Abstract
Modern turbofans with high bypass ratios, low blade passage frequencies and short nacelles require continuous development of acoustic linings to achieve the noise reductions expected by the international aviation authorities. Metamaterials and metafluids have been recently proposed as promising technologies for designing innovative acoustic treatments dedicated to reducing aeronautic turbofan noise emissions. In this work, a phase-gradient metasurface treatment is investigated as a way to tackle the noise radiation from an axially symmetric nacelle. This paper aims to study the potential benefits of the mentioned technology, and is not an attempt to design a complete new liner or nacelle. The metasurface is modelled through an equivalent metafluid, and a simulation-based optimisation is used in defining the design parameters. The tonal contribution of the blade passage frequency is considered, and the numerical results with the metafluid optimised on one azimuthal mode at a time show a significant effect in terms of acoustic levels and directivity over an arc of virtual receivers.
Highlights
Massive efforts have been devoted to alleviating noise pollution produced by aviation during the last decades by the aeroacoustic community
Metamaterials and metafluids have been recently proposed as promising technologies for designing innovative acoustic treatments dedicated to reducing aeronautic turbofan noise emissions
This paper aims to study the potential benefits of the mentioned technology, and is not an attempt to design a complete new liner or nacelle
Summary
Massive efforts have been devoted to alleviating noise pollution produced by aviation during the last decades by the aeroacoustic community. Urban areas surrounding airports are continuously expanding, and air traffic, even though momentary slowed by the Covid pandemic, is forecast to increase by all future-predicting scenarios. Both these issues have to be tackled for the sustainable development of the air transport system. One of the possible approaches to the issue of noise reduction is eliminating the main source or at least substituting it with a quieter one. Since their introduction in the early 1970s, turbofans contributed dramatically to increasing engine efficiency and reducing noise emissions. As higher bypass ratios led to decreasing importance of the jet component, the fan noise emerged as the new dominant source, during take-off operations [2]
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