Abstract
The fan noise characteristics of a turbofan engine and fan noise abatement technology have been studied by the authors.The study was concentrated on the theoretical analysis of the spinning mode propagating from the rotating fan of a high bypass-ratio turbofan engine and the practical technology to abate the fan noise and the practical technology to abate the fan noise acoustic linings on the engine inlet duct and the fan cowl. Through the above study, the procedures to determine the optimum impedance and necessary dimensions on the acoustic treatment for the reduction of the fan rotating noise under given conditions were obtained. Following those procedures, acoustically lined ducts were designed, manufactured and installed on a high bypass-ratio turbofan engine for a series of noise measuring tests. As a result, it was revealed that the acoustically lined duct attached to the engine inlet is effective for the reduction of the noise level so long as it is designed to the optimum impedance value for maximizing the attenuation rate of the spinning mode, and that some portions of the spinning mode can be cut off even by the solid wall duct, though this was not realized in the simulation test using a plane wave source.
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