Slanted septum and multiple folded cavity liners for broadband sound absorption

Abstract

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.

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

“…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.…”

“…Ref. [16] provides a detailed description of the numerical model used for predicting the liner impedances.…”

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

Aeroacoustics research in Europe: The CEAS-ASC report on 2020 & 2021 highlights

“…Emphasis was placed on achieving maximum insertion loss at representative sound pressure levels, for grazing incidence of multi-modal sound in the presence of flow [97]. The ISVR work involved numerical impedance modelling of novel cavity concepts, using the COMSOL FEM code [98]. Small scale test samples for no-flow normal impedance tests were also 3-D printed, and tested at the ISVR using the Rolls-Royce portable impedance tube.…”

Development of ultra-broadband sound absorber based on double-layered irregular honeycomb microperforated panel

“…Therefore, many researchers have combined honeycomb sandwich structures with perforated panels to realize multichannel perforated resonant acoustic superstructures or metamaterial [37,38]. In addition, honeycomb sandwich MPP structures have been widely used as acoustic liners [39][40][41][42] in aircraft engines to control noise. However, the nonlinear behavior of the acoustic liner with MPP at high sound pressure levels (generally above 130 dB) must be considered.…”