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Wavelet-based Analysis of Spectral and Temporal Structures in F404 Engine Jet Noise

AIAA 2023-3213
Session: Jet Aeroacoustics I: Noise Sources
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Spatiospectral lobes are features identified in the noise fields radiated from full-scale tactical aircraft that are unseen in lab-scale experiments. Prior studies have explored lobe frequency-domain characteristics, but a joint time-frequency domain (JTFD) analysis has the potential to further explore these phenomena and connect them to source-related events. This paper applies an event-based beamforming technique to acoustical data collected at a 120-microphone array near a T-7A-installed F404 engine. The algorithm correlates time domain events between pairs of adjacent microphones to find an event propagation direction and then ray traces to the jet centerline to identify an apparent source location. A wavelet transform is used to identify frequency triggers tied to the spatiospectral lobes to gain insights. It is shown that the source responsible for the spatiospectral lobes is composed of multiple, overlapping sources each with a different peak radiation angle. Events are also observed in the time-frequency domain, and it is found that there is an underlying temporal structure reminiscent of mode switching seen with lab-scale jet screech. Using a Markov-style analysis, this temporal structure is characterized. It is found that spectral peaks related to the spatiospectral lobes are composed of discrete but randomly distributed temporospectral events, each with a frequency-dependent directivity.