Lu Y., Yu Y., Hu Z.

By utilizing the acoustic mode matching between the source modes and the propagation modes, the theoretical prediction system of rotor-stator interaction noise generation and propagation and attenuation in an annular duct with multi-treatments is presented. That means the prediction of sound propagation and attenuation in the segmented ducts may no longer perfectly depend on the in-duct mode measurements, and the studies on the sound propagation and attenuation in ducts can be accomplished not only by modal measurements, but also by utilizing the source prediction to determine the source modes due to rotor-stator interaction. Numerical calculations are presented to show the effects of blade passing frequency (BPF), rotor blades/stator vanes number, axial distance between the rotor and stator rows on the in-duct sound attenuation and the resulting sound power levels. As for the given linings mounted in ducts on the fan/compressor facilities, there is an optimum operating condition when the in-duct sound attenuation due to acoustic treatments approaches the maximum. In addition, although increasing the axial distance between the rotor and stator will result in decrease of the in-duct generated noise, but increasing of the axial distance is often restricted by aerodynamic performance such as efficiency of the fan/compressor stage, etc, and weight of the engine. Perhaps in contrast to increasing the rotor-stator axial distance, choosing proper ratio of rotor blade number to that of stator vanes in order to produce the cut-off condition for acoustic modes would be more effective. The accuracy of the prediction system is restricted by the accuracy of the source prediction and that of the sound propagation analysis. With improvement of the restricting factors and the precision of source and in-duct propagation prediction, the accuracy of the theoretical prediction system will be further increased.