Finite element investigation of the influence of liner splices on duct modes

There is evidence that the sound field radiating from a turbofan engine inlet can be strongly modulated by the presence of intercostal strips in the liner. The major concern here is that this modulation might result in a significantly louder radiated sound field. To investigate this phenomenon and to establish possible techniques for minimising any adverse effects, a finite element model has been developed. The model represents a length of an infinite cylindrical duct, part of which is lined, and where the flowfield is treated as uniform and unidirectional. Using a modal coupling scheme, modal input magnitudes are specified as an input at one end of the duct and the magnitudes of the transmitted modes are obtained at the other end. In this paper, the method is used to analyse liners with different numbers of splices and splices of different widths for a range of low order incident circumferential modes. It is demonstrated that the transmitted modal spectrum can be significantly modulated by the presence of splices, but for the frequency range considered (ka<10), the overall transmitted power is not significantly affected.