Understanding and reduction of cruise jet noise at aircraft level

Within the LINFaN research project, Airbus and Rolls-Royce have jointly investigated the acoustic impact of supersonic jet noise on the fuselage of an Airbus A340 equipped with Rolls-Royce Trent 500 engines in cruise conditions. The main results are presented in this paper. The influence of chevron fan nozzles designed to reduce cruise jet noise on the rear fuselage is investigated. The characterization of both the acoustic field and the aerodynamic flow is carried out. The acoustic data are obtained from a microphone array on the rear fuselage. After specific data de-noising, the acoustic spectra radiated on the fuselage by only the right inner engine are estimated. Beamforming acoustic maps are also computed to localize the jet noise sources. Besides, the supersonic jet flows are characterized by RANS CFD approach. For the baseline round nozzle, the acoustic results show the presence of two distinct broadband shock-associated noise contributions on the fuselage. The first pattern is observed in the far aft section of the fuselage for Strouhal numbers based on the jet mixing diameter and velocity between St = 4 and 6. The other pattern, around St = 1, radiates preferably forward. In addition, the high frequency source is located between 4 and 5 mixed jet diameters past the secondary nozzle exhaust plane, while the low frequency source is located farther downstream between 7 and 8 mixed jet diameters. Shock-associated noise is usually associated with the interaction between shock cells and turbulent shear-layers. We postulate that the high-frequency noise component results from the interaction between the shock-cells in the secondary flow and the inner shear-layer, while the low-frequency component is due to the interaction between shock cells and the outer shear-layer. This interpretation is consistent with two observations. The first observation is a strong modification of the measured high frequency noise when the engine is pushed to high power. At this regime shock cells are present in the primary flow and the inner shear layer interacts with two supersonic flows, hence modifying emitted noise. The second observation is the reduction of low- and high-frequency noise by the introduction of chevron fan nozzles. The flow field results confirm the strong impact of chevrons on the structure of the shock cells in the fan stream: they are consistent with the experimental observations of the characteristic shock noise frequencies. For the Trent 500 engine, appropriate imniersive fan chevrons may bring noise reductions by as much as 6dB SPL integrated over a relevant Strouhal number range [0.7; 2]. However, the reduction level due to the chevrons varies with the engine operating conditions. A given chevron nozzle may be found more efficient at reducing noise levels at low engine operating regime than at high regime.