An alternative and industrial method using low frequency ultrasound enabling to measure quickly tortuosity and viscous characteristic length

Abstract

In the past, several different methods have been proposed in order to determine tortuosity and characteristic lengths, using various physical approaches. For instance, the BET (from S. Brunauer, P. Emmett and E. Teller) method used the adsorption of argon molecules within the surface of the pores to measure thermal characteristic length (or the so-called specific surface). Ultrasonic methods, during the two last decades, were efficiently implemented to measure these parameters, using large frequency bandwidth, different gazes including helium, and varying static pressures. The most standard and well-known method relies on phase spectrum measurements performed over short bursts, by plotting the inverse of the squared velocity as a function of the inverse of the square root of the angular frequency. Unfortunately, this method is quite sensitive on noise during the unwrapping procedure. We here propose a much simplier and efficient method, which is very robust with noisy signals, working at one single frequency, in most cases around 40 kHz. Measurements have been done onto a specific bench developped at CTTM, with some calibrated glass beads, enabling to recover the expected values of the physicalparameters. This method is particularly well suited for industrial and "on line" applications.