Christian Moyne

Soil water dynamics in an oak stand

Soil water dynamics under a mixed stand of mature sessile and pedunculate oaks were studied both under natural conditions and during imposed water shortages in a lysimeter. Root densities of each species were described in situ by counting roots in the trench surrounding the dry plot. Soil water contents and potentials, and pre-dawn leaf water potentials (Ψwp) were monitored during three successive years. Soil water retention characteristics were obtained from field measurements of water potential and water content. The decreasing rooting density with depth was strongly related to soil physical properties. The root system was separated into two compartments by a layer with a high clay content. The deepest soil compartment was mainly explored by fine roots. Neutron probe measurements allowed the detection of variations in water content down to a depth of 2.00 m. The distribution of water uptake among the different soil layers changed when drought increased. Water was extracted from the deepest reservoir, and capillary rises even occurred after partial water depletion in the upper part of the soil. Seasonal trends of pre-dawn leaf water potential generally matched those of soil water potential in the wettest rooted zone, which was at − 140 cm. In the upper, dry, horizons, the sharp loss of soil hydraulic conductivity reduced water transport to roots leading to impossible equilibrium between roots and soil at pre-dawn. Finally, Ψwp presented a low sensitivity to variations of total soil water content between 40% and 100% of extractable water. Below this threshold, Ψwp decreased sharply to a minimal value of about − 2.0 MPa.

Electro-chemo-mechanical couplings in swelling clays derived from a micro/macro-homogenization procedure

Abstract A macroscopic model for highly compacted expansive clays composed of a charged solid phase saturated by a binary monovalent aqueous electrolyte solution is derived based on a rigorous scale-up of the microstructural behavior. The homogenization technique is applied to propagate information available in the pore-scale model to the macroscale. Macroscopic electrokinetic phenomena such as electro-osmotic flow driven by streaming potential gradients, electrophoretic motion of mobile charges and osmotically induced swelling are derived by homogenizing the microscopic electro-hydrodynamics coupled with the Nernst–Planck and Poisson–Boltzmann equations governing the flow of the electrolyte solution, ion movement and electric potential distribution. A notable consequence of the upscaling procedure proposed herein are the micromechanical representations for the electrokinetic coefficients and swelling pressure. The two-scale model is discretized by the finite element method and applied to numerically simulate contaminant migration and electrokinetic attenuation through a compacted clay liner underneath a sanitary landfill.

DISPERSION IN TWO-DIMENSIONAL PERIODIC POROUS MEDIA. PART II. DISPERSION TENSOR

The dispersion tensor of two-dimensional periodic porous media is investigated numerically. The theory is first briefly reviewed using the volume averaging method. Then, with the help of the hydrodynamics determined in Part I of this study, the dispersion tensor is calculated both for ordered (in-line or staggered square cylinders) and disordered (randomly distributed square cylinders) varying both the particle Peclet number (0<Pep<104), the particle Reynolds number, from the Stokes flow to the laminar–inertial regime (0<Rep<100), and the direction of the average flow with the axes of the unit cell. The influence of order and spatial periodicity is discussed. Lastly the results are compared with those for “real” porous media.

Macroscopic behavior of swelling porous media derived from micromechanical analysis

A new macroscopic model for swelling porous media is derived based on a rigorous upscaling of the microstructure. Considering that at the microscale the medium is composed of a charged solid phase (e.g. clay platelets, bio-macromolecules, colloidal or polymeric particles) saturated by a binary monovalent aqueous electrolyte solution composed of cations ‘+’ and anions ‘−’ of an entirely dissociated salt, the homogenization procedure is applied to scale up the pore-scale model. The microscopic system of governing equations consists of the local electro-hydrodynamics governing the movement of the electrolyte solution (Poisson–Boltzmann coupled with a modified Stokes problem including an additional body force of Coulombic interaction) together with modified convection–diffusion equations governing cations and anions transport. This system is coupled with the elasticity problem which describes the deformation of the solid phase. Novel forms of Terzaghis effective principle and Darcys law are derived including the effects of swelling pressure and osmotically induced flows, respectively. Micromechanical representations are provided for the macroscopic physico-chemical quantities.

Thermal dispersion in porous media: one-equation model

Abstract In this work, the methods of volume average and multiple scale expansion are employed to obtain one macroscopic equation governing thermal dispersion in a rigid homogeneous porous medium. The structure of the real porous medium is described here by a spatially periodic model. The theoretical longitudinal thermal dispersion coefficient for a stratified system is compared with numerical data obtained from a random walk method, and good agreement is achieved.

Dispersion in two-dimensional periodic porous media. Part I. Hydrodynamics

In this paper we investigate the hydrodynamics of two-dimensional spatially periodic porous media. The finite volume method is used to compute the flow field in these media with the Navier–Stokes equations. Two particular points are described: how to deal with the periodic boundary conditions on the surface of a unit cell; and how to avoid numerical dispersion. The fluid flow is computed for the Stokes regime and for moderate values of the particle Reynolds number Rep, up to Rep=200. Four types of media are studied. Three of them are “ordered,” with in-line or staggered square cylinders and zigzag medium. The fourth is “disordered,” with randomly distributed square cylinders. The effect of the direction of the average flow velocity is analysed in all these cases. In the Stokes regime, the results for the pressure drop agree with the usual evaluation in terms of permeability. For higher particle Reynolds numbers, the non-linear correction to Darcy’s law is discussed. The correction term for moderate Reynol...

Dispersion in periodic porous media. Experience versus theory for two-dimensional systems

A method is presented for the experimental study of dispersion in saturated porous media for two-dimensional spatially periodic systems. The use of stereophotolithography laser as the basis of the porous media fabrication process made this investigation feasible. The porous media consist of a set of circular cylinders accurately positioned in an ordered (in line) or in a disordered (random) unit cell periodically reproduced in the plane of the study. For two directions of the average fluid velocity in the in-line array and one direction of the velocity vector in the disordered array, dye is injected in the form of a pulse at the entrance to the medium. The tracer concentration variations with time are measured by a video camera and are averaged over a unit cell. The measured time distributions are compared with computations using the macroscopic convection-diffusion equation in order to estimate the longitudinal dispersion coefficient. The variations of this coefficient with particulate Peclet number in the three geometries investigated are compared with the available numerical results. All the results agree, showing a significant influence of the direction of the average fluid velocity for the ordered medium. Concerning the random medium, the results appear to indicate that, in spite of its periodic character, this type of medium is capable of describing the behaviour of disordered porous media. This original technique is highly promising for the explanation of dispersion mechanisms in porous media.

Ablative melting of a solid cylinder perpendicularly pressed against a heated wall

Abstract This paper reports an analytical and experimental study on ablative melting of a solid cylinder perpendicularly pressed against a stationary heated surface. An explicit analytic solution is found for the rate of ablation in terms of temperature difference and pressure applied and of geometrical as well as physical properties of the solid and liquid. Data obtained in a limited number of rather crude experiments with rods of melting solids (ice, paraffin) and with rods of wood under flash pyrolysis conditions show a fair agreement with the predictions of the theoretical study thus confirming the “fusion model” of flash pyrolysis of wood in ablation regime.

Transport in PFSA Membranes

The proposed model allows estimating polymer membrane transport properties. It is based on a microscopic description of charge and water transport in a single capillary with a uniform distribution of charge at the wall. The equations are solved analytically for two geometries: cylindrical capillary and parallel plates. Macroscopic transport properties of the whole membrane are then deduced by upscaling the capillary model. Their variations with temperature and water content are evaluated and compared to data available in the literature. In addition, the influence of the pore size and of the effective charge density are studied.

Measurement of soil water diffusivity of an undisturbed forest soil using dual-energy gamma radiation technique

A procedure is described to measure the soil water diffusivity of a heterogeneous porous medium, a clay-loam forest soil. A dual-energy gamma-ray apparatus was used to determine nondestructively water content variations in soil samples during infiltration. The general principles of the dual-source technique (in particular correction of the Compton effect caused by cesium photons in the americium window) are reviewed, and a calibration method to determine the mass attenuation coefficients of soil is described. Infiltration measurements on disturbed soil samples were obtained by dual-energy gamma-ray technique to validate the calibration procedure. Imbibition tests were done on undisturbed soil core samples, and the soil water diffusivity of soil was calculated with a classical Boltzmann method. An optimization technique was used to determine the parameters of the hydraulic diffusivity model.