Determination of fan inlet duct mode structure using in-duct microphone array measurement

Abstract

A methodology is developed in which the amplitudes of propagating modes are extracted from the sound measured in the duct. The method is derived from a beamforming technique in which the weighting function is the wave description of a chosen mode. The modal distribution that arises from the analysis is used to synthesize the sound propagating in the duct which is projected to the far field to estimate the sound radiated from the duct inlet. The methodology is tested using an array of dynamic pressure transducers in the inlet duct wall of a turbofan engine. The data show that the modal structure of the blade passage frequency tone at lower fan speeds is predominately from the interaction of the rotor with downstream obstructions. At higher speeds, when the fan blade tip speed becomes supersonic, many modes are excited. Good agreement is found between measured and estimated far field sound levels in the forward quadrant of the engine at the lower engine speeds. Significant divergence between measured and calculated far field sound occurs when the blade tip speed becomes supersonic. The methodology is a useful identification of the noise generation mechanisms present in the fan.