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Coming dissertations at TekNat

  • Wind power wake modelling : Development and application of an actuator disc method for industrial utilization.

    Author: Nikolaos Simisiroglou
    Publication date: 2018-06-13 12:55

    As a wind turbine extracts energy from the wind it creates a region downstream where the wind velocity is decreased and the urbulence intensity is increased, this region is commonly called the wake region. Today’s wind farms include a large number of wind turbines position in tight layouts. These tight layouts result in increased power losses due to wakes, rendering accurate wind turbine wake modelling crucial in developing cost effective projects.

    The primary aim of this study is to create a method capable of conducting full-scale wind farm wake computations accurately in a time efficient manner by taking into account the computational resources and data availability of a typical industrial user. As a first step of this study, an actuator disc (ACD) method (old ACD) used within WindSim, is evaluated against power production data from the Lillgrund offshore wind. This study is followed by the development of a new ACD method. The new ACD method differs from the previous ACD method in terms of how the thrust distribution and the power production is calculated. A series of validation studies are performed on this newly introduced ACD method. These consist of validating the...

  • World oil supply and unconventional resources : Bottom-up perspectives on tight oil production

    Author: Henrik Wachtmeister
    Publication date: 2018-05-30 13:28

    Oil is the world’s largest primary energy source. It dominates the transportation sector which underpins the world economy. Yet, oil is a nonrenewable resource, destined not to last forever. In the mid-2000s global conventional oil production stagnated, leading to rising oil prices and fears of permanent oil scarcity. These fears, together with the high prices, receded with the unforeseen emergence of a new supply source: tight oil.

    This licentiate thesis investigates unconventional tight oil production and its impacts on world oil supply in terms of resource availability and oil market dynamics, and in turn briefly discusses some possible wider economic, political and environmental implications of these impacts. The thesis is based on three papers. The first investigates the usefulness of bottom-up modelling by a retrospective study of past oil projections. The second looks at how unconventional tight oil production can be modelled on the well level using decline curve analysis. The third derives typical production parameters for conventional offshore oil fields, a growing segment of conventional production and a useful comparison to tight oil.

    The results show...

  • Dispersal and environmental impact of contaminants in organic rich, fibrous sediments of industrial origin in the Baltic Sea

    Author: Anna Apler
    Publication date: 2018-05-29 08:10

    The health of the Baltic Sea is negatively affected by hazardous substances such as metals and persistent organic pollutants (POPs), which include legacy pollutants that were banned decades ago, but still circulate in the ecosystem. Elevated levels of legacy pollutants, identified by HELCOM as key hazardous substances, have been found in accumulations of fibrous sediments, so-called fiberbanks and fiber-rich sediments, which derive from old pulp mills along the Swedish north coast. The fiberbanks are deposited in shallow water and bathymetrical models show evidence of their erosion, potentially caused by propeller wash, submarine landslides and gas ebullition. This thesis addresses the potential dispersal of key substances from three fiberbank sites located in a non-tidal Swedish estuary, in which metals and POPs are present in concentrations that may pose a risk for benthic organisms. Metals and POPs are partitioned to organic material and, as expected, show the highest partitioning coefficients (KD) in fiberbanks that have higher TOC levels compared to adjacent areas with fiber-rich sediments (natural clay sediments mixed up with fibers) or relatively unaffected...

  • Pollinator-mediated selection and the evolution of floral traits in orchids

    Author: Judith Trunschke
    Publication date: 2018-05-24 11:15

    In this thesis, I combined manipulations of traits and pollination environment with analysis of phenotypic selection to examine causes of variation in strength and mode of selection on floral traits, and I conducted a reciprocal sowing experiment to test for local adaptation in germination success. I tested the following predictions (1) the opportunity for selection, and the strength of pollinator-mediated and net selection increase with increasing pollen limitation, (2) the effects of traits affecting pollinator attraction and traits affecting pollination efficiency are non-additive and this leads to pollinator-mediated correlational selection, (3) the effects of spur length on pollen removal, pollen receipt, and female fitness differ between populations with short-tongued and populations with long-tongued pollinators, and (4) local adaptation at the stage of germination contributes to the maintenance of ecotypes growing in grasslands and woodlands, respectively.

    A study including natural populations of 12 orchid species that varied widely in pollen limitation showed that opportunity for selection, pollinator-mediated selection and net selection were all positively...

  • Holographic Wilson Loops : Quantum String Corrections

    Author: Daniel Ricardo Medina Rincon
    Publication date: 2018-05-23 13:33

    The gauge-string duality has been one of the most exciting areas in theoretical physics as it connects strongly coupled field theories with weakly interacting strings. The present thesis concerns the study of Wilson loops in this correspondence. Wilson loops are observables arising in many physical situations like the propagation of particles in gauge fields, the problem of confinement, etc. In the gauge-string correspondence these observables have a known physical description at both sides, making them ideal probes for the duality. Remarkable progress from localization has lead to predictions at all orders in the coupling for certain Wilson loop configurations in supersymmetric field theories. Being the string theory weakly interacting, in principle we can use perturbation theory to calculate the corresponding quantities. However, current string calculations have only been successful at leading order and in rare cases, next to leading order. At next to leading order the difficulties encountered include divergences, ambiguous boundary conditions, mismatch with field theory results, etc. The research presented in this thesis aims at a better understanding of these issues. The...

  • Probing Magnetism at the Atomic Scale:  Non-Equilibrium Statistical Mechanics Theoretical Treatise

    Author: Juan David Vasquez Jaramillo
    Publication date: 2018-05-23 12:37

    Here, I present a theoretical study, based on non-equilibrium quantum statistical mechanics and on the non-equilibrium extension to the RKKY intveraction,where I investigate the emergence of magnetism at the atomic scale in adsorbed molecular complexes hosting localized spin moments, at the stake of being probed with scanning tunneling microscopy tip, and being driven by a temperature gradient and gated by an electric field. The scanning tunneling microscopy set up is modeled as a molecular junction with a magnetic molecule embedded within it, where the molecule consists in a set of electronic levels resembling the typical s-p orbitals of a metal hydride or an organometal, and a localized spin moment resembling the magnetic unit hosted by the latter and former type of molecules mentioned. The electronic levels and the magnetic units are coupled via the Kondo interaction. One of the electrodes in the junction plays the role of an scanning tunneling microscopy tip, and the other one, does it for the metal in which the molecule is adsorbed, and a bias voltage and a temperature gradient is applied across both metals, giving rise to the effect of the above mentioned experimental set...

  • Influence of Aromaticity on Excited State Structure, Reactivity and Properties

    Author: Kjell Jorner
    Publication date: 2018-05-22 14:52

    This thesis describes work that could help development of new photochemical reactions and light-absorbing materials. Focus is on the chemical concept "aromaticity" which is a proven conceptual tool in developing thermal chemical reactions. It is here shown that aromaticity is also valuable for photochemistry. The influence of aromaticity is discussed in terms of structure, reactivity and properties. With regard to structure, it is found that photoexcited molecules change their structure to attain aromatic stabilization (planarize, allow through-space conjugation) or avoid antiaromatic destabilization (pucker). As for reactivity, it is found that stabilization/destabilization of reactants decrease/increase photoreactivity, in accordance with the Bell-Evans-Polanyi relationship. Two photoreactions based on excited state antiaromatic destabilization of the substrates are reported. Finally, with respect to properties, it is shown that excited state energies can be tuned by considering aromatic effects of both the electronic ground state and the electronically excited states. The fundamental research presented in this thesis forms a foundation for the development of new...

  • Electronic excitation, luminescence and particle emission : Studying ion-induced phenomena in ToF-MEIS

    Author: Svenja Lohmann
    Publication date: 2018-05-22 13:45

    Medium energy ion scattering (MEIS) is an experimental technique for the high-resolution depth profiling of thin films. Commonly, ions with energies between several ten to a few hundred keV are employed as probes, and backscattered particles are detected. Apart from scattering, keV ions can upon their interaction with matter induce luminescence, electron emission and the sputtering of neutrals and ions. However, research on this secondary particle emission in the medium energy regime is scarce. Thus, this thesis aims to perform a systematic analysis of 1) ion-induced photon emission and 2) the sputtering of positive ions in a time-of-flight (ToF) MEIS set-up. A significant fraction of photons exhibits energies of only a few eV, which is on the order of typical valence transitions in solids. The dependence of the photon yield on several experimental parameters is studied. By analysing the dependence on the employed geometry, it is concluded that photons are produced along the whole trajectory of the incident ion. Furthermore, the photon yield shows a strong material dependence, which seems to be subject to matrix effects. To study the sputtering process, mass spectrometry was...

  • The effect of nano-confinement on hydrogen uptake in metallic superlattices

    Author: Sotirios A. Droulias
    Publication date: 2018-05-22 10:40

    The absorption of hydrogen is exothermic in vanadium whereas it is endothermic in iron and chromium. Investigations of the hydrogen uptake within Fe/V(001) and Cr/V(001) superlattices allow therefore a detailed exploration of finite size effects and the influence of boundaries on hydrogen absorption. Fe/V(001) and Cr/V(001) superlattices can be grown as single crystal structures with a small mosaic spread, as determined by X-ray reflectometry and diffraction. Furthermore when the thickness ratio of the constituents is kept constant the crystal quality can be retained in the range from a few up to 40 monolayer repeat distances (Λ). Neutron reflectometry was used to simultaneously determine the volume expansion and concentration of hydrogen in the vanadium layers. Large differences are found in the expansion of Fe/V(001) and Cr/V(001) superlattices, in good agreement with density functional theory (DFT) calculations. The findings are consistent with tetrahedral and octahedral site occupancy in Cr/V(001) and Fe/V(001) superlattices, respectively. Full fitting of the reflectivity pattern is required to obtainan accurate measure of expansion if the number of repeats is small. Under...

  • MATTER-ANTIMATTER INTERACTIONS : The hydrogen-antihydrogen system and antiproton-matter interactions

    Author: Henrik Stegeby
    Publication date: 2018-05-18 14:50

    Ever since antiparticles were discovered their nature has been something of a mystery. They were postulated to be identical to regular particles except for having opposite charge, but this would imply that an equal amount of antiparticles and particles should have been created at the beginning of time. However, everywhere we look the Universe seems to be constituted of regular particles, giving rise to the question whether there is something else that differentiates antiparticles from regular particles, or if there is something amiss in the Standard Model of particle physics.

    This thesis focuses on a central system of study in this field, the hydrogen-antihydrogen system and the theory surrounding it, as well as an expansion into systems with an antiproton interacting with small molecules, bridging the fields of quantum physics and quantum chemistry.

    Methods expanding on the Born-Oppenheimer approximation for the interaction between the two atoms are presented. The resulting 2-body interaction potential is then used for creating a part of the basis in a non-adiabatic 4-body method in order to look for resonance states whose existence could impact cross-sections of...