Acoustic Analysis of Counter-Rotating Open Rotors with a Locked Blade Row

Abstract

Counter Rotating Open Rotors (CROR) have the potential to reduce environmental emissions thanks to their high propulsive efficiency. However, there are a number of concerns surrounding their acoustic emissions. This contribution presents a novel multi-configuration CROR that offers considerable noise reductions. In particular, we consider locking either fore or aft rotor during takeoff , with the running rotor providing the required thrust. During cruise, both rotors are operated to retain the high efficiency of the CROR. A coupled Computational Fluid Dynamics-Computational Aeroacoustics analysis (CFD-CAA) has shown the potential of this multi-configuration concept to offer substantial noise reductions when compared to a baseline CROR. During a simulated constant-altitude flyover at takeoff conditions, reductions of 3.5 dBA and 7.9 dBA have been demonstrated when either fore or aft row is locked, respectively. Using the EPNL metric, this result corresponded to7 EPNLdB and 12 EPNLdB, respectively, for the same flyover.

Noise Source Analysis in Counter-Rotating Open Rotors

“…However, earlier work involving the Fairey Gannet [6,7] demonstrated the importance of the potential field interactions, particularly for the unsteady loading on the fore r otor. Furthermore, recent work by the present authors [8] also showed the importance of the upstream interactions. These results showed that with AIAA low-solidity blade designs the upstream interactions were as important as the downstream interactions.…”

“…The HMB3 solver has been validated for a range of rotor flow cases [47][48][49]. Furthermore, the authors have previously shown the ability of the HMB3 solver in predicting open rotor flows [8,50].…”

“…The present implementation of the Farassat formulation has previously shown the capability in predicting the noise emission for the particular case of CROR [8,50], and will therefore not be reproduced here.…”

Noise of Future Aircraft Propulsion: Sources and Design Considerations

“…Finally, a method to reduce noise emissions during takeoff is by locking the aft blade row (working as a stator) (Smith et al, 2020). This will decreases the noise generated by the aft rotor, in all directivities.…”

Parametric study of aerodynamic performance of equivalent ducted/un-ducted rotors

“…Refs. [33][34][35][36]. It has also been used in previous simulation and optimisation studies of ducted rotors [13,17].…”