An experimental analysis of acoustic liners utilising metamaterial space filling curves, applied to commercial jet engine inlets

Acoustic metamaterials research has grown exponentially in the past 10 years driven by the advances in manufacturing and an increased understanding of damaging environmental noise. The first noise reduction target, 2020, as set by Advisory Council for Aircraft Research and Innovation in Europe was a relative 50% decrease in aircraft noise. This was missed by current commercial jet engine noise control technology; however, metamaterials offer an encouraging alternative. Space Filling Curves (SFC) have the potential to provide a lightweight, thin, high performance acoustic liner. SFC have a history in mathematical geometry dating back to the 1890’s but, are a comparatively new addition to acoustics. They are designed with a sub-wavelength curled cross-section creating a maze-like pattern which slows acoustic wave propagation through the liner enabling characteristics such as negative refraction and low frequency attenuation. 

This thesis analysed the most promising SFC metamaterial acoustic liner designs, in terms of the fundamental theory of the design category and experimental reflection, absorption and transmission characteristics. The industry standard impedance tube testing captured a full operating condition profile through head on, transmissive and grazing flow conditions. The existing SFC design computer simulation and thesis research experimental testing compared the liners, enabling a bespoke design refinement for SFC liners specific to jet engine inlets. Hybrid SFC liner were proposed for the low frequency dominant noise profile of a jet engine with the unique HexSpi double 10mm liner with 8 hole top plate presented as solution to engine noise exceeding that of the traditional Helmholtz resonator (HR). 

Extensive research into, and iterations of, a simulation based low user input experimental validator, for acoustic liners capturing acoustic characteristic performance was undertaken. OpenFOAM a free to use CFD software was studied to provide a simulated acoustic domain, where liners could undergo standard impedance testing without the need for expensive experimental equipment or sample manufacture. The final simulation case provided a turbulent, incompressible domain with a random noise low flow input and anechoic output. Which demonstrated near experimental and theoretical HR resonance with both single and many HR units present. However, future progressions are required to improve simulation output convergence and accuracy.