Low-Frequency Intensity Modulation of High-Frequency Rotor Noise

Acoustic spectra of rotor noise yield frequency-distributions of energy within pressure time series. However, they are unable to reveal phase-relations between different frequency components, while the latter play a role in low-frequency intensity modulation of higher-frequency rotor noise. Baars et al. (AIAA Paper 2021-0713) outlined a methodology to quantify inter-frequency modulation, which in the current work is applied to a comprehensive acoustic dataset of a rotor operating at low Reynolds number at advance ratios ranging from $J = 0$ to $0.61$. The findings strengthen earlier observations for the case of a hovering rotor, in which the modulation of the high-frequency noise is strongest at angles of $\theta \approx -20^\circ$ (below the rotor plane). For the non-zero advance ratios, modulation becomes dominant in the sector $-45^\circ \lesssim \theta \lesssim 0^\circ$, and is maximum in strength for the highest advance ratio tested ($J = 0.61$). Intensity-modulation of high-frequency noise is primarily the consequence of a far-field observer experiencing a cyclic sweep through the noise directivity patterns of the relatively directive trailing-edge/shedding noise component. This noise becomes more intense with increasing J and is associated with the broadband features of the (partially) separated flow over the rotor blades.