Non-Destructive Visualization and Quantification of 3-D Microstructure of Granular Materials and Direct Numerical Simulations

摘要

This paper summarizes the key concepts from the recent published work of the authors on using both neutron and X-ray imaging techniques to study partially saturated sand and water flow through compacted sand. The goal of the manuscript is to serve as a review paper building on discrete contributions from cited publications for geomechanics community as the topic is rather new and concepts are connected. For this study, neutron and micro-CT based X-ray imaging was performed at Helmholtz-Zentrum-Berlin (HZB) in Germany. Due to different attenuation characteristics of neutrons and X-rays to three phases (silica, air and water) of partially saturated sand, radiation based images provide unique but complementary information in a non-destructive fashion. Water phase is very precisely identified with neutron radiation based images, and sand (silica) phase is well identified with X-ray images. An automatic image registration technique is implemented to combine neutron and X-ray images in the same coordinates for a detailed quantitative evaluation of micro-structural features in three dimensions. In-situ imaging experiment of flow through compacted sand was performed based on the dual modality imaging concept. The initial 3-D pore geometry was obtained from dry compacted sand specimen by using X-ray. The water flow pattern was monitored by using time-lapsed neutron radiography and tomography after a target water injection step. The initial microstructure obtained with X-ray tomography is also used to perform direct numerical simulations. Experiments based on using neutron and X-ray imaging technique thus provide unique opportunity to characterize partially saturated sand and investigate multi-phase flow behavior through porous media. Direct numerical simulation based on realistic geometry can account for complex pore geometry including heterogeneity of the pore structure.