Multiple pure tone generation in aeroengine fans at subsonic and supersonic relative tip speeds

Abstract

It has been generally assumed that the multiple pure tones (MPT's) exist only at supersonic relative tip speeds. However, recent narrowband analysis of fan noise at subsonic relative rotor tip speeds has shown that what was previously assumed to be a purely broadband noise, is, in fact, a broadband noise mixed with MPT's. Previous theories, based on shock wave propagation, need to be extended to explain this and other related phenomena. In this paper, the limitations of the linear acoustic equations are reviewed. These limitations make it essential to use a nonlinear model for the MPT generation problem. It is shown that there are three different MPT generation mechanisms. It is shown further that the characteristics given under the first kind of MPT are the ones which are often quoted as the characteristics of the buzzsaw noise. The second kind involves radial modes and would be manifest at higher supersonic relative tip speeds. The third kind is dependent upon the rotor-stator or rotorinflow interactions. It can and does exist at subsonic relative tip speeds. An example of an application of the theory to the analysis of narrowband data is provided.