The relationship between entrapped air content and the corresponding hydraulic conductivity was investigated experimentally for two coarse sands. Two packed samples of 5 cm height were prepared for each sand. Air entrapment was created by repeated infiltration and drainage cycles. The value of K was determined using repetitive falling-head infiltration experiments, which were evaluated using Darcy’s law. The entrapped air content was determined gravimetrically after each infiltration run. The amount and distribution of air bubbles were quantified by micro-computed X-ray tomography (CT) for selected runs. The obtained relationship between entrapped air content and satiated hydraulic conductivity agreed well with Faybishenko’s (1995) formula. CT imaging revealed that entrapped air contents and bubbles sizes were increasing with the height of the sample. It was found that the size of the air bubbles and clusters increased with each experimental cycle. The relationship between initial and residual gas saturation was successfully fitted with a linear model. The combination of X-ray computed tomography and infiltration experiments has a large potential to explore the effects of entrapped air on water flow.

<p>In this study, magnetic resonance imaging (MRI) was used to investigate the freezing and thawing process of a series of repacked samples of sand, soil, and sand-soil mixture. The samples were placed in a thermally insulated container inside a vertical bore MRI scanner and cooled by flowing cold gaseous nitrogen through a porous material at the top of the container. Temperatures were monitored in several points above the sample and at the sample surface, and a marker placed on the sample surface was used to measure sample deformation. A 4.7 T magnet was used for MRI and the Multiple-Slice Spin-Echo (MSME) and Zero Echo Time (ZTE) pulse sequences were employed to obtain the images. The contrast between the frozen and unfrozen water in the samples was given by the substantial difference in T1 and T2 relaxation times between the two states. The hydrogen in the frozen water does not produce any signal for both pulse sequences, thus all the signal represent the liquid/unfrozen water. The time-lapse three-dimensional (3D) imaging was performed during the entire course of the experiment with alternating use of the MSME and ZTE imaging techniques. Once the freezing front reached near the bottom of the sample, the thawing process was initiated by switching the inflow of cooling gas to the inflow of nitrogen at room temperature. The small changes in sand structure as a consequence of volumetric ice-water changes were studied using spatiotemporal analysis of the freezing front advancement and frozen water volume. The study detected interesting patterns of preferential thawing on the onset of thawing process in the case of sand. The MSME pulse sequence was successfully used to image the process in the sand, whereas the ZTE was capable of detecting water in the finer soil material. The data obtained in the study were used to develop two-phase ice-water simulation models to interpret the experimental results and better understand the freezing and thawing phenomena.</p>

<p>The research focused on the simulation of the previous experiment described by Princ et al. (2020). The relationship between entrapped air content (<em>ω</em>) and the corresponding satiated hydraulic conductivity (<em>K</em>) was investigated for two coarse sands, in the experiment. Additionally the amount and distribution of air bubbles were quantified by X-ray computed tomography.</p><p>The pore-network model based on OpenPNM platform (Gostick et al. 2016) was used to attempt simulation of a redistribution of the air bubbles after infiltration. Satiated hydraulic conductivity was determined to obtain the <em>K</em>(<em>ω</em>) relationship. The results from pore-network model were compared with the results from experiments.</p><p>Gostick et al. (2016). Computing in Science & Engineering. 18(4), p60-74.</p><p>Princ et al. (2020). Water. 12(2), p1-19.</p>

<p>Magnetic resonance imaging (MRI) of the freezing and thawing process was performed on a series of repacked samples of sand, soil, and sand-soil mixture. The freezing/thawing is performed in the sample container placed inside the vertical bore MRI scanner within the 66 mm inner diameter of the radiofrequency coil. The sample container was vacuum insulated from the sides and bottom to allow for the minimum thickness of the insulation layer. The vacuum was constantly maintained by a vacuum pump. The sample assembly was built from PMMA and other nonmetallic - MRI compatible materials. A porous material in the sample container was cooled at the top by the flow of cold gaseous nitrogen released from the liquid nitrogen stored in the Dewar flask. The cooling took place across the glass plate positioned at the top of the sample in the headspace above the sample. The temperature of the gas that was delivered to the headspace and leaving the headspace was monitored. Additionally, the temperature was monitored in the headspace above the glass disk and directly in the glass disk by fiber optics temperature probes. A 4.7 T magnet at the FZJ was used for MRI. Multiple-Slice Spin-Echo and Zero Echo Time pulse sequences were utilized. The contrast between the frozen and unfrozen water is given by the difference in T1 and T2 relaxation times. The time-lapse 3D imaging was done during the entire course of the experiment. Once the freezing front reached the bottom of the sample, the thawing process was induced. The small changes in sand structure as a consequence of volumetric ice-water changes were studied. The spatiotemporal analysis of the freezing front advancement and frozen water volume has been performed. The data are available for the development of two-phase ice-water simulation models.</p>