Modal Analysis of Acoustic Directivity in Turbulent Jets

The directivity of noise from three large-eddy simulations of turbulent jets at Mach 0.7, 0.9, and 1.5 is investigated using spectral proper orthogonal decomposition (SPOD). The most energetic patterns of acoustic radiation are extracted using the far-field pressure 2-norm. Specialization of the norm to the far field is accomplished through localized spatial weighting. Radiation patterns to specific jet inlet angles are isolated by further restricting the spatial weighting to small rectangular regions in the far field. The most energetic radiation pattern for all cases and relevant frequencies is a single superdirective acoustic beam in the downstream direction. The source region of these beams is traced back to the end of the potential core for low frequencies and the shear-layer region for higher frequencies. In the sideline direction, to low angles, the acoustic patterns consist of beams that propagate upstream or perpendicular to the jet axis. The sideline radiation patterns are found to originate from the same source locations as the dominant superdirective beams. Inspection of the SPOD modes reveals that sideline radiation is directly linked to directive downstream radiation. Within the restricted radial extent of the computational domain, these results indicate that the sources of sideline and downstream radiation are intimately linked.

Automated summary

The study investigates the directivity of noise from turbulent jets at Mach 0.7, 0.9, and 1.5 using spectral Proper Orthogonal Decomposition (SPOD), revealing superdirective acoustic beams originating from the end of the potential core and the shear-layer region. The most energetic radiation patterns are isolated and linked back to specific source regions, indicating an intimate connection between sideline and downstream radiation within the computational domain.