Universität Stuttgart
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Item Open Access Introduction to air-water flows(1991) Kobus, HelmutFor many hydraulic structures, safe operation can only be achieved if not only the characteristics of the water flow are considered, but due attention is also given to the simultaneous movement of air in the system. Although the difference in specific weight of air and water is so large that they are usually well separated by a sharp interface, a number of flow configurations lead to an intensive mixing across this surface. This process is called air entrainment. Consideration of the effects of entrained air upon water flow may be essential to provide for the safe operation of a hydraulic structure.Item Open Access Application of a non-parametric classification scheme to catchment hydrology(2008) He, Yi; Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing. habil.)Classification has been considered a fundamental step towards improved catchment hydrology science. Catchments classification has been traditionally carried out via Linnaeus-type cluster analysis, mainly represented by hierarchical approaches and methods based on partitioning of hydrological data set. This paper proposes a new scheme where the classification procedure is based on similarity interpreted as distances between catchments. The similarity or distance is defined under the following premises: 1. similar catchments behave similarly; 2. similarity can be described with catchments' characteristics; and 3. hydrological models are able to capture catchments' similarity. If many sets of model parameters lead to similar model performance for two catchments, they are considered as similar catchments. To implement the proposed scheme, two procedures, namely multidimensional scaling (MDS) and local variance reduction (LVR), are undertaken to construct a configuration of n catchments' characteristics in Euclidean space using information about similar performance between the catchments. The MDS is used to determine the appropriate dimension of the Euclidean space and the LVR is used to obtain the transformation matrix and the coordinates in the transformed Euclidean space. This scheme avoids the idea of parametric regression-based regionalization approaches where a regression function is pre-defined between model parameters and catchment descriptors. In the aforementioned approach, the function that is selected is usually subjective and arbitrary and one can also argue that a priori function is neither able to represent the highly complex hydrological processes nor consider the interdependences amongst model parameters. The proposed scheme is initially tested with a research version of the HBV-IWS model on a number of catchments within the Rhine Basin. Additionally a modified Xinanjiang model is applied to the same catchments to check if the assumption of invariant catchment similarity holds true. Invariant catchment similarity here assumes the catchments genuinely carry their similarities independent of the model used for simulation. This test is also a backstop measure to determine if the models under consideration are capturing the underlying simplified hydrological processes in a rational manner. The scheme will be extended to regional calibration of rainfall runoff models as well as regional drought or flood studies once similarity within catchments has been established. The proposed scheme will eventually contribute to the PUB (Predictions in Ungauged Basins) initiative.Item Open Access An example of a combined discharge-control and aeration structure(1983) Kobus, Helmut; Westrich, BernhardThe design of a hydraulic structure in a cooling water circuit is described which serves the dual purpose of controlling the water levels in the system upstream and of providing a maximum oxygen uptake without discharging large amounts of air into the subsequent pressure duct in order to avoid blow-out problems.Item Open Access Parallele Numerische Simulation instationärer Probleme mit adaptiven Methoden auf unstrukturierten Gittern(2001) Lang, Stefan; Helmig, Rainer (Prof. Dr. Ing.)Ziel dieser Arbeit ist die Numerische Simulation von partiellen Differentialgleichungen mit den aktuellen Methoden: Mehrgitterverfahren, lokale Gitteradaption und Parallelität. Besonders Aspekte des Softwareengineerings und der Implementierung werden im Detail betrachtet. Diese Methoden werden durch paralleles I/0 und parallele Graphik zu einer skalierbaren Toolkette erweitert. Anhand von drei Gleichungstypen: Dichtegetriebene Grundwasserströmung, Zweiphasen-Strömung und Elastoplastizität wird die Leistungsfähigkeit der realisierten Simulationsplattform untersucht. Beispielsweise konnte bei einer Simulation auf 512 Prozessoren eine Beschleungiung von mehr als 300 erzielt werden.Item Open Access Numerical investigations of the role of hysteresis in heterogeneous two-phase flow systems(2008) Papafotiou, Alexandros; Helmig, Rainer (Prof. Dr.-Ing.)Various groundwater applications often involve the flow of two immiscible fluids in heterogeneous porous media. In problems that involve the assessment of travel times of hazardous substances in the unsaturated zone or monitoring and predicting the fate of groundwater contaminations, efficient tools and approaches need to be developed to achieve accurate predictions of two-phase flow behavior in heterogeneous porous media. However, this is not an easy task, as heterogeneities -observed on different spatial scales- have a strong influence on the distribution of the fluid phases and therefore form a significant source of uncertainty. Moreover, the prediction of two-phase flow in heterogeneous porous media becomes complicated through alternating drainage and imbibition conditions taking place in the complex heterogeneous pore structure that lead to hysteresis effects in the capillary pressure-saturation relationship. Numerical simulations are widely used to predict hysteretic two-phase flow in heterogeneous porous media in lab or field applications. This approach, however, demands good knowledge on the hydraulic properties of the materials that form the heterogeneous structures involved in the application. Traditionally, the hydraulic properties and the hysteretic behavior of porous media are empirically determined on the local scale with lab experiments conducted on material samples. On the other hand, alternative methods suggest the direct determination of hydraulic properties, including hysteretic capillary pressure-saturation relationships, from a pore-scale consideration. This is done using available information on the pore structure of a material. Nevertheless, it remains unclear how accurate predictions can be in problems of hysteretic two-phase flow in porous media, even when advanced state-of-the-art methods are used on different scales for the determination of the hydraulic properties. The first part of this thesis deals with the implementation of two hysteresis concepts in a numerical model for the simulation of two-phase flow in heterogeneous porous media. Special attention is given on the combination of the hysteresis concepts with a capillary interface condition for the treatment of material interfaces and the approximation of saturation discontinuities due to heterogeneities. This provides an efficient and consistent approach for the prediction of hysteretic two-phase flow in heterogeneous porous media. In the second part, predictions made with the numerical implementations of the hysteresis concepts are compared to measurements from a 1-D monitored transient experiment, that involves successive alternating drainage and imbibition conditions. Conclusions related to the importance of hysteresis and the possibilities of the applied hysteresis concepts are drawn. Furthermore, the comparative study presents remarks on the beneficial combination of different approaches -from the modeling and the experimental viewpoint- that lead to reliable predictions on hysteretic two-phase flow. The last part of this work focuses on predictions of hysteretic two-phase flow made with hydraulic properties determined on different spatial scales. In this case, numerical simulations of drainage and imbibition are compared to experimental measurements in a 3-D heterogeneous structure. The hydraulic properties that are used as input for the numerical simulations are determined with two approaches: -On the local scale with multistep outflow/inflow experiments. -On the pore scale with advanced image analysis and lattice Boltzmann flow simulations in mapped sand geometries. The comparative study in this case reveals the possibilities for predictions of hysteretic two-phase flow made with hydraulic properties determined on different scales (local and pore scale), indicates sensitivities in such hydraulic properties, reveals the significant influence of material interfaces in heterogeneous structures and finally detects the apparent temporal- and spatial-scale dependency of non-wetting phase trapping effects during imbibition processes. Conclusions related to the observed hysteresis are drawn, considering the assumptions and the conceptual differences involved in the different approaches. Finally the comparison between simulations and experiment triggers a discussion on the potentials of our modeling approaches in the case of heterogeneous structures, shows how one needs to approach applications of hysteretic two-phase flow in heterogeneous porous media and what aspects must be taken into account when dealing with different scales.Item Open Access Numerische Modellierung des großräumigen Wärme- und Schadstofftransports im Grundwasser : Forschungsprogramm und Bedeutung für die Praxis(1992) Kobus, HelmutDie Planung wasserwirtschaftlicher Maßnahmen zum Grundwasserschutz setzt Kenntnisse über Ausbreitungs- und Transportvorgänge im Untergrund voraus. Eine interdisziplinäre Forschergruppe hat sich deshalb das Ziel gesetzt, Berechnungsgrundlagen für den Transport von Wärme und Inhaltsstoffen in Grundwasserleitern zu erarbeiten und diese an Fallbeispielen für die Anwendungsbereiche Wärme, leichtflüchtige chlorierte Kohlenwasserstoffe und Nitrat zu erproben. Die Forschungsarbeiten wurden in sechs Teilprojekten an den Universitäten Stuttgart, Hohenheim und Karlsruhe durchgeführt und durch drei stoffbezogene Fallstudien ergänzt. Primäres Ziel war die Entwicklung und Erprobung anwendungsorientierter numerischer Modelle für Strömungs- und Transportvorgänge im natürlichen Untergrund, welche mit der Art der verfügbaren Naturdaten kompatibel sind. Die Modellarbeiten wurden durch entsprechende Entwicklungen der Meß- und Erkundungstechnik ergänzt. Die Erkenntnisse aus den Forschungsarbeiten liefern Aussagen zur Einsatzfähigkeit von Transportmodellen im Grundwasserschutz und beleuchten deren Rolle als Entscheidungshilfe in der wasserwirtschaftlichen Planung, bei der Beurteilung von Grundwasserverunreinigungen und bei der Erstellung von Sanierungs- und Bewirtschaftungskonzepten.Item Open Access Quantitative evaluation of clogging phenomena in river beds(1982) Geldner, Peter; Kobus, HelmutClogging phenomena in rivers are of extremely complex nature, and a development of a clogging layer depends on a large variety of influences. In this paper a quantitative approach for the evaluation of a defined hydrodynamic clogging parameter is presented. This approach is based on the inverse use of type curves representing characteristics of the interaction between surface water bodies and the aquifer. The method is applied to field data of discharge measured in seepage ditches at levee protected sites. Since conventional approaches are limited to the assumption of quasi-steady-state flow, a stochastic method for the evaluation of time-variable hydrographs is also outlined.Item Open Access Simulation von Mehrphasenvorgängen in porösen Medien unter Einbeziehung von Hysterese-Effekten(1999) Sheta, Hussam; Helmig, Rainer (Prof. Dr.)Die vorliegende Arbeit entstand im Rahmen des Forschungsprogramms der Versuchseinrichtung zur Grundwasser- und Altlastensanierung VEGAS. Sie beschäftigt sich mit einem zentralen Thema zur Beschreibung von Transport und Schadstoffausbreitung im natürtlichen Untergrund.Item Open Access Numerical simulation of CO2 sequestration in geological formations(2007) Bielinski, Andreas; Helmig, Rainer (Prof. Dr.-Ing.)CO2 sequestration in geological formations is regarded as a measure for reducing the emissions of this greenhouse gas into the atmosphere. With this method, carbon dioxide is captured at large point sources, e.g. coal-fired power plants, and injected into subsurface reservoirs for long-term storage. Such formations can be exploited gas or oil reservoirs or saline aquifers. The latter have the advantage that they are abundant in many places and provide large storage volumes. In this study, the topic of CO2 sequestration in saline aquifers is addressed using numerical methods. A model concept is developed, taking into account a CO2-rich phase, a brine phase and the rock matrix as a rigid porous medium. The focus is on the correct description of the fluid properties including super- and subcritical properties of the CO2-rich phase. Mutual dissolution of the components in the phases and the effect of the salt content in the aqueous phase on dissolution behavior and fluid properties are considered. Non-isothermal effects are taken into account. Advective and diffusive flow processes are included into the model concept. Multi-phase behavior is considered using relative permeability and capillary pressure functions. The set-up of the mathematical model and its numerical implementation in the numerical simulator MUFTE-UG are described in detail. Various simulations show the capability of the developed model and give insight into principle processes that occur during CO2 sequestration in geological formations. The short-term carbon dioxide plume propagation during the injection process is investigated. The long-term behavior is studied in another example considering the dissolution of a CO2 plume in the subsurface over a period of 100 years. Furthermore, non-isothermal processes are investigated in a simulation example. This work provides a tool which is capable of reproducing and predicting many of the processes that occur during CO2 sequestration in geological formations. It can be used for principle studies as well as for the field cases that this technology is going to be applied to. It provides the foundation for further extensive research into many open questions to be addressed in the future.Item Open Access Hydrologische und hydraulische Entwurfsgrundlagen und Bemessungsrichtlinien : Vorwort zum Sonderheft(1988) Kobus, HelmutAm 21. Juli 1987 fand an der Universität Stuttgart eine Vortragsveranstaltung statt, die sich mit den hydrologischen und hydraulischen Aspekten der Sicherheit von Stauanlagen befaßte. Das Sonderheft der WASSERWIRTSCHAFT enthält die Beiträge dieser Veranstaltung, welche vom Ministerium für Umwelt Baden-Württemberg gemeinsam mit dem Institut für Wasserbau der Universität Stuttgart ausgerichtet und vom Lehrstuhl für Technische Hydromechanik und Wasserbauliches Versuchswesen organisatorisch betreut wurde.