Numerical Investigation on Tonal Noise Directivity in Distributed Propulsion Systems

Abstract

The potential of distributed electric propulsion to mitigate noise environmental impact has been increasingly explored for UAV and UAM applications. However, when rotors are placed in close proximity, strong aerodynamic interactions and complex acoustic phenomena are induced. Phase synchronization has been proposed as a tonal noise mitigation strategy. Significant reductions in tonal levels have been reported under various configurations, though the extent and nature of such reductions remain debated. Previous assessments have relied on sparse sound pressure measurements, potentially misrepresenting global noise attenuation due to altered directivity. To address this experimental limitation, highfidelity simulations are carried out for a multi-propeller configuration, focusing on the effects of phase synchronization. The influence of a 30°-phase offset between three co-rotating six-blade rotors is evaluated against an in-phase configuration. A spherical microphone array was used to determine the spatial directivity and total radiated sound power. Results show that phase-angle differences substantially redistribute the acoustic energy toward different spatial locations. Nevertheless, it also impacts the global noise emission, mitigating the sound power level of the opposite-phase configuration by 7.47 dB in acoustic power for the first BPF harmonic and 4.79 dB for the second.