity field, a characteristic that can be shown theoretically to be caused by the radial compactness of the sound source. This is confirmed by significant correlations, as high as 10 %, between the axisymmetric modes of the velocity and acoustic pressure fields, these values being significantly higher than those reported for two-point flow–acoustic correlations in subsonic jets. The axisymmetric and first helical modes of the velocity field are then compared with solutions of linear parabolized stability equations (PSE) to ascertain if these modes correspond to linear wavepackets. For all but the lowest frequencies close agreement is obtained for the spatial amplification, up to the end of the potential core. The radial shapes of the linear PSE solutions also agree with the experimental results over the same region. The results suggests that, despite the broadband character of the turbulence, the evolution of Strouhal numbers $0. 3\leq St\leq 0. 9$ and azimuthal modes 0 and 1 can be modelled as linear wavepackets, and these are associated with the sound radiated to low polar angles.