Didier Stemmelen

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.

Ébullition en milieu poreux: analyse expérimentale de la stabilité du front zone liquide-zone diphasique

Abstract We are interested in the problem of boiling in a porous column made up of glass beads saturated with water. The temperature of the plate at the bottom of the column is fixed at a value higher than the boiling point of water. The temperature at the top of the column is imposed at a value allowing a liquid zone to be present (by re-condensation), whereas the pressure is kept at the atmospheric value. We report in this paper different tests for which we have modified the size of the beads in order to obtain stable and unstable situations. In the last case, instabilities appear in the form of periodic oscillations of the temperature field and are accompanied by outlet then inlet of water through the permeable upper surface (‘Geyser effect’). After having mentioned the 1D treatment of stable situations, we examine unstable ones for which the convection in the liquid zone plays a disturbing effect. A simplified linear stability analysis taking into account the increase of effective conductivity of the liquid zone with free convection is carried out with a view to obtaining a new stability criterion.

Identification of the Hydraulic Diffusivity of a Soil by Inverse Method with Dual-Energy Gamma Ray Attenuation Measurements

Hydraulic diffusivity is fundamental to the study of a natural forestry site. Complementary field and laboratory experiments were carried out to identify hydraulic diffusivity. A transient physical process of water transport is required for this, for which a dynamic imbibition experiment was chosen. Time variations of moisture content are measured at various locations by dual-energy gamma ray attenuation. The diffusion transport occurring in this experiment has been described by the classical diffusion equation in terms of the Boltzmann variable with various models of non-linear diffusivity. An inverse method is then applied to determine the parameters of the hydraulic diffusivity model.

Electro-osmosis in kaolinite with pH-dependent surface charge modelling by homogenization

A new three-scale model to describe the coupling between pH-dependent flows and transient ion transport, including adsorption phenomena in kaolinite clays, is proposed. The kaolinite is characterized by three separate nano/micro and macroscopic length scales. The pore (micro)-scale is characterized by micro-pores saturated by an aqueous solution containing four monovalent ions and charged solid particles surrounded by thin electrical double layers. The movement of the ions is governed by the Nernst-Planck equations, and the influence of the double layers upon the flow is dictated by the Helmholtz-Smoluchowski slip boundary condition on the tangential velocity. In addition, an adsorption interface condition for the Na+ transport is postulated to capture its retention in the electrical double layer. The two-scale nano/micro model including salt adsorption and slip boundary condition is homogenized to the Darcy scale and leads to the derivation of macroscopic governing equations. One of the notable features of the three-scale model is there construction of the constitutive law of effective partition coefficient that governs the sodium adsorption in the double layer. To illustrate the feasibility of the three-scale model in simulating soil decontamination by electrokinetics, the macroscopic model is discretized by the finite volume method and the desalination of a kaolinite sample by electrokinetics is simulated.