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Because phenomena observed in noise radiated from high-performance military aircraft are not fully understood, different analysis techniques can yield complementary insights into their characteristics. Here, a coherence analysis from a 120-microphone near-field array is used to identify and characterize the noise radiation and source-related properties of a T-7A-installed F-404-GE-103 engine over a broad range of frequencies. Some of the noteworthy findings include: a) coherence trends for mixing noise and spatiospectral lobes generally match those observed for previously studied aircraft; and b) bands of coherence between upstream and downstream locations are observed at higher engine powers, suggesting a coherent interaction between upstream broadband shock-associated noise radiation and downstream Mach wave radiation. [Work supported by ONR Grant No. N00014-21-1-2069.]

Because the noise source mechanisms and radiation properties associated with high-thrust, tactical jet engines are not fully understood, full-scale measurements and analysis can shed significant insight. One method for examining spectral data is to compare them to empirical models for jet noise spectra. This paper compares measured near-field spectra from an installed F-404 engine with analytical similarity spectra for fine-scale mixing noise, large-scale mixing noise, and broadband-shock associated noise. This similarity spectral analysis enables us to determine spatial trends in overall level and peak frequency, and the relative importance of each type of noise radiation per location. This approach can be used to gain insights for different engine conditions as well as quickly compare to other aircraft.

Similarity spectra analysis has long been a popular method for identifying both the fine-scale and large-scale components of jet noise. For an imperfectly expanded supersonic jet, addition of a broadband shock-associated noise (BSN) model allows spectra to be better represented. A similarity spectra analysis of T-7A-installed GE-F404 engine noise has been performed at conditions ranging from intermediate to full afterburner. The BSN contributions are significant at military and afterburner conditions. The combined model captures the noise reasonably well with some caveats. First, spatiospectral lobes present in the measured noise are not well represented. Second, the measured low-frequency spectral slope is steeper at higher engine conditions in the region of maximum radiation. Third, the measured high-frequency slope is shallower across most of the radiation angles. Fourth, the noise at large inlet angles, beyond the peak radiation lobe, are not well represented by the combined model. The successes (and failures of the model) for different spatial regions and frequencies will aid in developing improved models for noise radiation. [Work supported by ONR.]