Noise Source Analysis in Counter-Rotating Open Rotors

Because of their potential to reduce aircraft emissions, counter-rotating open rotors (CRORs) have seen a renewed interest in applications, from civil aviation to urban air mobility. However, there are concerns surrounding their acoustic emissions. In the work presented, the authors investigated the noise sources for a range of CROR configurations for a general aviation class aircraft. Numerical models are used to compute the aerodynamic and aeroacoustic performance of these configurations. Further dissection of the various CROR noise sources is then carried out in order to better understand their contribution to CROR noise, and to direct the design of future low-noise CRORs. Noise reductions were observed when the rotor-rotor spacing was increased. Increased contraction of the wake requires the need for increased clipping to reduce the tip vortex interaction source. Noise reductions were also observed in the case of asynchronous rotational speeds. However, this was accompanied by a reduction in propulsive efficiency. Analysis of the CROR in nonaxial flight demonstrated the need to evaluate noise-reducing strategies at off-design conditions. Although the analysis proved the ability to reduce various CROR noise sources, it highlighted that tradeoffs with aerodynamic performance is necessary.