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The distribution and migration of artificial fallout radionuclides in natural soils has been profusely studied for assessing radioecological impacts and predicting their long-term behaviour, among other topics. Despite the standardized use of the analytical solutions of a simplified convection-diffusion equation (CDE), there are still some concerns and open questions. This work is aimed at contributing to the understanding of basic processes governing the distribution of fallout radionuclides in vegetated soils with rhizospheres. It studies 210Pb and 137Cs in soil cores and vegetal samples from Chréa National Park, in Algeria, along with other natural radionuclides and some major and trace elements. Results include surficial and depth distributions of radionuclide concentrations, and site and plant-specific concentration ratios (CR). Inventories of 137Cs (3620 ± 120 Bq m−2) and 210Pbexc (9000 ± 900 Bq m−2) in soils are typical from global fallout in high precipitation areas in the Northern Hemisphere. A simple model of a polyphasic soil, including rhizospheres, provides a realistic description in the studied case, where plant roots occupy about 45% of the volume in the 0–10 cm interval, with a high porosity around rhizomes. This composite soil matrix explains the different patterns observed in the depth distribution of the studied elements. The depth-distributions of 137Cs and 210Pbexc have been modelled with different approaches: i) analytical solution of the CDE with mean annual convection and large observation times; ii) as before, but with convection representing infiltration events and short observation times; iii) numerical modelling of the 137Cs profile in the mineral phase using CDE with fast initial distributions. The three approaches fit the empirical data, but they predict different time evolutions. The approach iii) provides a more realistic description. Results are questioning the common accepted analysis and its predictive use.

The sedimentary system of the Algiers Bay has been impacted in the last decades by the exponential increases in population, urbanism and industrial activities of the city of Algiers, and by episodic floods, mudflows, earthquakes and tsunamis. Activity concentrations of environmental gamma-emitters, namely 210Pb, 226Ra, 40K, 228Ra, 228Th and 137Cs, were measured in a sediment core sampled in 2015 at the central zone of this bay, at 73 m depth from the surface water. The 210Pb-based radiometric dating was challenging due to the incomplete recovery of the inventory, the absence of 137Cs peaks, and the failure of classical models. The analysis of clusters in the logarithmic plot of the excess 210Pb versus mass depth profile allowed for establishing a reliable chronology. Fluxes of excess 210Pb and sediment accumulation rates (SAR) have both varied over the last five decades. Recent SAR values are of 0.73 ± 0.21 g cm−2y−1. Major and trace elements were determined by X-Ray Fluorescence in some selected sediment slices. Moderate enrichments have been observed for Mn, Zn, Br, Rb, Sr, Y and Pb in the 0–1 cm sediment layer (when using Al as normalization element), being governed by Fe-rich carriers. The major impact is observed in the carbon content, which increased by two orders of magnitude (from their basal values) for three decades since early 70's. The dataset also allowed for a radioecological assessment in the area, performed using the ERICA software.

International audience; The subject matter of this investigation is that of establishing a better understanding of the rheological and hydrodynamic behaviour of a fully developed turbulent flow of the Non-Newtonian fluids through a cylindrical pipe, in additional to ascertaining the accuracy and reliability of the laboratory code present findings, via analysing and discussing the evolution of the main rheological and flow predicted results. Toward this end, a fully developed turbulent flow of a power-law fluid describing by a shear thinning fluid with a flow index of 0.75 in an isothermal axially stationary cylindrical pipe, with domain length of 20R and an adequate grid resolution of 65 3 gridpoints in the axial, radial and circumferential directions, has been performed numerically by means of a Large eddy simulation (LES) approach with an extended Smagorinsky model, at a simulation's Reynolds number of 4000. The main present findings show a good qualitative agreement with the numerical and experimental data, where these findings suggest that the increased shear rate of the shear thinning fluid results in a pronounced enhancement in the mean axial velocity compared to the Newtonian fluid especially in the logarithmic and the core regions. The apparent viscosity of the shear thinning fluid enhances gradually out of the viscous sublayer towards to the pipe core region where the shear thinning fluid becomes more and more viscous, in turn the friction factor exhibits a slightly attenuation.

International audience; A large eddy simulation (LES) with an extended Smagorinsky model has been carried to investigate numerically the fully developed turbulent flow of a shear thinning fluid (n=0.75) in a stationary pipe at a simulation's Reynolds number equals to 4000 with a grid resolution of 65 3 gridpoints in axial, radial and circumferential directions and a domain length of 20R. The present study set out to critically evaluate the influence of the Non-Newtonian rheological and hydrodynamic behaviour on the turbulence main features, as well as to ascertain the accuracy and reliability of the laboratory code predicted results. The turbulent flow statistics obtained compared reasonably well with the experimental data and DNS results. A reasonably good agreement has been obtained between the predicted results and available results of literature. The main findings suggest that the mean axial velocity fluctuations were generated in the wall vicinity and transferred to the radial and spanwise components, which resulted in to an enhancement in the kinetic energy of turbulent fluctuations along the radial direction. The energy transport from the axial fluctuations to the other fluctuations components between the flow layers was suppressed compared to that of the Newtonian fluid.

The present study set out to reveal the influence of the flow behaviour index of the power-law fluid on the main features of the turbulence in especially at the vicinity of the wall. Toward this end, a large eddy simulation (LES) with a standard dynamic model has been devoted to a fully developed turbulent flow of pseudoplastic and dilatant fluids through an isothermal stationary pipe, at a simulation's Reynolds number of 4000. The emerged of this study suggest that the decreased flow behaviour index induces a pronounced enhancement in the generation of the axial turbulence intensities and a reduction in the transfer of these axial intensities to the tangential and radial intensities, The decreased flow behaviour index leads also to decrease the turbulent kinetic energy and to ameliorate the transport of the axial radial and tangential turbulence intensities from the vicinity of the wall towards the core region. Moreover, the skewness and flatness of the axial velocity fluctuations are almost independent of the flow behaviour index along the radial direction.

A fully developed forced convection heat transfer of a thermally independent shear thinning (n=0.75) and Newtonian fluids flowing inside a uniform heated axially pipe has been carried out numerically by means of the large eddy simulation (LES) with an extended Smagorinsky model at a simulation's Reynolds and Prandtl numbers equal to Res=4000 and Prs=1 respectively. The present investigation aims to ascertain the accuracy and reliability of the LES laboratory code predicted results and evaluate the effectiveness of the extended Smagorinsky model to resolve and describe the evolution of the main mean flow quantities and thermal statistics in the present computational domain especially at the wall vicinity. The Computations are based on a finite difference scheme, second order accurate in space and in time, the numeric resolution is 65 3 grid points in r, θ and z direction respectively, with length of the domain of 20R. The predicted results show an excellent agreement with the DNS results, the main findings show that the present LES laboratory code can be considered as a powerful tool for predicting the mean thermal quantities of the Non-Newtonian fluid. The reduction in the fluid flow index (n) results in enhancement in the mean axial velocity and a pronounced attenuation in the mean temperature over the radial direction, where leads also to noticeable drop in the average Nusselt number.

The overall aims of the present investigation are to provide a detailed knowledge on the thermal turbulence statistics especially at the wall vicinity and to describe the thermal and rheological behaviour of the Non-Newtonian fluid, that in order to characterise the main features of the thermal turbulence and shed further light on the transfer mechanism of the turbulent energy. In addition to ascertain the accuracy and reliability of the present LES laboratory code by comparing the predicted findings with those available in the literature. Towards these purposes, a fully developed of the turbulent forced convection heat transfer of a thermally independent shear thinning (n=0.75) and Newtonian fluids in a heated pipe, has been carried out numerically by means of a Large eddy simulation (LES) with extended Smagorinsky model at simulation Reynolds and Prandtl numbers of 4000 and 1 respectively. The main findings suggest that the temperature fluctuations generation and the turbulent energy transport mechanism between the flow layers of the shear thinning fluid are pronounced compared to that of Newtonian fluid, the skewness and the flatness of the temperature fluctuations are almost independent of fluid flow index. Moreover, the present LES laboratory code can be considered as a powerful tool to resolve and predict the thermal turbulence statistics of such issue.

Colloque avec actes et comité de lecture. Internationale.; International audience; L’étude d’un écoulement monophasique dans un segment d'un contacteur de disque rotatif (RDC) d’une colonne d'extraction liquide-liquide ayant un diamètre nominale de 450 mm a été réalisée numériquement avec le code commercial ANSYS-CFX. La validation a été faite sur la base d’une confrontation de nos résultats à ceux obtenus par la PIV (Particle Image Velocimetry) dans le cadre des travaux de Christian Drumm et al. Notre contribution a consisté à tester la prédiction de la CFD de la réalité du système industriel.A cet effet différents modèles de turbulence : standard k-ε, RNG k-ε et EARSM ont été utilisés dans le cadre de ces simulations d’écoulements monophasiques. Les variations des champs de vitesse (en utilisant l’eau comme fluide) ont été comparées aux résultats expérimentaux. Les résultats obtenus à partir de cette étude sont en bon accord avec les résultats expérimentaux. Ceci montre bien que le code de calcul utilisé « ANSYS-CFX » peut prédire les phénomènes liés à ce type d’écoulements tournants telles que les instabilités hydrodynamiques.

Colloque avec actes et comité de lecture. Internationale.; International audience; Ce travail constitue une contribution à la dynamique des lits fluidisés, notre objectif est d’étudier la modélisation de l’effet de la chambre d’espace sur la qualité de fluidisation. La chambre d'espace joue un rôle essentiel dans la distribution du fluide et ce avant d’occuper l'espace d'air de fluidisation, car la mauvaise distribution affecte l'exécution du lit fluidisé. Elle joue également un rôle essentiel dans la répartition du fluide au niveau de l'entrefer du distributeur, cette action est nécessaire à l'exécution du lit fluidisé. L’étude numérique de l’écoulement tourbillonnant et de la distribution des vitesses dans la chambre d’espace a été réalisée à l’aide du logiciel commercial ANSYS-CFX. La validation de ces résultats a été faite suite à une confrontation à ceux obtenus expérimentalement. A cet effet deux géométries différentes ont été analysées afin de mieux cerner l’influence de la géométrie de la chambre d’espace sur l’écoulement tournant. Les résultats ainsi obtenus pourront servir comme moyens d’optimisation de la chambre d’espace dont les effets géométriques peuvent influer sur le rendement, l’uniformité de la vitesse à la sortie et la perte de charge.

Colloque avec actes et comité de lecture. Internationale.; International audience; Les différentes méthodes de visualisation des lignes de courant pariétales au voisinage d'un disque fixe ont constitué un outil important pour vérifier les résultats théoriques concernant le champ de vitesses. Cet article propose une comparaison entre deux techniques de visualisation, l'une par dépôt électrolytique et l'autre par filets colorés, appliquées à l'écoulement d'un fluide tournant au voisinage d'un disque fixe. Deux méthodes de visualisations sont ainsi confrontées à des résultats théoriques obtenus grâce à une méthode de Newton utilisant la technique itérative à tir corrigé.

Colloque avec actes et comité de lecture. Internationale.; International audience; La mise en œuvre de la technique de visualisation de l’écoulement pariétal par dépôt électrochimique a donné de bons résultas dans différents types d’écoulements. Les lignes de courant pariétales visualisées par dépôt électrochimique nous a permis de comprendre et de mettre en évidence les différents éléments caractérisant la nature centripète de l’écoulement au voisinage du disque fixe. Les résultats expérimentaux mettent en évidence les phénomènes d’entrée et de bord à travers l’écoulement.