An analysis of the generation of subharmonics in aeroengine ducts and methods of their prevention

When the fans of aero-engine ducts go supersonic they often produce radiation at the subharmonics of blade passage frequency known as Multiple Pure Tones. It has been shown that the conventional explanation that these MPTs are created by the shock waves is inadequate. An alternative mechanism based on the concept of 'strong interaction' between the harmonics during resonance is proposed. Expression for the governing equation for such an interaction is derived. The results show an improved agreement with observed data. The analysis has also led to several practical suggestions for the suppression of noise.