Der Einfluss von Kolloiden auf Wasserfluss und Stofftransport in Böden: Randaspekt oder Schlüsselprozess
Final Report Abstract
In the present project the influence of soil colloids on solute transport and fluid dynamics were studied by using the tool of systematic predictive simulation. Specifically, this includes the coupling to the fluid motion through the induced alteration of the pore space or the change of surface properties of the solid phase and the thus resulting coupling in the transport of solutes. The deterministic description of the considered physicochemical mechanisms on basis of the conservation laws for mass, momentum and energy results in coupled systems of time-dependent non-linear partial differential equations. The developed numerical models divide into two categories: Purely macroscopic ones and those which take both the macroscopic and the pore scale into account, the so-called two-scale models. Moreover, there are model components which can be treated scale-independent. A model describing the dissolution and precipitation of the immobile solid phase on the macroscopic scale by means of mass-conserving surface reactions was developed. This results in an increase or decrease of the porosity which in turn influences the fluid velocity. The rate of these reactions highly depend on the amount of present ions which was incorporated in the model by introducing effective ion activity coefficients. The resolution of different pore geometries or the consideration of a surface potential requires the pore scale to be explicitly taken into account. In this sense, two-scale models were developed to examine, e. g., the effect of electrostatically induced acceleration or retardation of the transport of charged colloids or the impact of pore geometry transformations on the fluid motion. In order to make the model operative with respect to the problem formulation, one has to approximate it by numerical methods and to implement those in appropriate software tools. In this sense, an existing simulation tool Richy1D which provides model components for fluid flow and colloid dynamics was extended by the newly developed model components. Further, HyPHM, a new software toolbox capable to deal with two dimensional two-scale models was built using accurate state-of-the-art numerical algorithms. The implemented numerical solvers were verified by the application to various test scenarios. The used high programming language Matlab together with broad code and user documentations provide the third party usability for the future. Numerical simulations were performed to examine the impact of the newly considered dependencies. In this manner, the influence of the ionic strength of a fluid on reaction rates was examined. Here it was shown, e. g., that the presence of ions may displace the chemical equilibria of liquid-phase reactions as well as surface reactions. The latter also encompass reactions describing the precipitation or dissolution of the immobile solid phase—a component which was also implemented. Numerically simulated column experiments demonstrate the hereby induced impact of porosity changes on the fluid velocity. Two-scale simulations enabled a more detailed description of the pore structure and the consequence of different pore geometries on the macroscopic water movement via upscaled soil conductivity tensors. A two-scale model which reflects the effect of electrostatically induced acceleration or retardation of the transport of charged colloids was implemented and numerical studies are in preparation.
Publications
- (2010): Numerical Simulations of a Two-Scale Model for Fluid Flow and Colloidal Transport in Porous Media. International Workshop on Multiscale Modeling, Simulation and Optimization, October 10-13, 2010, Fraunhofer IISB, Erlangen
F. Frank, N. Ray, K. U. Totsche, P. Knabner
- (2011): Variable Choices of Scaling in the Homogenization of a Nernst-Planck-Poisson Problem. Preprint No. 344 of Department of Mathematics, Friedrich-Alexander-University of Erlangen-Nuremberg
N. Ray, A. Muntean, Ch. Eck, P. Knabner
