Installation Effects on Contra-Rotating Open Rotor Noise at High-Speed

Abstract

A numerical strategy has been derived to assess the aeroacoustic installation effects of a Contra-Rotating Open Rotor (CROR) propulsion system at high cruise Mach number. Installation effects, which correspond to the interactions between the airframe and the rearmounted open rotors, may have a major impact on the noise (both cabin and far-field) generated by the aircraft. Numerical predictions of these installation effects are challenging as they imply simulations of various source mechanisms (such as shock-induced quadrupole sources or pylon wake-rotor interactions) along with complex propagation phenomena (namely the effect of fuselage boundary layer on acoustic waves) onto large-scale geometries. Therefore a careful validation process is proposed in two stages. In the first stage, the focus is on the validation of the methodology for computing the noise sources of an isolated CROR. In the second stage, the method is investigated in terms of its ability to capture the installation effects of a CROR at rig scale. The numerical strategy itself follows a two-step approach. Firstly, unsteady CFD simulations are performed to compute the unsteady flow across the rotors. Secondly, the Ffowcs Williams-Hawkings (FWH) analogy is applied to radiate the unsteady pressure to the near-field. The numerical procedure is validated via comparisons with experimental noise measurements taken from Rolls-Royce’s rig 145 (build 2) as part of a high-speed collaborative programme with Airbus that involved CROR systems of increasing complexity, ranging from isolated configurations to rear fuselagemounted engines. The study shows that the CFD approach is able to capture more accurately installation effects, which may account for a much better prediction of cabin noise levels.