Constantin Oltean

Numerical Efficiency Assessment of IB–LB Method for 3D Pore-Scale Modeling of Flow and Transport

In many earth science and environmental problems, the fluid–structure interactions can affect the hydrodynamics properties of the porous medium via the spatial evolution of its solid matrix. A significant insight into these properties can be obtained from pore-scale simulations. Using a 3D pore-scale domain with moving walls, we proposed in this paper a comparison of the numerical accuracy between different approaches in regard to flow and reactive transport. Two direct-forcing immersed boundary (IB) models coupled with lattice Boltzmann method (LBM) are evaluated for the flow calculation against an analytical solution. The IB–LBM showed improvement compared to the classical and interpolated bounce-back lattice Boltzmann model. Concerning the reactive transport, the most accurate IB–LB method was coupled with two non-boundary conforming finite volume methods (volume of fluid and reconstruction). The comparative study performed with different mesh sizes and Damköhler numbers demonstrates better results for the reconstruction method.

Gravity-driven fingers in fractures: experimental study and dispersion analysis by moment method for a point-source injection.

In this study, we investigate the behavior of a dense contaminant injected from a point-source in a fracture. Our experimental model is a transparent Hele-Shaw cell, 0.5 mm of aperture. A saline solution is injected locally representing the point-source pollution. A Laser Induced Fluorescence (LIF) method provides concentration measurement of the pollution plume. Two propagation patterns have been observed: one and two-finger plumes. If the upper part of the plume is stable over time regardless of the second configuration, the moment when the plume separates into two fingers is highly dependent on both injection flow-rate and contaminant concentration. To further investigate the dispersion process inside the fracture, experimental results are interpreted by the spatial and time moment methods. Resulting dispersivities and plume propagation mean velocity are compared to theoretical values derived from a modified Taylor-Aris dispersion tensor. The longitudinal macro-dispersion obtained suggests an asymptotical behavior of the plume spread regardless of the studied configurations. Experimental local dispersivities derived from time and space moments proved to be close at large times to theoretical values predicted by the density-dependent dispersion tensor (Oltéan et al., 2004). Based on those observations the mechanism behind the separation of the plume into two fingers is believed to be significantly impacted by the pre-asymptotic behavior of the dispersion tensor.

An Upscaled Model for Bio-Enhanced NAPL Dissolution in Porous Media

We develop a Darcy-scale model for multiphase transport in porous media colonized by biofilms. We start with the pore scale description of mass transfer within and between the phases (water, biofilm, and NAPL phases) and biologically mediated reactions. The macroscale mass balance equations under local mass equilibrium condition at the fluid–biofilm interface are derived from the pore scale problem, obtained by the method of volume averaging. The case of local mass equilibrium considered here finally provides one mass balance equation for the fluid and biological phases coupled with the NAPL-phase equation. We predict the effective dispersion tensor and the mass exchange coefficient that appear in the upscaled equation by solving closure problems on representative unit cells. The results of this model have been compared with pore scale simulations. Based on these comparisons, the validity domain of this model has been identified in terms of hydrodynamic and biochemical conditions of transport (i.e., Péclet and Damköhler numbers). This study should provide a better insight on the impact of biofilm dynamics near NAPL sources through the upscaling process.

Impact of biofilm‐induced heterogeneities on solute transport in porous media

In subsurface systems, biofilm may degrade organic or organometallic pollutants contributing to natural attenuation and soil bioremediation techniques. This increase of microbial activity leads to change the hydrodynamic properties of aquifers. The purpose of this work was to investigate the influence of biofilm-induced heterogeneities on solute transport in porous media and more specifically on dispersivity. We pursued this goal by (i) monitoring both spatial concentration fields and solute breakthrough curves from conservative tracer experiments in a biofilm-supporting porous medium, (ii) characterizing in situ the changes in biovolume and visualizing the dynamics of the biological material at the mesoscale. A series of experiments was carried out in a flow cell system (60 cm(3)) with a silica sand (Phi = 50-70 mesh) as solid carrier and Shewanella oneidensis MR-1 as bacterial strain. Biofilm growth was monitored by image acquisition with a digital camera. The biofilm volume fraction was estimated through tracer experiments with the Blue Dextran macromolecule as in size-exclusion chromatography, leading to a fair picture of the biocolonization within the flow cell. Biofilm growth was achieved in the whole flow cell in 29 days and up to 50% of void space volume was plugged. The influence of biofilm maturation on porous medium transport properties was evaluated from tracer experiments using Brilliant Blue FCF. An experimental correlation was found between effective (i.e., nonbiocolonized) porosity and biofilm-affected dispersivity. Comparison with values given by the theoretical model of Taylor and Jaffe (1990b) yields a fair agreement.

Comparison of theory and experiment for NAPL dissolution in porous media

Contamination of groundwater resources by an immiscible organic phase commonly called NAPL (Non Aqueous Phase Liquid) represents a major scientific challenge considering the residence time of such a pollutant. This contamination leads to the formation of NAPL blobs trapped in the soil and impact of this residual saturation cannot be ignored for correct predictions of the contaminant fate. In this paper, we present results of micromodel experiments on the dissolution of pure hydrocarbon phase (toluene). They were conducted for two values of the Péclet number. These experiments provide data for comparison and validation of a two-phase non-equilibrium theoretical model developed by Quintard and Whitaker (1994) using the volume averaging method. The model was directly upscaled from the averaged pore-scale mass balance equations. The effective properties of the macroscopic model were calculated over periodic unit cells designed from images of the experimental flow cell. Comparison of experimental and numerical results shows that the transport model predicts correctly - with no fitting parameters - the main mechanisms of NAPL mass transfer. The study highlights the crucial need of having a fair recovery of pore-scale characteristic lengths to predict the mass transfer coefficient with accuracy.

The permeability contrast influence on the plume propagation with variable physical properties

ABSTRACT This paper deals with the modelling of variable-density flow and solute transport in saturated porous media. The mathematical formulation for such problems is solved by a combination of the mixed hybrid finite element method and discontinuous finite element method. The specific point of interest in this study is the investigation of dense plume behaviour in presence of heterogeneity. The model under consideration called FRIPE is used: (i) to study the behaviour of a dense solution locally injected into a stratified porous medium without groundwater flow field and (ii) to simulate the movement of radioactive salt solutions in Lake Karachai (Russia).