MPT Prediction of Aircraft Engine Fan Noise

A collection of computer programs has been developed that implements a procedure for predicting multiple- puretone (MPT) noise generated by fan blades of an aircraft engine (e.g., a turbofan engine). MPT noise arises when the fan is operating with supersonic relative tip Mach No. Under this flow condition, there is a strong upstream running shock. The strength and position of this shock are very sensitive to blade geometry variations. For a fan where all the blades are identical, the primary tone observed upstream of the fan will be the blade passing frequency. If there are small variations in geometry between blades, then tones below the blade passing frequency arise — MPTs. Stagger angle differences as small as 0.1° can give rise to significant MPT. It is also noted that MPT noise is more pronounced when the fan is operating in an "unstarted" mode. Computational results using a three-dimensional flow solver to compute the complete annulus flow with non-uniform fans indicate that MPT noise can be estimated in a relatively simple way. Hence, once the effect of a typical geometry variation of one blade in an otherwise uniform blade row is known, the effect of all the blades being different can be quickly computed via superposition. Two computer programs that were developed as part of this work are used in conjunction with a user's computational fluid dynamics (CFD) code to predict MPT spectra for a fan with a specified set of geometric variations: