Prediction of the acoustic shielding by aircraft empennage for contra-rotating open rotors

Abstract

This paper presents a numerical study of the acoustic shielding of aeroengine noise by the aircraft structure. More specifically, the engines considered are contra-rotating open rotors in pusher mode which are installed at the rear of the aircraft. Several empennage designs and engine positions are evaluated. An original method for the calculation of engine noise installation effects is presented and its application to contra-rotating open rotor propulsion is demonstrated. It is based on the weak coupling between computational fluid dynamics, an integral method based on Lighthill's analogy for the calculation of acoustic radiation, and on a boundary element method for the calculation of the acoustic diffraction by the aircraft fuselage and empennage. The prediction of a pylon-installed (isolated) contra-rotating open rotor noise directivity is presented and compared to equivalent wind-tunnel measurements. In both cases, the simulation and experiment are performed at 1:7 scale. In addition, the installation effect of the wind-tunnel, being nonanechoic, is also assessed with the numerical approach presented. Finally, for each aircraft configuration of interest, equipped with two rear contra-rotating open rotors, the predicted noise impact on the ground is evaluated. Of these, the contra-rotating open rotor mounted above an L-tail empennage is found to be the most beneficial configuration in terms of acoustic shielding.

A parametric study of airframe effects on the noise emission from installed contra-rotating open rotors

“…These results compare well with the numerical simulations of Sanders et al., 6,7 which reported beneficial effects of increased pylon angle and for the L and U tails. The differences in directivity for the various BPF tones and the broadband noise demonstrate the challenge faced when designing an airframe with shielding benefits for CROR noise.…”

“…These experimental results are being utilized by GRA partners to develop and validate numerical codes for CROR noise prediction with initial results recently reported by Sanders et al. 6,7 The more recent paper by Sanders reviews the literature for installation effects.…”

“…The comparisons of the acoustic emissions of the pusher and the tractor configuration tested at the same flow velocities and angles of incidence favor slightly the tractor configuration at 2*BPF which are the dominant rotor tones when ignoring the influence of the pylon. These experimental results are being utilized by GRA partners to develop and validate numerical codes for CROR noise prediction with initial results recently reported by Sanders et al 6,7 The more recent paper by Sanders reviews the literature for installation effects.…”

“…The perpendicular distance from the center of the front blade plane to the linear microphone array varies depending on both engine pylon angle and also on the extension which ranges between a minimum distance of 6.28D and a maximum of 6.65D. The work of Sanders et al 6,7 conducted a numerical simulation of the WENEMOR engine and demonstrated that the farfield region begins at a distance of 1.5D for the fundamental BPF.…”

“…The work of Sanders et al. 6,7 conducted a numerical simulation of the WENEMOR engine and demonstrated that the far-field region begins at a distance of 1.5 D for the fundamental BPF.…”