Part-Span Approximation of Tone Noise Propagation in an Aeroengine Intake. Part I: Linear Propagation

Abstract

A part-span methodology is proposed for the approximation of fan tone noise propagation in aeroengine intake ducts. For fan rotor-associated tone noise sources in particular, but also rotor-stator interaction tones, much of the acoustic energy flux is concentrated towards the outer wall of the duct, offering the opportunity to reduce the computational expense of numerical predictions by neglecting the low-span regions of the duct. While conventional CFD methods can predict the acoustic field with reasonable accuracy, the computational expense is too high for iterative application during the design process. The research community also requires more efficient methods to understand modern challenges in aeroengine noise, such as the impact of increased flow distortion with UHBR (Ultra-High Bypass Ratio) designs. Part I of this study considers linear propagation. An analytic study is performed first using a uniform circular duct with uniform mean flow to assess the effect of the part-span approximation on individual Fourier-Bessel modes. This is followed by linear Computational AeroAcoustics (CAA) calculations of intake noise propagation and radiation using an axisymmetric but otherwise realistic geometry. Seven potential sources of error, associated with the part-span approximation, are identified and categorised into three characteristics: source truncation error, in-duct propagation error, and far-field radiation error.

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