Evaluation of Novel Liner Concepts for Fan and Airframe Noise Reduction

Abstract

This paper presents a review of four novel liner concepts: soft vanes, over-the-rotor liners, external liners, and flap side-edge liners. A number of similarities in the design and evaluation of these concepts emerged during these investigations. Since these were the first attempts to study these particular liner concepts, there was limited information to guide the design process. In all cases, the target frequencies (or frequency range) were known, but the optimum acoustic impedance and optimum liner placement were typically not known. For these cases, the maximum available surface was used and a ρc-impedance was targeted based on the assumption the sound field impinges on the surface at normal incidence. This choice proved fruitful for every application. An impedance prediction model was used to design variable-depth liner configurations, and a graphical design code (ILIAD) was developed to aid in this process. The ability to build increasingly complex liner configurations via additive manufacturing was key, such that multiple designs could quickly be tested in a normal incidence impedance tube. The Two-Thickness Method was used to evaluate available bulk materials, such that bulk liners could also be considered for each application. These novel liner concepts provide sufficient noise reduction to warrant further investigations.

Design of a single degree of freedom acoustic liner for a fan noise test rig

“…2 The acoustic liners are commonly employed at the nacelle internal walls, more specifically the inlet and bypass ducts, whereas over-the-rotor and external liners are currently under investigation. [4][5][6] The acoustic effectiveness of liners depends not only on its geometry, but also on the environmental conditions found in turbofan engines, namely high grazing flow velocity and high sound pressure levels. These two combined effects represent a striking challenge for liner modelling, predictive tools and experimental techniques.…”

Impedance modelling of acoustically treated circumferential grooves for over-tip-rotor fan noise suppression

“…at normalised groove depth of zero, for the whole range of frequencies of interest and azimuthal mode numbers is also critical. This comparison is shown in Figure 10 for the lined groove and modes (5,1), (15,1) and (20,1). The numerical results are plotted for each mode only for the cut-on frequencies.…”

“…A comparison of the analytical and numerical effective impedance is shown in Figure 9 for a hard and lined duct, limited to modes (5,1), (15,1) and (20,1). In the hard groove case, the resistance is zero and the reactance results are in good agreement, tending to minus infinity when moving towards the hard wall boundary condition.…”

“…The use of circumferential grooves in OTR liners has shown a reduction in the aerodynamic performance penalty of the fan attributed to a decrease of the dynamic pressure on the acoustic treatment. 1,2 This led to liner designs which consist of a casing of circumferential grooves with and without absorbing elements. Acoustic test results from a single stage axial compressor at NASA-GRC with and without OTR liners has shown a net PWL insertion loss benefit of 2.5-3.5 dB for forward propagating modes.…”

Investigations of three over-the-rotor liner concepts at various technology readiness levels

“…Each of the concepts mentioned above was investigated 4,5 at the NASA Glenn and Langley Research Centers. Those liners were evaluated for their respective absorptive characteristics when they were placed in close proximity or over the rotor.…”