Application of Duct Propagation Codes to the Prediction of Far Field Turbofan Generated Noise

Fan noise from a turbofan engine is a key contributor to the noise signature of an aircraft on both approach and take-off. Predicting the propagation and far field radiation of this source at various operating conditions and with and without nacelle acoustic treatment is essential to performing noise certification assessments, developing efficient noise reduction technologies, and satisfying customer concerns. Three acoustic propagation codes are used to generate far field predictions of the blade pass frequency (BPF) tone noise and its harmonics radiated from the inlet of a Honeywell TFE731-60 engine. Where internal modal data is not available to characterize the noise source, assumed mode distributions were utilized. Predictions generated with the Eversman code, LEE code, and ACTRAN/TM code are compared to measured engine data at 60% power at the BPF. The predicted far field directivities from the different codes compare reasonably well with each other although the LEE code possibly indicates more radial scattering due to the non-uniform mean flow around the nacelle lip. Other engine power settings are investigated using the Eversman code and the predicted results can be matched to the data when the modal structure of the sound field is considered. A genetic algorithm is utilized to find the optimum linear combination of modes that fit the measured far field data. Although the uniqueness of the solution needs further examination, the optimization method can potentially provide a better alternative to the equal-energy-per-mode assumption. The results of this investigation indicate the need for improved mode measurements and more refined far field measurements to enable further validations with hardwall and acoustically treated data.