1,286 research outputs found
Trauma and suicidality in war affected communities
No full textAvailable online 15 September 2012 as article in press. Cite as: Jankovic J, et al. Trauma and suicidality in war affected communities. European Psychiatry (2012),
http://dx.doi.org/10.1016/j.eurpsy.2012.06.001Journal homepage: http://www.elsevier.com/journals/european-psychiatry/0924-933
Effective conductivity of an anisotropic heterogeneous medium of random conductivity distribution
No full text"\"The paper deals with the effective conductivity tensor K(ef) of anisotropic random media subject to mean uniform flux. The hydraulic conductivity K field is modeled as a collection of spheroidal disjoint inclusions of different, isotropic and independent Y = ln K; the latter is a random variable with given distribution of variance sigma(2)(Y). Inclusions are embedded in homogeneous background of anisotropic conductivity K(0). The K field is anisotropic, characterized by the anisotropy ratio f, ratio of the vertical and horizontal integral scales of K. We derive K(ef) by accurate numerical simulations; the numerical model for anisotropic media is presented here for the first time, and it generalizes a previously developed model for isotropic formations [I. Jankovic, A. Fiori, and G. Dagan, Multiscale Model. Simul., 1 (2003), pp. 40-56]. The numerical model is capable of solving complex three-dimensional flow problems with high accuracy for any configuration of the spheroidal inclusions and arbitrary K distribution. The numerically derived K(ef) for a normal Y is compared with its prediction by (i) the self-consistent solution K(sc), (ii) the first-order approximation in sigma(2)(Y), and (iii) the exponential conjecture [L. J. Gelhar and C. L. Axness Water. Resour. Res., 19 (1983), pp. 161-180]. It is found that the self-consistent solution K(sc) is very accurate for a broad range of the values of the parameters sigma(2)(Y), f and for the densest inclusions packing. In contrast, the first-order solution strongly deviates from K(ef) for large sigma(2)(Y), as expected, and the exponential conjecture is generally unable to correctly represent the effective conductivity. The numerical solution for the potential is expressed as an infinite series of spheroidal harmonics, attached to the interior and exterior of each inclusion, which accounts for the nonlinear interaction between neighboring inclusions.\"
Flow and transport in highly heterogeneous formations: 1. Conceptual framework and validity of first-order approximations RID A-2321-2010
No full text[1] Flow of uniform mean velocity U takes place in a formation of spatially variable, random conductivity K(x). Advective transport of a plume of an inert solute is investigated by the Lagrangean approach. The aim of the study is to determine the spatial moments of the plume, i.e., of fluid particle trajectories, for highly heterogeneous aquifers, for which sigma(Y) > 1, where Y = ln K. A multi-indicator model of the permeability structure, which is different from the common multi-Gaussian one, is proposed: the formation is modeled as a collection of N blocks of different K-(j). The structure is defined by the distribution of K-(j), the blocks' shape, and the coordinates of their centroids. The following simplifications are adopted: the blocks are inclusions of a regular shape (circles, spheres for isotropic media investigated here) defined by the radius A, and the inclusions are not overlapping, and their centroids are distributed uniformly and independently in space. At the continuous limit the model is characterized by the joint pdf f(Y, A). The model is shown to be quite general and to comprise binary, bimodal, indicator variograms and unimodal distributions of Y as particular cases. The study is focused on the latter case, with Y normal N [[Y], sigma(Y)(2)] and stationary covariance of given integral scale I-Y; these are the parameters commonly estimated for sedimentary formations. This leaves still freedom in selecting the pdf f(A). The simple model selected for semianalytical and numerical analysis is that of inclusions of radius R and volume fraction n, submerged in a matrix of effective conductivity K-ef. The latter represents the effect of inclusions of much smaller radius, which appear as a nugget in the log conductivity two-point covariance. An approximate analytical solution of the flow is obtained by using a self-consistent approximation, while a fully numerical one is derived in part 3 [Jankovic et al., 2003a]. Transport is solved by particle tracking, and the time-dependent spatial moments (trajectories variance, skewness, kurtosis) are presented in part 2 [Fiori et al., 2003]. In the self-consistent approximation the asymptotic longitudinal macrodispersivity alpha(L), which is a function of Y, shows strong nonlinear effects: inclusions of large positive Y lead to a finite alpha(L), whereas alpha(L) grows unbounded for those of negative Y. This effect is not captured by the common first-order approximation in sigma(Y), which is symmetrical and overestimates alpha(L) for Y > 0 and underestimates it for Y < 0. As a result, the second spatial moment is predicted accurately by the first-order approximation, by cancellation of errors, provided that f(Y) is symmetrical. However, the transient regime and higher-order moments are not captured by the first-order approximation
Can we determine the transverse macrodispersivity by using the method of moments? RID A-2321-2010
No full textOne of the main assumptions that renders the stochastic theories applicable to real aquifers is the ergodic hypothesis, i.e. the possibility to exchange ensemble and spatial averages of a variable of interest. The principal aim of this paper is to elucidate the conditions that allow for an exchange between ensemble and spatially averaged second moments of concentration field (S-ij); the fulfillment of the ergodic condition underlies the applicability of the dispersion coefficients or other related quantities obtained by the stochastic theories to actual aquifers. The fulfillment of the ergodic hypothesis is assessed here by analyzing the diminishing of the variance of S-ij as the initial size of the plume V-0 grows, i.e. the tendency of S-ij toward its expected value (S-ij). It is shown that it is not always possible to assume ergodicity for solute plumes in heterogeneous aquifers. For the typical plume configurations encountered in applications, transverse and vertical spreading are the most problematic in this respect. In particular, satisfying the ergodic hypothesis depends largely on the initial plume configuration and size, on one hand, and the direction of the moment of interest, on the other. Numerical simulations based on the analytic element method elucidate the results. The relevance of the results is mostly felt for the inference of macrodispersive parameters, which are often derived through S-ij. The work indicates that S-ij may be a distorted and inadequate measure of the plume spread. This should serve as a warning against application of results based on ensemble averages to real-life plumes, particularly when estimating macrodispersion coefficient from field experiment. © 2005 Elsevier Ltd. All rights reserved
Analysis of the impact of injection mode in transport through strongly heterogeneous aquifers
No full textLarge-scale advective transport through highly heterogeneous 3D formations is investigated using highly resolved numerical simulations and simple analytic models. Investigations are focused on impacts of two types of contaminant injection on transport through isotropic formations where flow conditions are uniform in the average. Transport is quantified by analyzing breakthrough curves for control planes at various distances from the injection zone. In flux-proportional injection mode local mass in injection zone is proportional to local groundwater flux; this setup models many practical cases such as contaminant injection through wells. In resident concentration mode local concentration in injection zone is constant. Results show that impacts of injection mode on breakthrough curves and their moments are strong and they persist for hundreds of correlation scales. The resident concentration mode leads to a fatter tails of the breakthrough curves, while the peaks are generally underpredicted. For a synthetic porous medium with logconductivity variance of 8, dispersivity computed using resident concentration mode at control plane 100 integral scales away from the injection zone was about 10 times larger than corresponding one for flux-proportional mode. Hence, injection mode impacts on transport through highly heterogeneous formations are strong and they persist for large distances from the injection zone. (C) 2010 Elsevier Ltd. All rights reserved
Effective conductivity of 2D isotropic formation: performance of the effective medium approximation
No full textAnalysis of the longitudinal macrodispersivity of highly heterogeneous aquifers in presence of local diffusion
No full text
The impact of local diffusion on longitudinal macrodispersivity and its major effect upon anomalous transport in highly heterogeneous aquifers RID A-2321-2010
No full textFlow and transport are solved for a heterogeneous medium modeled as an ensemble of spherical inclusions of uniform radius R and of conductivities K, drawn from a pdff(K) (Fig. 1). This can be regarded as a particular discretization scheme, allowing for accurate numerical and semi-analytical solutions, for any given univariate f (Y) (Y = In K) and integral scale I(y). The transport is quantified by the longitudinal equivalent macrodispersivity alpha(Leq), for uniform mean flow of velocity U and for a large (ergodic) plume of a conservative solute injected in a vertical plane (x = 0) and moving past a control plane at x >> I(y). In the past we have solved transport for advection solely for highly heterogeneous media of sigma(2)(Y) > 1. (C) 2008 Elsevier Ltd. All rights reserved
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