Transport Modeling Framework Research Articles

Hybrid numerical methods for multiscale simulations of subsurface biogeochemical processes

Many subsurface flow and transport problems of importance today involve coupled non-linear flow, transport, and reaction in media exhibiting complex heterogeneity. In particular, problems involving biological mediation of reactions fall into this class of problems. Recent experimental research has revealed important details about the physical, chemical, and biological mechanisms involved in these processes at a variety of scales ranging from molecular to laboratory scales. However, it has not been practical or possible to translate detailed knowledge at small scales into reliable predictions of field-scale phenomena important for environmental management applications. A large assortment of numerical simulation tools have been developed, each with its own characteristic scale. Important examples include 1. molecular simulations (e.g., molecular dynamics); 2. simulation of microbial processes at the cell level (e.g., cellular automata or particle individual-based models); 3. pore-scale simulations (e.g., lattice-Boltzmann, pore network models, and discrete particle methods such as smoothed particle hydrodynamics); and 4. macroscopic continuum-scale simulations (e.g., traditional partial differential equations solved by finite difference or finite element methods). While many problems can be effectively addressed by one of these models at a single scale, some problems may require explicit integration of models across multiple scales. We are developing a hybrid multi-scale subsurface reactive transport modeling framework that integrates models with diverse representations of physics, chemistry and biology at different scales (sub-pore, pore and continuum). The modeling framework is being designed to take advantage of advanced computational technologies including parallel code components using the Common Component Architecture, parallel solvers, gridding, data and workflow management, and visualization. This paper describes the specific methods/codes being used at each scale, techniques used to directly and adaptively couple across model scales, and preliminary results of application to a multi-scale model of mineral precipitation at a solute mixing interface.

Modelization of the EOS

This article summarizes theoretical predictions for the density and isospin dependence of the nuclear mean field and the corresponding nuclear equation of state. We compare predictions from microscopic and phenomenological approaches. An application to heavy ion reactions requires to incorporate these forces into the framework of dynamical transport models. Constraints on the nuclear equation of state derived from finite nuclei and from heavy ion reactions are discussed.

The foundations of diffusion revisited

Diffusion is essentially the macroscopic manifestation of random (Brownian) microscopic motion. This idea has been generalized in the continuous time random walk formalism, which under quite general conditions leads to a generalized master equation (GME) that provides a useful modelling framework for transport. Here we review some of the basic ideas underlying this formalism from the perspective of transport in (magnetic confinement) plasmas.Under some specific conditions, the fluid limit of the GME corresponds to the Fokker–Planck (FP) diffusion equation in inhomogeneous systems, which reduces to Fick's law when the system is homogeneous. It is suggested that the FP equation may be preferable in fusion plasmas due to the inhomogeneity of the system, which would imply that part of the observed inward convection (‘pinch’) can be ascribed to this inhomogeneity.The GME also permits a mathematically sound approach to more complex transport issues, such as the incorporation of critical gradients and non-local transport mechanisms. A toy model incorporating these ingredients was shown to possess behaviour that bears a striking similarity to certain unusual phenomena observed in fusion plasmas.

Divalent inorganic reactive gaseous mercury emissions from a mercury cell chlor-alkali plant and its impact on near-field atmospheric dry deposition

Abstract The emission of inorganic divalent reactive gaseous mercury (RGM) from a mercury cell chlor-alkali plant (MCCAP) cell building and the impact on near field (100 km) dry deposition was investigated as part of a larger collaborative study between EPA, University of Michigan, Oak Ridge National Laboratory, Chlorine Institute, and Olin Corporation in February 2000. Measurements in the cell building roof vent showed that RGM constituted 2.1±0.7% (median±variance) of the concurrently measured elemental gaseous mercury (Hg 0 ). This relationship was used to calculate an estimated RGM emission rate from the cell building roof vent of 10.4 g day −1 . The percentage of RGM/Hg 0 at ambient monitoring sites 350 m (1.5%) and 800 m (1.3%) away while being impacted by cell building emissions suggests the rapid deposition of RGM species. The observed 2% relative emission of RGM/Hg 0 was substantially lower than the 30% estimate utilized by EPA to model the impact of MCCAPs for the 1997 Mercury Report to Congress. However, the MCCAP was still found to present a significant impact on near field mercury atmospheric dry deposition. A Lagrangian transport and deposition modeling framework using only emissions from the MCCAP found the mean annualized dry deposition of mercury within a 10 km radius of the facility contributed the annual equivalent of 4.6 μg m −2 . For comparison, the total annual wet mercury deposition measured at the Savannah River National Mercury Deposition Network sampling site ∼30 km away was 9.8 μg m −2 .

Kaon and antikaon production in dense nuclear matter

The production and propagation of kaons and antikaons in relativistic heavy-ion collisions have been systematically investigated with the Kaon spectrometer at SIS/GSI. Experimental results on the K+ and K− yield and on the azimuthal emission pattern of K+ mesons are presented. Within the framework of transport models the data can be explained assuming in-medium kaon–nucleon potentials. The comparison of K+ production excitation functions obtained for Au+Au and C+C collisions with results of transport model calculations favours a soft nuclear equation-of-state.

Electron transport through a metal-molecule-metal junction

Molecules of bisthiolterthiophene have been adsorbed on the two facing gold electrodes of a mechanically controllable break junction in order to form metal-molecule(s)-metal junctions. Current-voltage (I-V) characteristics have been recorded at room temperature. Zero bias conductances were measured in the 10-100 nS range and different kinds of non-linear I-V curves with step-like features were reproducibly obtained. Switching between different kinds of I-V curves could be induced by varying the distance between the two metallic electrodes. The experimental results are discussed within the framework of tunneling transport models explicitly taking into account the discrete nature of the electronic spectrum of the molecule.

Secondary electron emission near the electronic stopping power maximum

We report on measurements of light ion $(Z=2)$ and heavy ion $(Z=14)$ induced electron emission yields from the entrance and exit surfaces of thin carbon foils near the electronic energy loss per unit path length $(dE/dx)$ maximum. At constant $dE/dx,$ secondary electron yields are lower for high projectile velocities than for lower projectile velocities. This velocity effect, which is well known for secondary ion emission, is observed for backward electron emission for both He and Si. In the case of Si projectiles such a velocity effect is also observed for forward electron emission. The results can be qualitatively understood in the framework of recent electron transport and nuclear track models.

Velocity effect in secondary electron emission below and above the electronic stopping power maximum

We studied secondary electron emission yields γ from a carbon surface with helium projectiles near the electronic stopping power maximum (0.2–2 MeV). We ask the question: “Does γ depend on the electronic stopping power (dE/dx)e only or is there an additional dependence on the projectile velocity VP?” A velocity effect is indeed found for backward emission. The results are discussed in the framework of recent electron transport and track models and compared with the yield of secondary ions from the same collision system.

Hall-Effect Studies on Microcrystalline Silicon with Different Structural Composition and Doping

AbstractHall-effect experiments on <n>-type microcrystalline silicon samples with a wide range of structural composition and doping have been performed. For highly doped samples the conductivity Σ and the mobility μ show a non-singly activated behaviour while the carrier density is almost temperature independent. The comparison of the carrier density with the phosphorous concentration in conjunction with the conductivity gives strong evidence that the Hall-effect data have to be corrected with the crystalline volume fraction Xc. Furthermore, the increase of the mobility with Xc, which is linked in our case to the grain size, can be explained when the length of the transport paths is taken into account. Our results will be discussed in the framework of different transport models. It is concluded that transport in μc-Si:H can not be explained in terms of thermionic emission over barriers with a well defined barrier height; instead a distribution of barrier heights have to be considered. A transport model is suggested where μc-Si:H is viewed as an interconnected network.

ChemInform Abstract: Transport Processes in Solids During Ion Implantation

Abstract There are many important effects of fast charged particle irradiation on solid state rate processes in solids: (1) sputtering; (2) enhanced diffusion; (3) break-up of clusters of precipitated atoms; (4) electric field formation promoted by electronic excitation. In most irradiation studies of solids, different combinations of these effects, depending on the target electronic density, occur together. In this paper we present a survey on transport phenomena occurring in solids during irradiation, stressing possible common features as well as peculiarities arising solely from the nature of the irradiated solid: metal, semiconductor or insulator. In particular, we comment on the different role of electronic excitations in dielectric solids as opposed to metallic ones. Basic phenomena occurring during fast charged particle irradiation of solids are described in the framework of transport models. Particular emphasis is placed upon recent theoretical and experimental developments concerning correlation processes during atomic migration in implanted solids.

Transport processes in solids during ion implantation

There are many important effects of fast charged particle irradiation on solid state rate processes in solids: (1) sputtering; (2) enhanced diffusion; (3) break-up of clusters of precipitated atoms; (4) electric field formation promoted by electronic excitation. In most irradiation studies of solids, different combinations of these effects, depending on the target electronic density, occur together. In this paper we present a survey on transport phenomena occurring in solids during irradiation, stressing possible common features as well as peculiarities arising solely from the nature of the irradiated solid: metal, semiconductor or insulator. In particular, we comment on the different role of electronic excitations in dielectric solids as opposed to metallic ones. Basic phenomena occurring during fast charged particle irradiation of solids are described in the framework of transport models. Particular emphasis is placed upon recent theoretical and experimental developments concerning correlation processes during atomic migration in implanted solids.