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PORE-SCALE MODELING OF FLOW AND REACTIVE TRANSPORT IN POROUS MEDIA
No full textFlow and reactive transport of fluids in porous media are observed in a wide variety of fields and applications such as hydrology, contaminated site remediation and petroleum engineering. Concerning the environmental issues, fate and transport of dissolved contaminants in natural porous media is a fundamental aspect to understand pollutants migration in groundwater and to identify the most appropriate technologies to remove these compounds from subsurface (i.e. the vadose zone and the groundwater). The traditional approach used to study the motion of fluids (single or multi-phase) and transport of dissolved substances within porous media is based on a macroscopic representation, founded on the continuum hypothesis (Bear, 1972). At this scale, pore-scale effects are embedded into the model through a set of constitutive equations such as the phenomenological Darcy’s law for flow in saturated porous media (Hubbert, 1956; Bear, 1972), Richard’s equation in variable saturated media (Richards, 1931; Van Genuchten, 1980) and the advection-dispersion equation for solute transport (Bear, 1972). When the continuum description breaks down, the need to investigate a more detailed scale as the pore-scale brings to use the Navier-Stokes equations (NSE) and the advection-diffusion (and reaction) equation (ADE), able to capture microscopic-scale gradients in concentration resulting from transport and a non-uniform distribution of reactive material (Steefel et al., 2005; Blunt et al., 2013).
Pore-scale modeling has developed rapidly over the last decades, thanks to the development of both direct 3D imaging of the pore space and faster and more efficiency computing tools. It can be used to predict macroscopic properties of porous media that are difficult to obtain experimentally and provides the opportunity to investigate phenomena impossible to be obtained by laboratory experiments for single and multi-phase fluids. In the framework of pore-scale modeling, the lattice Boltzmann method (LBM) is able to solve the NSE for incompressible fluids and ADE in porous media, which is emerged over the last decades as an alternative approach for computational fluid dynamics (CFD) (Chen and Doolen, 1998; Succi, 2001; Aidun and Clausen, 2010).
Unlike the conventional CFD schemes based on discretization of macroscopic continuum equations, the LBM is based on microscopic models and mesoscopic kinetic equation. The main features are the relatively ease to code, versatility to model different process, handle complex boundary conditions and its efficiency for parallel platforms (Latt, 2009, Coon et al., 2014).
For the above-mentioned reasons, the goal of my research is the study of pore-scale effects on different flow and transport processes, which have a close relationship with contaminant dynamics at a macroscopic scale (i.e. laboratory and field scale). Each of these processes is addressed in the following chapters of the thesis. The thesis consists of a collection of scientific papers, except for the first chapter, submitted to or already published in international journals and is organized as follows.
In chapter I, the basics of the LB algorithm are introduced. The numerical schemes for the simulation of fluid flow and transport process of a concentration field are described.
In chapter II, the effects of the pore spatial distribution on seepage velocity through numerical simulations of 3D fluid flow performed by the lattice Boltzmann method are investigated. The goal of this work is to evaluate the sensitivity of the flow, through the seepage velocity, inside a porous medium to the spatial distribution of the pores size and to address the uncertainty associated to the sample size and resolution. To this scope, we generate 3D porous media using a geostatistical method based on random spatially correlated fields applied at the pore-scale (typically from tens to hundreds μm).
The use of 3D domains allows handling porous media with realistic porosity compared to 2D structures. Finally, a sensitivity analysis of the macroscopic velocity is carried out in relation to two semi-variograms models (or correlation functions) and different spatial resolutions.
In Chapter III, the impact of heterogeneity through pore-scale flux and transport LBM simulations are carried out. One of the effects that heterogeneities produce in the framework of contaminated sites remediation is associated with the retention of pollutants in the finest (or less mobile for an effective transport) regions of the porous media, where contaminants are released by diffusion to more mobile zones after the concentration in the latter is significantly reduced because of efficient transport. Despite its microscopic nature, it may have important implications for macroscale pollutants transport. The process of mass transfer from low to high mobility regions at the back end of a contaminant plume has referred to back diffusion (also defined as matrix diffusion). The main question that we address in this study is the extent to which spatial heterogeneities in the structure (porosity and permeability) of the host porous medium influences the rate of migration of dissolved species (e.g. contaminants).
Chapter IV is dedicated to reactive transport at the pore-scale. A lattice Boltzmann model is developed for surface reaction taking place at the interface between solid-fluids and multiphase fluids (Di Palma et al., 2015). The phase-field approach is used to identify the interface and its orientation, the concentration of reactant at the interface is then calculated iteratively to impose the correct reactive flux condition. The main advantages of the model is that interfaces are considered part of the bulk dynamics and the reactive scheme is introduced in the classical LBM algorithm; as a consequence, the model’s implementation and performance is independent of the interface geometry and orientation. Results obtained with the developed model are compared to analytical solution for three different benchmark tests
A new lattice Boltzmann model for interface reactions between immiscible fluids
Full text linkIn this paper, we describe a lattice Boltzmann model to simulate chemical reactions taking place at the interface between two immiscible fluids. The phase-field approach is used to identify the interface and its orientation, the concentration of reactant at the interface is then calculated iteratively to impose the correct reactive flux condition. The main advantages of the model is that interfaces are considered part of the bulk dynamics with the corrective reactive flux introduced as a source/sink term in the collision step, and, as a consequence, the model’s implementation and performance is independent of the interface geometry and orientation. Results obtained with the proposed model are compared to analytical solution for three different benchmark tests (stationary flat boundary, moving flat boundary and dissolving droplet). We find an excellent agreement between analytical and numerical solutions in all cases. Finally, we present a simulation coupling the Shan Chen multiphase model and the interface reactive model to simulate the dissolution of a collection of immiscible droplets with different sizes rising by buoyancy in a stagnant fluid
Use of a reactive transport model to describe reductive dechlorination (RD) as a remediation design tool: application at a CAH-contaminated site
No full textIn this paper, a numerical model is presented that is capable of describing the complex set of biochemical processes that occur in chlorinated aliphatic hydrocarbon (CAH)-contaminated groundwater when an exogenous electron donor is added. The reactive pattern is based on the degradation pathways of both chlorinated ethanes and ethenes, and it includes electron donor production (H2 and acetate) from the fermentation of an organic substrate as well as rate-limiting processes related to electron acceptor competition. Coupling of the kinetic model to a convection-dispersion module is described. The calibration phase was carried out using data obtained at a real CAH-contaminated site in the north of Italy. Model simulations of different application scenarios are presented to draw general conclusions on the effectiveness of reductive dechlorination (RD) as a possible cleanup strategy. Early outcomes indicate that cleanup targets can only be achieved if source longevity is reduced. Therefore, metabolic RD is expected to produce beneficial effects because it is known to induce bioenhanced degradation and transformation of CAHs
A Gis Tool for the Fast Identification of Drinking Water Abstraction Points Possibly Reached by a Contamination EventWater Contamination Emergencies
No full textPore-scale simulations of concentration tails in heterogeneous porous media
No full textThe retention of contaminants in the finest and less-conductive regions of natural aquifer is known to strongly affect the decontamination of polluted aquifers. In fact, contaminant transfer from low to high mobility regions at the back end of a contaminant plume (i.e. back diffusion) is responsible for the long-term release of contaminants during remediation operation. In this paper, we perform pore-scale calculations for the transport of contaminant through heterogeneous porous media composed of low and high mobility regions with two objectives: (i) study the effect of permeability contrast and solute transport conditions on the exchange of solutes between mobile and immobile regions and (ii) estimate the mass of contaminants sequestered in low mobility regions based on concentration breakthrough curves
Best Practices Protocol for Drinking Water Supply Contamination Emergencies
No full textAbstractA procedure for the emergency management of accidental pollution of groundwaters exploited for human consumption is being developed in the framework of a collaboration between the Water Research Institute and the National Civil Protection Department. The aim is to produce a “Best Practices Protocol” (BPP) targeted to water managers, public water agencies and boards. In this paper the part of the BPP concerning fast screening modelling is discussed: different methods for the assessment of the arrival time and concentration at the withdrawal point are compared, using a hypothetical contamination scenario applied to a real well field in central Italy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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