The influence of intercellular misalignment in multilayer acoustic liners

Current turbofan engine liners incorporate double-layer resonator type geometric configurations. These liners are usually designed on the assumption of locally reacting behaviour, which is rarely valid. Cells in adjacent layers may not line up, either because they are of different size or due to manufacturing difficulties. Unless explicitly designed for, misalignments create undesired acoustic paths, thus admitting intercellular acoustic interaction and rendering the design nonlocally reacting. A 3D finite element prediction technique which can represent a liner sample consisting of an array of individual cells, each of arbitrary shape and configuration, has been developed. Porous surfaces of arbitrary impedance distribution may be modelled, including nonlinear effects, and the solution provides a detailed output of the acoustic field and performance. The two-layer model is verified, and the method is applied to single mode device configurations. Two double-layer, misaligned liner configurations are analysed and compared with a datum, aligned, double-layer geometry. Unexpectedly, the predicted insertion loss spectra for these three configurations were not very different, and peak attenuation occurred at around the same frequency in all cases. The results show that the behaviour of these liners is almost locally reacting. Considering the extent of interconnection between cells this is a surprising outcome. Possible reasons are outlined, and suggestions for further study are discussed.