Innovative Acoustic Treatments of Nacelle Intakes Based on Optimised Metamaterials

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.

Efficient broadband sound absorption exploiting rainbow labyrinthine metamaterials

“…In conventional ideal fluid environments, sound propagation is typically described by a lossless wave equation [35][36][37]. However, when fluids are confined in small regions, like narrow tubes or slits, it is necessary to account for structural losses [30,33]. There are two primary loss mechanisms: thermal losses, mainly arising from heat diffusion at the boundaries within the sound field, and viscous losses, due to friction with the UC walls.…”

“…This kind of adaptability could largely benefit noise absorption applications at small to medium scale, where restrictions on structural size of the absorbers impose trade-offs between efficiency and encumbrance. Sufficiently thick conventional acoustic absorbing materials, such as glass wool or foams within sandwich panels with an average density of 50-75 kg m −3 [33] can absorb acoustic wave energy in wide frequency ranges, but their bulky characteristics limit their broad application for low frequency absorption. Moreover, lightweight characteristics become crucial when dealing with devices in the aerospace and automotive industry or other technological domains [8].…”

Numerical and Experimental Investigation of a Phase Gradient Metasurface Lining

“…The equivalent length is the fundamental parameter of such cells, directly defining the expected phase delay introduced in the reflected field compared to a flat boundary ∆Φ = 4πleq λ0 or, equivalently, the effective relative refractive index n eff = l eq /t. The acoustic modelling of the cells is obtained in this work by means of an equivalent metafluid model [20][21][22]. An acoustic metafluid is a particular case of metacontinuum that behaves acoustically as a fluid due to the null shear modulus exhibited by its structure (or near-zero in practical realizations [23]), potentially including anisotropy in its response.…”

Subsonic installed Jet noise scattering prediction using BEM and near-field-data-optimized Wave-Packet model

“…The research on noise reduction devices is nowadays very active in all the aircraft areas, involving relatively mature technologies for quieter high lift devices [3], chevrons for jet exhaust [4,5], the evolution of acoustic liners [6][7][8][9][10][11] for turbofans ducts, and also more innovative treatments with lower Technology Readiness Level [12]. Jet noise has always been a dominant noise source for turbojets and turbofans especially during take-off operations.…”

“…Innovative configurations such as Blended and Hybrid Wing Body (BWB and HWB) aircraft are probably the most promising alternative to the well-known tube-and-wing configuration in terms of aerodynamic efficiency and community noise reduction [13][14][15][16]. The most popular interpretation of these innovative configurations involves the upper installation of the propulsion system on top of the large center body surface, offering interesting acoustic shielding capability to be exploited for engine-related community noise reduction [12,17,18]. The propulsion-airframe acoustic interaction is an aspect of growing research interest for future aircraft and should be accounted for since the beginning of the design process.…”