Aeroacoustic Analysis of Ducted Contra-Rotating-Rotor Unmanned Aerial Vehicles in Hover

This work investigates a ducted contra-rotating-rotor unmanned aerial vehicle (UAV) whose aeroacoustic properties are less known compared to quadrotor UAVs. Two factors complicate the aeroacoustic analysis of this new design: the aerodynamic interaction among coaxial contra-rotating rotors and motor struts introduces additional source mechanisms, and acoustic scattering on duct walls further complicates the acoustic propagation process. A hybrid computational aeroacoustics (CAA) method is developed to project the aeroacoustic sources onto the in-duct modal space, allowing one to link various aeroacoustic sources to the acoustic far field through duct modes. An intriguing finding is that rotor–rotor interactions, the main source mechanism in open contra-rotating rotor propulsion systems, dominate acoustic far field only at even harmonic orders of blade passing frequencies (BPFs). A modified Tyler–Sofrin criterion has been derived to explain the underlying physics, where it is shown that the rotor–rotor interaction noise excites plane wave mode that couples to the acoustic far field. Conversely, at odd harmonic orders of BPFs, the rotor–strut interaction dominates the acoustic far field through the lower-order duct modes following the classical Tyler–Sofrin criterion. These results suggest the potential of exploiting mode scattering effects among rotors and struts, as well as the duct cutoff effect for significant noise reduction in ducted UAVs.