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, whereas neighboring mode orders are suppressed. The mode distribution and intermode 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 structures of the generated sound field than a classical spectral analysis.