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

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 take-off, with the running rotor providing the required thrust. Duringcruise, 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 take-off 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 12EPNLdB, respectively, for the same flyover.