Prediction of model scale turbofan exhaust tone noise

Abstract

This paper describes in detail the process for predicting, from first principles, the rotor–stator interaction tone levels for two realistic model-scale fans that are representative of high bypass ratio and ultrahigh bypass ratio turbofans. The prediction scheme relies on a suite of three-dimensional computational tools that include Reynolds Averaged Navier–Stokes aerodynamic models and linearized inviscid aeroacoustic models. The goal of the study was to assess the accuracy of tone-level predictions for realistic fans operating under realistic conditions. The predictions were carried out over a wide range of operating conditions that include, but were not limited to the approach, cutback, and sideline conditions for each of the two fans. The in-duct and external tonal sound fields were computed at a representative blade passing frequency harmonic tone. The predicted tone sound pressure level and sound power level have been compared with the measurements acquired at a NASA anechoic wind tunnel. The data–theory comparisons are primarily focused on the exhaust tone levels due to lack of validated models for predicting the three-dimensional tone acoustic transmission through a rotor. The data–theory comparisons show that it is possible to make accurate predictions of the exhaust rotor–stator interaction tone power levels starting with the geometry of the fan stage. Specifically, the results demonstrate that the exhaust rotor–stator interaction tone power levels can be predicted to within $\pm 1$ dB on a consistent basis for rotor at subsonic tip relative speeds by including the three-dimensional geometry of the fan stage and its flowfield. Furthermore, using the predicted in-duct sound pressure levels, the basic trends in the directivity of the exhaust tone sound pressure level can also be predicted despite the complex nature of tone directivity for realistic fans and the complicated physics of sound refraction through the exhaust shear layer.