Noise interference in phase-synchronised rotors

The aeroacoustic interference between two co-rotating, phase-synchronized rotors is investigated theoretically and experimentally. An analytical model predicting the azimuthal location of constructive interference is developed and compared with dedicated measurements. For the dominant blade-passing-frequency mode and the single spacing, radius, and operating condition tested here, the near-field formulation predicts the measured peak directions within the angular resolution of the microphone array. The measured interference pattern is systematically rotated relative to the far-field prediction owing to the helical phase structure of the rotating-source acoustic mode. The physical basis and limitations of the model are interpreted through a spherical-harmonic framework, which explains the characteristic two-lobe interference pattern, identifies a transition beyond which it breaks down, and provides a spacing-dependent regime framework consistent with prior experimental literature. The full framework is not independently tested by the present single-spacing campaign. By relating this parameter to the tip Mach number, a propulsion-oriented classification of interference modes is proposed.