Comparison Between Band-Stop and Broadband Noise Absorbers for Propellers in Non-Uniform Inflow

With the advent of urban air mobility and distributed electric propulsion in aviation, the need for effective noise mitigation strategies for propeller systems has become pressing. The presently-reported study concerned the application of noise-mitigatingmaterials, in the bottom wall of the UTwente’s wind-tunnel test section, underneath a non-shrouded propeller subject to a non-uniform inflow. The objective was to experimentally assess the noise mitigating materials impact on tonal noise reduction, emitted noise levels, and sound-emission directiv- ity. Two types of noise-mitigating materials were applied: arrays of additive-manufactured quarter-wavelength resonators and a slab of porous material (metal foam). In the case of the quarter-wavelength resonators, the effect of several geometrical configurations on noise reduction was investigated. An optimal configuration was found; viz., with the cells of the quarter-wavelength oscillator arrays distributed everywhere on the instrumentation surface, except right underneath the propeller blades. Indeed, it was observed that applying quarter- wavelength resonators immediately underneath the propeller in combination with a critical clearance induces a spurious hydrodynamic interaction, which amplifies the propeller’s tonal noise. The latter is markedly true for higher harmonics. In addition, the quarter-wavelength resonators’ performance was compared to a flat plate (reference) and the metal foam (broad-band absorber). Findings show that the efficiency of the quarter-wavelength-resonator solution is highly dependent on the configuration. Moreover, the tuned-quarter-wavelength-resonator outperformed the broadband porous material in noise reduction. The polar directivity of the recorded emitted sound is affected by the configuration. It was found that the presence of any sound-mitigating material underneath the propeller, is consistently detrimental to the azimuthal directivity.