Computation of local permeability in granular soils using spatial time domain reflectometer

This letter proposes semi-analytical methods to obtain the local permeability for granular soils based on indirect measurements of the local porosity profile in a large coaxial cell permeameter using spatial time domain reflectometry. The porosity profile is used to obtain the local permeability using the modified Kozeny-Carman and Katz-Thompson equations, which incorporated an effective particle diameter that accounted for particle migration within the permeameter. The profiles of the local permeability obtained from the proposed methods are compared with experimentally obtained permeability distributions using pressure measurements and flow rate. The permeabilities obtained with the proposed methods are comparable with the experimentally obtained permeabilities and are within one order of magnitude deviation, which is an acceptable range for practical applications.

Mixed-Gas Selectivity Based on Pure Gas Permeation Measurements: An Approximate Model

An approximate model based on friction-coefficient formalism is developed to predict the mixed-gas permeability and selectivity of polymeric membranes. More specifically, the model is a modification of Kedem’s approach to flux coupling. The crucial assumption of the developed model is the division of the inverse local permeability of the mixture component into two terms: the inverse local permeability of the corresponding pure gas and the term proportional to the friction between penetrants. Analytical expressions for permeability and selectivity of polymeric membranes in mixed-gas conditions were obtained within the model. The input parameters for the model are ideal selectivity and solubility coefficients for pure gases. Calculations have shown that, depending on the input parameters and the value of the membrane Peclét number (the measure of coupling), there can be both a reduction and an enhancement of selectivity compared to the ideal selectivity. The deviation between real and ideal selectivity increases at higher Peclét numbers; in the limit of large Peclét numbers, the mixed-gas selectivity tends to the value of the ideal solubility selectivity. The model has been validated using literature data on mixed-gas separation of n-butane/methane and propylene/propane through polymeric membranes.

Permeability heterogeneity during sandstone compaction

<p>Compaction of porous sandstones is generally associated with a reduction in permeability. Depending on porosity and other microstructural characteristics, compaction may be diffuse or localised in bands. Compaction bands have been shown to act as barriers to fluid flow and therefore reduce permeability perpendicular to the band orentiation, and thus also introduce permeability anisotropy. Additionally, the localised nature of compaction bands should also introduce strong permeability heterogeneity. We present new experimental data on sandstone compaction combining acoustic emission monitoring and spatially distributed pore fluid pressure measurements, allowing us to establish how permeability heterogeneity develops during progressive compaction. Three sandstones were tested in the compactant regime: Locharbriggs sandstone, which is microstructurally heterogeneous with beds of higher and lower initial permeability; a low porosity (21%) Bleurville sandstone, which is microstructurally homogeneous and produces localised compaction bands; and a high porosity (24%) Bleurville sandstone, which is also homogeneous but produces compaction in a more diffuse pattern. At regular intervals during compactive deformation, a constant pore pressure difference was imposed at the upper and lower boundaries of the cylindrical samples, and steady-state flow allowed to become established. Following this, local pore pressure measurements were made at four locations, allowing us to derive estimates of the local permeability. In all samples, progressive compaction produced overall reductions in permeability. In addition, localised compaction also produced internal reorganisation of the permeability structure. Localised compaction bands caused local decreases in permeability, while more diffuse compaction produced a more homogeneous overall reduction in permeability.</p><p> </p>

Cockade-bearing breccias, cataclasites and gouges in a single fault zone: Microstructures and geochemisty

<p>The seismic-interseismic cycle strongly relates to the interplay between dilation owing to fracturing and frictional granular flow on one hand side and hydrothermal cementation processes on the other side. This study investigates different fault rocks of a crustal-scale fault zone in the Central Alps (Switzerland). We combine microstructural with geochemical approaches to decipher the interaction of grain size reduction via frictional processes with precipitation and resulting particle size increases. The three major fault rocks, i.e. (1) cockade-bearing breccias, (2) cataclasites, and (3) fault gouges, differ in their microstructure. The chemical data clearly demonstrate a decreasing gain of volume along this group of tectonites. Their different precipitation volumes most likely relate to dynamic changed of the local permeability of these rocks. The fluid pathways control the precipitation at different localities and times, which affect the healing of these fault rocks inducing a gain in rock strength. During the next deformation event, the extent of healing therefore directly controls the mechanical behavior of the rock. The estimated volume gain (~+110%) in cockade-bearing breccias is consistent with the seismic dilatant behavior of these frictional rocks as already proposed from other arguments (Berger and Herwegh 2019). This is in contrast to the fault-gouges with only minor gains in volume and mass resulting in a predominantly non-cohesive deformation style. This example indicates that permeability evolution (and related hydrothermal processes) strongly influences the mechanical behavior of such faults. This shows the highly dynamic behavior with time in long-lived fault systems. These dynamic changes in precipitation and resulting different strengths occur at different timescales from minutes (seismic events) to thousands of years.</p><p>Ref.: Berger, A., Herwegh, M., 2019. Cockade structures as a paleo-earthquake proxy in upper crustal hydrothermal systems. Nature Scientific Reports, 9, 9209.</p>

Modelling the permeability of random discontinuous carbon fibre preforms

A force-directed algorithm was developed to create representative geometrical models of fibre distributions in directed carbon fibre preforms. Local permeability values were calculated for the preform models depending on the local fibre orientation, distribution and volume fraction. The effect of binder content was incorporated by adjusting the principal permeability values of the meso-scale discontinuous fibre bundles, using corresponding experimental data obtained for unidirectional non-crimp fabrics. The model provides an upper boundary for the permeability of directed carbon fibre preform architectures, where predictions are within one standard deviation of the experimental mean for all architectures studied.

Hematopoietic Stem and Progenitor Cells Use Podosomes to Transcellularly Home to the Bone Marrow

ABSTRACTBone marrow (BM) endothelium plays an important role in homing of hematopoietic stem and progenitor cells (HSPCs) upon transplantation, but surprisingly little is known on how endothelial cells regulate local permeability and HSPC transmigration. We show that temporal loss of VE-cadherin function promotes vascular permeability in BM, even upon low dose irradiation and strongly enhanced homing of transplanted HSPCs to BM of irradiated mice. Intriguingly, stabilizing junctional VE-cadherinin vivoreduced BM permeability, but did not prevent HSPC migration into the BM, suggesting that HSPCs enter the bone marrow by transcellularly crossing the endothelium. Indeed, HSPCs induce podosomes to cross human BM endothelial monolayers in a transcellular manner. By contrast, HSPC rather use the paracellular route when VE-cadherin function is inhibited. Taken together, VE-cadherin is crucial for BM vascular homeostasis and HSPC homing, and may therefore serve as a potential therapeutic target to improve HSPC homing strategies.