Optimisation of slanted septum core and multiple folded cavity acoustic liners for aero-engines

Abstract

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.

Experimental and numerical assessment of novel acoustic liners for aero-engine applications

“…This paper extends earlier work presented in Ref. [9], which considered liner insertion loss prediction in the presence of uniform flow. The primary aim of this paper is to present the results of a detailed evaluation of two novel liner concepts, named Slanted Septum Core (SSC) and a MULTIple FOlded CAvity Liner (MultiFOCAL), while accounting for the influence of boundary-layer refraction, and the impact of the boundary layer profile.…”

“…The MultiFOCAL's insertion loss drops a little around 0.7 kHz. This is because the optimum impedance and the maximum possible insertion loss are very sensitive to small changes in impedance at low frequencies [9]. Additionally, it has been shown in Refs.…”

“…Further details of the optimization methodology are provided in Ref. [9,16]. A laser sintering additive manufacturing method has been used to fabricate the liner panels.…”

“…In the finite element model, the interior boundary condition for the perforated layer is given by Equation 1 in Ref. [9] to predict the impedance spectra for each liner configuration. The predicted impedance spectra are compared with those measured using the NLR's high-intensity impedance test set-up.…”

“…As stated earlier, the details of the liner design and optimisation are provided in Refs. [9,16], and summarised in Table . 1.…”