Ferrous Fibre Network Materials for Jet Noise Reduction in Aeroengines Part I: Acoustic Effects

Abstract

Noise emitted by gas turbine aeroengines is of major concern. For turbofan engines, it comes from two main sourcesthe rotational motion of the system (fan, compressor and turbine) and gas mixing phenomena associated with the jet emerging from the rear of the engine ("jet noise"). Most rotation-generated noise occurs at well-defined frequencies (dependent on engine speed), whereas jet noise tends to be more broadband. Fan/compressor/turbine noise normally dominates during landing and is of comparable amplitude to jet noise during cruising. Fan noise can be reduced by using acoustic liners and by redesigning the inlet lip to make the inner wall of the nacelle sound-absorbent, and by reshaping the fan blades -for example so that they tend to dissipate local shockwaves.Jet noise (including any contributions from mixing of core/combustion and by-pass streams) is mainly associated with turbulent flow in the exhaust region. It is the dominant acoustic source during take-off, and is hence of prime concern. Unfortunately, jet noise is hard to control, since any measures must exert their effect before the exhaust leaves the engine. In fact, Lighthill's eighth power law (linking noise intensity to gas velocity), and its consequence of high-bypass ratio (low gas velocity) engines tending to allow lower noise levels for a given thrust, remains a pre-eminent design guideline. Other design changes aimed at reducing jet noise have had limited success. Recent developments include nozzle designs incorporating chevrons [1,2] to control mixing of core and by-pass streams and exhaust designs promoting mixing of core, by-pass and ambient airflows, both of which are reported to give up to ∼ 3 dB noise reduction. Broadband noise reduction can also be achieved using active absorption systems, [3,4] but these rely on rapid and complex feedback control. In general, they are considered unlikely to be effective on most aircraft, at least in the near future.Increased exhaust containment length, with a module made of a suitable acoustic attenuation material in the enclosed region, has good potential for reduction of jet noise. Open cell porous materials are promising candidates. Acoustic absorption within them [5][6][7] results mainly from drag on acoustic waves as they propagate through narrow, tortuous channels. The frequency characteristics of such absorption should be taken into account, and related to the frequency sensitivity of the human ear. The well known Fletcher-Munson curves [8] indicate maximum sensitivity at around 1-6 kHz. Frequencies below a few hundred Hz and above ∼ 10 kHz are unlikely to be problematic. There have been relatively few studies on the effect of the architecture of porous materials on the frequency dependence of their sound absorption characteristics. Xie et al. [7] did, however, report that, in their work on directionally-solidified porous copper with porosity in the approximate range of 40-60 %, sound absorption increased with increasing frequency, from a low level below ∼ 1 kHz to a broad peak somewhere betw...

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