Analysis of the influence of inflow distortions on turbofan rotor noise

Abstract

Acoustic data measured in a fan test facility with and without an inflow control device (ICD) over the inlet were analysed using the cyclostationary analysis technique, which separates the total power into a rotor locked and a fluctuating component. Measurements were taken with an inlet mode ring upstream of the rotor and with far-field microphones for configurations with subsonic and with supersonic fan blade tip speeds. The ICD reduces the fluctuating component and stabilises the rotor locked component of the frequency spectrum at the blade passing frequency and its harmonics. The mode distribution in the inlet and the far-field directivity are also changed when an ICD is installed. The cyclostationary analysis of the mode spectrum in the inlet duct and of the far-field directivity can help to understand the effect of an ICD on the sound field in more detail than a classical spectral analysis.