In recent years, urban air mobility (UAM) vehicles have seen a surge in academic and industry interest. Their propulsive systems are composed of rotors, and they are the main source of noise, which is a concern as they will operate close to the population. Ducting the propeller can provide significant advantages, such as enhanced operational safety, improved aerodynamic performance, and the potential to reduce noise emissions. In this thesis, the geometric design of a ducted propeller is undertaken, analyzing the aerodynamic performance, flow mechanisms, and aeroacoustic properties with numerical methods and experimental techniques. Furthermore, the performance of the ducted propeller is investigated for a variety of flying and flow conditions, and porous materials are used to reduce p...[ Read more ]

In recent years, urban air mobility (UAM) vehicles have seen a surge in academic and industry interest. Their propulsive systems are composed of rotors, and they are the main source of noise, which is a concern as they will operate close to the population. Ducting the propeller can provide significant advantages, such as enhanced operational safety, improved aerodynamic performance, and the potential to reduce noise emissions. In this thesis, the geometric design of a ducted propeller is undertaken, analyzing the aerodynamic performance, flow mechanisms, and aeroacoustic properties with numerical methods and experimental techniques. Furthermore, the performance of the ducted propeller is investigated for a variety of flying and flow conditions, and porous materials are used to reduce propeller noise and propeller-airframe interaction noise. First, the effect of different geometric parameters, such as the duct’s lip profile, expansion ratio, tip clearance, and propeller’s axial placement, are thoroughly investigated in hovering conditions. The primary noise sources are identified near the propeller’s tip and leading edge, with significant pressure fluctuations at the duct’s lip. The flow results show that adjusting the curvature and size of the duct’s lip can enhance aerodynamic performance by increasing suction at the duct’s lip, and slightly divergent ducts can increase the total thrust by creating a lower-speed wake and increasing suction at the lip, with reduced tonal noise but increased broadband noise levels. Reducing the tip clearance can also increase the duct’s thrust significantly without penalizing noise generation, and positioning the propeller at a certain distance from the duct’s lip towards the center maximizes the system’s aerodynamic efficiency while minimizing noise. When the ducted propeller operates in axial flow, the aerodynamic performance of the propeller and the duct degrades. The incoming flow helps maintain attached flow on the duct’s inner wall, reducing the turbulent kinetic energy, although it is higher than for the unducted propeller. The noise spectrum results indicate a reduction in noise at the blade-passing frequency (BPF) with increasing flow speed, and the broadband noise is higher for the ducted propeller but decreases with the flow speed. A scaling law is proposed for the broadband noise. With oblique flow, the thrust decreases with the flow angle, and the broadband and tonal noise at higher BPF harmonics increase with oblique flow. Compared to the unducted propeller, the ducted propeller reduces noise at 2 × BPF under oblique flow, likely due to shielding and shear effects. Finally, the experimental results in descending flight conditions show that the propeller’s thrust increases monotonically with the descent rate. In contrast, the duct’s thrust presents a minimum at higher descent rates linked to the vortex ring state. Noise at the BPF follows the aerodynamic loading trends, while noise at higher-order BPF harmonics decreases with higher descent rates. Broadband noise increases with the descent rate for mid-frequencies and decreases at high frequencies. The use of a partially porous duct is also investigated. The results show that high-frequency broadband noise can be reduced due to weakened noise sources towards the propeller’s tip. Despite the noise reduction, the aerodynamic performance remains practically unaltered, with the additional benefit of weight reduction. In practice, the propeller will be mounted on a supporting structure close to the blades, which introduces large unsteady load fluctuations and tonal noise at the harmonics of the BPF. When using a porous airframe, the numerical results show significant reductions in the unsteady loading on the propeller and airframe’s surfaces, in addition to tonal noise reductions of up to 20 dB, especially at higher BPF harmonics. These findings suggest that porous treatments could be an effective passive method for mitigating propeller noise and propeller-airframe interaction noise in UAM vehicles.