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Coming dissertations at TekNat

  • Forecasting of Icing Related Wind Energy Production Losses : Probabilistic and Machine Learning Approaches

    Author: Jennie Molinder
    Publication date: 2021-01-15 12:24

    Icing on wind turbine blades causes significant production losses for wind energy in cold climate. Next-day forecasts of these production losses are crucial for the power balance in the electrical grid and for the trading process, but they are uncertain due to lack of understanding of, and simplifications, in the modelling chain. In the present work, uncertainties in the modelling chain for icing related production losses are addressed with the aim to increase the utility of next-day production loss forecasts. Probabilistic and machine learning methods are applied both to improve the forecast skill and to estimate reliable forecast uncertainties. The different methods enable uncertainties in different parts of the chain to be addressed. A Numerical Weather Prediction (NWP) ensemble captures uncertainties in the initial conditions of the forecasts while a neighbourhood method describes uncertainties in the spatial representation of the NWP forecast at the exact locations of the wind parks. An icing model ensemble is generated in order to address uncertainties in the icing model parameters. Finally, machine learning approaches are employed to both deterministically and...

  • Aeroacoustic Prediction for Vertical Axis Wind Turbines

    Author: Aya Aihara
    Publication date: 2021-01-14 12:42

    This thesis investigates the aerodynamic and aeroacoustic prediction of vertical axis wind turbines, using computational fluid dynamics simulations. Noise pollution from wind turbines is one of the disadvantages of wind energy, calling for strategies to reduce noise levels. Yet for vertical axis wind turbines in particular, there is insufficient knowledge of how to identify sound sources and mitigate the sound level. The aim of this study is to predict aerodynamic noise, using large eddy simulation and acoustic analogy, so as to better understand the mechanism of sound generation for vertical axis wind turbines. First, the prediction method is validated for a static single blade in stall. This model is able to capture the dominant frequency, but it does not well reproduce the broadband characteristics. Next, the aerodynamic behavior of the 12 kW H-rotor vertical axis wind turbine is studied, whereby the focus is on the importance of properly modeling the strut influence for an accurate prediction of the blade forces. To achieve this, the flow field is solved for three different tip speed ratios. The results show that the struts significantly affect on the force distribution...

  • Bright Needles in a Haystack : A Search for Magnetic Monopoles Using the IceCube Neutrino Observatory

    Author: Alexander Burgman
    Publication date: 2021-01-12 13:47

    The IceCube Neutrino Observatory at the geographic South Pole is designed to detect the light produced by the daughter-particles of in-ice neutrino-nucleon interactions, using one cubic kilometer of ice instrumented with more than 5000 optical sensors.

    Magnetic monopoles are hypothetical particles with non-zero magnetic charge, predicted to exist in many extensions of the Standard Model of particle physics. The monopole mass is allowed within a wide range, depending on the production mechanism. A cosmic flux of magnetic monopoles would be accelerated by extraterrestrial magnetic fields to a broad final velocity distribution that depends on the monopole mass.

    The analysis presented in this thesis constitutes a search for magnetic monopoles with a speed in the range [0.750;0.995] in units of the speed of light. A monopole within this speed range would produce Cherenkov light when traversing the IceCube detector, with a smooth and elongated light signature, and a high brightness.

    This analysis is divided into two main steps. Step I is based on a previous IceCube analysis, developed for a cosmogenic neutrino search, with similar signal event characteristics as in...

  • Microfluidics for High-Pressure Inertial Focusing : Focusing, Separation and Concentration of Micro and Sub-micron Particles

    Author: Javier Cruz
    Publication date: 2021-01-12 13:34

    The birth of microsystems set the ground for technologies never imagined before, for it is not only the small size what characterizes the miniaturized systems, but unique phenomena arise in the micro scale. This thesis relates to one such unique phenomenon, inertial focusing, a phenomenon that occurs in microfluidic systems if very special conditions are met and that allows for fine manipulation of particles in fluid samples. This ability is key in a bigger picture: the analysis of complex fluids, where rare particles of interest may be present in very few numbers amongst a myriad of others, making the task difficult – if not impossible. A system exploiting inertial focusing allows, for instance, to focus, separate, isolate and concentrate such rare particles of interest, and even to transfer them to another fluid, thereby enabling/facilitating their detection and analysis. Examples of rare particles of interest in complex fluids are circulating tumor cells in blood, that give away the presence of cancer, extracellular vesicles also in blood, that contain biomarkers with physiological and pathological information about the patient, or bacteria in natural water, where the...

  • Amorphous Magnetic Materials : A Versatile Foundation for Tomorrow’s Applications

    Author: Sebastian George
    Publication date: 2021-01-12 09:33

    Amorphous magnetic materials exhibit a number of key differentiating properties with respect to crystalline magnets. In some cases, the differences may simply be in the values of macroscopic properties such as saturation magnetization, coercivity, Curie temperature, and electrical conductivity. Other cases are more fundamental, such as the possibility for many amorphous alloys to be produced with nearly arbitrary composition, something that is not always possible in crystal structures that may only be stable for certain specific compositions.

    Fundamentally, these properties arise due to the disordered arrangement of atoms in amorphous materials. However, this structure is challenging to probe and characterize, either experimentally or theoretically. A significant contribution of this thesis is the development of a new approach for studying the local atomic structure of amorphous materials, specifically amorphous SmCo and FeZr alloys. The strategy combines extended x-ray absorption spectroscopy (EXAFS) measurements with stochastic quenching (SQ) simulations in a way that provides more information than either method can offer alone. Additionally, this approach offers the...

  • On Solving String Constraints

    Author: Phi Diep Bui
    Publication date: 2021-01-11 13:21

    Software systems are deeply involved in diverse human activities as everyone uses a variety of software systems on a daily basis. It is essential to guarantee that software systems all work correctly. Two popular methods for finding failures of software systems are testing and model checking. Various efficient testing and model checking approaches are satisfiability-based, where the core of the approaches is Satisfiability Modulo Theories (SMT) solvers for solving the path feasibility and/or reachability problems. The significant growth of string manipulating programs in modern programming languages, including Python and JavaScript, demands SMT solvers being capable of analysing string constraints. This thesis proposes two frameworks for checking the satisfiability of extensive classes of string constraints, discovers a new decidable fragment of string constraints, and introduces efficient solvers for solving string constraints.

    The first framework for checking the satisfiability of string constraints is based on Counter-Example Guided Abstract Refinement (Cegar) procedure, and applicable to diverse classes of string constraints. It is worth mentioning that the framework...

  • Criticality of fast failures in the High Luminosity Large Hadron Collider

    Author: Björn Lindström
    Publication date: 2021-01-05 10:38

    Each of the two Large Hadron Collider (LHC) beams contain 362 MJ of energy. This will be further increased to 678 MJ in the upcoming upgrade to the High Luminosity LHC (HL-LHC). In the event of an uncontrolled beam loss, a significant hazard occurs, that can damage the machine components. This thesis is focused on failures that can lead to a fast increase of beam losses, with a focus on the new optics and equipment in the HL-LHC. The criticality for a number of failure scenarios is studied, under different optics configurations of the machine. Mitigation strategies, involving dedicated interlocking and a reduction of the impact that the failures have on the beam are proposed for the most critical scenarios. For a number of less critical failures it is determined that current interlock strategies are sufficient.

    Failures involving the magnet protection and the crab cavities constitute the most severe hazards. The former consists of quench heaters and a new system known as coupling loss induced quench (CLIQ). A new connection scheme is proposed for these, in order to limit their effect on the beam. Dedicated interlocks for detecting spurious discharges of these systems are...

  • Parallelization of dynamic algorithms for electronic structure calculations

    Author: Anton G. Artemov
    Publication date: 2020-12-21 13:18

    The aim of electronic structure calculations is to simulate behavior of complex materials by resolving interactions between electrons and nuclei in atoms at the level of quantum mechanics. Progress in the field allows to reduce the computational complexity of the solution methods to linear so that the computational time scales proportionally to the size of the physical system. To solve large scale problems one uses parallel computers and scalable codes. Often the scalability is limited by the data distribution.

    This thesis focuses on a number of problems arising in electronic structure calculations, such as inverse factorization of Hermitian positive definite matrices, approximate sparse matrix multiplication, and density matrix purification methods. No assumptions are made about the data distribution, instead, it is explored dynamically.

    The thesis consists of an introduction and five papers. Particularly, in Paper I we present a new theoretical framework for localized matrices with exponential decay of elements. We describe a new localized method for inverse factorization of Hermitian positive definite matrices. We show that it has reduced communication costs...

  • Rosetta Observations of Plasma and Dust at Comet 67P

    Author: Fredrik Leffe Johansson
    Publication date: 2020-12-21 13:09

    In-situ observations of cometary plasma are not made because they are easy. The historic ESA Rosetta mission was launched in 2004 and traversed space for ten years before arriving at comet 67P/Churyumov-Gerasimenko, which it studied in unprecedented detail for two years. For the Rosetta Dual Langmuir Probe Experiment (LAP), the challenge was increased by the sensors being situated on short booms near a significantly negatively charged spacecraft, which deflects low-energy charged particles away from our instrument. To disentangle the cometary plasma signature in our signal, we create a charging model for the particular design of the Rosetta spacecraft through 3D Particle-in-Cell/hybrid spacecraft-plasma interaction simulations, which also can be applicable to similarly designed spacecraft in cold plasma environments. By virtue of this model, we find a way to cross-calibrate (with the Mutual Impedance probe, MIP) the LAP spacecraft potential to a plasma density estimate with increased temporal resolution and dynamic range than any single plasma instrument alone.

    To characterise and disentangle the Sun-driven photoelectric current from the positive cometary ion current...

  • Expression and synthetic activation of [FeFe]-hydrogenases in cyanobacteria

    Author: Adam Wegelius
    Publication date: 2020-12-21 07:46

    Photosynthetic microbes can be utilized for hydrogen production, generating a clean, carbon neutral energy carrier from abundant substrates. Cyanobacteria are photosynthetic prokaryotes with large potential for biotechnological energy applications and several strains are capable of hydrogen production. This production is catalysed by a bi-directional [NiFe]-hydrogenase, or by nitrogenase during nitrogen fixation. However, nature’s foremost hydrogen producing enzymes, the [FeFe]-hydrogenases, are not present in these organisms. Many [FeFe]-hydrogenases boast incredible catalytic activities and high bias towards proton reduction. Introduction of a suitable [FeFe]-hydrogenase in a cyanobacterial host could greatly improve the hydrogen production capacity. Unfortunately, generation and characterisation of cyanobacterial strains carrying active [FeFe]-hydrogenases is stalled by the intricate maturation process associated with these enzymes.

    In this thesis, I investigate heterologous expression and artificial maturation of [FeFe]-hydrogenases in cyanobacteria. Genetic tools to reliably express [FeFe]-hydrogenases were developed and tested in the unicellular cyanobacterium ...