Effects of phase synchronization on the tonal sound directivity of distributed propellers

Recent studies on distributed electric propulsion systems suggest phase synchronization between rotors as a noise reduction strategy. However, the aerodynamic interactions between propellers' near fields and their influence on far-field tonal noise remain poorly understood, partly due to experimental limitations in microphone placement. This paper addresses this gap through lattice Boltzmann very large eddy simulations of three adjacent, co-rotating rotors, spaced radially at 2% of their diameter, to investigate how relative phase angle affects tonal noise directivity. Results reveal that proximity-induced aerodynamic interactions generate dominant tonal noise in most spatial directions, driven by two mechanisms: time-averaged inflow distortion from nearby propellers and impulsive local effects at blade tips, with the latter influenced by phase angle. While the directivity pattern of the blade-passing frequency harmonic tone remains consistent across phase angles, comparing cases with zero relative phase (blades aligned) and opposite-phase conditions shows sound pressure level shifts of up to 4.5 dB along the primary noise axis, namely, along the inflow direction. Conversely, acoustic interference significantly alters noise directivity, especially in opposite-phase conditions where sound is nearly canceled in specific directions. These findings highlight rotor synchronization as a promising strategy for reducing noise emissions toward sensitive areas.