Mixing noise from a jet engine originates from an extended spatial region downstream of the nozzle and is partially correlated both spatially and temporally. Previously, the coherence properties in the downstream direction of the sound field of a tethered military aircraft were investigated, resulting in the identification of different spatial regions based on coherence length [B. M. Harker et al., AIAA J. 54, 1551-1566 (2016)]. In this study, a vertical array of microphones to the side of the jet plume is used to obtain the azimuthal coherence of the sound field. Although multipath interference effects and a limited angular aperture make coherence length calculation impossible, information about upper and lower bounds can be extracted. The measured azimuthal coherence as a function of downstream distance and frequency is then compared to that predicted by sound field reconstructions using multisource, statistically-optimized near-field acoustical holography (M-SONAH) [A. T. Wall et al., J. Acoust. Soc. Am., 139, 1938-1950 (2016)]. This comparison helps to benchmark the performance of a reduced-order M-SONAH algorithm that employs only axisymmetric cylindrical basis functions to represent the sound field.

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