Spatiotemporal Correlation Analysis of Jet Noise from a Round-Nozzle Supersonic Aircraft

Spatiotemporal analysis of noise from a tethered F-35B provides insight into the spatial, spectral and temporal relationships within the sound field. Six engine power conditions ranging from 25% to 150% engine thrust request were measured using a 71-microphone linear ground array located approximately 8 m from the estimated shear layer. Mixing noise trends with engine power for the round-nozzle F-35B are similar to those of a nominally rectangular-nozzle high-performance jet aircraft [Harker et al, AIAA, 2016]. Cross-correlation and coherence measures are used to corroborate and confirm identifications of fineand large-scale turbulent mixing noise contributions from a concurrent study of the F-35B dataset [Neilsen et al., AIAA, 2018]. The relationships observed between multiple spatiospectral lobes seen in the maximum radiation regions of prior and concurrent high-performance aircraft noise studies [Leete et al., AIAA, 2018] are confirmed and expanded upon . Correlograms help identify how the multiple spatiospectral lobes have different apparent phase speeds across the array, corresponding to different directionality, some components of which also change with engine power. Increased overlap of lobes with increased engine power appears to drive global decreases in field coherence. Finally, the structure of the spatiospectral lobes appears to be more visible in nondimensionalized coherence length than in the spectrum itself. Broadband shock-associated noise (BBSAN) is found in the upstream direction at engine powers of 75% engine thrust request and above. Coherence is also used to separate BBSAN from jet mixing noise because the BBSAN is coherent within the relevant frequency range while adjacent fine-scale mixing noise is not. However, correlation and coherence analyses show that the upstream BBSAN signature is related to sound received in the peak radiation region dominated by the spatiospectral lobes. Possible links between the shock-associated noise and the spatiospectral lobes are discussed.

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