Optimisation of axially segmented liners for aeroengine broadband noise

A practical two-stage method for optimising an acoustic liner divided into axial segments for aeroengine broadband noise is presented. The principles are explained based on a three segment design but extension to more segments is straightforward. Initially, the optimisation focuses on the material properties of the individual segments. The second stage determines the optimum axial segment lengths and the optimum permutation of segments along the duct by evaluating the fully scattered acoustic field. This method is deployed to optimise an absorber for a broadband source noise of bandwidth 900 Hz–1.8 kHz. The axial order of the segments is found to have a dramatic influence on the optimum axial segment lengths. In the case study, comparisons with optimised uniform liners over this frequency range demonstrate only a small noise reduction. A simple optimisation that neglects scattering between axial segments significantly overestimates the available attenuation and provides a poor estimate of the optimum segment lengths.