Abstract
As building blocks of acoustic metamaterials, resonant scatterers have demonstrated their ability to modulate the effective fluid parameters, which subsequently possess extreme properties ...such as negative bulk modulus or negative mass density. Promising applications have been shown such as extraordinary absorption, focusing, and abnormal refraction for instance. However, acoustic waves can be further controlled in Willis materials by harnessing the coupling parameters. In this work, we derive the closed forms of the effective parameters from the transfer matrix in three asymmetric and reciprocal one-dimensional resonant configurations and exhibit the differences in terms of coupling coefficients. The way in which Willis coupling occurs in spatially asymmetric unit cells is highlighted. In addition, the analysis shows the absence of odd Willis coupling for reciprocal configurations. These effective parameters are validated against experimental and numerical results in the three configurations. This article paves the way of a novel physical understanding and engineering use of Willis acoustic materials.
The thermodynamic properties of fluids play a crucial role in many engineering applications, particularly in the context of energy. Fluids with multistable thermodynamic properties may offer new ...paths for harvesting and storing energy via transitions between equilibria states. Such artificial multistable fluids can be created using the approach employed in metamaterials, which controls macro‐properties through micro‐structure composition. In this work, the dynamics of such “metafluids” is examined for a configuration of calorically‐perfect compressible gas contained within multistable elastic capsules flowing in a fluid‐filled tube. The velocity‐, pressure‐, and temperature‐fields of multistable compressible metafluids is studied by both analytically and experimentally, focusing on transitions between different equilibria. The dynamics of a single capsule is first examine, which may move or change equilibrium state, due to fluidic forces. The interaction and motion of multiple capsules within a fluid‐filled tube is then studied. It shows that such a system can be used to harvest energy from external temperature variations in either time or space. Thus, fluidic multistability allows specific quanta of energy to be captured and stored indefinitely as well as transported as a fluid, via tubes, at standard atmospheric conditions without the need for thermal isolation.
This study proposes a novel approach to energy harvesting, storage, and on‐demand release by integrating compressible gas within a multistable structure. An analytical and experimental investigation is carried out into this “metafluid.” We examine its potential for energy harvesting from temperature fluctuations (such as day and night cycles), store the energy internally, and convert it into mechanical energy when needed.
The paper presents a method to modify the mechanical properties of a statically–designed metamaterial device in order to preserve its performance when operating in a non–uniform aerodynamic flow. The ...objective of the research is to contribute to the disclosure of the acoustic metamaterial potential in aeroacoustic applications, where acoustic propagation and scattering are deeply affected by aerodynamic convection. The emphasis is on aeronautical applications aiming at the development of methods for the design of breakthrough solutions for aircraft noise mitigation. The present approach is based on the application of the inverse Taylor transformation to the static design space in order to obtain the properties of an equivalent metamaterial in the convected space. The metamaterial so–obtained is capable to guarantee the target acoustic response in presence of a non–uniform background flow at low Mach number and with negligible vorticity. Numerical results obtained through finite–element simulations are presented for a free stream Mach number ≤0.35, which is compatible with the take–off and landing operation of a commercial airplane. The benchmark used is the widely assessed problem of the scattering cancellation (cloaking) of a circular obstacle. The acoustic disturbance is here assumed to be generated by an isotropic point source located in the vicinity of the scatterer and co–moving with it. The numerical results reveal that the effect of the proposed correction strongly depends on the relative position of the source and the metamaterial device, with the worst performance obtained when they are aligned with the free stream. The reason of this dependence is analytically explained and verified numerically. When the alignment of the source and the treated object is orthogonal to the flow, the Taylor–corrected metamaterial recovers almost completely its expected behaviour. This result makes the approach appealing for all those engineering applications where the relative position of sources and moving boundaries is fixed and compatible with the most favourable conditions.
Spherical silica xerogels are efficient acoustic Mie resonators. When these sub‐wavelength inclusions are dispersed in a matrix, the final metafluid may display a negative acoustic refractive index ...upon a set of precise constraints concerning material properties, concentration, size, and dispersity of the inclusions. Because xerogels may sustain both pressure and shear waves, several bands with negative index can be tailored.
Optical metafluids have held a special position among the platforms of metamaterials, because other than the lithography-based hard approaches, the soft fluidity-based solution process not only ...enables their immediate practical utility but also allows for reconfigurable and adaptable nanophotonic systems. However, the fundamental limits of the available effective parameters of optical metafluids are not yet clearly defined. Of particular interest is the accessible range of the refractive index under a practically available volume fraction ϕ and the structural motifs of building blocks. In addition, previously reported theoretical works are based on an effective medium theory that excludes dipolar coupling between building blocks. Using these initial approaches, the interaction between the building blocks at a relatively higher ϕ was not accurately rationalized. In this work, we advance an effective medium theory by using the 3D dressed polarizability. Then, we successfully rationalize the dipolar coupling between each of the building blocks and systematically exploit the fundamental limits of optical metafluids in terms of accessible effective parameters. Also, for the first time, we discuss both the phase transition of metafluids and uniaxial characteristics of fluidic crystals in terms of engineering effective parameters. Thereby, the practically available range of effective parameters from the concept of an optical metafluid is realistically defined. It is revealed that an unnaturally near-zero refractive index and an ultrahigh refractive index can be attainable through optical metafluids. Given the fundamental limits defined by 3D dressed polarizability, a comprehensive perspective of the limits and merits of optical metafluids is provided.
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.
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Mechanical metamaterials are man-made structures with counterintuitive mechanical properties that originate in the geometry of their unit cell instead of the properties of each ...component. The typical mechanical metamaterials are generally associated with the four elastic constants, the Young's modulus E, shear modulus G, bulk modulus K and Poisson's ratio υ, the former three of which correspond to the stiffness, rigidity, and compressibility of a material from an engineering point of view. Here we review the important advancements in structural topology optimisation of the underlying design principles, coupled with experimental fabrication, thereby to obtain various counterintuitive mechanical properties. Further, a clear classification of mechanical metamaterials have been established based on the fundamental material mechanics. Consequently, mechanical metamaterials can be divide into strong-lightweight (E/ρ), pattern transformation with tunable stiffness, negative compressibility (−4G/3 < K < 0), Pentamode metamaterials (G ≪ K) and auxetic metamaterials (G ≫ K), simultaneously using topology optimisation to share various fancy but feasible mechanical properties, ultralight, ultra-stiffness, well-controllable stiffness, vanishing shear modulus, negative compressibility and negative Poisson’s ratio. We provide here a broad overview of significant potential mechanical metamaterials together with the upcoming challenges in the intriguing and promising research field.
•The transfer matrix method is used to characterize built-up acoustic black hole duct terminations.•A differential equation describes wave propagation inside ideal acoustic black holes.•Acoustic ...waves in a metafluid with a power-law increasing density obey the differential acoustic black hole equation.•An acoustic black hole made of rings and cavities corresponds to a realization of the metafluid.•The transfer matrix solution in the metafluid formally tends to the solution of the differential acoustic black hole equation.
The transfer matrix method has been proposed to analyze the acoustic black hole effect in duct terminations. The latter is achieved by placing a retarding waveguide structure inside the duct, which consists in a set of rings whose inner radii decrease to zero following a power law. The rings are separated by thin air cavities. If the number of ring-cavity ensembles is large enough, wave propagation inside the waveguide can be treated as a continuous problem. A governing differential equation can be derived for the acoustic black hole which intrinsically relies on assumptions from transfer matrix theory. However, no formal demonstration exists showing that the transfer matrix solution is consistent and formally tends to the solution of the continuous problem. Proving such consistency is the main goal of the paper and an original option has been adopted to this purpose. First, we prove the differential equation for the acoustic black hole is identical to the wave equation for a metafluid with a power-law varying density. Transfer matrices are then applied to solve wave propagation in a discretization of the metafluid into layers of constant density. It is shown that when the number of layers tends to infinity and their thicknesses to zero, the transfer matrix solution satisfies the metafluid equation and therefore the acoustic black hole one. The transfer matrices for the metafluid discrete layers take a particularly simple form, which is a great advantage. This work allows one to interpret the retarding waveguide structure as a particular realization of the metafluid.
Assembling nanoparticles into well-defined structures is an important way to create and tailor the optical properties of materials. Most advances in metamaterials research to date have been based on ...structures fabricated in two-dimensional planar geometries. Here, we show an efficient method for assembling noble metal nanoparticles into stable, three-dimensional (3-D) clusters, whose optical properties can be highly sensitive or remarkably independent of cluster orientation, depending on particle number and cluster geometry. Some of the clusters, such as tetrahedra and icosahedra, could serve as the optical kernels for metafluids, imparting metamaterial optical properties into disordered media such as liquids, glasses, or plastics, free from the requirement of nanostructure orientation.
The design and chemical synthesis of plasmonic nanoresonators exhibiting a strong magnetic response in the visible is a key requirement to the realization of efficient functional and self-assembled ...metamaterials. However, novel applications like Huygens’ metasurfaces or mu-near-zero materials require stronger magnetic responses than those currently reported. Our numerical simulations demonstrate that the specific dodecahedral morphology, whereby 12 silver satellites are located on the faces of a nanosized dielectric dodecahedron, provides sufficiently large electric and magnetic dipolar and quadrupolar responses that interfere to produce so-called generalized Huygens’ sources, fulfilling the generalized Kerker condition. Using a multistep colloidal engineering approach, we synthesize highly symmetric plasmonic nanoclusters with a controlled silver satellite size and show that they exhibit a strong forward scattering that may be used in various applications such as metasurfaces or perfect absorbers.
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