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Coming dissertations at TekNat

  • Change Point Detection with Applications to Wireless Sensor Networks

    Author: Markus Eriksson
    Publication date: 2019-03-21 14:51

    In this thesis work we develop a new algorithm for detecting joint changes in statistical behavior of multiple, simultaneously recorded, signals. Such signal analysis is commonly known as multivariate change point (CP) detection (CPD) and is of interest in many scientific and engineering applications.

    First we review some of the existing CPD algorithms, where special attention is given to the Bayesian methods. Traditionally, many of the previous works on Bayesian CPD have focused on sampling based methods using Markov Chain Monte Carlo (MCMC). More recent work has shown that it is possible to avoid the computationally expensive MCMC methods by using a technique that is reminiscent of the forward-backward algorithm used for hidden Markov models. We revisit that technique and extend it to a multivariate CPD scenario where subsets of the monitored signals are affected at each CP. The extended algorithm has excellent CPD accuracy, but unfortunately, this fully Bayesian approach quickly becomes intractable when the size of the data set increases.

    For large data sets, we propose a two-stage algorithm which, instead of considering all possible combinations of joint CPs as...

  • Exact Results in Supersymmetric Field Theories : A dissertation on the defect and deformed

    Author: Erik Widen
    Publication date: 2019-03-12 13:45

    Quantum field theories (QFTs) are the most precise descriptions of the physical reality that humanity has found. Yet exact predictions are often missing as most computations are notoriously difficult to carry out. One generally resorts to perturbation theory which immediately limits the regime of validity. The need of better computational techniques and a deeper understanding of quantum field theory is evident.

    The highly symmetric N=4 SYM theory offers guidance in this quest. The theory's maximal supersymmetry and conformal invariance have allowed for the development of several computational techniques, most notably the AdS/CFT correspondence, supersymmetric localization and applications of integrability. These methods provide the rarity of exact results in a fully interacting QFT and shine light on regimes inaccessible by traditional computations.

    The insights drawn from N=4 SYM can be extended into more general settings through deformations and modifications. Three such modifications are the β-deformation, the massive deformation N=2* and N=4 SYM with a defect. This thesis summarizes a number of exact results for these three settings through: i)...

  • Micro-grids supplied by renewable energy : Improving technical and social feasibility

    Author: Caroline Bastholm
    Publication date: 2019-03-08 14:06

    Universal access to electricity stands high on the global agenda and is regarded as essential for positive development in sectors such as health care, education, poverty reduction, food production and climate change. Decentralized, off-grid electrification is deemed an important complement to centralized grid extension. By utilizing a renewable energy source, solar technology for the generation of electricity, photovoltaics (PV) is being considered as a way forward to minimize the environmental problems related to energy use.

    This thesis aims to contribute to improving the technical and social feasibility of PV and PV-diesel hybrid micro-grids for the purpose of providing access to electricity to people in rural areas of countries with low level access to electricity. In line with these general aims, the focus has been to address three questions related to challenges in three phases of rural electrification. The work has a multi-disciplinary approach, addressing mainly technical and social aspects of long-term sustainability of micro-grids, in a local context, and the changes these are intended to generate. One specific micro-grid in Tanzania has been used as a major case...

  • Toward fully automatic earthquake detection and processing for tomography in the Hengill area

    Author: Frederic Wagner
    Publication date: 2019-03-05 09:38

    This thesis focuses on the automation of seismic data analysis, in particular, event detection, quality assessment of detected events, and preparation of an earthquake catalogue for seismic tomography.

    The developed event detector uses back-propagation and stacking of a seismic trace attribute with a known velocity model to detect and locate events. A four-dimensional volume in space and time is probed for local maxima of coherently stacking signals. These local maxima define event location and origin time. Application of the detection algorithm to data from a dense 26-station 3-component seismic network in the Hengill area, SW Iceland, produced an increased true-to-false detection ratio compared to the local detection routine.

    The detected events were analysed using inter-event cross-correlation with a manually picked reference catalogue to determine their similarity with real events. Automatic P- and S-phase picks were derived using the time delay information from highly correlated events. Relocation with the determined phase picks improves hypocentre uncertainty. A multi-stage selection process is implemented to categorise the detected events into different...

  • Multidisciplinary Near-Surface Investigation of a Quick-Clay Landslide Prone Area in Southwest Sweden

    Author: Silvia Salas Romero
    Publication date: 2019-03-05 09:37

    Quick-clay landslides are considered one of the most important geohazards in Sweden, Norway, and Canada. The deposits involved are glacial and postglacial clays and silty clays, which are very sensitive to increased stress that may collapse their structure and cause liquefaction.

    A multidisciplinary approach was adopted in this study of quick clays in an area of southwest Sweden that is prone to landslides. It was mainly based on geophysical methods, but was complemented and validated with geotechnical, geological, and hydrological data. Downhole geophysics, land and river reflection seismics, radio magnetotellurics, P-wave refraction tomography, magnetics, and multichannel analysis of surface waves comprised the main geophysical methods used in this research. Laboratory measurements of core samples, such as grain size analysis, mineral magnetic properties, fossil content, X-ray fluorescence, cation exchange capacity, X-ray powder diffraction, electrical conductivity, and pH were also taken. Hydrological modelling was used to obtain information on the properties of groundwater within a coarse-grained layer, given the importance of this information in the formation of...

  • Moisture-Induced Strains and Stresses in Wood

    Author: Sabina Huč
    Publication date: 2019-02-27 11:57

    To design safe, reliable and durable timber structures subjected to varying natural outdoor or indoor climates, understanding the long-term behavior of wood when mechanically loaded or restrained to deform is crucial. The present thesis focuses on the numerical modeling of the long-term mechanical behavior of wood. The numerical analysis is divided in the moisture transport and the mechanical analyses. In the moisture analysis, the multi-Fickian moisture transport model is used to determine spatial and temporal moisture content fields over the analyzed domain due to changing relative humidity (RH) of the ambient air. The obtained moisture contents are taken into the mechanical analysis where a new mechanical model is applied for predicting rheological response of wood in three orthotropic directions simultaneously. Experimental results of different authors are used to support numerous numerical analyses performed for various wood species, deformation and loading modes in constant or changing RH conditions. The performed analyses show that the new mechanical model adequately predicts the viscoelastic behavior of hardwood and softwood species in two orthotropic...

  • Problematizing Sustainable ICT

    Author: Per Fors
    Publication date: 2019-02-26 09:20

    How should we understand the relationship between information and communication technology (ICT) and sustainability? Generally, it is assumed that while ICT products contribute to many environmental and social problems as they are produced and disposed of, the potential of using ICT to achieve a more sustainable society is immense. However, despite the fact that such a discourse is favored not only in the industrial but also in the political and academic spheres, we have yet to see this presumed sustainability-related potential of ICT fully exploited.

    This thesis argues that conventional assumptions and understandings related to three abstractions in sustainable ICT research and practice – namely the technological, the social, and the sustainable – contribute to an overly optimistic discourse of sustainable ICT, which favors certain research approaches and practical applications. Adhering to such a discourse risks reinforcing, rather than breaking loose from, an unsustainable status quo. Through problematization, this thesis aims to unveil and challenge such underlying assumptions and understandings, based on insights from the social...

  • Functional Traits in Sphagnum

    Author: Fia Bengtsson
    Publication date: 2019-02-21 10:04

    Peat mosses (Sphagnum) are ecosystem engineers that largely govern carbon sequestration in northern hemisphere peatlands. I investigated functional traits in Sphagnum species and addressed the questions: (I) Are growth, photosynthesis and decomposition and the trade-offs between these traits related to habitat or phylogeny?, (II) Which are the determinants of decomposition and are there trade-offs between metabolites that affect decomposition?, (III) How do macro-climate and local environment determine growth in Sphagnum across the Holarctic?, (IV) How does N2 fixation vary among different species and habitats?, (V) How do species from different microtopographic niches avoid or tolerate desiccation, and are leaf and structural traits adaptations to growth high above the water table?

    Photosynthetic rate and decomposition in laboratory conditions (innate growth and decay resistance) were related to growth and decomposition in their natural habitats. We found support for a trade-off between growth and decay resistance, but innate qualities translated differently to field responses in different species. There were no trade-offs between production of...

  • Insights into Covalent Chemistry for the Developmen­t of Biomaterials

    Author: Daniel Bermejo-Velasco
    Publication date: 2019-02-21 08:12

    Covalent cross-linking chemistry is currently exploited in the preparation of biomaterial for biomedical applications. Choice of these chemistries for the preparation of biomaterials and bioconjugates strongly influences the biological output of these materials. Therefore, this thesis aims to develop novel bioconjugation strategies understanding their advantages and drawbacks. Our results provide new insight to adapt these chemical transformations for a specific application.

    The first part of this thesis points out the relevance of tuning different properties of biomaterials with specific emphasis on the development of hyaluronic acid (HA) hydrogels. The second part of the thesis describes how different chemical transformations including hydrazone formation (Paper I), thiazolidine formation (Paper II), cross-aldol addition reaction (Paper III) and disulfide formation (Paper IV) dictate material properties.

    This thesis explores both basic organic reaction mechanism and application of these reactions to influence material characteristics. The detailed study of the reaction conditions, kinetics, and...

  • Knowledge elicitation as abstraction of purposive behaviour

    Author: Anton Axelsson
    Publication date: 2019-02-15 08:43

    Researchers use knowledge elicitation methods to document expert knowledge for the primary purpose of understanding cognitive processes and with this understanding, technical solutions to resolve human factors issues can be produced. This dissertation offers a novel perspective on knowledge elicitation as an abstraction process. Such a theoretical framework has emerged by consolidating the ecological approach of Brunswikian psychology with the ideas of tacit and personal knowledge of Polanyian epistemology. Traditionally, knowledge elicitation has been considered an extraction process in which knowledge can be readily transferred from one individual to another. Here, this traditional position is rejected in favour of Polanyi’s premise that much of the knowledge individuals possess is tacit in nature, which implies that it cannot be documented easily, expressed in explicit form or explained. In this dissertation, knowledge is characterised as a personal process of knowing, highlighting context as a subjective knowledge structure of personal experiences that is formulated implicitly and indirectly over time through a dynamic interaction with the environment. Therefore, tacit...

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