Ultrasound propagation in concentrated suspensions: shear‐mediated contributions to multiple scattering

Ultrasonic techniques offer many advantages for process monitoring for suspensions of particles; this type of material occurs in a wide variety of process industries. However, the application of ultrasonic techniques has been limited by the inaccuracy of the models used to interpret the measured ultrasonic propagation parameters (typically speed and attenuation spectra) in terms of particle size, concentration and physical properties. Multiple scattering models have been used with great success in relatively dilute suspensions (up to 10%w/w) for colloidal particles, but are inadequate at higher concentrations, smaller particles, and low frequencies. The principal reason is believed to be the shear‐mediated contributions to multiple scattering which have previously been neglected. We report analytical and numerical results for a modified scattering model that includes these shear‐mediated effects. The model is a development of the multiple scattering formulation published by Luppé, Conoir and Norris (J Acoust Soc Am, 2012 (131) 1113) which incorporates thermal and shear wave mode conversions to and from the compressional wave mode. We consider a silica‐in‐water suspension, identify the dominant scattering contributions and develop analytical forms for them. Numerical calculations demonstrate the contribution of the additional shearmediated effects to the compressional wave speed and attenuation through the suspension. The calculations are compared with previously published experimental data. The work follows a similar formulation to a recently published model for concentrated emulsions in which thermally‐mediated effects are considered (Pinfield, J Acoust Soc Am, 2014 (136) 3008)

The coherent shear wave in suspensions

We consider a disordered suspension of spherical silica particles in water. For a particle size of a few hundred nanometres and concentration (volume fraction) around 0.15 to 0.2, experiments conducted in the MHz range have shown that the non ideal nature of water must be taken into account for the "longitudinal" coherent wave attenuation to be understood, because of wave conversions, from longitudinal to shear and then back to longitudinal, occurring at each pair of scattering events. We are interested here in the properties of the "shear" coherent wave that are given by the expansion of its squared wavenumber, around that in the absence of particles, in powers of the concentration. At 1 MHz and a particle radius of 0.05 m, we show that convergence of the modal series involved in that expansion may be reached after three terms: we use ten terms subsequently. We study the evolution of both the effective shear velocity and attenuation with concentration, as well as that of the effective shear viscosity deduced therefrom

Effective dynamic properties of random complex media with spherical particles

The effective dynamic bulk modulus and density are presented for random media consisting of particles in a viscous host fluid, using a core-shell, self-consistent effective medium model, under the large compressional wavelength assumption. These properties are relevant to acoustic or dynamic processes in nano- and microparticle fluids including particle density determination, resonant acoustic mixing and acoustic characterisation. Analytical expressions are obtained for the effective bulk modulus and mass density, incorporating the viscous nature of the fluid host into the core-shell model through wave mode conversion phenomena. The effective density is derived in terms of particle concentration, particle and host densities, particle size, and the acoustic and shear wavenumbers of the liquid host. The analytical expressions obtained agree with prior known results in the limit of both static and inviscid cases; the ratio of the effective bulk modulus to that of the fluid is found to be quasi-static. Numerical calculations demonstrate the dependence of the effective mass density on frequency, particle size (from nano- to micro-regime) and concentration. Herein it is demonstrated both theoretically and numerically that the viscosity, often neglected in the literature, indeed plays a significant role in the effective properties of nanofluids

Experimental verification of nanofluid shear-wave reconversion in ultrasonic fields

Here we present the verification of shear-mediated contributions to multiple scattering of ultrasound in suspensions. Acoustic spectroscopy was carried out with suspensions of silica of differing particle sizes and concentrations in water to find the attenuation at a broad range of frequencies. As the particle sizes approach the nanoscale, commonly used multiple scattering models fail to match experimental results. We develop a new model, taking into account shear mediated contributions, and find excellent agreement with the attenuation spectra obtained using two types of spectrometer. The results determine that shear-wave phenomena must be considered in ultrasound characterisation of nanofluids at even relatively low concentrations of scatterers that are smaller than one micrometre in diameter

Ultrasonic spectrometry and modelling that verifies shear-wave reconversion in micro- and nano-fluids

Ultrasonic spectrometry and modelling that verifies shear-wave reconversion in micro- and nano-fluid

Contribution a la resolution du probleme inverse dans le cas de cylindres excites par des impulsions ultrasonores de haute frequence. Etude du probleme direct dans le cas de l'incidence oblique

SIGLET 54574 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Ondes guidées dans une plaque poroélastique immergée dans l'eau

Le comportement acoustique d'un matériau poroélastique est décrit par le modèle de Biot. La première partie présente une étude analytique aboutissant à l'écriture sous forme compacte des coefficients de réflexion et de transmission de la plaque immergée et saturée d'eau. La deuxième partie est consacrée à l'étude numérique pour 2 matériaux différents. Il existe 2 types de modes guidés. Les premiers, issus d'un couplage entre les ondes, rapide et transverse (élastique), sont propagatifs. Les seconds, dus à un couplage des 3 ondes, ne se propagent que si l'onde n'est pas diffusive. La dernière partie traite de l'étude numérique des coefficients de réflexion et de transmission. Dans le cadre de la RST, l'utilisation des termes de transmission montre qu'un lien existe entre les résonances angulaires et modes guidés de type élastique. Une étude expérimentale permet de déterminer le domaine fréquentiel dans lequel le QF-20 obéit au modèle de Biot.The poroelstic material acoustic behaviour is modeled via the Biot model. In the first part, an analytic study allows to derive the reflection and transmission coefficients of the loaded water saturated poroelastic plate in compact new forms. The second part presents the numerical study for 2 different materials, "QF-20" and "Stoll sand". 2 kinds of leaky lamb waves exist. The first ones, due to a coupling between the fast and transverse waves (elastic), propagate. The second ones, due to a coupling between the 3 waves existing in the infinite porous medium, propagate only if the slow wave itself is propagative. The third part deals with the numerical study of the reflection and transmission coefficients. As part of the RST theory, the introduction of transmission terms shows the link between some angular resonances and the first type of leaky lamb waves. An esperimental study shows in which frequency domain the "QF-20" material obeys the Biot theory.LE HAVRE-BU Centrale (763512101) / SudocSudocFranceF

Diffusion multiple par des cibles élastiques immergées (propagation d'ondes cohérentes et interactions résonantes)

LE HAVRE-BU Centrale (763512101) / SudocSudocFranceF

Propagation d'ondes guidées dans des plaques élastiques d'épaisseur linéairement variable

Etude expérimentale et numérique de la propagation des ondes guidées dans des plaques élastiques d'épaisseur linéairement variable. La première partie situe l'étude dans son contexte bibliographique et présente le support théorique utilisé pour l'analyse des résultat expérimentaux. La deuxième partie est consacrée à la description des protocole des expériences et des simulations numériques ainsi qu'à celles des phénomènes observés. Propagation et mise en évidence de la reflexion et de la conversion des modes adiabatiques.Guided waves in elastic plates with a linearly variable thickness are studied experimentally and numerically.. In the first part, a non-exhaustive review of published work is done, followed by the description of some results obtained by use of two theories. In the second part, the experimental and numerical methods used are explained, and the observed phenomena are discussed. Evidence of adiabatic modes propagation is shown, and ther are shown to be partially reflested and/or converted.LE HAVRE-BU Centrale (763512101) / SudocSudocFranceF

The coherent shear wave in suspensions [Abstract]

The coherent shear wave in suspensions [Abstract