Ultrasonic waves Reflected at Oblique Incidence by Porous Rigid Medium.

International audienceAn ultrasonic reflected wave at oblique incidence by porous medium with rigid frames is considered using equivalent fluid model. The viscous and thermal losses of the medium are described by two susceptibility kernels which depend on the viscous and thermal characteristic lengths. Analytical derivation of reflection coefficient is given in frequency domain. The simulated reflected wave is obtained at time domain by convolution between the reflected operator and the incident field. Experimental results for plastic foam samples of air-saturated porous media are given and compared with theoretical prediction

Détermination de l'épaisseur et la résistivité au passage de l'air d'un matériau poreux à structure rigide en utilisant les ondes transmises.

National audienceUne méthode acoustique est proposée pour mesurer la résistivité au passage de l’air et l’épaisseur d’un échantillon poreux à structure rigide. Les méthodes classiques 3,4 permettant la mesure de la résistivité (où la perméabilité visqueuse) nécessitent la connaissance préalable de la porosité. La méthode présentée dans ce travail est basée sur un modèle temporel du problème direct dans lequel une expression simplifiée (indépendante de la fréquence et de la porosité) du coefficient de transmission dans le régime de Darcy (très basses fréquences) est établie. Cette expression ne dépend que de la perméabilité visqueuse (où la résistivité au passage de l’air) et de l’épaisseur d’un échantillon poreux. Le problème inverse est résolu en minimisant, l’écart entre le signal transmis théorique et expérimentale, permettant ainsi la détermination de l’épaisseur et de la perméabilité visqueuse (où la résistivité) d’une mousse en plastique. Cette méthode présente l’avantage d’être simple, rapide et efficace

Ultrasonic propagation of reflected waves in cancellous bone: Application of Biot theory.

International audienceAn ultrasonic propagation in cancellous bone is considered using the Biot theory modified by the Johnson et al. Numerical simulations of reflected waves in the time domain are worked out by varying the modified Biot parameters. The sensitivity of different mechanical parameters : Young modulus and the Poisson ratio of the skeletal frame and physical parameters , porosity , tortuosity and viscous characteristic length are studied showing their effect on the reflected ultrasonic waves of the bone sample. The sensitivity of the modified Biot parameters with respect to the reflected wave depends strongly on the coupling between the solid and fluid phases of the cancellous bone. We show from these simulations that some parameters such as porosity and tortuosity play an important role on reflected wave ; the remaining parameters have low sensitivity compared with the porosity and tortuosity. Experimental results for reflected waves by human cancellous bone samples are given and compared with theoretical predictions

Pseudo-Gaussian cylindrical acoustical beam – Axial scattering and radiation force on an elastic cylinder

International audiencea b s t r a c t Making use of the addition theorem for the cylindrical wave functions and the complex-source-point method in cylindrical coordinates, an exact solution to the Helmholtz equation is derived, which corresponds to a tightly focused (or collimated) cylindrical quasi-Gaussian beam with arbitrary waist. The solution is termed "quasi-Gaussian" to make a distinction from the standard Gaussian beam solution obtained in the paraxial approximation. The advantage of introducing this new solution is the efficient and fast computational modeling of tightly focused or quasi-collimated cylindrical wave-fronts depending on the dimensionless waist parameter kw 0 , where k is the wavenumber of the acoustical radiation. Moreover, a closed-form partial-wave series expansion is obtained for the incident field, which has the property that the axial scattering (i.e. along the direction of wave propagation) and the axial acoustic radiation force (which is a time-averaged quantity) on a cylinder, can be calculated without any approximations in the limit of linear acoustical waves in a nonviscous fluid. Examples are found where the extinction in the radiation force function plot is found to be correlated with conditions giving reduction of the backscattering from an elastic cylinder. Those results are useful in beam-forming design, particle manipulation in acoustic tweezers operating with focused cylindrical beams, and the prediction of the scattering and radiation forces on a cylindrical particle or liquid bridges

Determination of the flow resistivity and thickness of porous materials with rigid frames via transmitted waves at Darcy's regime

International audienceAn acoustic method is proposed for measuring the flow resistivity and the thickness of air-saturated porous materials. The conventional methods [14, 16, 17] for the measurement of the flow resistivity (or the viscous permeability) require the prior knowledge of the porosity. The method presented in this work is based on a temporal model of the direct problem in which a simplified expression (independent of frequency and porosity) of the transmission coefficient at the Darcy’s regime (low frequency range) is established, this expression depends only on the viscous permeability (or the flow resistivity) and the thickness of a porous sample. The inverse problem is solved based on the leastsquare numerical method using experimental transmitted wave in time domain. Tests are performed using two samples of different thicknesses to same industrial plastic foam, thereby enabling the determination the thickness and flow resistivity of foam plastic. This method has the advantage of being simple, fast and efficient

Characterization of rigid porous medium via ultrasonic reflected waves at oblique incidence

International audienceIn this paper, an enhanced method is proposed for measuring porosity, tortuosity, viscous and thermal characteristic length of porous materials having a rigid frame via reflected ultrasonic waves at oblique incidence using the equivalent fluid model. The advantage of the proposed method is that the four parameters are determined simultaneously just using reflected experimental waves for a porous material saturated by air. The inverse problem is solved based on the least-square numerical method using experimental reflected waves in time domain. Tests are performed using industrial plastic foams. Experimental and numerical validation results of this method are presented

Inverse estimation of the permeability of porous materials using experimental data via reflected waves at low frequencies

International audienceAn acoustic reflectivity method is proposed for measuring the permeability or flow resistivity of air-saturated porous materials. In this method, a simplified expression of the reflection coefficient is derived in the Darcy's regime (low frequency range), which does not depend on frequency and porosity. Numerical simulations show that the reflection coefficient of a porous material can be approximated by its simplified expression obtained from its Taylor development to the first order. This approximation is good especially for resistive materials (of low permeability) and for the lower frequencies. The permeability is reconstructed by solving the inverse problem using waves reflected by plastic foam samples, at different frequency bandwidths in the Darcy regime. The proposed method has the advantage of being simple compared to the conventional methods that use experimental reflected data, and is complementary to the transmissivity method which is more adapted to low resistive materials (high permeability)

Measuring static viscous permeability of porous absorbing materials

International audienceConventional acoustical methods for measuring the permeability or flow resistivity of a porous material require a priori estimation of the porosity. In this work, an acoustical method is presented in which a simplified expression (independent of both the frequency and porosity) for the transmitted waves at the Darcy’s regime (low frequency range) is derived, and used for the inverse determination of both the viscous static permeability (or flow resistivity) and the thickness of air-saturated porous materials. The inverse problem is solved based on the least-square numerical method using transmitted waves in time domain. Tests are performed using industrial plastic foams. Experimental and numerical validation results of this method are presented, which show theadvantage of measuring the viscous permeability and thickness of a porous slab, without the required prior knowledge of the porosity, but by simply using the transmitted waves

Reflected wave by the first interface of rigid porous medium at Darcy’s regime

International audienceIn this paper, reflection waves by the first interface of porous medium having rigid frame is considered. The method presented in this work is based on a temporal model of the direct problem in which a simplified expression of the reflection coefficient at the Darcy's regime (low frequency range) is established using equivalent fluid model, this expression depends only on the porosity and the viscous permeability (or the flow resistivity) of the medium. The simulated reflected wave is obtained in time domain by convolution between the reflected operator and the incident field. Experimental results are given for samples of air-saturated plastic foams and compared with theoretical predictions

Full characterization of rigid porous material through ultrasonic reflected waves at oblique incidence.

International audienceAn improved method is proposed for measuring porosity, tortuosity, viscous and thermal characteristic length of porous materials having a rigid frame via reflected ultrasonic waves at oblique incidence. The conventional ultrasonic approach can be used to determine all the parameters via transmitted waves [1] or using the rst and second reflected waves at normal incidence[2] (the ratio between the viscous and thermal characteristic lengths is xed as in classical acoustic methods [3,4]). The advantage of the proposed method is that the four parameters are determined simultaneously just using reflected experimental waves for a porous material saturated by air. In addition, no relationship is assumed between the two characteristic lengths. The inverse problem is solved based on the least-square numerical method using experimental refllected waves in time domain. Tests are performed using industrial plastic foams. Experimental and numerical validation results of this method are presented.Reference[1] ZEA Fellah, M Sadouki, M Fellah, F. G Mitri, E Ogam, C Depollier." Simultaneous determination of porosity, tortuosity, viscous and thermal characteristic lengths of rigid porous materials" J. Appl. Phys, 114, 204902 (2013); [2] Z. E. A. Fellah, M. Fellah, W. Lauriks, and C. Depollier, "Direct and inverse scattering of transient acoustic waves by a slab of rigid porous material" J. Acoust. Soc. Am. 113, 61-73 (2003). [3] C. Ayrault, A. Moussatov, B. Castagnede, and D. Lafarge, "Ultrasonic characterization of plastic foams via measurements with static pressure variations" Appl. Phys. Lett. 74, 3224-3226 (1999). [4] A. Moussatov, C. Ayrault, and B. Castagne`de, "ultrasonic method for estimation of tortuosity and characteristic length using a barometric chamber"Ultrasonics 39, 195-202 (2001)