Cyclostationary analysis of fan noise influenced by an inflow control device

Acoustic data measured in a fan test facility with and without an inflow control device (ICD) over the inlet were analyzed 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 detection 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 stabilizes the rotor locked component of the frequency spectrum at the blade passing frequency and its harmonics in the way that the mode amplitudes of the Tyler-Sofrin modes increase, while neighboring mode orders are suppressed. The mode distribution and inter-mode coherence in the inlet are also changed, which changes the far-field directivity when an ICD is installed. It is demonstrated that cyclostationary analysis is a powerful tool to investigate the acoustic effect of an ICD on turbofan noise, revealing more details about the temporal and spatial structure of the generated sound field than a classical spectral analysis