Conservative Transport Model Research Articles

Conservative transport model for surfactant on the interface based on the phase-field method

We propose a new surfactant transport model on the interface based on the phase-field method. The new model has two features: (a) preventing surfactants on the interface from leaking out of the interface due to numerical diffusion even when the diffusion coefficient is extremely low, and (b) satisfying the conservation of surfactant mass. The concentration profile on the interface is matched to the surface delta function to prevent numerical diffusion, and the finite volume method is applied to the resulting model to satisfy the conservation of surfactant mass. The proposed model is incorporated in a weakly compressible scheme and applied in several numerical tests related to incompressible two-phase flows with surfactants. Numerical results show that the proposed model can be applied to various incompressible two-phase flow simulations with surfactant.

A three‐dimensional positivity‐preserving and conservative multimoment finite‐volume transport model on a cubed‐sphere grid

AbstractA three‐dimensional positivity‐preserving and conservative transport model on a cubed‐sphere grid is presented using a multimoment finite‐volume (MMFV) method. The three‐point fourth‐order MMFV method with boundary gradient switching projection is utilized to ensure nonoscillatory numerical solutions in the horizontal, while the simple and computationally efficient semi‐Lagrangian piecewise rational method is used to allow a large time step in the vertical. By means of horizontal and vertical separation, a Strang‐type splitting technique is adopted to advance the numerical solutions in time. Non‐negative corrections are made to achieve positive‐definite preservation exactly. The resulting transport model is inherently conservative, high‐accuracy, and positive‐definite, and allows a large Courant number no matter how dense the vertical grid is. Various benchmark test cases, including three representative advection tests and an idealized terminator chemistry test, are conducted to validate the performance of the model presented. The numerical results indicate that the model is quite competitive in comparison with the existing models in the literature and looks very promising in real atmospheric simulations.

Modelling of Matrix Diffusion in a Tracer Test in Concrete

A laboratory-scale tracer test has been carried out to improve the characterization of the transport properties of the concrete from the radioactive waste disposal facility at El Cabril (Spain). High entry pressure was employed in order to perform the experiment in a reasonable time span. Lithium, bromide and deuterium were used as tracers. The conceptual model considered matrix diffusion between a mobile pore domain, where water can flow, and an immobile zone without any advective transport. Three geometries have been compared, considering the immobile zone as slabs, spheres or tubes. Porosity of the mobile zone and characteristic time was estimated by calibrating the model results to the measured breakthrough curves of deuterium and bromide. The calculated values showed that the characteristic time depends on the geometry, and similar porosity of the mobile zone was estimated for all geometries. The double-porosity conservative transport model could reproduce the deuterium breakthrough curve. However, the bromide behaviour could not be reproduced even when linear retardation was applied.

Combined application of conservative transport modelling and compound-specific carbon isotope analyses to assess in situ attenuation of benzene, toluene, and o-xylene

In recent years, compound specific isotope analyses (CSIA) have developed into one of the most powerful tools for the quantification of in situ biodegradation of organic contaminants. In this approach, the calculation of the extent of biodegradation of organic contaminants in aquifers is usually based on the Rayleigh equation, and thus neglects physical transport processes such as dispersion that contribute to contaminant dilution in aquifers. Here we combine compound specific isotope analyses with a conservative transport model to study the attenuation of aromatic hydrocarbons at a former gasworks site. The conservative transport model was first used to simulate concentration reductions caused by dilution at wells downgradient of a BTEX source. In a second step, the diluted concentrations, together with the available stable carbon isotope ratios and carbon fractionation factors for benzene, toluene and o-xylene were applied in the Rayleigh equation to quantify the degree of biodegradation at each of those wells. At the investigated site, where other attenuation processes such as sorption and volatilisation were proven to be negligible, the combined approach is recommended for benzene, which represents a compound for which the effect of biodegradation is comparable to or less than the effect of dilution. As demonstrated for toluene and o-xylene, the application of the Rayleigh equation alone is sufficient if dilution can be proved to be insignificant in comparison to biodegradation. The analysis also suggests that the source width and the position of the observation wells relative to the plume center line are significantly related to the degree of dilution.

Noninvasive Quantitative Measurement of Colloid Transport in Mesoscale Porous Media Using Time Lapse Fluorescence Imaging

We demonstrate noninvasive quantitative imaging of colloid and solute transport at millimeter to decimeter (meso-) scale. Ultraviolet (UV) excited fluorescent solute and colloid tracers were independently measured simultaneously during co-advection through saturated quartz sand. Pulse-input experiments were conducted at constant flow rates and ionic strengths 10(-3), 10(-2) and 10(-1) M NaCl. Tracers were 1.9 microm carboxylate latex microspheres and disodium fluorescein. Spatial moments analysis was used to quantify relative changes in mass distribution of the colloid and solute tracers over time. The solute advected through the sand at a constant velocity proportional to flow rate and was described well by a conservative transport model (CXTFIT). In unfavorable deposition conditions increasing ionic strength produced significant reduction in colloid center of mass transport velocity over time. Velocity trends correlated with the increasing fraction of colloid mass retained along the flowpath. Attachment efficiencies (defined by colloid filtration theory) calculated from nondestructive retained mass data were 0.013 +/- 0.03, 0.09 +/- 0.02, and 0.22 +/- 0.05 at 10(-3), 10(-2), and 10(-1) M ionic strength, respectively, which compared well with previously published data from breakthrough curves and destructive sampling. Mesoscale imaging of colloid mass dynamics can quantify key deposition and transport parameters based on noninvasive, nondestructive, spatially high-resolution data.

Backfill Performance Requirements—Estimates from Transport Models

Since backfill is just one part of a waste package that might contain five or more components, its performance has been considered as part of the performance of the whole package. A simple method has been developed for estimating the required behavior of this complete system. The method is based on: federal regulations concerning permissible concentrations in solution at the point of discharge to the accessible environment; a simple and conservative transport model; and baseline and potential ''worst-case'' release scenarios. Use of the transport model enables calculation of maximum permissible release rates within a repository for each of the scenarios. The maximum permissible release rates correspond to performance requirements for the engineered barrier system. It has been determined from the model that the most important nuclides in spent fuel located in a repository in basalt at Hanford are: /sup 129/I, /sup 79/Se, /sup 99/Tc, /sup 107/Pd, /sup 237/Np, /sup 239/Pu, /sup 240/Pu, /sup 242/Pu /sup 241/Am, /sup 243/Am, and members of the /sup 238/U decay chain. The maximum permissible release rates for the waste package have been translated into concentrations at the inner and outer surfaces of the backfill under low- and high-flow conditions. It appears that solubility constraints shouldmore » control the concentrations of uranium and possibly of plutonium, americium, neptunium, and selenium below the required levels.« less