Investigation of multi-lobed fighter jet noise sources using acoustical holography and partial field decomposition methods

Full-scale tactical aircraft noise exhibits multiple radiation lobes not seen in laboratoryscale jets. These lobes have different radiation directions yet appear to have similar, overlapping source regions. Near-field acoustical holography (NAH) source reconstructions, in conjunction with partial field decomposition (PFD) methods that produce physically meaningful partial fields, are used in the current work to investigate the nature of these radiation patterns. First, it is shown that the two main radiation lobes are highly incoherent, suggesting independent partial sources. Second, these lobes are isolated as mutually orthogonal partial fields. In this representation, the lobes seem to be generated by independent yet spatially coincident extended partial sources. Source comparisons are made between non-afterburner and afterburner engine powers to investigate whether afterburner combustion produces any sources that are fundamentally different from those of nonafterburner operations. The current results show no qualitative changes occur due to the addition of the afterburner thrust aside from minor variations in source distribution, level, and the nature of the overlap between the multiple lobes.