X-ray tomographic image post-processing and a new 2D LBM simulation for the determination of the porosity and the static airflow resistivity of an acoustic fibrous material

Abstract

A set of X-ray tomographic images of a highly porous material composed of air-saturated coconut fibers is considered and used to estimate intrinsic characteristics of the material. Two physical properties are of interest: porosity and static airflow resistivity. For the porosity, two-dimensional gray-scale tomography images are obtained and post-processed to produce approximative black and white ones, unambiguously attributing distinct regions to fibers and air locations. The porosity is then directly deduced counting the black and white pixels. Several image processing algorithms are compared and associated porosities range from 0.76 to 0.97, depending on the method employed, while it is estimated to be 0.86 from our analysis. For the airflow resistivity, the idea followed here is to use the pattern of the post-processed images as the lattice in a Lattice Boltzmann Method (LBM) fluid dynamics computation. To our knowledge, the LBM has not been used in this context before. A new 2D implementation of the method is therefore developed and studied. After tuning computational parameters, we have estimated the airflow resistivity using ten images of our sample to be 1382 +/- 12 Pa. s/m³. Both porosity and resistivity results are fully consistent with measurements obtained from a porosity-meter and a resistivity-meter, demonstrating the pertinence of X-ray tomography and the associated proposed methods.