Turbulent-boundary-layer-propeller-interaction noise with an exploration of noise mitigation strategies

The aeronautical industry is shifting towards unconventional aircraft designs to enhance operational flexibility and reduce emissions. However, integrating propulsive systems with airframes introduces complex aeroacoustic challenges. While conventional aircraft noise mechanisms are well understood, novel configurations require fundamental research to improve noise prediction and mitigation. This study presents an aeroacoustic experimental analysis of a propeller ingesting a turbulent boundary layer. A simplified boundary layer ingesting propulsion system was modeled using a propeller mounted on a flat plate, with boundary layer thickness manipulated via upstream tripping devices. The impact of different boundary-layer-thickness-to-diameter immersion ratios was evaluated in terms of aerodynamic performance and noise generation. Performance analysis revealed a discrepancy between experimental results and blade element momentum (BEM) predictions, with BEM overestimating efficiency, potentially due to setup vibrations and wind tunnel influences. Aeroacoustic measurements showed immersion-induced changes in noise directivity, increasing noise levels by up to 9 dB in the upstream region. Noise-mitigation strategies employing quarter-wavelength resonators (QWRs) and a broadband metal-foam absorber demonstrated that optimal QWR placement reduced tonal and broadband noise, outperforming the metal-foam solution. These insights contribute to the growing body of knowledge necessary to develop more efficient noise mitigation strategies in novel aircraft designs, especially those involving boundary layer ingestion. The results also offer valuable data for validating computational models and optimizing future propulsive systems to improve acoustic performance in advanced aviation applications.