02 Fakultät Bau- und Umweltingenieurwissenschaften
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/3
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Item Open Access On the computational modeling of micromechanical phenomena in solid materials(2013) Linder, Christian; Miehe, Christian (Prof. Dr.-Ing. habil.)This work aims to contribute to the research on the constitutive modeling of solid materials, by investigating three particular micromechanical phenomena on three different length scales. The first microscopic phenomenon to be considered on the macroscopic scale is the process of failure in solid materials. Its characteristic non-smoothness in the displacement field results in the need for sophisticated numerical techniques in case one aims to capture those failure zones in a discrete way. One of the few finite element based methods successfully applied to such challenging problems is the so called strong discontinuity approach, for which failure can be described within the individual finite elements. To avoid stress locking, a higher order approximation of the resulting strong discontinuities is developed in the first part of this work for both, purely mechanical as well as electromechanical coupled materials. A sophisticated crack propagation concept relying on a combination of the widely used global tracking algorithm and the computer graphics based marching cubes algorithm is employed to obtain realistic crack paths in three dimensional simulations. Secondly, materials with an inherent network microstructures such as elastomers, hydrogels, non-woven fabrics or biological tissues are considered. The development of advanced homogenization principles accounting for such microstructures is the main focus in the second part of this work to better understand the mechanical and time-dependent effects displayed by such soft materials. Finally, the incorporation of wave functions into finite element based electronic structure calculations at the microscopic scale aims to account for the fact that the properties of condensed matter as for example electric conductivity, magnetism as well as the mechanical response upon external excitations are determined by the electronic structure of a material.Item Open Access Microbial biostabilization in fine sediments(2022) Gerbersdorf, Sabine Ulrike; Wieprecht, Silke (Prof. Dr.-Ing.)Microbial biostabilization has increasingly received attention over the last years due to its significance for the dynamics of fine sediments in fluvial and coastal systems with implications for ecology, economy and human-health. This habilitation thesis highlights the contributions of the applicant and her team to this multi-disciplinary research area and is based on eight core publications that are presented in seven chapters. First, the topic of biofilm and biostabilization is introduced and second, the materials and methods applied are presented before own research findings are discussed. To start with, the stabilization potential of heterotrophic bacterial assemblages has been emphasised as well as the adhesive properties of the protein moieties within the EPS (extracellular polymeric substances) that are more significant than previously thought. Furthermore, the engineering potential of estuarine prokaryotic and eukaryotic assemblages has been studied separately and combined to reveal the effective cooperation of mixed biofilm that resulted in highest substratum stabilization although the effects were not clearly synergistic (=more than additive). The significance of biostabilization could be evidenced as well for freshwaters where highest adhesive capacity and sediment stability occurred during spring. Microbial community composition differed accordingly to result in mechanically highly diverse biofilm. Moreover, the importance of two of the most influential abiotic conditions, light intensity and hydrodynamics, was shown for biofilm growth, species composition and functionality - here biostabilization. In order to test adhesive properties at the relevant mesoscale (mm-cm) but non-destructively and highly sensitive, MagPI (Magnetic Particle Induction) has been applied. The last chapter concerns technical aspects to further improve its performance while demonstrating the impact of material and geometry and the importance of both, magnetic field strength and field gradient for the physics of the MagPI approach.Item Open Access Rottweil - Untersuchungen zur Stadtbaugeschichte im Hochmittelalter(1970) Meckseper, Cord; Hanson, Harald (Prof.)Item Open Access Modeling the long-term behavior of structural timber for typical serviceclass-II-conditions in South-West Germany(2010) Schänzlin, Jörg; Kuhlmann, Ulrike (Prof. Dr.-Ing.)Creep deformation influences the serviceability limit state as well as the ultimate limit state of timber structures. In order to consider this time-dependent behavior, creep coefficients and rheological models have been developed by various researchers. Comparing the rheological models, quite different temporal deformations are evaluated for a duration of load of 50 ears. In order to find the model, which is most suitable to the situation in the region of Tübingen, South-West Germany, the existing deformations of several beams in roof structures in opened, protected but not heated buildings are measured. By loading the structure the elastic global stiffness of the particular element is determined. So creep coefficients can be evaluated, which should have been used by the engineer in order to get the existing deflection fter 50 years. Within the region of Tübingen, on average a creep coefficient of 2.23 was found based on these measurements. However, the standard deviation of 0.97 is quite large. For the numerical evaluation of the time-dependent behavior Toratti’s model is modified, so that it matches the measured deformations. This modified model is verified by an additional set of measurements in the region of Breisgau-Hochschwarzwald, where the influence of the snow on the creep coefficient has to be taken into account. However, the application of the modified model takes too much time due to the numerical solutions of the single time steps. By means of a case study, functions are fitted to the results of the models in order to develop “simple” functions for the determination of the creep coefficient with respect to the main influences. The creep deformation influences the ultimate limit state especially in composite structures or elements subjected to compression. For this reason, the influence of the increased creep strain is approximated for columns, in order to reach the same safety level as proposed in the current regulations in DIN 1052. Additionally, the design procedure for timber-concrete-composite structures is modified in order to consider the increased creep coefficients.Item Open Access Advanced methods for a sustainable sediment management of reservoirs(Stuttgart : Eigenverlag des Instituts für Wasser- und Umweltsystemmodellierung der Universität Stuttgart, 2022) Haun, Stefan; Wieprecht, Silke (Prof. Dr.-Ing.)As a result of an increasing demand on storing water, sustainable reservoir management will become more and more important in the future. Minimizing, or in the best case avoiding, the loss of storage due to sedimentation is a challenging task because each reservoir has unique boundary conditions. Hence, not every management strategy is suitable for a given reservoir. Due to the combination of state of the art measurement methods and hydro‐morphodynamic models, reservoir sedimentation can be better predicted in the future and the success of sediment management strategies can be assessed. The development of advanced measurement methods makes it possible to obtain data with a high accuracy, but also with a high spatial and temporal resolution. The combination of recent measurement approaches with reliable hydro‐morphodynamic numerical prediction models, enhances a highly accurate prediction and understanding of governing processes. This opens new possibilities for an objective selection of important parameters, essential spatial domains as well as for the temporal resolution of measurements. This will finally lead to more reliable predictions about the future of reservoirs that provide water for human life, health and wealth. The presented scientific work gives an overview of recent developments to investigate hydromorphological processes in reservoirs.Item Open Access Mikropolare Zweiphasenmodelle : Modellierung auf der Basis der Theorie Poröser Medien(2000) Diebels, Stefan; Ehlers, Wolfgang (Prof. Dr.)In der vorliegenden Arbeit wird eine Theorie zur Beschreibung von Mehrphasenmaterialien vorgestellt. Grundlage der Modellierung ist die Theorie Poröser Medien, die um Elemente der mikropolaren Theorie erweitert wird, um den Einfluß der Mikrostruktur erfassen zu können. Dazu werden für Mischungen aus einer beliebigen Anzahl von Konstituierenden die kinematischen Beziehungen einer finiten Theorie und die notwendigen Bilanzgleichungen diskutiert. Besondere Berücksichtigung kommt dabei den Erweiterungen aufgrund der vom Verschiebungsfeld unabhängigen Rotationen zu, die durch die mikropolare Erweiterung Eingang in die Theorie finden. Aus einer allgemeinen Bilanz können in einfacher Weise die Strukturen abgeleitet werden, die die Partialbilanzen der einzelnen Konstituierenden mit den Bilanzen der Mischung als Ganzes verknüpfen. Für ein Zweiphasenmodell bestehend aus einem elastischen porösen Festkörperskelett und einem viskosen Porenfluid wird dann durch die Auswertung der Clausius-Duhem-Ungleichung ein thermodynamisch konsistentes Modell formuliert. Dabei kommt der Aufstellung einer Evolutionsgleichung für die Volumenanteile besondere Bedeutung zu, da diese im Fall kompressiblen Materialverhaltens der Konstituierenden nicht aus kinematischen Überlegungen folgt. Schließlich wird das Modell konkretisiert. Für das Festkörperskelett wird eine Verzerrungsenergiefunktion angegeben, während anhand einer Dimensionsanalyse gezeigt wird, daß die Extraspannungen des Porenfluids gegenüber den Wechselwirkungskräften zwischen Fluid und Festkörper vernachlässigbar sind. Einige numerische Beispiele zeigen die Anwendbarkeit des Modells und demonstrieren den Einfluß der verschiedenen bei der Modellierung berücksichtigten Effekte.Item Open Access Weak or strong : on coupled problems in continuum mechanics(2010) Markert, Bernd; Ehlers, Wolfgang (Prof. Dr.-Ing.)The present work aims at giving a concise introduction to the vast field of coupled problems, particularly to those of importance in engineering and physics. Therefore, the common terminology and an appropriate classification of coupled equation systems is presented accompanied by some mathematical and computational issues. Attention is focused on volumetrically coupled multi-field formulations arising from the continuum mechanical treatment of multi-physics problems, but also geometrically coupled problems are addressed. Based on actual problems in the areas of poroelastodynamics, continuum biomechanics, and fluid-saturated porous media in general both the theoretical modeling by means of coupled continuum equations as well as the efficient numerical solution in the context of the finite element method (FEM) are presented and discussed in a problem-oriented fashion.Item Open Access The role of interfacial areas in two-phase flow in porous media : bridging scales and coupling models(2010) Niessner, Jennifer; Helmig, Rainer (Prof. Dr.-Ing. habil.)This habilitation deals with a thermodynamically consistent modeling of two-phase flow in porous media which is extremely relevant for the understanding, the prediction, and optimization of the processes in many environmental, technical, and biological systems. Among these are the storage of carbon dioxide in the subsurface, methane migration from abandoned coal mines, the migration of radioactive gases from nuclear waste disposal sites (environmental systems), the processes in fuel cells and heat exchangers (technical systems) or the interaction between blood vessels and interstitial space (biological systems) which is very important for cancer therapy. The presented thermodynamically consistent model of two-phase flow in porous media is the first to numerically account for the extremely important role of phase-interfacial areas. This is put into practice through use of a rational thermodynamics approach by Hassanizadeh and Gray [1990] which not only includes interfaces as parameter in the equations, but additionally as entities allowing the formulation of conservation equations for interfaces. To be exact, conservation equations of mass, momentum, energy, and entropy are formulated on the pore scale for phases and interfaces and volume-averaged to the macro scale. The entropy productions of the entropy conservation equations are used to formulate the second law of thermodynamics. A speciality of the approach is the fact that thus, constitutive relationships do not need to be empirically formulated, but can be obtained by exploiting the residual entropy inequality. The aim of this work is to make the thermodynamically consistent and physically-based model accessible to numerical modeling allowing to represent effects which could otherwise not (or only using completely empirical approaches) be described. Among these are capillary hysteresis as well as the kinetics of mass and energy transfer between phases as these transfer processes take place across interfaces and thus, are highly dependent on them. Based on indicators and dimensionless quantities, the integration of the interfacial-area-based model into a multi-scale multi-physics framework is shown. This allows for the solution of the physically-based and thermodynamically consistent model whenever this is necessary and the solution of the empirical, but less costly, classical model wherever and whenever the physical situation allows. With such an approach, computing times and the amount of data needed can be drastically reduced.Item Open Access Scale dependence of flow and transport parameters in porous media(2006) Neuweiler, Insa; Helmig, Rainer (Prof. Dr.)The study discusses the problem of the influence of spatial scales on modeling of flow and transport processes in porous media. As soil and rock formations are usually heterogeneous, this problem is strongly coupled to the question as to how information about structure is transported over length scales. On different length scales different driving forces determine flow and transport processes. In order to derive an appropriate model for a certain scale of interest it is crucial to include information about the impact of structure on smaller scales into the model. The transfer from a detailed heterogeneous model to an equivalent model on a larger scale, where averaged properties are described, is called upscaling. To derive an upscaled model the following questions have to be addressed: • What are the relevant time scales and length scales? • What are the relevant processes on the scale, where heterogeneities are resolved? • How does the upscaled model look like? • What are the effective parameters for the upscaled model? As detailed information about the parameter distribution in the heterogeneous model is in most cases not known or it is not possible to deal with the exact distribution, further questions have to be addressed: • How can the effective parameters be approximated based on incomplete knowledge about the parameter distribution? • What are the relevant quantities to characterize the heterogeneous distribution and how can they be taken into account? Upscaling for different two-phase flow problems in porous media is discussed in the study.Item Open Access Tackling coupled problems in porous media : development of numerical models and an open source simulator(2013) Flemisch, Bernd; Helmig, Rainer (Prof. Dr.-Ing.)Flow and transport processes in porous media are the governing processes in a large variety of geological, technical and biological systems. For many interesting and important applications, these processes cannot be treated in an isolated manner or adequately described by means of a single-scale, single-physics mathematical model, and the coupling of two or more models is required. The development of coupled numerical models poses severe challenges on the conceptual, analytical and computational level. This habilitation thesis aims to describe a number of these challenges and solve some of the problems they pose. It is divided into three parts: Part A "Model Coupling" deals with uncoupled and coupled porous-media models in general and describes some of these models in detail. "Locally Conservative Discretization Methods," as treated in Part B, are a fundamental ingredient of reasonable numerical models for porous media flow and transport processes. A numerical model is realized by its implementation in the form of computer code. Part C "Open-Source Porous-Media Simulation" deals with the idea of developing such a computer code by means of open-source development techniques.