Sound radiation of fan tones from an installed turbofan aero-engine: fuselage boundary-layer refraction effects

Abstract

A distributed source model to predict fan tone noise levels of an installed turbofan aero-engine is extended to include the refraction effects caused by the fuselage boundary layer. The model is a simple representation of an installed turbofan, where fan tones are represented in terms of spinning modes radiated from a semi-infinite circular duct, and the aircraft's fuselage is represented by an infinitely long, rigid cylinder. The distributed source is a disc, formed by integrating infinitesimal volume sources located on the intake duct termination. The cylinder is located adjacent to the disc. There is uniform axial flow, aligned with the axis of the cylinder, everywhere except close to the cylinder where there is a constant thickness boundary layer. The aim is to predict the near-field acoustic pressure, and to quantify the effect of the boundary layer on the sound pressure levels on the cylinder's surface. Thus no far-field approximations are included in the modelling. The sound propagation through the boundary layer is calculated by solving the Pridmore-Brown equation. Results from the theoretical method show that the boundary layer has a significant, nuanced effect on the sound pressure levels on the cylindrical fuselage, owing to sound radiation of fan tones from an installed turbofan aero-engine.