Formulation and analysis of a diffusion-velocity particle model for transport-dispersion equations

The modelling of diffusive terms in particle methods is a delicate matter and several models were proposed in the literature to take such terms into account. The diffusion velocity method (DVM), originally designed for the diffusion of passive scalars, turns diffusive terms into convective ones by expressing them as a divergence involving a so-called diffusion velocity. In this paper, DVM is extended to the diffusion of vectorial quantities in the three-dimensional Navier–Stokes equations, in their incompressible, velocity–vorticity formulation. The integration of a large eddy simulation (LES) turbulence model is investigated and a DVM general formulation is proposed. Either with or without LES, a novel expression of the diffusion velocity is derived, which makes it easier to approximate and which highlights the analogy with the original formulation for scalar transport. From this statement, DVM is then analysed in one dimension, both analytically and numerically on test cases to point out its good behaviour.

Metal measurement in aquatic environments by passive sampling methods: Lessons learning from an in situ intercomparison exercise

Passive sampling devices (PS) are widely used for pollutant monitoring in water, but estimation of measurement uncertainties by PS has seldom been undertaken. The aim of this work was to identify key parameters governing PS measurements of metals and their dispersion. We report the results of an in situ intercomparison exercise on diffusive gradient in thin films (DGT) in surface waters. Interlaboratory uncertainties of time-weighted average (TWA) concentrations were satisfactory (from 28% to 112%) given the number of participating laboratories (10) and ultra-trace metal concentrations involved. Data dispersion of TWA concentrations was mainly explained by uncertainties generated during DGT handling and analytical procedure steps. We highlight that DGT handling is critical for metals such as Cd, Cr and Zn, implying that DGT assembly/dismantling should be performed in very clean conditions. Using a unique dataset, we demonstrated that DGT markedly lowered the LOQ in comparison to spot sampling and stressed the need for accurate data calculation.

Modélisation hydro-sédimentaire de la rade de Toulon : application à l’étude de la dispersion de radionucléides

L’objectif de ce travail est mettre en place un modèle hydro-sédimentaire de dispersion des radionucléides, comme outil d’aide à la décision suite à une hypothétique contamination marine accidentelle. Appliqué à la Rade de Toulon, il utilise le modèle MARS-3D pour l’étude hydrodynamique et le module MIXSED pour la dynamique des différentes classes de sédiments. Les cas d’application présentés concernent le césium et le plutonium, qui ont des affinités très différentes avec les sédiments fins. Les simulations de rejet, effectuées en fond de baie, montrent une variabilité saisonnière marquée, due aux conditions météorologiques typiques, mais aussi aux conditions de débit et de charge sédimentaire associée du fleuve Las, qui se jette dans la Rade. Ainsi, les simulations par forts vents, qui favorisent la circulation et les échanges de masses d’eau, présentent une diminution rapide des activités dissoutes. Par ailleurs, le Vent d’Est hivernal, qui engendre d’importantes crues du Las et un apport de sédiments à la Rade, favorise le piégeage des radionucléides dans les sédiments de fond, où les deuxtiers de la contamination initiale sont piégés après une simulation de deux mois.