Aerodynamics and aeroacoustics of ducted propellers: A study on the design and geometry effects

Ducted propellers present broad applicability in urban air mobility vehicles due to their enhanced operational safety, improved aerodynamic performance, and potential to mitigate noise emissions. This study proposes a numerical approach for designing adequate duct geometries, focusing on the duct's lip profile, expansion ratio, and tip clearance, aiming to provide valuable design guidance for ducted propellers. The simulations are validated through experimental data, showing reasonable agreement in terms of thrust generation and far-field noise. The mean flow and generated thrust are characterized with a parametric study using steady simulations, while delayed detached eddy simulations are employed to capture transient flow characteristics and investigate noise generation. The noise levels were computed using the integral solution of the Ffowcs-Williams and Hawkings equation. The lip geometry impacts the flow distribution and generated thrust, modifying the tonal noise. Furthermore, slightly divergent ducts can increase the total thrust by minimizing flow separation on the duct wall while increasing the suction on the duct lip. The primary noise sources are identified at the propeller's leading edge and tip. The results reveal that divergent ducts effectively reduce tonal noise at all observer angles but increase broadband noise, attributed to the noise sources at the leading edge of the propeller and the interaction with the duct lip. Additionally, reducing the tip clearance from 2 to 1 mm enhances the total thrust by more than 20% without causing extra noise generation.