Numerical Aeroacoustic Study of Small Ducted Coaxial Counter-rotating UAV Propellers

The increasing interest in electric vertical takeoff and landing (eVTOL) aircraft has highlighted a need to understand aerodynamic and acoustic characteristics of open versus ducted rotors. This numerical study investigates the aerodynamic and acoustic characteristics of open and ducted coaxial counter-rotating propellers, and analyzes the effects of a duct on the frequency spectrum and directivity of small UAV propeller noise. We use a high-fidelity numerical flow solver, charLES, which performs a compressible, wall-modeled LES capable of resolving turbulence and acoustics. Geometries of a 9.4-inch diameter UAV propeller and an encompassing aerodynamic duct are modeled in detail. Far-field noise propagation is computed using the permeable Ffowcs-Williams & Hawkings technique. The ducted configuration demonstrates a 4% increase in total thrust compared to the open rotors, attributed to reduced tip leakages and improved inflow. Acoustic simulations reveal that the duct can effectively shield only the blade passing frequency (BPF, 200 Hz) tone, although duct mode analysis indicates that the BPF, 3BPF and 5BPF tones become all cut-off. At frequencies above 1, 500 Hz, the ducted configuration generates higher broadband noise levels due to interaction between the duct boundary layer and the rotor tip vortices with higher turbulent intensity. The ducted configuration exhibits higher overall sound pressure levels (OASPL) and a distinct two-lobe pattern with a sharp valley at 90, while the open configuration has a smeared directivity with lower OASPL in the fore arc due to the BPF tone dominance.