1,720,958 research outputs found
Simulation of DNAPLs migration in saturated porous media by means of Lattice Boltzmann Models
No full textThe aim of the research is to implement a numerical model inspired by the Lattice Boltzmann (LB) theory able to simulate the complex scenario of contamination of DNAPLs (Dense Non Aqueous Phase Liquids) in saturated porous media.
Groundwater contamination is nowadays recognized as a serious environmental problem. Among different polluting substances, the contamination by NAPLs continues to persist as a significant problem in industrialized nations. More specifically, an accidental release at the ground surface of DNAPLs, which include a variety of organic compounds denser than water, can lead to long-term contamination of both the unsaturated and saturated zones.
Once released, the DNAPLs tend to migrate mainly vertically in the subsurface under gravity and capillary forces. Since they are slightly soluble in both water and air, they can exist in the subsurface as a separate and immiscible fluid phases and migrate to significant depths below the water table giving rise to aqueous phase plumes depending on the slow dissolution kinetics with persistence of the source for a long period. Furthermore, during DNAPLS migration in both unsaturated and saturated media, DNAPLs can be trapped by capillary forces in the porous medium (i.e. residual DNAPLS saturation).
Predicting the source localization, often unknown, and the fate of these organic chemicals in the subsurface is challenging. Because of the difficulty in defining the separate phases migration in porous media and the further complication of the soil heterogeneity, a significant effort has been carried out for the investigation of DNAPLs motion and transport by means of experimental tests and image analysis and fate and transport modeling mainly of the dissolved phases. Nevertheless, further effort is requested to model the separate phase motion in porous media.
On the basis of this context, numerical methods can be a support for understanding and investigating the fluids behavior in complex systems such as porous media. Different approaches to modeling flows through porous media can be considered. The classical macro-scale multiphase models however do not explicitly account for many important physical phenomena in which the phases are involved. As an alternative, a mesoscopic model known as Lattice Boltzmann (LB) has been successfully applied to saturated porous media and it has been widely used for its ability to deal with complex geometries as well as its capacity to simulate fluid flow and transport at the pore-scale. The LBM originates from the kinetic theory of gases and represents the microscopic phenomena by means of a statistical (macroscopic) description. The LBM has been shown to recover the conservation laws of mass, momentum and energy and the CFD community agrees that it has reached an high degree of maturity to solve the Navier- Stokes equations for incompressible fluids in single and multiphase environments.
Compared to traditional CFD, the LBMs lead to an easier implementation of multi-phase and multi-component flows and they are applied in many fields such as geologic storage of CO2, Petroleum Engineering or reactive and melting/dissolution phenomena. Even more dealing with multiphase fluids, LBMs provides several advantages compared to the traditional CFD, such as the ability to model the interface dynamics between the different phases and to simply handle the forcing terms involved in the non- wetting phase migration in a porous media e.g. capillary forces, viscous forces and buoyancy forces.
In the thesis, a multiphase multicomponent LB model is therefore presented to simulate DNAPLs migration in saturated porous media. Once described the main DNAPLs physical-chemical properties and the multiphase multicomponent LB theory, the implemented model is validated by means of different analytical solutions. Firstly, the interfacial tension phenomenon is analyzed by means of the bubble coalescence and the spinodal decomposition simulations: a sharp interface of few lattice nodes thickness is present between the two fluid phases in function of the tension parameter. The model has proved to recover the Young-Laplace Law, which correlates the pressure difference (at equilibrium) between the regions inside and outside a bubble of fluid 1 surrounded by the fluid 2, to 1/R, where R is the radius of the bubble. Then, the wettability effects are analyzed and validated by determining the contact angle measure and comparing it with the analytical solution in Huang et al. (2007). Finally, the multiphase Poiseuille flow is simulated and compared to the results obtained from Dou et al. (2012) .Different simulations varying the forces applied to the two fluids are then carried out to investigate the impacts of the drag forces, the viscosity effects and the velocity profiles in the channel.
Regarding the simulations of the DNAPLs migration in saturated porous media, the results obtained aimed to investigate the influence of the gravity, viscous and capillary forces on DNAPLs motion. Specifically, the role of these forces is characterized by means of different dimensionless numbers, which vary during the conducted simulation tests; the simulations results confirm that, once the DNAPL is released, an increase in contaminant amount and a more radial migration of DNAPL occurs in proximity of the release point when the capillary number “Ca” (ratio between the viscosity and the capillary forces) increases. Contrarily, when the Bond number “Bo” (ratio between the buoyancy and the capillary forces) exceeds the “Ca” number, the gravity assumes a “destabilizing” role leading to the formation of gravity driven fingers.
The influence of the hydraulic gradient on DNAPLs migration is also investigated. When a hydraulic gradient is applied, the separate phase shift towards the water flow direction occurs and the pooled DNAPL accumulated above the fine lenses is less. However, the DNAPL phase is also able to move backwards the flow direction when the hydraulic gradient does not let it to exceed the capillary pressure between the pores situated downgradient, explaining the not totally removal of pools over the fine lens even with higher hydraulic gradients. Finally, “trapped DNAPLs” are present: the gravity-driven fingers extend between the pores until they are too long and break. The disconnected mass can be immobilize as blobs and ganglia for the capillary forces.
It can be concluded that the development of a model that reproduces the motion of immiscible fluids starting from the microscale- mesoscopic scale is of particular interest and importance. On one hand, it can be useful to investigate the DNAPLs scenario of contamination as well as the reactions and physical microscale processes that significantly influence their macroscale behavior; on the other hand, it is worth to underline its potential use as a tool to support any projects of environmental remediation, in which the identification of the source of contamination and the prediction migration dynamics of contaminants in porous media are essential in selecting the best remediation technology to apply. Although further research and investigation are needed to simulate the DNAPLs migration paths in real contaminated sites, the LB method can provide good results in such a field. The model can therefore be applied to investigate the migration behavior of these separate phases in saturated porous media. However, further effort has to be spent in order to provide a tool able to simulate greater contaminated domains: the potential use of upscaling techniques, without losing the intrinsic microscopic information provided by the LB model, can be suggested
Oxygen control and improved denitrification efficiency by dosing ferrous ions in the anoxic reactor
No full textAbstract: Small concentrations of dissolved oxygen (DO) in the range 0.2–0.4 mg L−1 normally are present in biological pre-denitrification reactors. This situation causes adverse effects on denitrification rate and, consequently, on the process efficiency. The results presented show the possibility to control the DO in the anoxic reactor by dosing ferrous Fe(II) ions. The experiments were carried out on both batch samples and a pilot plant and proved that oxidation of Fe(II) to Fe(III) is very efficient in the DO control. Moreover, Fe(III) reacts with phosphorus which precipitates as ferric orthophosphate. A dose of 6 mgFe2+ L−1 decreased the mean DO concentration from 0.45 to 0.28 mg L−1; as a consequence, the denitrification efficiency (ηDEN) increased from about 65–77%. ηDEN reached up to 89% with 9 mgFe2+ L−1 (50% over the stoichiometric for phosphorus removal) thanks to an average DO concentration of 0.08 mgO2 L−1 in the denitrification stage. The results also highlighted the strong influence of DO (and consequently the dosage of Fe2+) on the specific denitrification rate suggesting to maintain DO concentration in the pre-denitrification reactors lower than 0.2 mg L−1 in order to achieve high operation efficiencies
Oxygen control and improved denitrification efficiency by dosing ferrous ions in the anoxic reactor
Full text linkSmall concentrations of dissolved oxygen (DO) in the range 0.2–0.4 mg L−1 normally are present in biological pre-denitrification reactors. This situation causes adverse effects on denitrification rate and, consequently, on the process efficiency. The results presented show the possibility to control the DO in the anoxic reactor by dosing ferrous Fe(II) ions. The experiments were carried out on both batch samples and a pilot plant and proved that oxidation of Fe(II) to Fe(III) is very efficient in the DO control. Moreover, Fe(III) reacts with phosphorus which recipitates as ferric orthophosphate. A dose of 6 mgFe2+ L−1 decreased the mean DO concentration from 0.45 to 0.28 mg L−1; as a consequence, the denitrification efficiency (ηDEN) increased from about 65–77%. ηDEN reached up to 89% with 9 mgFe2+ L−1 (50% over the stoichiometric for phosphorus removal) thanks to an average DO concentration of 0.08 mgO2 L−1 in the denitrification stage. The results also highlighted the strong influence of DO (and consequently the dosage of Fe2+) on the specific denitrification rate
suggesting to maintain DO concentration in the pre-denitrification reactors lower than 0.2 mg L−1 in order to achieve high operation efficiencies
NUMERICAL MODEL FOR CHELATING AGENTS ENHANCED SOIL FLUSHING
No full text“In situ” remediation technologies, if applied without careful evaluation of the process parameters,
may themselves be a source of pollution. The soil flushing is a useful technology for in situ
remediation of a heavy metal contaminated soil and is particularly effective when the washing
solution is added with a chelating agent such as EDTA.
Excessive use of chelating agent may, however, be harmful to the environment for several
reasons: the chelating solution, if incompletely intercepted by extraction wells, can reach the water
table; an excessive concentration of chelator can damage soil structure and finally the metal
transported by the chelating solution can be reabsorbed into the deeper layers of the soil.
This paper presents the results of a soil flushing treatment with a EDTA solution carried out on
an experimental soil column that consists of a surface layer contaminated with lead and a not
contaminated deeper layer.
The purpose of the study is to determine whether a part of the metal contained into the superficial
contaminated soil can be transferred, by the chelating solution, in the uncontaminated soil;
furthermore the concentration of the chelating agent and the volume of the solution that minimizes
the risk for the soil are determined by 1D model that simulates the chelation process.
The experimental results confirm the possibility that a small amount of metal can be transferred
and absorbed into the not contaminated soil. Operationally this information can be used to define
the correct positioning of the extraction wells used in the soil flushing; at the same simulation
model can be used to determine the concentration and volume of chelating solution that optimize
the treatment
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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