On the effect of unsteady motions on the rotor aerodynamic noise

This work presents a parametric study of the effects of rotational speed variation and blade vibration on the aeroacoustic characteristics of the fixed-pitch rotors. The study is based on an efficient prediction of the rotor noise with the effect of unsteady motions, including the fluctuations in the rotation speed and blade vibration, considered. The flow variables are computed using a cost-effective blade element and momentum theory method to evaluate the equivalent sound sources. The amplitudes of the unsteady motions are small. The computation results showed that periodic rotation speed fluctuations and blade vibrations could produce multiple tones in the combination of the blade passing frequency and the frequency of the fluctuation. By contrast, random rotation speed fluctuation or blade vibration can increase the noise level in a wide frequency range, despite that the sources due to turbulent flows are not modelled. In addition, the influence of these unsteady effects is observer angle-dependent, and the influences in the perpendicular directions are more significant than that on the rotational plane of the rotor. Similar observations are found for cases with oncoming flows in the axial direction.