Recent findings regarding noise from tactical jet engines

Abstract

This paper reviews the outcomes of several recent investigations into full-scale supersonic jet noise. First, concordance has been shown between a T-7A-installed GE F404 engine’s sound power and classical jet noise theory in both the subsonic and supersonic regimes [M. A. Christian et al., JASA Express Lett. 3, 073601 (2023)]. The results indicate approximate agreement between the “eighth-power” law for subsonic engine conditions and the “third-power” law for supersonic engine conditions, with a radiation efficiency of ∼0.5–0.6% at afterburner. However, a sudden jump in radiation efficiency in the jet’s transonic region is not presently explained. Second, normalized far field-derived sound power spectra for the F404 do not agree well with a curve found in NASA SP-8072 (Eldred, 1971), likely indicating differences between the importance of different Strouhal number regimes in military jet and rocket noise and, possibly, different radiation phenomena. Third, the maximum directivity angle for overall level appears to be primarily driven by the convective Mach number associated with Kelvin–Helmholtz instability waves (M. A. Christian et al., AIAA Paper 2023-3351). Together, these recent findings represent improved understanding of full-scale tactical jet noise phenomena. [Work supported by ONR.]