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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Electronic correlation and magnetism in multi-band Kondo lattice models

Sharma, Anand 09 July 2009 (has links)
Die vorliegende Arbeit untersucht elektronische Korrelationseffekte in Realsubstanzen wie Europium-Sulphid (EuS) und Gadolinium-Nitrid (GdN). Es wird dazu eine Kombination von vielteilchentheoretischen Analysen der Spin-Austauschwechselwirkung zwischen itineranten Bandelektronen und lokalisierten 4f-Momenten, durchgefuehrt im Rahmen eines Mehr-Band-Kondo-Gitter-Modells (KLM), mit first-principles (T=0) Bandstrukturrechnungen vorgeschlagen. Die Ein-Teilchen-Energien (hopping-Integrale), die als Energie-Matrix in den Mehr-Band-Hamilton-Operator eingehen, werden einer TB-LMTO-ASA entnommen. Die interessierenden physikalischen Eigenschaften wie die Quasiteilchen-Spektraldichte und die Quasiteilchen-Zustandsdichte werden mit der Bewegu-ngsgleichungs-Methode Greencher Funktionen berechnet. Dazu wird fuer die gesuchte Mehr-Band-Selbstenergie der itinerantenLadungstraeger als Verallgemeinerung des sogenannten Interpolating Selfenergy Approach (ISA) ein Ansatz vorgeschlagen. Es stellt sich heraus, dassdas elektronische Anregungsspektrum durch die Austausch-Kopplung an das lokalisierte Momenten-System eine spektakulaere Temperaturabhaengigkeit aufweist, in Uebereinstimmung mit vorliegenden experimentellen Beobachtungen. Stark temperaturbestimmte Korrelationseffekte werden registriert, z.B. eine mit fallender Temperatur in der ferromagnetischen Phase auftretende Rotverschiebung der unteren Leitungsbandkante in guter Uebereinstimmung mit experimentellen Daten. Um die reinen f-Spin-Korrelationen zu beschreiben, wird eine modifizierte RKKY-Theorie fuer Mehr-Band-Systeme entwickelt, wobei durch Ausmitteln der elektronischen Freiheitsgrade das Mehr-Band KLM auf ein effektives Heisenberg-Modell abgebildet wird. Mit einer RPA-Theorie wird das effektive Heisenberg-Modell auf Aussagen zu zentralen magnetischen Eigenschaften wie Curie-Temperatur und Magnetisierungskurve analysiert. Durch gezielte Variation der Systemparameter wird die Brauchbarkeit des Modells getestet. / This dissertation deals with a combination of many-body evaluation of a spin exchange interaction between the itinerant electrons and localized 4f moments on a periodic lattice, i.e. within the so-called multi-band Kondo lattice model (KLM), and the T=0 first principles calculations in order to study the electronic correlation effects in real materials like Europium Sulphide (EuS) and Gadolinium Nitride (GdN).The single-particle ground state energy or hopping integral acting as an input in the many-body part is obtained using tight binding linear muffin-tin orbital within atomic sphere approximation (TB-LMTO-ASA) program and is a matrix in general. The physical properties of interest like the quasi-particle spectral density and quasi-particle density of states are calculated within the Green function theory and the equation of motion method. In order to do so the required multi-band self-energy of the band electrons istaken as an ansatz, i.e. the so-called interpolating self-energy approach (ISA). The electronic excitation spectrum gets a striking temperature dependence by its exchange coupling to the localized spin system. We observe very strong temperature dependent electronic correlation effects in GdN and the calculated red-shift of the lower conduction band is in close comparison with experiment. In order to determine the pure f-spin correlations, we develop the multi-band modified RKKY theory. The central idea of this theory beingto average out the itinerant electron degrees of freedom from the spin-exchange interaction and map the latter on to an effective Heisenberg model. Using this procedure, we determine the magnetic properties of the system like Curie temperature (within Random Phase Approximation) while calculating the chemical potential and magnetization within a self consistent scheme for various configurations of system parameters.
32

Volumetrie des rechten Leberlappens vor und während der Lebendspende

Brinkmann, Martin Julius 31 October 2005 (has links)
Die Lebendspende von Leberlappen wird in der Zukunft gerade vor dem Hintergrund des stets weiter steigenden Bedarfs und des sich dazu diskrepant entwickelnden Mangels an Leichenorganen zur Transplantation eine zunehmend wichtige Rolle einnehmen, um Patienten im Endstadium einer Lebererkrankung kurativ zu versorgen. Umso mehr spielen Überlegungen zur Gewährleistung insbesondere der Sicherheit für einen gesunden Lebendspender eine Rolle, ohne Risiken für ihn eliminieren zu können. In diese Überlegungen gehen Weiterentwicklungen der Möglichkeiten für die spezielle Evaluation der Leber eines potenziellen Spenders anhand bildgebender Verfahren ein. Hier nehmen Methoden zur präoperativen Abschätzung der Gewichts- und Volumenverhältnisse einer potenziellen Spenderleber und ihrer Lappen einen besonderen Stellenwert ein, da bei entsprechend ungünstigen Voraussetzungen ein gesunder Mensch aus Gründen der Sicherheit für eine Lebendspende nicht in Frage kommt. Die vorliegende Arbeit zeigt anhand einer prospektiven Studie unterschiedliche Methoden der präoperativen CT-gestützten Volumetrie zur Evaluation von Lebern und ihrer beiden Lappen von potenziellen Lebendspendern auf. Dabei wurde ein neu entwickeltes Volumetrieverfahren klinisch erprobt und mit einem etablierten Verfahren verglichen. Als Referenzgrößen wurden erstmalig gleichermaßen intraoperativ gemessene Gewichte und Volumina der transplantieren rechten Leberlappen herangezogen. Hinsichtlich der auf CT-gestützter Volumetrie basierenden, präoperativen Abschätzung von intraoperativ zu erwartendem Gewicht und Volumen von rechten Leberlappen im Rahmen einer Lebendspende erwies sich das etablierte Verfahren bezüglich des Gewichts dem neu entwickelten Verfahren geringgradig überlegen, während das neu entwickelte Verfahren bezüglich des Volumens gegenüber dem etablierten Verfahren geringgradig besser abschnitt. Darüber hinaus resultierte aus den intraoperativ erhobenen Daten die Erkenntnis, dass die physikalische Dichte von gesundem Lebergewebe bei einer relativ hohen interindividuellen Streuung im Mittel um knapp 12% höher liegt als zumeist angenommen. In Zukunft werden Fortschritte technischer Verfahren sehr genaue virtuelle Trennungen von Lebern in ihre beiden Lappen ermöglichen. Gleichzeitig werden chirurgische Resektionstechniken verfeinert. Sowohl der virtuelle als auch der reale Ansatz haben den Anspruch, die avaskuläre und somit ideale Resektionsfläche zwischen beiden Leberlappen aufzusuchen, um gleichzeitig präoperativ exakte Gewichts- und Volumenabschätzungen zu ermöglichen und intraoperativ Risiken zu minimieren. Welchem dieser beiden Ansätze die stärkste Annäherung an diesen Anspruch oder dessen Vollendung zuerst gelingt, wird sich als Referenzmethode behaupten, an der sich der unterlegene Ansatz wird messen lassen müssen. / The increasing need of cadaveric liver grafts and the scarcity of living related liver transplants (LRLT) will play a critical role in the future treatment of patients suffering from end stage liver disease. Various considerations, including especially a safe outcome for the donor, are essential. However, risks can not be eliminated. These considerations can be influenced in the evaluation of a potential living donor. Accurate methods, including imaging modalities, for the preoperative estimation of the potential donor liver’s weight and volume are essential as an adverse condition would preclude a living donation for safety reasons. This thesis presents different methods of preoperative CT-based volumetric analyses for the evaluation the liver and both its lobes in potential living donors. A newly developed method of volumetric analysis was clinically tested and compared with an established method. Intraoperatively measured weights and volumes of transplanted right hepatic lobes were used as reference values. With regards to the weight, the established method proved to be mildly superior, while the newer method was slightly more accurate for volume. Additionally, it was discovered that the mean density of healthy liver tissue is approximately 12 percent higher than generally assumed but with a relatively high individual variation. Progress in technical methods will render possible very exact virtual divisions of the liver in both of its lobes. Both the virtual and surgical approach have a claim for finding the appropriate avascular and consequently ideal resection plane in order to minimize risks intraoperatively.
33

Bandwidth Selection in Nonparametric Kernel Estimation / Bandweitenwahl bei nichtparametrischer Kernschätzung

Schindler, Anja 29 September 2011 (has links)
No description available.
34

Interplay of magnetic, orthorhombic, and superconducting phase transitions in iron-based superconductors

Schmiedt, Jacob 29 October 2014 (has links) (PDF)
The physics of iron pnictides has been the subject of intense research for half a decade since the discovery of superconductivity in doped LaFeAsO in 2008. By now there exists a large number of different materials that are summarized under the term "pnictides'' with significant differences in their crystal structure, electronic properties, and their phase diagrams. This thesis is concerned with the investigation of the various phase transitions that are observed in the underdoped compounds of the pnictide subgroups RFeAsO, where R is a rare-earth element, and AFe_2As_2, where A is an alkaline-earth element. These compounds display two closely bound transitions from a tetragonal to an orthorhombic phase and from a paramagnetic to an antiferromagnetic metal. Both symmetry-broken phases are suppressed by doping or pressure and close to their disappearance superconductivity sets in. The superconducting state is stabilized until some optimal doping or pressure is reached and gets suppressed thereafter. The central goal of this thesis is to improve our understanding of the interplay between these three phases and to describe the various phase transitions. We start from an itinerant picture that explains the magnetism as a result of an excitonic instability and show how the other phases can be included into this picture. This approach is based on the the observation that the compounds we are interested in have a Fermi surface with multiple nested electron and hole pockets and that they have small to intermediate interaction strengths. The thesis starts with a study of the doping dependence of the antiferromagnetic phase transition in four different five-orbital models. We use the random-phase approximation to determine the transition temperature, the dominant ordering vector, and the contribution of the different orbitals to the ordering. This allows us to identify the more realistic models, which give results that are in good agreement with experimental observations. In addition to the frequently made assumption of orbital-independent interaction potentials we study the effect of a reduction of the interaction strengths that involve the d_{xy} orbital. We find that this tunes the system between two different nesting instabilities. A reduction of the interactions that involve the d_{xy} orbital also enhances the tendency towards incommensurate (IC) order. For a weak reduction this tendency is compensated by the presence of the orthorhombic phase. However, for a reduction of 30%, as it is suggested by constrained random-phase-approximation calculations, we always find large doping ranges, where a state with IC order has the highest transition temperature. We continue the investigation of the magnetic phase transition by studying the competition of different possible types of antiferromagnetic order that arises from the presence of two degenerate nesting instabilities with the ordering vectors (pi,0) and (0,pi). We derive a Ginzburg-Landau free energy from a microscopic two-band model and find that the presence of the experimentally observed stripe phase strongly depends on the number and size of the hole pockets in the system and on the doping. We show that within the picture of a purely magnetically driven nematic phase transition, which breaks the C_4 symmetry and induces the orthorhombic distortion, the nematic phase displays exactly the same dependence on the model parameters as the magnetic stripe phase. We propose that in addition to the purely magnetically driven nematic instability there is a ferro-orbital instability in the system that stabilizes the nematic transition and, thus, explains the experimentally observed robustness of the orthorhombic transition. We argue that including a ferro-orbital instability into the picture may also be necessary to reproduce the transition from simultaneous first-order transitions into an orthorhombic antiferromagnetic state to two separate second-order transitions, which is observed as a function of doping. Finally, a study of the superconducting phase transition inside the antiferromagnetic phase that is observed in some pnictide compounds is presented. We present an approach to calculate the fluctuation-mediated pairing interaction in the spin-density-wave phase of a multiband system, which is based on the random-phase approximation. This approach is applied to a minimal two-band model for the pnictides to study the effect of the various symmetry-allowed bare on-site interactions on the gap symmetry and structure. We find a competition between various even- and odd-parity states and over a limited parameter range a p_x-wave state is the dominant instability. The largest part of the parameter space is dominated by even parity states but the gap structure sensitively depends on the bare interactions. We propose that the experimentally observed transition from a nodeless to a nodal gap can be due to changes in the on-site interaction potentials.
35

A biocompatible and functional adhesive aminerich coating based on dopamine polymerization

Yang, Ying, Qi, Pengkai, Ding, Yonghui, Maitz, Manfred F., Yang, Zhilu, Tu, Qiufen, Xiong, Kaiqin, Leng, Yang, Huang, Nan 07 January 2020 (has links)
Amine groups physiologically play an important role in regulating the growth behavior of cells and they have technological advantages for the conjugation of biomolecules. In this work, we present a method to deposit a copolymerized coating of dopamine and hexamethylendiamine (HD) (PDAM/HD) rich in amine groups onto a target substrate. This method only consists of a simple dip-coating step of the substrate in an aqueous solution consisting of dopamine and HD. Using the technique of PDAM/HD coating, a high density of amine groups of about 30 nmol cm⁻² was obtained on the target substrate surface. The PDAM/HD coating showed a high cross-linking degree that is robust enough to resist hydrolysis and swelling. As a vascular stent coating, the PDAM/HD presented good adhesion strength to the substrate and resistance to the deformation behavior of compression and expansion of a stent. Meanwhile, the PDAM/HD coating exhibited good biocompatibility and attenuated the tissue response compared with 316L stainless steel (SS). The primary amine groups of the PDAM/HD coating could be used to effectively immobilize biomolecules containing carboxylic groups such as heparin. These data suggested the promising potential of this PDAM/HD coating for application in the surface modification of biomedical devices.
36

A family of 2D and 3D coordination polymers involving a trigonal tritopic linker

Hauptvogel, Ines Maria, Bon, Volodymyr, Grünker, Ronny, Baburin, Igor A., Senkovska, Irena, Müller, Uwe, Kaskel, Stefan January 2012 (has links)
Five new coordination polymers, namely, [Zn2(H2O)2(BBC)](NO3)(DEF)6 (DUT-40), [Zn3(H2O)3(BBC)2] (DUT-41), [(C2H5)2NH2][Zn2(BBC)(TDC)](DEF)6(H2O)7 (DUT-42), [Zn10(BBC)5(BPDC)2(H2O)10](NO3)(DEF)28(H2O)8 (DUT-43), and [Co2(BBC)(NO3)(DEF)2(H2O)](DEF)6(H2O) (DUT-44), where BBC – 4,4′,4′′-(benzene-1,3,5-triyl-tris(benzene-4,1-diyl))tribenzoate, TDC – 2,5-thiophenedicarboxylate, BPDC – 4,4′-biphenyldicarboxylate, DEF – N,N-diethylformamide, were obtained under solvothermal conditions and structurally characterized. It has been shown that compounds DUT-40, DUT-41 and DUT-44 exhibit 2D layered structures with large hexagonal channels. Utilization of additional angular dicarboxylic TDC linker led to the formation of the DUT-42 compound with the structure consisting of three interpenetrated 3D networks. Using the linear co-linker dicarboxylic BPDC, DUT-43 was obtained which forms a complicated 3D architecture arising from the polycatenation of triple-layered 2D building units and 2D single layer units. The pore accessibility of the synthesized compounds in the liquid phase was proved by the adsorption of dye molecules. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.
37

Computergestützte Simulation und Analyse zufälliger dichter Kugelpackungen

Elsner, Antje 19 November 2009 (has links)
In dieser interdisziplinär geprägten Arbeit wird zunächst eine Übersicht über kugelbasierte Modelle und die algorithmischen Ansätze zur Generierung zufälliger Kugelpackungen gegeben. Ein Algorithmus aus der Gruppe der Kollektiven-Umordnungs-Algorithmen -- der Force-Biased-Algorithmus -- wird ausführlich erläutert und untersucht. Dabei werden die für den Force-Biased-Algorithmus als essenziell geltenden Verschiebungsfunktionen bezüglich ihres Einflusses auf den erreichbaren Volumenanteil der Packungen untersucht. Nicht nur aus der Literatur bekannte, sondern auch neu entwickelte Verschiebungsfunktionen werden hierbei betrachtet. Daran anschließend werden Empfehlungen zur Auswahl geeigneter Verschiebungsfunktionen gegeben. Einige mit dem Force-Biased-Algorithmus generierte Kugelpackungen, zum Beispiel hochdichte monodisperse Packungen, lassen den Schluss zu, dass insbesondere strukturelle Umbildungsvorgänge an solchen Packungen sehr gut zu untersuchen sind. Aus diesem Grund besitzt das Modell der mit dem Force-Biased-Algorithmus dicht gepackten harten Kugeln große Bedeutung in der Materialwissenschaft, insbesondere in der Strukturforschung. In einem weiteren Kapitel werden wichtige Kenngrößen kugelbasierter Modelle erläutert, wie z. B. spezifische Oberfläche, Volumenanteil und die Kontaktverteilungsfunktionen. Für einige besonders anwendungsrelevante Kenngrößen (z. B. die spezifische Oberfläche) werden Näherungsformeln entwickelt, an Modellsystemen untersucht und mit bekannten Näherungen aus der Literatur verglichen. Zur Generierung und Analyse der Kugelpackungen wurde im Rahmen dieser Arbeit die Simulationssoftware „SpherePack“ entwickelt, deren Aufbau unter dem Aspekt des Softwareengineerings betrachtet wird. Die Anforderungen an dieses Simulationssystem sowie dessen Architektur werden hier beschrieben, einschließlich der Erläuterung einzelner Berechnungsmodule. An ausgewählten praxisnahen Beispielen aus der Materialwissenschaft kann die Vielfalt der Einsatzmöglichkeiten eines Simulationssystems zur Generierung und Analyse von zufälligen dicht gepackten Kugelsystemen gezeigt werden. Vor allem die hohe Aussagekraft der Untersuchungen in Bezug auf Materialeigenschaften unterstreicht die Bedeutung des Modells zufällig dicht gepackter harter Kugeln in der Materialforschung und verwandten Forschungsgebieten.
38

Self-interaction corrected SCAN functional for molecules and solids in the numeric atom-center orbital framework

Bi, Sheng 12 May 2023 (has links)
Das „Strongly Constrained and Appropriately Normed“ (SCAN) Austausch-Korrelations-Funktional gehört zur Familie der meta-GGA (generalized gradient approximation) Funktionale. Es gibt aber auch Nachteile Zum einen leiden SCAN Rechnungen oft unter numerischen Instabilitäten, wodurch sehr viele Iteration zum Erreichen von Selbst-Konsistenz benötigt werden. Zum anderen leidet SCAN unter dem von GGA Methoden bekannten Selbstwechselwirkung-Fehler. Im ersten Teil der Arbeit habe ich die numerischen Stabilitätsprobleme in SCAN Rechnungen im Rahmen der numerischen Realraum-Integrationsroutinen im Code FHI-aims untersucht. Diese Analyse zeigt, dass die genannte Probleme durch Anwendung von standardisierten Dichte-Mischalgorithmen für die kinetische Energiedichte abgemildert werden können. Dadurch wird auch in SCAN-Rechnungen eine schnelle und stabile Konvergenz zur selbstkonsistenten Lösung ermöglicht. Im zweiten Teil der Arbeit habe ich untersucht, in welchem Rahmen sich der Selbstwechselwirkung-Fehler in SCAN mittels des von Perdew und Zunger vorgeschlagenen Selbstinteraktionskorrekturalgorithmus (PZ-SIC) verringern lässt. Es wurden aber auch Optimierungen für die PZ-SIC Methode entwickelt. Inspiriert von den ursprünglichen Argumenten in der PZ-SIC-Methode und anderen lokalisierten Methoden, wird in dieser Arbeit eine neuartige Randbedingung (orbital density constraint) vorgeschlagen, die sicherstellt, dass die PZ-SIC Orbitale während des Selbstkonsistenzzyklus lokalisiert bleiben. Dies mildert die Anfangswertabhängigkeit deutlich ab und hilft dabei, in die korrekte selbst-konsistente Lösung mit minimaler Energie zu konvergieren, unabhängig davon ob reelle oder komplexe SIC Orbitale verwendet werden. Die in dieser Arbeit getägtigen Entwicklungen und Untersuchungen sind Wegbereiter dafür, in Zukunft mit SIC-SCAN Rechnungen deutlich genauere ab initio Rechnungen mit nur gering höherem Rechenaufwand durchführen zu können. / The state-of-the-art “Strongly Constrained and Appropriately Normed” (SCAN) functional pertains to the family of meta-generalized-gradient approximation (meta-GGA) exchange-correlation functionals. Nonetheless, SCAN suffers from some well-documented deficiencies. In the first part of this thesis, I revisited the known numerical instability problems of the SCAN functional in the context of the numerical, real-space integration framework used in the FHI-aims code. This analysis revealed that applying standard density-mixing algorithms to the kinetic energy density attenuates and largely cures these numerical issues. By this means, SCAN calculations converge towards the self-consistent solution as fast and as efficiently as lower-order GGA calculations. In the second part of the thesis, I investigated strategies to alleviate the self-interaction error in SCAN calculations by using the self-interaction correction algorithm proposed by Perdew and Zunger (PZ-SIC). Inspired by the original arguments in PZ-SIC and other localized methods, I introduced a mathematical constraint, i.e., the orbital density constraint, that forces the orbitals to retain their localization throughout the self-consistency cycle. In turn, this alleviates the multiple-solutions problem and facilitates the convergence towards the correct, lowest-energy solution both for complex and real SIC orbitals. The developments and investigations performed in this thesis pave the road towards a more wide-spread use of SIC-SCAN calculations in the future, allowing more accurate predictions within only moderate increases of computational cost.
39

Variable-Density Flow Processes in Porous Media On Small, Medium and Regional Scales

Walther, Marc 03 November 2014 (has links) (PDF)
Nowadays society strongly depends on its available resources and the long term stability of the surrounding ecosystem. Numerical modelling has become a general standard for evaluating past, current or future system states for a large number of applications supporting decision makers in proper management. In order to ensure the correct representation of the investigated processes and results of a simulation, verification examples (benchmarks), that are based on observation data or analytical solutions, are utilized to evaluate the numerical modelling tool. In many parts of the world, groundwater is an important resource for freshwater. While it is not only limited in quantity, subsurface water bodies are often in danger of contamination from various natural or anthropogenic sources. Especially in arid regions, marine saltwater intrusion poses a major threat to groundwater aquifers which mostly are the exclusive source of freshwater in these dry climates. In contrast to common numerical groundwater modelling, density-driven flow and mass transport have to be considered as vital processes in the system and in scenario simulations for fresh-saltwater interactions. In the beginning of this thesis, the capabilities of the modelling tool OpenGeoSys are verified with selected benchmarks to represent the relevant non-linear process coupling. Afterwards, variable-density application and process studies on different scales are presented. Application studies comprehend regional groundwater modelling of a coastal aquifer system extensively used for agricultural irrigation, as well as hydro-geological model development and parametrization. In two process studies, firstly, a novel method to model gelation of a solute in porous media is developed and verified on small scale laboratory observation data, and secondly, investigations of thermohaline double-diffusive Rayleigh regimes on medium scale are carried out. With the growing world population and, thus, increasing pressure on non-renewable resources, intelligent management strategies intensify demand for potent simulation tools and development of novel methods. In that way, this thesis highlights not only OpenGeoSys’ potential of density-dependent process modelling, but the comprehensive importance of variable-density flow and transport processes connecting, both, avant-garde scientific research, and real-world application challenges.
40

Variable-Density Flow Processes in Porous Media On Small, Medium and Regional Scales

Walther, Marc 07 May 2014 (has links)
Nowadays society strongly depends on its available resources and the long term stability of the surrounding ecosystem. Numerical modelling has become a general standard for evaluating past, current or future system states for a large number of applications supporting decision makers in proper management. In order to ensure the correct representation of the investigated processes and results of a simulation, verification examples (benchmarks), that are based on observation data or analytical solutions, are utilized to evaluate the numerical modelling tool. In many parts of the world, groundwater is an important resource for freshwater. While it is not only limited in quantity, subsurface water bodies are often in danger of contamination from various natural or anthropogenic sources. Especially in arid regions, marine saltwater intrusion poses a major threat to groundwater aquifers which mostly are the exclusive source of freshwater in these dry climates. In contrast to common numerical groundwater modelling, density-driven flow and mass transport have to be considered as vital processes in the system and in scenario simulations for fresh-saltwater interactions. In the beginning of this thesis, the capabilities of the modelling tool OpenGeoSys are verified with selected benchmarks to represent the relevant non-linear process coupling. Afterwards, variable-density application and process studies on different scales are presented. Application studies comprehend regional groundwater modelling of a coastal aquifer system extensively used for agricultural irrigation, as well as hydro-geological model development and parametrization. In two process studies, firstly, a novel method to model gelation of a solute in porous media is developed and verified on small scale laboratory observation data, and secondly, investigations of thermohaline double-diffusive Rayleigh regimes on medium scale are carried out. With the growing world population and, thus, increasing pressure on non-renewable resources, intelligent management strategies intensify demand for potent simulation tools and development of novel methods. In that way, this thesis highlights not only OpenGeoSys’ potential of density-dependent process modelling, but the comprehensive importance of variable-density flow and transport processes connecting, both, avant-garde scientific research, and real-world application challenges.:Abstract Zusammenfassung Nomenclature List of Figures List of Tables I Background and Fundamentals 1 Introduction 1.1 Motivation 1.2 Structure of the Thesis 1.3 Variable-Density Flow in Literature 2 Theory and Methods 2.1 Governing Equations 2.2 Fluid Properties 2.3 Modelling and Visualization Tools 3 Benchmarks 3.1 Steady-state Unconfined Groundwater Table 3.2 Theis Transient Pumping Test 3.3 Transient Saltwater Intrusion 3.4 Development of a Freshwater Lens II Applications 4 Extended Inverse Distance Weighting Interpolation 4.1 Motivation 4.2 Extension of IDW Method 4.3 Artificial Test and Regional Scale Application 4.4 Summary and Conclusions 5 Modelling Transient Saltwater Intrusion 5.1 Background and Motivation 5.2 Methods and Model Setup 5.3 Simulation Results and Discussion 5.4 Summary, Conclusion and Outlook 6 Gelation of a Dense Fluid 6.1 Motivation 6.2 Methods and Model Setup 6.3 Results and Conclusions 7 Delineating Double-Diffusive Rayleigh Regimes 7.1 Background and Motivation 7.2 Methods and Model Setup 7.3 Results 7.4 Conclusions and Outlook III Summary and Conclusions 8 Important Achievements 9 Conclusions and Outlook Bibliography Publications Acknowledgements Appendix

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