Predicting the effect of flow distortion on in-duct acoustic propagation

Abstract

Accurate prediction of the propagation and attenuation of fan noise in turbofan engine intakes is crucial for designing quieter aircraft engines. Predictions are often performed by using numerical or analytic models simplified to an axisymmetric geometry despite the fact that most aircraft engine intakes are non-axisymmetric, typically drooped and scarfed. Due to such geometric features and the off-axis angle at which flow enters the intake, the mean flow within the intake is not axisymmetric but varies in the circumferential direction. Duct modes cannot then be represented by a single azimuthal order but contain components of a number of adjacent orders. Although full three-dimensional models are required for accurate predictions, an approximate solution is obtained by considering modes in an axisymmetric duct with axially uniform but azimuthally varying mean flow. In this work a novel model is proposed in which duct modes, which are solutions of the Pridmore-Brown equation with a Fourier azimuthal basis, are obtained for a circumferentially varying axial flow. A weak variational formulation of the problem yields a third order eigenvalue problem in the axial wave number. Eigenvalue solutions are then used to perform a mode-matching analysis to predict the evolution of the azimuthal components along the intake duct. The method is applied to a straight annular duct with axially evolving non-axisymmetric flow. The resulting solutions are shown to be in reasonable agreement with a full three dimensional solution obtained by using the finite element code Actran TM.

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Identifiers

ORCID for Rie Sugimoto: orcid.org/0000-0003-2426-2382

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