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.

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This research is supported by a private sponsorship (Mike Newton) and a U.K. Research Council Grant EP/S010092/1 (Methods and Experiments for Novel Rotorcraft). The authors would like to acknowledge the use of the Cirrus UK National Tier-2 HPC Service at Edinburgh Parallel Computing Centre (EPCC) (http://www.cirrus.ac.uk) funded by the University of Edinburgh and Engineering and Physical Sciences Research Council (EPSRC) (EP/P020267/1). The authors would also like to acknowledge the assistance given by Research IT, and the use of The High Performance Computing (HPC) Pool funded by the Research Life cycle Programme at The University of Manchester. This work also benefitted from the aeroacoustics research within the UK Acoustics Network, EPSRC Grant EP/V007866/1.

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Received6 May 2021
Accepted8 August 2021
Published online13 October 2021

Noise Source Analysis in Counter-Rotating Open Rotors

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