4,499 research outputs found
Multiple waves propagate in random particulate materials
© 2019 Society for Industrial and Applied Mathematics Publications. All rights reserved. For over 70 years it has been assumed that scalar wave propagation in (ensembleaveraged) random particulate materials can be characterized by a single effective wavenumber. Here, however, we show that there exist many effective wavenumbers, each contributing to the effective transmitted wave field. Most of these contributions rapidly attenuate away from boundaries, but they make a significant contribution to the reflected and total transmitted field beyond the low-frequency regime. In some cases at least two effective wavenumbers have the same order of attenuation. In these cases a single effective wavenumber does not accurately describe wave propagation even far away from boundaries. We develop an efficient method to calculate all of the contributions to the wave field for the scalar wave equation in two spatial dimensions, and then compare results with numerical finite-difference calculations. This new method is, to the best of the authors' knowledge, the first of its kind to give such accurate predictions across a broad frequency range and for general particle volume fractions
Time reversal of ultrasound in granular media
Time reversal (TR) focusing of ultrasound in granular packings is
experimentally investigated. Pulsed elastic waves transmitted from a
compressional or shear transducer source are measured by a TR mirror, reversed
in time and back-propagated. We find that TR of ballistic coherent waves onto
the source position is very robust regardless driving amplitude but provides
poor spatial resolution. By contrast, the multiply scattered coda waves offer a
finer TR focusing at small amplitude by a lens effect. However, at large
amplitude, these TR focusing signals decrease significantly due to the
vibration-induced rearrangement of the contact networks, leading to the
breakdown of TR invariance. Our observations reveal that granular acoustics is
in between particle motion and wave propagation in terms of sensitivity to
perturbations. These laboratory experiments are supported by numerical
simulations of elastic wave propagation in disordered 2D percolation networks
of masses and springs, and should be helpful for source location problems in
natural processes.Comment: 15 pages, 6 figure
The average transmitted wave in random particulate materials
Microwave remote sensing is significantly altered when passing through clouds
or dense ice. This phenomenon isn't unique to microwaves; for instance,
ultrasound is also disrupted when traversing through heterogeneous tissues.
Understanding the average transmission in particle-filled environments is
central to improve data extraction or even to create materials that can
selectively block or absorb certain wave frequencies. Most methods that
calculate the average transmitted field assume that it satisfies a wave
equation with a complex effective wavenumber. However, recent theoretical work
has predicted more than one effective wave propagating even in a material which
is statistical isotropic and for scalar waves. In this work we provide the
first clear evidence of these predicted multiple effective waves by using high
fidelity Monte-Carlo simulations that do not make any statistical assumptions.
To achieve this, we also had to fill in a missing link in the theory for
particulate materials: we prove that the incident wave does not propagate
within the material, which is usually taken as an assumption called the
Ewald-Oseen extinction theorem. By proving this we conclude that the extinction
length - the distance it takes for the incident wave to be extinct - is equal
to the correlation length between the particles
Effective wavenumbers and reflection coefficients for an elastic medium containing random configurations of cylindrical scatterers
Propagation of P and SV waves in an elastic solid containing randomly
distributed inclusions in a half-space is investigated. The approach is based
on a multiple scattering analysis similar to the one proposed by Fikioris and
Waterman for scalar waves. The characteristic equation, the solution of which
yields the effective wave numbers of coherent elastic waves, is obtained in an
explicit form without the use of any renormalisation methods. Two
approximations are considered. First, formulae are derived for the effective
wave numbers in a dilute random distribution of identical scatterers. These
equations generalize the formula obtained by Linton and Martin for scalar
coherent waves. Second, the high frequency approximation is compared with the
Waterman and Truell approach derived here for elastic waves. The Fikioris and
Waterman approach, in contrast with Waterman and Truell's method, shows that P
and SV waves are coupled even at relatively low concentration of scatterers.
Simple expressions for the reflected coefficients of P and SV waves incident on
the interface of the half space containing randomly distributed inclusions are
also derived. These expressions depend on frequency, concentration of
scatterers, and the two effective wave numbers of the coherent waves
propagating in the elastic multiple scattering medium.Comment: 24 page
Multiple scattering by a collection of randomly located obstacles Part IV: The effect of the pair correlation function
The effect of two different pair correlation functions, used to model multiple scattering in a slab filled with randomly located spherical particles, is investigated. Specifically, the Percus-Yevick approximation is employed and a comparison with the simple hole correction is made. The kernel entries of the hole correction have an analytic solution, which makes the numerical solution of the integral equations possible. The kernel entries of Percus-Yevick approximation are integrated numerically after a subtraction of the slowly converging part in the integrand. Several numerical examples illustrate the effect of the two pair correlation functions, and we also make a comparison with the predictions Bouguer-Beer law gives
Multiple scattering theory for heterogeneous elastic continua with strong property fluctuation: theoretical fundamentals and applications
In this work, the author developed a multiple scattering model for
heterogeneous elastic continua with strong property fluctuation and obtained
the exact solution to the dispersion equation derived from the Dyson equation
under the first-order smoothing approximation. The model establishes accurate
quantitative relation between the microstructural properties and the coherent
wave propagation parameters and can be used for characterization or inversion
of microstructures. As applications of the new model, dispersion and
attenuation curves for coherent waves in the Earth lithosphere, the porous and
two-phase alloys, and human cortical bone are calculated. Detailed analysis
shows the model can capture the major dispersion and attenuation
characteristics, such as the longitudinal and transverse wave Q-factors and
their ratios, existence of two propagation modes, anomalous negative
dispersion, nonlinear attenuation-frequency relation, and even the
disappearance of coherent waves. Additionally, it helps gain new insights into
a series of longstanding problems, such as the dominant mechanism of seismic
attenuation and the existence of the Mohorovicic discontinuity. This work
provides a general and accurate theoretical framework for quantitative
characterization of microstructures in a broad spectrum of heterogeneous
materials and it is anticipated to have vital applications in seismology,
ultrasonic nondestructive evaluation and biomedical ultrasound.Comment: 70 pages, 42 figure
Ioffe-Regel criterion of Anderson localization in the model of resonant point scatterers
We establish a phase diagram of a model in which scalar waves are scattered
by resonant point scatterers pinned at random positions in the free
three-dimensional (3D) space. A transition to Anderson localization takes place
in a narrow frequency band near the resonance frequency provided that the
number density of scatterers exceeds a critical value , where is the wave number in the free space. The
localization condition can be rewritten as ,
where is the on-resonance mean free path in the independent-scattering
approximation. At mobility edges, the decay of the average amplitude of a
monochromatic plane wave is not purely exponential and the growth of its phase
is nonlinear with the propagation distance. This makes it impossible to define
the mean free path and the effective wave number in a usual way. If
the latter are defined as an effective decay length of the intensity and an
effective growth rate of the phase of the average wave field, the Ioffe-Regel
parameter at the mobility edges can be calculated and takes values
from 0.3 to 1.2 depending on . Thus, the Ioffe-Regel criterion of
localization is valid only
qualitatively and cannot be used as a quantitative condition of Anderson
localization in 3D.Comment: Revised and extended version. 9 pages, 6 figure
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