The influence of drainage slots on locally reacting acoustic liners

Current turbofan engine liners incorporate drainage slots to prevent any liquid build-up in the resonator cavities, which would affect the acoustic performance and structural integrity. Unless explicitly designed for, drainage slots create undesired acoustic paths, thus admitting intercellular acoustic interaction. 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 included, and the solution provides a detailed output of the acoustic field and performance. The essentially linear model is extended to encompass the influences of acoustic apertures with nonlinear resistance characteristics, such as may occur in perforate face sheets, cellular misalignment and drainage slots. A semi-empirical orifice impedance model is devised, and nonlinearity is handled by an iterative solution procedure. Large apertures are discretised in the conventional way, and small openings are treated using an impedance coupling. The new models are verified, and the method is applied to single mode device configurations. Predictions for single layer arrays of square cells with longitudinal slots show that drainage slots render the liner nonlocally reacting, and cause significant changes in the insertion loss curve. Numerical results for hexagonal cells with longitudinal and lateral slots compare well with experimental data in most cases.