This paper reports an assessment of the performance of two novel broadband liners for aero-engine applications, under both high sound pressure level and grazing flow. Duct insertion loss predictions are made for the novel liners for downstream sound propagation, with the results compared with that of optimised conventional Single Degree-of-Freedom (SDOF) perforate liners. The novel liner configurations include a slanted porous septum concept with varying open area, and a MultiFOCAL (Multiple FOlded CAvity Liner) concept. A numerical model of the flow duct test facility is developed using the commercial finite element code, COMSOL Multiphysics. This model is used to predict the liner insertion loss in the flow duct test facility. A multi-modal sound source is used, employing the assumption of uncorrelated modes with equal energy per mode. For downstream sound propagation, a simple convected wave equation model with the standard Ingard-Myers boundary condition is shown to be sufficient to predict the liner attenuation accurately. For upstream sound propagation, predictions and measurements for a SDOF linear liner highlight the need to account for boundary-layer refraction in the numerical modelling. The refraction effect due to the mean flow boundary layer will be modelled in a follow-up paper.

The design of acoustic liners with complex cavities for a wide frequency range of attenuation using numerical method is investigated in this paper. Three novel liner concepts are presented, demonstrating predicted improvements in broadband sound absorption when compared with that for conventional designs. The liners include a slanted septum core, a slanted septum core with varying percentage open area, and a MultiFOCAL concept. A finite element model of a normal incidence impedance tube is developed using COMSOL Multiphysics modeling software to predict the acoustic properties (resistance and reactance) of liners at medium and high sound pressure levels, and to study the impact of variations in the liner design parameters. The impedance tube finite element model incorporates non-linear semi-empirical impedance equations, validated by comparing numerical results with measurements performed on a single-degree-of-freedom liner, with a perforated face sheet, at high sound pressure level. The design variables of the novel liner concepts are optimized using a hybrid automated optimisation procedure. The low-frequency optimum slanted septum core concept with an open area of 4.5% for the face sheet and 18% for the short slanted septum is predicted to have an absorption level of at least 14 dB in the frequency range of 400–1000 Hz for normally incident pure tone excitations at 150 dB. The slanted septum core concept with varying percentage open area, with broadband optimum design variables, is predicted to have good broadband sound absorption levels of at least 10 dB in the frequency range of 570–3800 Hz. Finally, the MultiFOCAL liner concept with optimised percentage open areas is predicted to have an excellent broadband sound absorption levels of at least 14 dB, for pure tone excitations at 150 dB, in the frequency range of 900–5300 Hz. This work will be followed by optimisation of the face sheet geometries of these novel liner designs in order to maximise lined duct attenuation for aircraft engine applications.

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