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 inte rcostal strips or splices. Measurements indicate that the duct modal spectr um can be considerably altered by such circumferential variations in acoust ic properties, with energy being transferred from an incident mode to sever al others. To investigate and quantify this phenomenon, a three-dimensional , frequency domain finite element method has been developed. The computatio nal domain represents a length of infinite, hard-walled duct of circular cr oss-section fitted with a region of longitudinally spliced liner. A modal c oupling scheme is used, in which complex modal input amplitudes are specifi ed across a section at one end of the computational duct domain and the amp litudes 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 w ith splices. It is demonstrated that incident modes are significantly affec ted by the circumferential variations in impedance and as such, the influen ce of liner splices on the transmitted acoustic field ought to be further i nvestigated. (C) 1999 Academic Press.