Airframe scattering of engine fan noise

This paper presents a study on aircraft engine fan noise scattering by the airframe structure, using a methodology in the framework of geometric acoustics with extensions to account for features that are important for aircraft noise but are absent in classic geometric acoustics. Methods to parameterize engine fan noise sources for scattering calculation are presented and the important feature of source coherence is discussed, with examples given to demonstrate its effects in experimental data and in computation. The scattering results calculated for the Boeing 787 aircraft are presented and analyzed, for both the inlet and the aft fan component, and for both the broadband and the tonal noise. Many features are shown to be consistent with, and/or provide explanations for, observations in flight test data. The methodology and capabilities described in this study are written into the NASA PAASc code and represent a necessary and significant improvement in the accuracy and capabilities for acoustic scattering prediction while meeting the rigorous requirements of aircraft system noise assessments and design studies.