Effect of Rotor Axial Spacing on Ducted Counter-rotating UAV Propeller Noise

Ducted configurations for counter-rotating propellers are increasingly being adopted for small UAVs due to their potential efficiency and redundancy benefits. However, the acoustic characteristics of these systems, particularly the effects of the axial rotor spacing on noise emissions, remain largely unexplored. We numerically investigate the aerodynamic and aeroacoustic effects of the axial spacing between the front and rear rotors of ducted counter rotating propellers using a high-fidelity compressible, wall-modeled LES solver. Geometries of 9.4-inch-diameter UAV propellers and an aerodynamic duct are modeled, and far-field noise is computed using the permeable Ffowcs-Williams in contrast, the first harmonic tone at 400 Hz depicts a distinctive two-lobe pattern indicating an equally-split interaction tone between the front- and rear-rotor harmonics, except for the smallest spacing case. Both of the tone intensities decrease with the rotor spacing. The rotor spacing predominantly affects low-frequency tones (200 − 1500 Hz), which decrease their amplitude with increasing spacing, while high-frequency noise (2500 − 4000 Hz) dominated by broadband shows a little sensitivity to the rotor spacing. This suggests that the rotor spacing primarily impacts on large-scale structures in the wakes rather than small-scale features, such as turbulence and tip vortices in the wall boundary layer.