2,962 research outputs found
Biharmonic Split Ring Resonator Metamaterial: Artificially dispersive effective density in thin periodically perforated plates
We present in this paper a theoretical and numerical analysis of bending
waves localized on the boundary of a platonic crystal whose building blocks are
split ring resonators (SRR). We first derive the homogenized parameters of the
structured plate using a three-scale asymptotic expansion in the linearized
biharmonic equation. In the limit when the wavelength of the bending wave is
much larger than the typical heterogeneity size of the platonic crystal, we
show that it behaves as an artificial plate with an anisotropic effective Young
modulus and a dispersive effective mass density. We then analyze dispersion
diagrams associated with bending waves propagating within an infinite array of
SRR, for which eigen-solutions are sought in the form of Floquet-Bloch waves.
We finally demonstrate that this structure displays the hallmarks of
All-Angle-Negative-Refraction(AANR) and it leads to superlensing and
ultrarefraction effects, interpreted thanks to our homogenization model as a
consequence of negative and vanishing effective density, respectively.Comment: 17 pages, 6 figure
Subwavelength sound screening by coupling space-coiled Fabry-Perot resonators
We explore broadband and omnidirectional low frequency sound screening based
on locally resonant acoustic metamaterials. We show that the coupling of
different resonant modes supported by Fabry-Perot cavities can efficiently
generate asymmetric lineshapes in the transmission spectrum, leading to a
broadband sound opacity. The Fabry-Perot cavities are space-coiled in order to
shift the resonant modes under the diffraction edge, which guaranty the opacity
band for all incident angles. Indeed, the deep subwavelength feature of the
cavities leads to avoid diffraction that have been proved to be the main
limitation of omnidirectional capabilities of locally resonant perforated
plates. We experimentally reach an attenuation of few tens of dB at low
frequency, with a metamaterial thickness fifteen times smaller than the
wavelength (lambda / 15). The proposed design can be considered as a new
building block for acoustic metasurfaces having a high level of manipulation of
acoustic waves.Comment: 7 pages, 8 figure
Finite elements modelling of scattering problems for flexural waves in thin plates: Application to elliptic invisibility cloaks, rotators and the mirage effect
We propose a finite elements algorithm to solve a fourth order partial
differential equation governing the propagation of time-harmonic bending waves
in thin elastic plates. Specially designed perfectly matched layers are
implemented to deal with the infinite extent of the plates. These are deduced
from a geometric transform in the biharmonic equation. To numerically
illustrate the power of elastodynamic transformations, we analyse the elastic
response of an elliptic invisibility cloak surrounding a clamped obstacle in
the presence of a cylindrical excitation i.e. a concentrated point force.
Elliptic cloaking for flexural waves involves a density and an orthotropic
Young's modulus which depend on the radial and azimuthal positions, as deduced
from a coordinates transformation for circular cloaks in the spirit of Pendry
et al. [Science {\bf 312}, 1780 (2006)], but with a further stretch of a
coordinate axis. We find that a wave radiated by a concentrated point force
located a couple of wavelengths away from the cloak is almost unperturbed in
magnitude and in phase. However, when the point force lies within the coating,
it seems to radiate from a shifted location. Finally, we emphasize the
versatility of transformation elastodynamics with the design of an elliptic
cloak which rotates the polarization of a flexural wave within its core.Comment: 14 pages, 5 figure
Cavitation Induction by Projectile Impacting on a Water Jet
The present paper focuses on the simulation of the high-velocity impact of a projectile impacting on a water-jet, causing the onset, development and collapse of cavitation. The simulation of the fluid motion is carried out using an explicit, compressible, density-based solver developed by the authors using the OpenFOAM library. It employs a barotropic two-phase flow model that simulates the phase-change due to cavitation and considers the co-existence of non-condensable and immiscible air. The projectile is considered to be rigid while its motion through the computational domain is modelled through a direct-forcing Immersed Boundary Method. Model validation is performed against the experiments of Field et al. [Field, J., Camus, J. J., Tinguely, M., Obreschkow, D., Farhat, M., 2012. Cavitation in impacted drops and jets and the effect on erosion damage thresholds. Wear 290–291, 154–160. doi:10.1016/j.wear.2012.03.006. URL http://www.sciencedirect.com/science/article/pii/S0043164812000968 ], who visualised cavity formation and shock propagation in liquid impacts at high velocities. Simulations unveil the shock structures and capture the high-speed jetting forming at the impact location, in addition to the subsequent cavitation induction and vapour formation due to refraction waves. Moreover, model predictions provide quantitative information and a better insight on the flow physics that has not been identified from the reported experimental data, such as shock-wave propagation, vapour formation quantity and induced pressures. Furthermore, evidence of the Richtmyer-Meshkov instability developing on the liquid-air interface are predicted when sufficient dense grid resolution is utilised
Cloaking and anamorphism for light and mass diffusion
We first review classical results on cloaking and mirage effects for
electromagnetic waves. We then show that transformation optics allows the
masking of objects or produces mirages in diffusive regimes. In order to
achieve this, we consider the equation for diffusive photon density in
transformed coordinates, which is valid for diffusive light in scattering
media. More precisely, generalizing transformations for star domains introduced
in [Diatta and Guenneau, J. Opt. 13, 024012, 2011] for matter waves, we
numerically demonstrate that infinite conducting objects of different shapes
scatter diffusive light in exactly the same way. We also propose a design of
external light-diffusion cloak with spatially varying sign-shifting parameters
that hides a finite size scatterer outside the cloak. We next analyse
non-physical parameter in the transformed Fick's equation derived in [Guenneau
and Puvirajesinghe, R. Soc. Interface 10, 20130106, 2013], and propose to use a
non-linear transform that overcomes this problem. We finally investigate other
form invariant transformed diffusion-like equations in the time domain, and
touch upon conformal mappings and non-Euclidean cloaking applied to diffusion
processes.Comment: 42 pages, Latex, 14 figures. V2: Major changes : some formulas
corrected, some extra cases added, overall length extended from 21 pages (V1)
to 42 pages (present version V2). The last version will appear at Journal of
Optic
Tunable Graphene Antennas for Selective Enhancement of THz-Emission
In this paper, we will introduce THz graphene antennas that strongly enhance
the emission rate of quantum systems at specific frequencies. The tunability of
these antennas can be used to selectively enhance individual spectral features.
We will show as an example that any weak transition in the spectrum of coronene
can become the dominant contribution. This selective and tunable enhancement
establishes a new class of graphene-based THz devices, which will find
applications in sensors, novel light sources, spectroscopy, and quantum
communication devices
Techniques for Generating Centimetric Drops in Microgravity and Application to Cavitation Studies
This paper describes the techniques and physical parameters used to produce
stable centimetric water drops in microgravity, and to study single cavitation
bubbles inside such drops (Parabolic Flight Campaigns, European Space Agency
ESA). While the main scientific results have been presented in a previous
paper, we shall herein provide the necessary technical background, with
potential applications to other experiments. First, we present an original
method to produce and capture large stable drops in microgravity. This
technique succeeded in generating quasi-spherical water drops with volumes up
to 8 ml, despite the residual g-jitter. We find that the equilibrium of the
drops is essentially dictated by the ratio between the drop volume and the
contact surface used to capture the drop, and formulate a simple stability
criterion. In a second part, we present a setup for creating and studying
single cavitation bubbles inside those drops. In addition, we analyze the
influence of the bubble size and position on the drop behaviour after collapse,
i.e. jets and surface perturbations
Numerical Analysis of Three-dimensional Acoustic Cloaks and Carpets
We start by a review of the chronology of mathematical results on the
Dirichlet-to-Neumann map which paved the way towards the physics of
transformational acoustics. We then rederive the expression for the
(anisotropic) density and bulk modulus appearing in the pressure wave equation
written in the transformed coordinates. A spherical acoustic cloak consisting
of an alternation of homogeneous isotropic concentric layers is further
proposed based on the effective medium theory. This cloak is characterised by a
low reflection and good efficiency over a large bandwidth for both near and far
fields, which approximates the ideal cloak with a inhomogeneous and anisotropic
distribution of material parameters. The latter suffers from singular material
parameters on its inner surface. This singularity depends upon the sharpness of
corners, if the cloak has an irregular boundary, e.g. a polyhedron cloak
becomes more and more singular when the number of vertices increases if it is
star shaped. We thus analyse the acoustic response of a non-singular spherical
cloak designed by blowing up a small ball instead of a point, as proposed in
[Kohn, Shen, Vogelius, Weinstein, Inverse Problems 24, 015016, 2008]. The
multilayered approximation of this cloak requires less extreme densities
(especially for the lowest bound). Finally, we investigate another type of
non-singular cloaks, known as invisibility carpets [Li and Pendry, Phys. Rev.
Lett. 101, 203901, 2008], which mimic the reflection by a flat ground.Comment: Latex, 21 pages, 7 Figures, last version submitted to Wave Motion.
OCIS Codes: (000.3860) Mathematical methods in physics; (260.2110)
Electromagnetic theory; (160.3918) Metamaterials; (160.1190) Anisotropic
optical materials; (350.7420) Waves; (230.1040) Acousto-optical devices;
(160.1050) Acousto-optical materials; (290.5839) Scattering,invisibility;
(230.3205) Invisibility cloak
Analytical Approximations for the Collapse of an Empty Spherical Bubble
The Rayleigh equation 3/2 R'+RR"+p/rho=0 with initial conditions R(0)=Rmax,
R'(0)=0 models the collapse of an empty spherical bubble of radius R(T) in an
ideal, infinite liquid with far-field pressure p and density rho. The solution
for r=R/Rmax as a function of time t=T/Tcollapse, where R(Tcollapse)=0, is
independent of Rmax, p, and rho. While no closed-form expression for r(t) is
known we find that s(t)=(1-t^2)^(2/5) approximates r(t) with an error below 1%.
A systematic development in orders of t^2 further yields the
0.001%-approximation r*(t)=s(t)[1-a Li(2.21,t^2)], where a=-0.01832099 is a
constant and Li is the polylogarithm. The usefulness of these approximations is
demonstrated by comparison to high-precision cavitation data obtained in
microgravity.Comment: 5 pages, 2 figure
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