The Effect of Steady Flow Distortion on Mode Propagation in a Turbofan Intake

Abstract

Steady flow distortion in turbofan intakes is caused by non-axisymmetric nacelle geometry (`droop' and `scarf') and by incident flow which is not necessarily aligned with the nacelle axis. Predictions of propagation and attenuation in turbofan nacelles are commonly based on an assumption that the nacelle is straight and axisymmetric and that the mean flow does not vary azimuthally about the axis. Analytic and numerical solutions for the acoustic field are then obtained by assuming that each azimuthal Fourier component of the field can be treated separately and that no scattering occurs between them. When a numerical scheme is used, a solution can be obtained by discretising each azimuthal component on a two-dimensional slice through the duct axis and by solving separately for each azimuthal order (a `2.5D' model). This avoids the need to solve the discrete problem on a three-dimensional mesh. When three-dimensional geometric features and fully three dimensional mean flow are taken into account, such assumptions no longer hold. The propagation of fan noise through the intake then becomes a more challenging three-dimensional problem. In the current article, a numerical study is presented in which a three-dimensional numerical simulation is performed to assess the effect of steady flow distortion on the propagation and attenuation of fan tones in a lined, drooped intake. The objective of the study is to assess whether such effects are significant in determining acoustic power and directivity for current and future turbofan designs. The results presented here focus on the effect of flow distortion at take-off when strong rotor-locked tones dominate in the forward arc. A sequence of fan intake models of increasing geometrical complexity is presented and these are used to determine the degree of fidelity which is required in the description of the geometry and mean flow to make useful acoustic predictions. The results indicate that quite modest levels of flow distortion distribute the acoustic power of a dominant engineorder fan tone into a much broader spectrum of adjacent azimuthal orders at the throat, and significantly modify the far field directivity. These predictions appear to be consistent with existing measured test rig data which is also presented.