Two-point measurements on the acoustic field of subsonic turbulent jets

Two-point acoustic measurements are made for subsonic, isothermal turbulent jets over a range of Mach numbers, spanning from 0.4 to 0.9. The measurements are made using two antennae, each with 18 azimuthally distributed microphones, that can be independently displaced in the streamwise direction. The measurement system is used to construct CSD matrices of the sound field for each Mach number. We report a detailed analysis of the two-point structure of the sound field, which is decomposed using a Fourier series in azimuth, Fourier transform in time, and Spectral Proper Orthogonal Decomposition in the inhomogeneous streamwise direction. Analysed in this way, Mach- and Strouhal-number dependencies of the low-rank structure of the sound field are explored. Based on Crow’s model [1], we use a wavepacket source to explore features of the axisymmetric mode of the radiated sound. The model considers all components of Lighthill’s stress tensor to understand the physics behind the observed trends, and this suggests a modification to Crow’s wavepacket. The model proposed reproduces qualitatively some of the different scalings observed at low and high polar angles.