Computational Aeroacoustic Investigation of Co-rotating rotors for Urban Air Mobility

Abstract

Noise is a major issue in Urban Air Mobility vehicles, especially due to its operation in the urban environment. One of the major sources of noise is the rotor, and thus, there is a need to optimize the rotor design for aerodynamics and aeroacoustics. A possible technological solution is represented by Co-rotating rotors (i.e., two rotors connected through the same shaft with a prescribed axial and azimuthal separation). Such configuration can offer significant potential in noise reduction without affecting thrust and power distribution in a vehicle. At present, there is a lack of knowledge on the mutual interaction between the two rotors which limits physics based optimal designs.
The purpose of the present study was to investigate the behaviour of a co-rotating rotor when its geometrical parameters are varied, such as: axial and azimuthal separation, collective pitch, etc. The simulation was performed on commercial software 3DS PowerFLOW which uses Lattice-Boltzmann Very Large Eddy Simulations (LB-VLES) for flow-field simulation and Ffowcs-Williams and Hawkings (FWH) analogy for far-field aeroacoustic post-processing. Results show that, a higher angular separation results in higher thrust and lower noise, while a higher axial separation increases thrust but with noise increase with respect to a single rotor. However, thrust of co-rotating rotors is still smaller than the sum of two isolated rotors. This is due to the potential effect due to mutual interaction of the streamtubes of each rotor, blade-vortex interaction and flap effect (i.e. the lower rotor acts as a flap for the upper rotor when angular separation is small). Noise reduction is instead caused by destructive interference between the sound scattered by each rotor. Thus, a co-rotating rotor with higher azimuthal angle will produce less noise than a single rotor, both producing the same thrust. These findings suggest that the design of the two rotors can be optimized to account for the interaction effects, thus potentially doubling the effective thrust but still reducing noise.