Felix H. Kim

High-Resolution Neutron and X-Ray Imaging of Granular Materials

AbstractHigh spatial resolution (∼13.7 mm/pixel) neutron tomography was performed on partially water-saturated compacted silica sand specimens with two different grain morphologies (round and angular) at Helmholtz Zentrum Berlin using cold neutrons at the cold neutron radiography and tomography beam line. A specimen mixed with heavy water was imaged for contrast comparison purposes. Microfocus X-ray imaging was also performed on these specimens with slightly higher resolution (∼11.2 mm/pixel) using geometric magnification to locate the solid phase (silica particle boundaries) more precisely. Image processing was performed to remove unwanted gammas detected because of the gadox scintillator used for the high-resolution neutron imaging system. The visualization of solid, gas, and liquid phases for different grain morphologies is presented at the grain level. Using dual-modal contrast possible from simultaneous use of neutrons and X-rays, the authors introduce, for the first time, an improved ability to dist...

Characterizing Partially Saturated Compacted-Sand Specimen Using 3D Image Registration of High-Resolution Neutron and X-Ray Tomography

AbstractPartially saturated compacted-sand specimens were characterized by using three-dimensional (3D) image registration of dual-modal (neutron and X-ray) tomography data. Neutron and X-ray imaging provide complementary information for precisely identifying the three phases (silica sand, air, and water) of a compacted sand specimen that is partially saturated. Neutron tomography provides high contrast of the water phase, whereas X-ray tomography provides high contrast of the silica sand phase due to different fundamental interaction mechanisms of neutron and X-ray with matters. X-ray interacts with the electron cloud surrounding the nucleus, whereas neutron radiation interacts with the nucleus of an atom. In this paper, a computational technique was developed to unify digital images of dual-modal data obtained at different image resolution and specimen orientations based on the maximization of the normalized mutual information to combine the information from the water phase inferred from a neutron image...

Pore Size Distribution and Soil Water Suction Curve from Micro-tomography Measurements and Real 3-D Digital Microstructure of a Compacted Granular Media by Using Direct Numerical Simulation Technique

Predictive measurement of capillary pressure – saturation relationship of a porous media is obtained based on the actual microstructure obtained from high resolution tomography data. X-ray micro-tomography provided a high contrast for silica phase, and actual geometry of sand particles and void distribution. The morphological opening (erosion + dilation) is used to get a pore size distribution using the concept of granulometry. Full-morphology approach is used to model the quasi-static wetting and non-wetting phase distribution of a primary drainage process. Predicted soil water suction curves for a compacted silica sand sample is presented along with the effects of assumed contact angle between water and silica surface.

Lattice Boltzmann Simulation of Two Phase Flow through Porous Media and Verification Using High Resolution X-ray and Neutron Tomography Data

Two phase flow simulations are predicted for a compacted silica sand specimen whose microstructure is obtained from three-dimensional X-ray microtomography. Direct numerical simulations using a simple full morphology method based on the measured three-dimensional geometry of the pore space is demonstrated. Additionally, an advanced two-phase lattice-Boltzmann based model is applied solving the Navier-Stokes equation numerically to simulate multi-phase flow. Experiments utilized a micro-focus X-ray system at Helmholtz-Zentrum-Berlin (HZB). Using a cold neutron imaging beam line (CONRAD) at HZB, in-situ imaging of water flow through compacted sand is performed. Water is allowed to flow from bottom to top of compacted sand specimen under controlled conditions. The flow of water is precisely controlled by using a syringe pump to a flow rate that corresponds to capillary fingering flow regime. Two-dimensional neutron radiography based cinematography was performed during the injection of water into porous media, and three-dimensional neutron tomography was performed at target states after reaching flow equilibrium. The simulation results are directly compared with the experimental results obtained from neutron tomography data. Preliminary results of the latticeBoltzmann simulation in 2D did not provide a close match to the measured water phase distribution and advancing front. This paper introduces the innovative concept of direct numerical simulations of complex multi-phase flow using realistic microstructures measured non-destructively using radiation based imaging. The technical approach used in this study shows promise for applications in diverse fields involving fluid transport and porous media.

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

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 nondestructive 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 microstructural 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 providing a unique opportunity to characterize partially saturated sand and investigate multiphase flow behavior through porous media. Direct numerical simulation based on realistic geometry can account for complex pore geometry including heterogeneity of the pore structure.