Modelling techniques to inform the design of sub-wavelength periodic acoustic structures

Abstract

Minimising the environmental impact of aircraft noise and ensuring the sustainability of air travel in the future is crucial for the development of the industry. The ad- vancement in additive manufacturing paves the way for new materials to develop noise reduction technologies. This document aims to apply the use of 3D-printed acoustic metamaterials for tackling fan noise emissions. This work focuses on developing metamaterials based on coupled resonators. A detailed numerical investigation on the absorptive performances of a first geometry is carried out with an emphasis on predicting the viscothermal losses which are crucial in this application. The periodic nature of this geometry (unit cells composed of spherical cavities interlinked by cylindrical openings) is found to be well reproduced by additive manufacturing. The resultant microporosity of these printed structures causes deviations with the numerical predictions of their acoustic performance. Therefore, further development in designing low frequency high absorptive structures using more controlled techniques is explored. A second configuration composed of multilayered plates is considered. This system exhibits multiple band gaps with subwavelength absorptive behaviour for a sample be- ing 0.04λ thick. The repeating cellular structure enables efficiencies in the viscothermal modelling with the use of the transfer matrix approach. For further modelling efficien- cies reduced order modelling is used to approximate losses in the gaps between plates. The absorptive results are evaluated against full viscothermal models and experimental measurements which show good agreements. The satisfactory results obtained formed the basis to design more complex multi- layered plate structures with acoustic resonances acting in parallel which lowered the first resonant frequency peak below 500 Hz and a broader absorption spectrum.