Numerical and Experimental Investigation of the Effect of Rotation Speed Fluctuations on the Rotor Noise for Unmanned Aircraft Systems

The rotors for unmanned aircraft systems can undergo time-varying rotation speeds in practical operations. This work investigates the aeroacoustic effect of the rotor's rotation speed fluctuation using both numerical and experimental approaches. First, the real-time angular speed signal is experimentally measured, and the results suggest that the time history of the rotor's angular speed is highly deterministic and depends on the motor. The acoustic data are also acquired by a line array of microphones. The measured spectra show high-order tones at harmonics of the blade passing frequency in the downstream direction. Then, we perform delayed detached eddy simulations and realize the fluctuating rotation speed by controlling the mesh displacement at each time step. The results suggest that significant additional tones are produced at the excitation frequency and neighbour frequencies, even though the amplitudes of the rotation speed fluctuations are much smaller than the mean rotation speed. The numerical simulations reproduce the aeroacoustic observations in the experiments, indicating that the rotation speed fluctuation is likely responsible for the generation of middle-frequency tonal noise at harmonics of the blade passing frequency. An aeroacoustic source analysis is also conducted to isolate the contribution of various source types and correlate the far-field noise with the near-field source distribution.