Determination of transport parameters in air-saturated porous materials via reflected ultrasonic waves

An ultrasonic reflectivity method of evaluating the acoustic parameters of porous materials saturated by air (or any other gas) is discussed. The method is based on experimental detection of waves reflected at normal incidence by the first and second interface of the material. This method is based on a temporal model of direct and inverse scattering problems for the propagation of transient ultrasonic waves in a homogeneous isotropic slab of porous material with a rigid frame [Fellah et al., J. Acoust. Soc. Am. 113, 61-73 (2003)]. Generally, the conventional ultrasonic approach can be used to determine tortuosity, and viscous and thermal characteristic lengths via transmitted waves. Porosity cannot be estimated in transmitted mode because of its very weak sensitivity. First interface use of the reflected wave at oblique incidence leads to the determination of porosity and tortuosity [Fellah et al., J. Acoust. Soc. Am. 113, 2424-2433 (2003)] but this is not possible at normal incidence. Using experimental data of reflected waves by the first and second interface at normal incidence simultaneously leads to the determination of porosity, tortuosity, viscous and thermal characteristic lengths. As with the classic ultrasonic approach for characterizing porous material saturated with one gas, both characteristic lengths are estimated individually by assuming a given ratio between them. Tests are performed using weakly resistive industrial plastic foams. Experimental and numerical results, and prospects are discussed.     

Separation of viscothermal losses and scattering in ultrasonic characterization of porous media

This paper presents a method for separating viscothermal and scattering losses in ultrasonic characterization of porous media. This method is based on variations of the static pressure of the saturating fluid. Experimental results were already presented in previous papers and the losses separation was verified experimentally. The aim of this paper is to present an analytic justification of this losses separation in the case of this experimental method and to show that it is possible to estimate acoustic parameters without the knowledge of scattering characteristics. The standard scattering length is used to renormalize speed and transmission through the porous medium, described as an equivalent fluid. Under certain assumptions corresponding to a weak scattering regime, it is shown how viscothermal and scattering losses can be separated easily without knowing scattering characteristics. Application of this model is presented in the case of weak scattering in a polyurethane foam and in the limit case of stronger scattering in a glass beads sample.     

Ultrasonic measurements of the two characteristic lengths in fibrous materials

In a method for measuring the two characteristic lengths in rigid frame porous media proposed by Leclaire et al. [Ph. Leclaire, L. Keiders, W. Lauriks. Determination of the viscous and thermal characteristic lengths of plastic foams by ultrasonic measurements in helium and air. J Appl Phys 1996;80:2009–12] ultrasonic transmission measurements are made through a sample of material when it is saturated with two different gases. It is shown that the accuracy of this method can be limited when the porous medium in saturated with helium as proposed originally. A considerable improvement in the signal-to-noise ratio can be achieved if argon is used instead of helium because the characteristic impedance is higher and the viscous skin depth in argon is 3 times less than that in helium. This paper also discusses the measurement of the tortuosity and the sound velocity in glass wool.     

Measuring flow resistivity of porous materials at low frequencies range via acoustic transmitted waves

An acoustic transmissivity method is proposed for measuring flow resistivity of porous materials having rigid frame. Flow resistivity of porous material is defined as the ratio between the pressure difference across a sample and the velocity of flow of air through that sample per unit cube. The proposed method is based on a temporal model of the direct and inverse scattering problem for the diffusion of transient low-frequency waves in a homogeneous isotropic slab of porous material having a rigid frame. The transmission scattering operator for a slab of porous material is derived from the response of the medium to an incident acoustic pulse. The flow resistivity is determined from the solution of the inverse problem. The minimization between experiment and theory is made in the time domain. Tests are performed using industrial plastic foams. Experimental and numerical results, and prospects are discussed.     

Direct and inverse scattering of transient acoustic waves by a slab of rigid porous material

This paper provides a temporal model of the direct and inverse scattering problem for the propagation of transient ultrasonic waves in a homogeneous isotropic slab of porous material having a rigid frame. This new time domain model of wave propagation takes into account the viscous and thermal losses of the medium as described by the model of Johnson et al. [D. L. Johnson, J. Koplik, and R. Dashen, J. Fluid. Mech. 176, 379 (1987)] and Allard [J. F. Allard (Chapman and Hall, London, 1993)] modified by a fractional calculus based method applied in the time domain. This paper is devoted to the analytical calculus of acoustic field in a slab of porous material. The main result is the derivation of the expression of the scattering operators (reflection and transmission) which are the responses of the medium to an incident acoustic pulse. In this model the reflection operator is the sum of two contributions: the first interface and the bulk of the medium. Experimental and numerical results are given as a validation of our model.     

An ultrasonic testing method for gas pressure welded joints of reinforcing steel bars

Gas pressure welding for jointing of reinforcing steel bars is most commonly used in Japan and the Far Eastern regions, where there are frequent earthquakes. An ultrasonic testing method has been developed for gas pressure welded joints. An intrinsic feature of the ultrasonic testing method is that there is a correlation between the ultrasonic testing results and the mechanical property (tensile strength) of joints. This feature enables non-destructive detection of gas pressure welded joints with tensile strength below a reference value. The theoretical justification for the correlation between the ultrasonic testing results and the tensile strength of the joints is discussed.     

Room temperature NO_2 gas sensing of Au-loaded tungsten oxide nanowires/porous silicon hybrid structure

In this work, we report an enhanced nitrogen dioxide(NO_2) gas sensor based on tungsten oxide(WO_3)nanowires/porous silicon(PS) decorated with gold(Au) nanoparticles. Au-loaded WO_3 nanowires with diameters of 10 nm–25 nm and lengths of 300 nm–500 nm are fabricated by the sputtering method on a porous silicon substrate. The high-resolution transmission electron microscopy(HRTEM) micrographs show that Au nanoparticles are uniformly distributed on the surfaces of WO_3 nanowires. The effect of the Au nanoparticles on the NO_2-sensing performance of WO_3 nanowires/porous silicon is investigated over a low concentration range of 0.2 ppm–5 ppm of NO_2 at room temperature(25℃). It is found that the 10-? Au-loaded WO_3 nanowires/porous silicon-based sensor possesses the highest gas response characteristic. The underlying mechanism of the enhanced sensing properties of the Au-loaded WO_3 nanowires/porous silicon is also discussed.     

Measuring static thermal permeability and inertial factor of rigid porous materials (L)

An acoustic method based on sound transmission is proposed for deducing the static thermal permeability and the inertial factor of porous materials having a rigid frame at low frequencies. The static thermal permeability of porous material is a geometrical parameter equal to the inverse trapping constant of the solid frame [Lafarge et al., J. Acoust. Soc. Am. 102, 1995 (1997)] and is an important characteristic of the porous material. The inertial factor [Norris., J. Wave Mat. Interact. 1, 365 (1986)] describes the fluid structure interactions in the low frequency range (1-3 kHz). The proposed method is based on a temporal model of the direct and inverse scattering problems for the propagation of transient audible frequency waves in a homogeneous isotropic slab of porous material having a rigid frame. The static thermal permeability and the inertial factor are determined from the solution of the inverse problem. The minimization between experiment and theory is made in the time domain. Tests are performed using industrial plastic foams. Experimental and theoretical data are in good agreement. Furthermore, the prospects are discussed. This method has the advantage of being simple, rapid, and efficient.     

Mechanisms of ultrasonic modulation of multiply scattered coherent light: a Monte Carlo model

A Monte Carlo model of the ultrasonic modulation of multiply scattered coherent light in scattering media is provided. The model is based on two mechanisms: the ultrasonic modulation of the index of refraction, which causes a modulation of the optical path lengths between consecutive scattering events, and the ultrasonic modulation of the displacements of scatterers, which causes a modulation of optical path lengths with each scattering event. Multiply scattered light accumulates modulated optical path lengths along its path. Consequently, the intensity of the speckles that are formed by the multiply scattered light is modulated. The contribution from the index of refraction is comparable with the contribution from displacement when the acoustic-wave vector is less than a critical fraction of the transport mean free path and becomes increasingly greater than the contribution from displacement beyond this critical point. This Monte Carlo model agrees well with an independent analytical model for isotropically scattering media. Both mechanisms are coherent phenomena, requiring the use of a coherent light source.     

Light propagation in porous ceramics: porosity and optical property studies using tunable diode laser spectroscopy

By integrating gas in scattering media absorption spectroscopy and frequency domain photon migration a new method is developed for the study of optical porosity and optical properties of porous media, in our case ceramics. The optical porosity is defined as the ratio of the path length through the gas-filled pores and the physical path length through the whole medium. The effective refractive index of the porous ceramics is also retrieved based on the optical porosity, which is then used to evaluate the reduced scattering coefficients of the porous ceramics. The combined method provides a new way to study light propagation in porous media. A modified Looyenga model is proposed to study the relationship between the physical porosity and the effective refractive index of the porous medium, which also connects the optical and physical porosities, and provides the possibility to use the present method for porosimetry analysis.     

Theoretical study of the continuous evolution of a resonance fluorescence spectrum towards a resonance Raman spectrum

A theoretical model is proposed to interpret the continuous transition from a resonance fluorescence spectrum to a resonance Raman spectrum, recently observed when the scattering molecule is progressively perturbed by its environment. An analytical expression for the resonant scattering tensor is given, which describes the evolution of the resonance fluorescence spectrum of a diatomic molecule perturbed by a foreign gas. The variations of the intensities and polarizations are quantitatively studied in the case of iodine, and it is shown how the resonance fluorescence spectrum is progressively transformed into a resonance Raman spectrum when the pressure of the perturbing gas is increased.     

Influence of gas law on ultrasonic behaviour of porous media under pressure

This paper deals with the influence of gas law on ultrasonic behaviour of porous media when the saturating fluid is high pressured. Previous works have demonstrated that ultrasonic transmission through a porous sample with variations of the static pressure (up to 18 bars) of the saturating fluid allows the characterization of high damping materials. In these studies, the perfect gas law was used to link static pressure and density, which is disputable for high pressures. This paper compares the effects of real and perfect gas laws on modeled transmission coefficient for porous foams at these pressures. Direct simulations and a mechanical parameters estimation from minimization show that results are very similar in both cases. The real gas law is thus not necessary to describe the acoustic behaviour of porous media at low ultrasonic frequencies (100 kHz) up to 20 bars.     

Account of photoelectron elastic determination of overlayer thickness,in-depth profiling,escape depth,attenuation coefficients and intensities in surface systems

An analytical connection is found between parameters of Monte Carlo and transport theory methods to account for elastic electron scattering in solids. A simple analytical method is proposed for the intensity calculations in X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) with account of elastic scattering. The method is applied for solution of many problems in XPS including determination of the overlayer thickness, correction factors for the in-depth profiling, escape depth, attenuation lengths, intensities in layered systems.     

Inverse estimation of the acoustic impedance of a porous woven hose from measured transmission coefficients

A porous tube, comprised of a resin-coated woven fabric has recently been used as an effective component for use in intake systems of internal combustion engines to reduce the intake noise. For the prediction of the acoustic performance of an engine intake system with a porous woven hose, the acoustic wall impedance of the hose must be known. However, the accurate measurement of the wall impedance of a porous woven hose is not easy because of its peculiar acoustical and structural characteristics. A new measurement technique is proposed herein, that is valid over the low to mid frequency ranges. The acoustics impedance is inversely estimated from an overdetermined set of measured pressure transmission coefficients for specimens of different lengths and the reflection coefficient of end termination. The method involves only one measurement setup, and, as a result, it is very simple. A variation of the proposed method, an inverse estimation method using one of the four-pole parameters is also proposed. An error sensitivity analysis was performed to investigate the effect of measurement error on the accuracy of the final result. The measured TL for samples with arbitrary lengths and arbitrary porous frequency are in reasonably good agreement with values predicted from curve-fitted impedance data.     

Analytical Independent-Particle Model for Electron Scattering by Argon at Low Energy

We show that values of the electron-argon scattering lengths and very low-energy phase shifts computed by Endrédi and Apagyi [Few-Body Systems 17, 199 (1994)] in two variants of the analytical independent-particle model are not converged. To prove this we employ an accurate method of calculating the scattering lengths we proposed previously. Applied to the model used by Endrédi and Apagyi, the method combines numerical computation of a short-range contribution to the scattering length and analytical evaluation of a long-range contribution due to the electron-atom polarization interaction. Outside the low-energy domain the models of Endrédi and Apagyi appear reliable and the corresponding results are not invalidated by the present findings. Received July 10, 1995; accepted in final form October 23, 1995     

基于激光布里渊散射的高速气流测试技术

激光布里渊散射是由介质密度涨落引起的一种非弹性散射现象,其散射谱特征参数（频移、线宽）取决于散射角和散射介质压强、温度等参数,通过探测高速气流激光布里渊散射谱,可反演获得高速气流压强、温度等参数。介绍了基于激光布里渊散射原理的高速气流参数测试原理,设计了一种基于法布里 珀罗干涉仪的高速气流参数测试装置,搭建实验平台获取了高速氮气气流的激光布里渊散射谱。实验结果表明,通过激光布里渊散射谱获得的高速气流压强、温度值与理论值相对误差在15%以内。