Computational aeroacoustics study of propellers with vibrational motion

In this work, we conduct a numerical investigation of aerodynamics and aeroacoustics of propeller blades with vibrations, which could occur in practical unmanned aerial vehicles as the blades are often made of lightweight materials. The simulations resolve the sound generation from the unsteady turbulent flows using an acoustic-wave preserved artificial compressibility method. Then, the sound projection to the far-field observers is made using an integral solution of the Ffowcs-Williams and Hawkings equation. The study shows that periodic blade vibrations with small amplitudes can lead to aerodynamic thrust fluctuations. The blade vibration also affects the generation of tip vortices and the near-blade flow structures due to the periodic change of the effective angle of attack. Consequently, significant tonal noise at the harmonics of rotational frequency is produced, and the noise can propagate to both upstream and downstream directions of the rotor disc plane. A noise source analysis is performed to identify the contribution of different noise components. Results show that the extra tonal noise is mainly caused by the Doppler effect due to the blade axial motion and the influence of the thrust fluctuations. Moreover, the study also suggests that the high-frequency broadband noise seems to be insensitive to the blade vibration.