Modal Analysis of Acoustic Directivity in Turbulent Jets

What is it about?

The reduction of jet noise is an important objective for the aviation community. A modal decomposition technique called the spectral proper orthogonal decomposition (SPOD) was used to investigate the sources of jet noise in different various directions for subsonic, transonic, and supersonic jets. Till now, it is understood that the jet noise in the downstream direction is generated by the large-scale coherent structures and fine-scale structures are the sources of noise in the sideline direction. Using empirical evidence, we show that the sideline radiations are associated with the same large-scale coherent structures as the dominant downstream radiations.

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Photo by Timothy Newman on Unsplash

Photo by Timothy Newman on Unsplash

Why is it important?

Jet noise is a serious concern for the military and the aviation industry. It can affect the hearing of personnel working near aircraft. It is important to understand the underlying mechanisms that generate noise and develop strategies to reduce the jet noise. In contrast to the previous studies, our finding show that the source of jet noise in both the downstream and the sideline radiations are the large-scale coherent structures. This indicates that a single mechanism, the instability of the Kelvin-Helmhotz wavepacket governs the generation of jet noise in different directions.