The effect of steady flow distortion on mode propagation in a turbofan intake

Astley, Richard Jeremy, Sugimoto, Rie, Gabard, Gwenael, Norde, Ellen, Grift, Ernst Jan and Bocquier, Marc (2014) The effect of steady flow distortion on mode propagation in a turbofan intake. In 20th AIAA/CEAS Aeroacoustics Conference. American Institute of Aeronautics and Astronautics. 22 pp . (doi:10.2514/6.2014-3113).

Abstract

Steady flow distortion in turbofan intakes is caused by non-axisymmetric nacelle geometry (`droop' and `scarf') and by non-zero flow incidence to the nacelle axis. Much analysis of sound propagation and attenuation in turbofan ducts, both analytic and numerical, is based on assumptions that the local cross-section of the intake is annular or circular and the mean flow is axially symmetric. Separable modal solutions then exist and the radial and azimuthal orders of the modes correlate with cut-on ratio, mode angle and modal attenuation. While the cross-section close to the fan face is necessarily axisymmetric to accommodate rotating elements, the nacelle itself is usually drooped and scarfed to ensure that at flight incidence the flow onto the fan is as axially and radially uniform as possible. As a consequence, steady azimuthal flow distortion occurs in the throat of the nacelle. In the current article, a model is proposed to predict the properties of modes propagating through such a region of azimuthal flow distortion. The mean flow is decomposed into a Fourier superposition of low order azimuthal components, and the eigen-modes propagating on the flow are then calculated by using a spectral representation in the azimuthal direction. In the radial direction, a numerical, Finite Element model is used. A second approach in which a radial expansion is used in terms of hard-walled symmetric eigenmodes, rather than finite element radial shape functions, has also been implemented for hard-walled ducts. Both methods give consistent results for modes propagating on distorted Hows in the absence of acoustic liners. Results are presented also for the case when a locally reacting acoustic liner is present at the outer wall. Levels of steady How distortion typical of those found in realistic intakes are shown to have a significant effect on modeshapes and on the azimuthal components which contribute to a single mode. Finally predictions are made for the distribution of azimuthal components in modes propagating in a drooped fan rig intake. These are compared to measured data from a circumferential mode detection array at the throat of such an intake.

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Contributors

Author: Rie Sugimoto

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