Numerical aeroacoustic investigation of the phase shift method for multirotor directional noise reduction

This thesis presents a numerical investigation of the phase shift method applied to directional noise reduction in multirotor configurations with collective pitch control. To this end, a simulation toolchain was developed, integrating the unsteady free-wake panel code UPM for aerodynamic analysis and the Ffowcs Williams-Hawkings acoustic solver APSIM for noise prediction, developed by the German Aerospace Center (DLR) Institute of Aerodynamics and Flow Technology. The methodology was validated by reproducing experimental results from the NASA Langley research center on rotor phase shifting for hovering propellers, accurately capturing the polar distribution of sound pressure levels and the radiated sound power. The validated toolchain was then applied to manned size quad-rotor and hexa-rotor configurations in trimmed forward flight. Rotor phase angles were optimized to reduce noise in designated ground regions. Acoustic pressure calculations at a ground plane 150m below the vehicle demonstrated that optimized phase angles effectively redirected noise away from targeted areas, achieving up to 10dB reductions in average sound pressure level compared to a baseline of 70dB corresponding to the case with unsynchronized rotors. Spectral and time-domain analyses revealed that the phase shift method predominantly affects the blade passing frequency and its first three harmonics. The time histories of pressure fluctuations at selected observers, showing the contribution of the single rotors, revealed that noise canceling is enhanced by the symmetry of the configuration when applied to the front region, while its efficacy is somewhat reduced in the side regions. These findings support rotor phase synchronization as a viable noise control strategy for multirotor aircraft, capable of steering sound emission away from sensitive areas without affecting the aircraft performance and avoiding the need to modify the flight path.