Marc Walther

OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media

In this paper we describe the OpenGeoSys (OGS) project, which is a scientific open-source initiative for numerical simulation of thermo-hydro-mechanical-chemical processes in porous media. The basic concept is to provide a flexible numerical framework (using primarily the Finite Element Method (FEM)) for solving multifield problems in porous and fractured media for applications in geoscience and hydrology. To this purpose OGS is based on an object-oriented FEM concept including a broad spectrum of interfaces for pre- and postprocessing. The OGS idea has been in development since the mid-eighties. We provide a short historical note about the continuous process of concept and software development having evolved through Fortran, C, and C++ implementations. The idea behind OGS is to provide an open platform to the community, outfitted with professional software-engineering tools such as platform-independent compiling and automated benchmarking. A comprehensive benchmarking book has been prepared for publication. Benchmarking has been proven to be a valuable tool for cooperation between different developer teams, for example, for code comparison and validation purposes (DEVOVALEX and CO2 BENCH projects). On one hand, object-orientation (OO) provides a suitable framework for distributed code development; however, the parallelization of OO codes still lacks efficiency. High-performance-computing efficiency of OO codes is subject to future research.

The IWAS-ToolBox: Software coupling for an integrated water resources management

Numerical modeling of interacting flow and transport processes between different hydrological compartments, such as the atmosphere/land surface/vegetation/soil/groundwater systems, is essential for understanding the comprehensive processes, especially if quantity and quality of water resources are in acute danger, like e.g. in semi-arid areas and regions with environmental contaminations. The computational models used for system and scenario analysis in the framework of an integrated water resources management are rapidly developing instruments. In particular, advances in computational mathematics have revolutionized the variety and the nature of the problems that can be addressed by environmental scientists and engineers. It is certainly true that for each hydro-compartment, there exists many excellent simulation codes, but traditionally their development has been isolated within the different disciplines. A new generation of coupled tools based on the profound scientific background is needed for integrated modeling of hydrosystems. The objective of the IWAS-ToolBox is to develop innovative methods to combine and extend existing modeling software to address coupled processes in the hydrosphere, especially for the analysis of hydrological systems in sensitive regions. This involves, e.g. the provision of models for the prediction of water availability, water quality and/or the ecological situation under changing natural and socio-economic boundary conditions such as climate change, land use or population growth in the future.

Saltwater intrusion modeling: Verification and application to an agricultural coastal arid region in Oman

This paper deals with numerical modeling of density-dependent flow of saltwater intrusion in coastal groundwater systems. We present the implementation of an approach to solve a moving boundary problem for a dynamic water table within an invariant finite element mesh. The model is successfully validated against laboratory experiment data for an unconfined, density-dependent benchmark. The validated software is applied to a regional-scale study area and sufficiently calibrated for a steady state of pre-development conditions. Transient mass transport scenario simulations show good concordance with salinity measurements satisfyingly supporting the model setup.

TESSIN VISLab—laboratory for scientific visualization

Scientific visualization is an integral part of the modeling workflow, enabling researchers to understand complex or large data sets and simulation results. A high-resolution stereoscopic virtual reality (VR) environment further enhances the possibilities of visualization. Such an environment also allows collaboration in work groups including people of different backgrounds and to present results of research projects to stakeholders or the public. The requirements for the computing equipment driving the VR environment demand specialized software applications which can be run in a parallel fashion on a set of interconnected machines. Another challenge is to devise a useful data workflow from source data sets onto the display system. Therefore, we develop software applications like the OpenGeoSys Data Explorer, custom data conversion tools for established visualization packages such as ParaView and Visualization Toolkit as well as presentation and interaction techniques for 3D applications like Unity. We demonstrate our workflow by presenting visualization results for case studies from a broad range of applications. An outlook on how visualization techniques can be deeply integrated into the simulation process is given and future technical improvements such as a simplified hardware setup are outlined.

Assessing the saltwater remediation potential of a three-dimensional, heterogeneous, coastal aquifer system

This paper evaluates the remediation potential of a salinized coastal aquifer by utilizing a scenario simulation. Therefore, the numerical model OpenGeoSys is first validated against analytical and experimental data to represent transient groundwater level development and variable density saline intrusion. Afterwards, a regional scale model with a three-dimensional, heterogeneous hydrogeology is calibrated for a transient state and used to simulate a best-case scenario. Water balances are evaluated in both the transient calibration and scenario run. Visualization techniques help to assess the complex model output providing valuable insight in the occurring density-driven flow processes. Furthermore, modeling and visualization results give information on the time scale for remediation activities and, due to limitations in data quality and quantity reveal potential for model improvement.

Comprehensive assessment of eutrophication status based on Monte Carlo–triangular fuzzy numbers model: site study of Dongting Lake, Mid-South China

For the assessment of lake’s eutrophication status, a great deal of data uncertainties exists under the circumstances that monitoring data often are scarce and inaccuracy or the variation intervals are wide. In order to process these uncertainties of data and provide more valuable information for the decision makers, a methodology for assessing the eutrophication status was established by coupling Monte Carlo and triangular fuzzy numbers approaches and further combining it with the trophic level index method. This developed methodology was illustrated by a case study of evaluating the eutrophication status of Dongting Lake in Mid-South China (Hunan Province). The results indicated that the quantitative information of possible intervals of trophic level index, their corresponding probabilities and the comprehensive eutrophication statuses can be obtained. The eutrophication status of the East Dongting Lake was more serious than the southern and western parts. Portions of both East and South Dongting Lake showed a greater probability to light-eutrophic status, but with a worsening tendency, i.e., becoming mid-eutrophic in the 2010 year. By processing the data fuzzily and simulating their distribution characteristics stochastically, the presented methodology can be employed to process the uncertainties of the data evaluation and obtain a better early detection/warning of eutrophication levels with less requirement of time. Therefore, more reliable/valuable information can be provided to the decision makers, e.g., lake management authorities.

Description and verification of a novel flow and transport model for silicate-gel emplacement

We present a novel approach for the numerical simulation of the gelation of silicate solutions under density-dependent flow conditions. The method utilizes an auxiliary, not density-dependent solute that is subject to a linear decay function to provide temporal information that is used to describe the viscosity change of the fluid. By comparing the modeling results to experimental data, we are able to simulate the behavior and the gelation process of the injected solute for three different compositions, including long-term stability of the gelated area, and non-gelation of low concentrations due to hydro-dynamic dispersion. This approach can also be used for other types of solutes with this gelling property and is useful in a variety of applications in geological, civil and environmental engineering.

HT (Convection) Processes

In a geothermal system, unstable fluid density profiles due to temperature variations can trigger the onset and development of free thermal convective processes (J.W. Elder. Transient convection in a porous medium. (Elder in J Fluid Mech 27: 609–623 , 1967, Elder 1967). Early studies on the problem showed that the development of free thermal convection in the Earth’s crust require a relatively high permeability of the porous rocks (Lapwood in Math Proc Camb Philos Soc, 44:508–52, 1948, Lapwood 1948). Since the permeability inside the damaged area of major fault zones can far exceed the permeability of the enclosing rocks (Wallace, Morris in PAGEOPH, 124:107–125, 1986, Wallace and Morris 1986), one can expect the development of free thermal convective instabilities to occur in such tectonically perturbed rocks. The onset of thermal convection of a single-phase fluid in a vertical fault enclosed in impermeable rocks was considered in a full 3D approximation by Wang, Kassoy, Weidman (Int J Heat Mass Trans, 30:1331–1341, 1987, Wang et al. (1987)). A fundamental result of those investigations was that highly permeable faults allow for onset of free thermal convection even under a normal (e.g. 30 \(^{\circ }\mathrm{C}\cdot \mathrm{km}^{-1}\)) geothermal gradient. In contrast to simple homogenous 1D and 2D systems, no appropriate analytical solutions can be derived to test numerical models for more complex 3D systems that account for variable fluid density and viscosity as well as permeability heterogeneity (e.g. presence of faults). Owing to the efficacy of thermal convection for the transport of thermal energy and dissolved minerals in the moving fluid, a benchmark case study for density/viscosity driven flow is crucial to ensure that the applied numerical model accurately simulates the physical processes.

Stationary Groundwater Model

The mesh creation process via the OGS Data Explorer requires the selection of geometric data sets and the parametrization of those elements by the graphical user interface of the OGS Data Explorer.

Reactive Nitrate Transport Model

One of the main applications of numerical groundwater models is to predict future field behavior.