Numerical Study on Dual-Rotor Phase Synchronization for Tonal Noise Counteraction

Urban Air Mobility (UAM) is considered as a powerful way to relieve urban congestion, but the noise problem is still a major obstacle to its wide application. In this paper, noise generation and propagation from dual rotors under the hovering condition are investigated without and with a phase difference of a quarter of the rotation period. The flows are simulated using large eddy simulation (LES), and the noise prediction was made through the Ffowcs Williams–Hawkings (FW-H) acoustic analogy. The results show that the rotation phase difference attenuates interactions of vortices in the wakes induced by the rotors, especially tip vortices. Furthermore, it significantly changes the spatial distribution and directivity of the noise propagation. The primary tonal noise at the first blade passing frequency is completely counteracted underneath the rotors. Noise mapping shows that a reduction of overall sound pressure levels up to 5 dB(A) is achieved in propagation directions vertical to the ground plane where the original noise levels without the rotor phase difference are large. Although a penalty to increase 3∼4 dB(A) also arises in oblique directions, the resultant noise is still low because of the original insignificant levels. The study shows that the rotor phase synchronization has important potential in the directional noise mitigation for dual-rotor systems.