Prediction and control of sound propagation in turbofan engine bypass ducts

This thesis contains original research into the propagation of sound in acoustically lined ducts<br/>with flow. The motivation for this work is the requirement to predict the sound attenuation of<br/>acoustic liners in the bypass duct of modern turbofan aeroengines. The liners provide the most<br/>effective means with which to suppress the rear fan noise. It is therefore important to make<br/>the best possible use of the available lined area by optimising the liner configuration. A set<br/>of analytic and numerical methods for predicting the liner attenuation performance have been<br/>developed, which are suitable for use in intensive liner optimisation studies, or as preliminary<br/>design tools.<br/>Eigenvalue solvers have been developed to find modal solutions in rectangular ducts with<br/>uniform flow and annular ducts with sheared flow. The solvers are validated by replicating<br/>results from the scientific literature and the Finite Element method. The effect of mean core<br/>flow radial profile and boundary layers on the mode eigenfunctions and axial decay rates are<br/>considered. It is shown that solutions for thin boundary layer flows converge to those based on<br/>the commonly used slip flow boundary condition. It is demonstrated that realistic flow profiles<br/>should be used to assess acoustic mode propagation in bypass ducts. The flow profile can have<br/>strong effects upon low order modes and surface waves, and in fact at high frequencies, the<br/>profile can affect all the modes.<br/>Mode-matching schemes are developed to assess the power attenuation performance and<br/>modal scattering of finite length liners. The results of the schemes are used to show that refraction<br/>of sound by boundary layers increases attenuation at high frequency. Power attenuation<br/>is higher where the mean core flow gradient refracts sound towards the liner. It is found that<br/>asymmetric liners can provide improved attenuation, depending on the direction of the mean<br/>flow shear gradient.<br/>The optimisation of axially-segmented liners for single and multi-mode sources is demonstrated.<br/>It is found that potentially large improvements in the attenuation of tonal noise is possible,<br/>whilst benefits for broadband noise are more difficult to achieve.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2007

Author dc:creator

• Brooks, Christopher James

Advisors dc:contributor.advisor

• McAlpine, Alan
• Kempton, Andrew