Experimental investigation of the aeroacoustic performance of shrouded propeller systems

In recent years there has been a growing interest towards Unmanned Aerial Vehicles (UAVs). They play a central role in a wide array of industries, with examples in the medical sector, search and rescue operations and parcel delivery services. While they are attractive thanks to several of their characteristics, such as cost-efficiency and manoeuvrability, the noise they generate is still an obstacle to their widespread acceptance. Several propulsion configurations have been assessed to tackle this problem, and a substantial research effort is directed towards shrouded propeller systems. While the increase in safety due to the presence of the duct is clear, there are still open questions related to their aerodynamic performances and noise signature. Experimental campaigns are of key relevance in the research and development of ducted propellers and are at the core of this thesis' work. Adding on top of already existing research body, acoustic measurements are taken from inside the duct, offering an insight into the near-field sound propagation. Two different ducts, differing in length but with identical intake geometry, are tested and compared using both near-field and far-field measurements. These tests highlight that a non aerodinamically-optimized lip intake shape and the presence of the shroud alter the inflow characteristics into the propeller and thus its noise sources. The results, such as the increased far-field noise level for longer ducts, indicate a need for additional data to comprehensively understand the impact of phenomena like blade-boundary layer and blade-turbulent structures interactions on noise generation in shrouded propeller systems.