Determination of the viscous characteristic length in air-filled porous materials by ultrasonic attenuation measurements

Abstract

The concept of viscous characteristic length is used to describe the acoustical behavior of fluid-saturated porous media in the high-frequency regime. A method to determine this parameter consists of measuring the wave attenuation in the high-frequency limit. This method has already been used for porous materials saturated by superfluid 2He. It is tested in the case of air-filled absorbent materials in a frequency range of [50–600 kHz]. The thermal characteristic length is assumed to be known or measured independently. Two examples are presented. In the first one the method is usable and the viscous characteristic length Λ is deduced from the high-frequency behavior of the attenuation per cycle. In the second example, an additional attenuation occurs at high frequencies and only an estimate of Λ can be given. Nevertheless, the estimation appears to be rather accurate. The values obtained by this method are compared to those determined by a nonlinear fit of the dispersion curves.

A Review of Acoustical Methods for Porous Material Characterisation

“…22 (Eq. (6) in 30 defines properly the high-frequency asymptotic limit for k b (ω)). Because very high frequency data for the refraction index are usually unattainable, then the intercept of the function n 2 (ω −1/2 ) with the ordinate axis is found through a linear fitting procedure as illustrated in Fig.…”

Transient Acoustic Wave Propagation in Porous Media

“…Porosity cannot be determined from transmitted waves because of its weak sensitivity in this mode [4]. The advantage of the proposed method is that all the parameters can be determined at normal incidence (the ratio between viscous and thermal lengths is fixed as in classical methods [21][22][23][24] based on transmission measurement).…”

“…Solving the inverse problem, using the transmitted waves allowed the determination of the tortuosity and viscous and thermal characteristic lengths. Note that the ratio of two lengths was set at 3 [20][21][22][23]. Currently the only method for the separate determination of the two lengths is based on the saturation of the porous by two different fluids [24].…”

A time-domain model of transient acoustic wave propagation in double-layered porous media

“…3,4,[8][9][10][11] When a broadband ultrasound pulse passes through a layer of a medium, the pulse waveform changes as a result of attenuation and dispersion of the medium. The classic method for predicting a change in the waveform of a signal passing through a medium relies on the system's impulse response.…”

“…Ultrasonic characterization of materials is often achieved by measuring the attenuation coefficient and phase velocity in the frequency domain, 3,4 or by solving the direct and inverse problems directly in the time domain. [5][6][7][8][9][10][11][12][13] In the frequency domain, measurements of the attenuation coefficient may be more robust than measurements of phase velocity.…”