The effects of internal nozzle blockage in a single stream jet on far-field jet mixing noise

Wellman, Matthew, Proenca, Anderson, Lawrence, Jack and Self, Rod (2022) The effects of internal nozzle blockage in a single stream jet on far-field jet mixing noise. In 28th AIAA/CEAS Aeroacoustics 2022 Conference. American Institute of Aeronautics and Astronautics.. (doi:10.2514/6.2022-2913).

Abstract

Over the last half century, major reductions in jet mixing noise from aircraft engines have been possible from the study of isolated, axisymmetric jets. However, the impact on noise due to asymmetric effects, like the presence of a pylon connecting the wing to the engine, is not yet fully understood. In this study, the authors seek to establish whether azimuthal changes to the far-field jet noise introduced by the pylon can be predicted using a method called Lighthill's analogy with Ray Tracing (LRT) which is based on a k-ε RANS CFD model. One baseline annular and three increasingly internally blocked single-stream jets are studied and compared to model-scale experimental data. The k-ε RANS model is observed to sufficiently capture the deficit in the axial velocity and turbulent intensity, to within 1 and 0.5%, respectively, from the wake that forms immediately behind the blockage. The RANS CFD does, however, slightly under-predict the TI after 5 effective diameters compared to the experiment data. The increase in pylon thickness is seen to increase the mixing noise present at a polar angle of ninety degrees. At this polar angle, the high frequency modifications to the far field sound pressure level as a function of percentage blockage are captured to within 1 dB of the experimental data. The largest blockage is seen to introduce an additional vortex noise source at mid-Strouhal numbers, which is not modelled by LRT. This blockage is also observed to locally increase the turbulence levels and, therefore, the strength of the fine-scale acoustic sources at St≥2. The authors believe this to be due to the additional mixing of counter-rotating axial vortices. RANS-based noise predictions, using LRT, are shown to be capable of both indicating and diagnosing potentially noisy geometries quickly within an industrial context at the preliminary engine design stage.

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Contributors

Author: Matthew Wellman

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