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A new hybrid finite-difference (FD) and pseudospectral (PS) method adapted to the modeling of piezoelectric transducers (PZTs) is presented. The time-dependent equations of propagation are solved using the PS method while the electric field induced in the piezoelectric material is determined through a FD representation. The purpose of this combination is to keep the advantages of both methods in one model: the adaptability of FD representation to model piezoelectric elements with various geometries and materials, and the low number of nodes per wavelength required by the PS method. This approach is implemented to obtain an accurate algorithm to simulate the propagation of acoustic waves over large distances, directly coupled to the calculation of the electric field created inside the piezoelectric material, which is difficult with classical algorithms. These operations are computed using variables located on spatially and temporally staggered grids, which attenuate Gibbs phenomenon and increase the algorithm’s accuracy. The two-dimensional modeling of a PZT plate excited by a 50 MHz sinusoidal electrical signal is performed. The results are successfully compared to those obtained using the finite-element (FE) algorithm of ATILA™ software with configurations spatially and temporally adapted to the FE requirements. The cost efficiency of the FD-PS time-domain method is quantified and verified. [ABSTRACT FROM AUTHOR]

Abstract: The explosion ability of wood dust was characterized by a 20 L explosion sphere (Kühner). The overpressure inside the sphere is recorded during the explosion. The results show that the violence of the explosion is all the more important that the particle size is low. A model based on balances on chemical reaction, kinetics and thermodynamics leads to the representation of the pressure change during the explosion. There is a good agreement between the calculations and the experiments. [Copyright &y& Elsevier]

The collection and cleaning cycles of the filter media used in industrial air cleaning were analysed experimentally. Examination of the simultaneous pressure drop and filtration efficiency changes established that the efficiency of filters increases sharply during the collection phase only to fall back during cleaning. The amplitude of this reduction of efficiency is attained as the cycles progress. In contrast, the residual pressure drop rises. Our study has shown that treating the surface with thermobonding or covering the surface with a fine layer of submicronic fibres promotes detachment of the dust cake and prevents particle penetration inside the initial porous media. [Copyright &y& Elsevier]

The vibro-acoustography imaging method consists of forming an image of the deformability of a tissue submitted to a low frequency fLF stress field. This sound field can be created locally by means of a focused annular array emitting two primary beams driven at two close frequencies fa and fb=fa+fLF. In the existing literature, the origin of this stress field has been identified as the low frequency radiation pressure of the two primary beams. However, this work intends to show that another contribution to this internal stress is the low frequency field distributed in the object volume and created by the nonlinear interferences of the two primary beams. This nonlinear field was calculated in the case of multiple ring annular arrays and compared with the qLF beam experimentally measured in a water tank. The agreement between the theoretical and experimental curves provides information on the possibility that this nonlinear effect takes place in vibro-acoustography. [Copyright &y& Elsevier]

The filter media used in industrial air cleaning, if they are regenerable, are subjected to clogging and cleaning cycles. Their performance is closely linked to these two operating phases. A model of the phenomena based on the assumption of patchy cleaning is presented. The aim of the study was to determine the pattern of a parameter representative of cleaning, namely the cleaned fraction, from experimental values of the residual pressure drop of a filter medium subjected to given filtration conditions. This characterisation then allows the pressure drop pattern of the filter medium over a number of cycles to be determined. A comparison was made with the experimental results obtained on a laboratory test rig. Close agreement was observed, which confirms the hypothesis of patchy cleaning in this approach to describe the filtration and cleaning phenomena of a filter medium. [Copyright &y& Elsevier]

A measuring method of the emission rate of an atmospheric pollutant source, based on the use of a tracer gas (helium) and developed in the case of a gaseous source, was tested for an aerosol source. The influence of both particle sedimentation and wall depositions was studied. The transport coefficients of the tracer gas and of alumina particles of various particle sizes (MMAD from 8 to 36 Θ m) were measured on a vertical axis close to the source, in a 71 m[sup 3] room swept by a piston flow. The measurements clearly demonstrated the predominant influence of sedimentation in the case of particles with aerodynamic diameters greater than 10 Θ m. Particle wall deposition was determined by measuring the gas and particle concentration decay in the ventilated room. To do this, a new tracing method using a fluorescent aerosol was developed. The measured aerosol deposition rates are much higher than those calculated from the formula of Corner for a cubical volume. Aerosol sedimentation and wall deposition are two phenomena limiting the use of a tracer gas to measure the aerosol emission rate. The chemical substances and materials used in work premises are likely to be released into the atmosphere and lead to the formation of pollutants. These emissions stem from either physical or chemical processes (evaporation of a solvent) or from mechanical processes (dispersion of oil droplets at the source of mists). [ABSTRACT FROM AUTHOR]

A model for calculating the filtration efficiency and the pressure drop of a fiber filter media in dynamic regime was used and modified to take account of both the fiber and the particle size distributions. Measurements were carried out on two medias employed in industry and two loading aerosols to test the possibilities offered by the model to predict the evolution of both the efficiency and pressure drop characteristics. The results show that the model satisfactorily reflects the variations in efficiency and pressure drop of a media with respect to the loading if its structure is homogeneous and if the deposit of particles takes place within the thickness of the filtering layer. On the other hand, the divergence between the model and practical experience becomes significant as soon as surface filtration regime occurs or when the media has a heterogeneous structure. A test rig was developed to determine the filtering characteristics, such as fractional efficiency and pressure drop in relation to the degree of loading, from aerosols of various particle sizes. This study has highlighted the necessity of taking into account the influence of loading in the methods for testing filters, especially those used in the industry, and demonstrates that the particle size of the test aerosol is a very sensitive parameter. [ABSTRACT FROM AUTHOR]

Abstract: It is now accepted that an effective way to investigate the elastic properties of soft tissues is to generate a localized transient acoustic radiation force and to follow the associated displacements in the time/space domain. Shear waves induced by this stress field are particularly interesting in this kind of medium because they are governed by the shear elastic modulus μ, which is directly linked to the Young modulus, and spatial distribution and temporal evolution of the transient motion induced must therefore be obtained in detail. We report here a model based on the elastodynamic Green’s function formalism to describe these displacements. 3D simulation of radiation force in homogenous elastic media was performed and the displacement curves computed at different radial distances for different temporal force profiles. Amplitude and duration of displacement were found to be reliable parameters to characterize the elastic properties of the medium. Experimental measurements were performed in a homogeneous agar-gelatin tissue-mimicking phantom, and two transducers were used to generate the radiation force and follow the induced displacements. Displacements obtained from different lateral locations around the applied force axis were then used to reconstruct the shear-wave propagation in a scan plane as a function of time. The experimental displacements/curves agreed with the theoretical profiles obtained by the elastodynamic Green’s function formalism. [Copyright &y& Elsevier]

Dynamic acoustoelastic testing provides a more complete insight into the acoustic nonlinearity exhibited by micro-inhomogeneous media like granular and cracked materials. This method consists of measuring time of flight and energy modulations of pulsed ultrasonic waves induced by a low-frequency standing wave. Here pulsed ultrasonic head waves were employed to assess elastic and dissipative nonlinearities in a region near the surface of a solid. Synchronization of the ultrasound pulse sequence with the low-frequency excitation provided instantaneous variations in the elastic modulus and the attenuation as functions of the instantaneous low-frequency strain. Weak quadratic elastic nonlinearity and no dissipative nonlinearity were detected in duralumin. In limestone, distinction between tensile and compressive behaviors revealed an asymmetry in the acoustic nonlinearity and hysteresis in both the elastic modulus and the attenuation variations. Measured nonlinear acoustical parameters are in good agreement with values obtained by different techniques. Reversible acoustically induced conditioning modified the acoustic nonlinearity both quantitatively and qualitatively. It reduced tension-compression asymmetry, suggesting a nonequilibrium modification of the sources of acoustic nonlinearity. Additionally to the metrology of the acoustic nonlinearity, head wave based dynamic acoustoelastic testing may be a useful tool to monitor changes in the microstructure or the accumulation of damage in solids. [ABSTRACT FROM AUTHOR]

• Seven vibrators generate noises at [80–300] Hz and then a complex acoustic field. • S-wave speed is extracted with a 2D FFT to estimate viscoelastic parameters. • 10 iterations permit to remove outliers and ensure the repeatability of our method. In order to limit and slow the development of diseases, they have to be diagnosed early as possible to treat patients in a better and more rapid manner. In this paper, we focus on a noninvasive and quick method based on diffuse fields in elastography to detect diseases that affect the stiffness of organs. To validate our method, a phantom experiment numerically pre-validated is designed. It consists of seven vibrators that generate white noises in a bandwidth of [80–300] Hz and then a complex acoustic field in a phantom. Waves are tracked by a linear ultrasound probe L11-4v linked to a Verasonics Vantage System and are converted into a particle velocity 2D map as a function of time. The phase velocity of the shear waves is calculated using a temporal and 2D spatial Fourier transform and an adapted signal-processing method. Shear wave velocity dispersion measurement in the frequency bandwidth of the vibrators enables one to characterize the dynamic hardness of the material through the viscoelastic parameters μ and η in an acquisition time shorter than a second (T acq = 300 ms). With the aim of estimating the consistency of the method, the experiment is performed N = 10 times. The measured elastic modulus and viscous parameter that quantify the dynamic properties of the medium correspond to the expected values: μ = 1.23 ± 0.05 kPa and η = 0.51 ± 0.09 Pa∙s. [ABSTRACT FROM AUTHOR]

Abstract: In non-destructive evaluation, non-linear elastic wave spectroscopy (NEWS) methods represent powerful tools to explore damaged zones in a sample. The combination of these methods with time reversal (TR) process can be used to either increase the stress on the retrofocusing position or to retrofocuse elastic waves on the defect, when only the non-linear components of the received signal are time reversed. In this paper, we propose two different approaches to detect cracks in metallic samples in coupling NEWS methods and TR process. The first one, NEWS-TR, is defined by sending back only the non-linear components which are preliminary time reversed. Two different techniques to filter non-linear components have been numerically studied: classical harmonic filter and pulse inversion. In these two cases, performances of retrofocusing on different defect positions are analyzed and compared. The second approach, TR-NEWS, consists in measuring the non-linear signature on the focal spot. An experimental study of parametric interaction between two reversed signals ( and ) is led. Measurements of components at and around a crack are performed and discussed. [Copyright &y& Elsevier]

The aim of this article is to contribute to the qualitative description of the phenomena occurring during the clogging of fibre filters by submicron liquid particles. The changes in the pressure drop and efficiency of the filtration media in relation to the generated aerosol mass per unit area were monitored simultaneously. The different experimental approaches described converge on the same description of filter clogging, the scenario being broken down into four steps. A study of the influence of the operating conditions was carried out which highlighted the significant roles of filtration velocity, the nature of the filter, and the physico-chemical characteristics of the aerosol. [Copyright &y& Elsevier]