Modelling the influence of acoustic liner non-uniformities on duct modes

In most turbofan engine intakes, the acoustically lined region consists of several liner segments which are separated by longitudinal hard-walled intercostal strips or splices. Measurements indicate that the duct modal spectrum can be considerably altered by such circumferential variations in acoustic properties, with energy being transferred from an incident mode to several others. To investigate and quantify this phenomenon, a three-dimensional, frequency domain finite element method has been developed. The computational domain represents a length of infinite, hard-walled duct of circular cross-section fitted with a region of longitudinally spliced liner. A modal coupling scheme is used, in which complex modal input amplitudes are specified across a section at one end of the computational duct domain and the amplitudes and phases of the transmitted modes are captured at the other end. In addition, to model the convective effect of flow in the duct, a uniform, unidirectional flow field is treated. The method is verified by comparing predictions with analytical solutions and is then used to analyze a liner with splices. It is demonstrated that incident modes are significantly affected by the circumferential variations in impedance and as such, the influence of liner splices on the transmitted acoustic field ought to be further investigated.