Effect of noise control treatments on surfaces near a non-uniform inflow propeller

With the advent of urban air mobility and distributed electric propulsion in aviation, effective noise mitigation for propeller systems has become critical. This study explores the application of noise-mitigating materials in the bottom wall of UTwente's wind tunnel test section, beneath a non-shrouded propeller subject to non-uniform inflow. The objective was to assess the impact of these materials on tonal noise reduction, emitted noise levels, and directivity. Two materials were tested: arrays of additive-manufactured quarter-wavelength resonators (band-stop mitigator) and a slab of metal foam (broadband mitigator). Various geometrical configurations were investigated for the resonators, with the optimal configuration placing resonators everywhere except directly beneath the propeller blades. Applying resonators immediately underneath the propeller, with a critical clearance, induced a spurious hydrodynamic interaction, amplifying tonal noise at higher frequencies. The efficiency of the quarter-wavelength-resonator solution depends heavily on configuration. Performance was compared with a flat plate and the metal foam, with the tuned resonators outperforming the broadband material in noise reduction. The polar directivity of the emitted sound is influenced by the configuration, and the presence of any sound-mitigating material underneath the propeller consistently degrades upstream azimuthal directivity.