A REVIEW OF ACOUSTIC SPACE FILLING CURVE METAMATERIALS FOR JET ENGINE INLETS

Aircraft are notorious for their large sound profile, yet noise is one of the most significant limiting factors for aviation growth. In 2011 the World Health Organisation identified environmental noise as the second largest environmental health risk in Western Europe [1]. Prolonged exposure to unacceptable noise levels (≥55 dB Lden (day evening-night level)) is linked to conditions including tinnitus, hypertension, coronary heart disease and myocardial infarctions [2]. Factor such as these have led organisations like the Advisory Council for Aircraft Research and Innovation in Europe (ACARE) to target a reduction in perceived noise by 50% for new aircraft entering into service in 2020 relative to the year-2000 [3]. Moreover, airport operations are often restricted at night, due to factors such as noise therefore noise control offers a potential increased capacity. The ambitious ACARE targets are not being met, as of 2018 the percentage reduction is at 32% and is only projected to reach 37.5% [4]. The dominant noise source on an aircraft is the engine; approximately 68% at approach and at take-off approximately 98% [5]; however current technology and improvement attempts are not having the desired impact. This research project aims to introduce this innovation with metamaterial acoustic liners for the jet engine inlet. Metamaterials are a class of material that are engineered to produce properties that do not naturally occur. In practical terms this means additional properties of attenuation can be added to traditionally used aerospace materials by changing the internal structure. Metamaterials are not governed by the traditional bulk modulus and density characteristics of materials thus capable of controlling acoustic patterns through zero or even negative refraction; making acoustic cloaking possible and subwavelength low frequency attenuation [6]. In particular Space Filling Curves (SFC) have the potential to provide a lightweight, thin, high performance acoustic liner [7]. This paper contains a comparison of some of the most promising metamaterial acoustic liner designs which utilise Space Filling Curves, in terms of the fundamental theory of the design category and a discussion of the reflection and absorption characteristics. Computer simulation and impedance tube based experimental testing compares and contrasts the designs and the paper concludes with future application for aeroacoustics with particular focus on the engine inlet.