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Sökning: WFRF:(Persson Daniel) > Doktorsavhandling

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1.
  • Ask, Per, 1978- (författare)
  • Invasion of top and intermediate consumers in a size structured fish community
  • 2010
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • In this thesis I have investigated the effects of invading top and intermediate consumers in a size-structured fish community, using a combination of field studies, a lake invasion experiment and smaller scale pond and aquaria experiments. The lake invasion experiment was based on introductions of an intermediate consumer, ninespine stickleback (Pungitius pungitius L.), in to allopatric populations of an omnivorous top predator, Arctic char (Salvelinus alpinus L.). The invasion experiment was performed in two tundra lakes and in two birch forest lakes to investigate the effect of climate on the invasion success. I found that the effect of sticklebacks on char was size dependent. Small char suffered reduced growth from resource competition with sticklebacks whereas the maximum size of adult char increased from the addition of a larger prey resource, stickleback. The negative effect of sticklebacks on the growth of small char suggests that sticklebacks may be a better resource competitor than char, which was also supported by the pond and aquaria experiments. The pond experiments also suggested that char were more efficient cannibals than interspecific predators on sticklebacks. Cannibalism in char may limit the recruitment of char and decrease both their predatory and competitive effect on coexisting species and thereby also promote the coexistence of char and sticklebacks. The successful invasion by sticklebacks and their subsequent increases in density suggest that the absence of sticklebacks in char lakes in this region is not caused by biotic interactions with char. Instead, it may be suggested that co-occurrence of sticklebacks and char in the region is limited by dispersal. The char – stickleback system resembles an intraguild predation system with char as the top consumer and stickleback as the intermediate consumer. The effects of the stickleback invasion is also contrasted with a field study of a northern pike (Esox lucius L.) invasion into a system with coexisting char and stickleback, where pike can be viewed as the top consumer and char as the intermediate consumer both feeding on sticklebacks. In this case pike excluded char. The identity of the invading species and the relative strength of the predatory and competitive interactions in the two contrasting systems are discussed in relation to coexistence in intraguild predation systems. I found that the identity of the invading species is of crucial importance for the response at the ecosystem level, and that the inherent size dependency of competitive and predatory interactions in fish communities is important for attaining a mechanistical understanding of the effects of invasive species in lake ecosystems.
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2.
  • Azeez, Ahmed, 1991- (författare)
  • High-Temperature Durability Prediction of Ferritic-Martensitic Steel
  • 2023
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Materials used for high-temperature steam turbine sections are generally subjected to harsh environments with temperatures up to 625 °C. The superior creep resistance of 9–12 % Cr ferritic-martensitic steels makes them desirable for those critical steam turbine components. Additionally, the demand for fast and frequent steam turbine start-ups, i.e. flexible operations, causes accelerated fatigue damage in critical locations, such as grooves and notches, at the high-temperature inner steam turbine casing. A durability assessment is necessary to understand the material behaviour under such high temperatures and repeated loading, and it is essential for life prediction. An accurate and less conservative fatigue life prediction approach is achieved by going past the crack initiation stage and allowing controlled growth of cracks within safe limits. Besides, beneficial load-temperature history effects, i.e. warm pre-stressing, must be utilised to enhance the fracture resistance to cracks. This dissertation presents the high-temperature durability assessment of FB2 steel, a 9-12 % Cr ferritic-martensitic steam turbine steel.Initially, isothermal low-cycle fatigue testing was performed on FB2 steel samples. A fatigue life model based on finite element strain range partitioning was utilised to predict fatigue life within the crack initiation phase. Two fatigue damage regimes were identified, i.e. plastic- and creep-dominated damage, and the transition between them depended on temperature and applied total strain. Cyclic deformation and stress relaxation behaviour were investigated to produce an elastic-plastic and creep material model that predicts the initial and mid-life cyclic behaviour of the FB2 steel.Furthermore, the thermomechanical fatigue crack growth behaviour of FB2 steel was studied. Crack closure behaviour was observed and accounted for numerically and experimentally, where crack growth rate curves collapsed into a single curve. Interestingly, the collapsed crack growth curves coincided with isothermal crack growth tests performed at the minimum temperature of the thermomechanical crack growth tests. In addition, hold times and changes in the minimum temperature of the thermomechanical fatigue cycle did not influence crack closure behaviour.Finally, warm pre-stressing effects were explored for FB2 steel. A numerical prediction model was produced to predict the increase in the apparent fracture toughness. Warm pre-stressing effects can benefit the turbine life by enhancing fracture resistance and allowing longer fatigue cracks to grow within safe limits.
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3.
  • Cumming, Joshua, 1991- (författare)
  • The identification and functional evaluation of novel cancer-associated fibroblast subtypes and matrisome proteins in pancreatic cancer
  • 2023
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal malignancy characterized by an extensive desmoplastic stroma. The stroma is the site of intricate communication between malignant cells and their surrounding environment. This tissue microenvironment (TME) is populated by a heterogenous mixture of cell types and extracellular matrix proteins. Distinct stromal elements confer tumour-restraining or tumour-promoting influences on tumorigenesis. Characterizing stromal composition therefore represents an opportunity to identify candidates for therapeutic intervention to facilitate improved clinical outcomes. In this thesis we identify galectin-4 as an extracellular matrix protein which is upregulated in PDAC. We find that galectin-4 exerts a pro-tumorigenic influence in PDAC through promoting immune suppression, highlighting its potential as a novel therapeutic target. We subsequently provide a comprehensive characterization of mesenchymal cell diversity in PDAC including cancer-associated fibroblasts (CAFs) which represent one of the dominant stromal cellular components. We identify inflammatory CAF (iCAF) and myofibroblastic CAF (myCAF) subtypes in addition to defining a novel interferon-response CAF (ifCAF) subtype. In addition, we demonstrate that pancreatic stellate cells (PSCs) are capable of forming iCAFs, myCAFs and ifCAFs in response to tumour-derived signals using an organoid-based co-culture model and define biological pathways regulating CAF subtype formation. We then perform a high-throughput drug-screen using this co-culture model to identify compounds which can suppress tumour growth indirectly through modifying CAFs. One such compound is GNF-5 which we show can suppress cancer cell proliferation indirectly through manipulating CAF phenotype. Taken together, this thesis augments our understanding of the composition of the PDAC stroma and identifies potential therapeutic targets as well as developing an approach to discover drugs which yield a therapeutic benefit through targeting the PDAC stroma.   
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4.
  • Daniel, Aemiro Melkamu, 1988- (författare)
  • Towards sustainable energy consumption : Electricity demand flexibility and household fuel choice
  • 2020
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Paper [I] investigates household heterogeneity in valuing electricity contract attributes that include various load controls and information sharing to induce demand flexibility. Using a stated preference choice experiment conducted with Swedish households, this paper shows that, although a large proportion of households ask for substantial compensation, some households are willing to share their electricity consumption information and require relatively lower compensation to allow load controls. In addition, this paper finds that some households that are willing to provide flexibility by accepting load controls at a relatively low compensation ask for sizable compensation to share their electricity consumption information, and vice versa. From the perspective of the contract providers,Paper [II] uses a flexible model to accommodate heterogeneous decision rules in analysing data obtained from a discrete choice experiment aimed at eliciting Swedish households’ willingness to accept compensation for restrictions on household electricity and heating use during peak hours. The model combines behavioural processes based on random utility maximization with an elimination-by-aspects strategy, where the latter involves a two-stage decision process. In the first stage, respondents are allowed to eliminate from their choice set alternatives that contain an unacceptable level, in this case restrictions on the use of heating and electricity. In the second stage, respondents choose between the remaining alternatives in a rational utility maximizing manner. Our results show that about half of the respondents choose according to an elimination-by-aspects strategy, and considering elimination-by-aspects behaviour leads to a downward shift in elicited willingness-to-accept.Paper [III] tests the effect of a pro-environmental framing on households’ stated willingness to accept restrictions on their electricity use. We use a split-sample choice experiment and ask respondents to choose between their current electricity contract and hypothetical contracts featuring various load controls and monetary compensation. Our results indicate that the pro-environmental framing has little impact on the respondents’ choices. We observe a significant framing effect on choices and marginal willingness-to-accept for only a few contract attributes. The results further suggest that there is no significant framing effect among households that are already engaged in pro-environmental activities.Paper [IV] explores the socio-demographic and housing characteristics that influence household fuel choice and fuel use decisions in urban Ethiopia. The results indicate that, whereas households with a female head are more likely to combine traditional solid (firewood and charcoal) and modern (electricity) fuels for different uses, households with less-educated heads, many family members, and poor living conditions (fewer rooms) tend to use traditional solid biomass fuels. We find that households with an individual electricity meter are significantly less likely to use charcoal. Further, the results show the satiation effect of the increasing use of a fuel by households is relatively higher for firewood and lower for electricity.
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5.
  • Ferm, Lisa, 1984- (författare)
  • Vocational Students’ Agency in Identity Formation as Industrial Workers
  • 2021
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The overall aim of this thesis is to contribute knowledge about vocational identity formation among students within the industrial programme in Swedish upper secondary education, with a particular focus on their workplace-based learning. To break down the aim, three research questions have been formulated and each is addressed in one or two specific articles. These questions are: (1) What learning strategies do vocational students use to become part of a work community, and how do these strategies relate to the formation of a vocational identity at the workplace? (2) How do vocational students experience their identity formation in relation to a vocation within the industrial sector? (3) How do vocational students handle the division between theoretical and practical knowledge as they learn to become skilled industrial workers? The thesis builds on 53 semi-structured qualitative interviews with Swedish upper secondary vocational students enrolled on the industrial programme. The interviews revolve around the students’ vocational identity formation, with a focus on their workplace-based learning. The students are between 18 and 20 years old and the majority are boys. The findings are analysed through the theoretical lens of situated learning, where identity formation is viewed as a social learning process that takes place through participation in communities of practice. In addition, the concepts of habitus, gender and social categorisation are used as analytical tools to provide a deeper understanding of issues concerning status, power and exclusion in relation to vocational identity formation. The findings reveal that the students’ vocational identity formation is closely connected to the social aspects of participating in workplace communities. Knowledge about the jargon and social norms of the workplace seem to be of more importance for vocational identity formation than knowledge about the concrete working tasks. The study follows the students’ vocational identity formation throughout their vocational learning trajectories, which reveal that vocational identities are formed in heterogenic ways. The students may adopt a committed, flexible or ambivalent approach towards industrial work. Aspects concerning agency and status seem to be crucial for the vocational identification process. The forming of a vocational identity also implies positioning oneself in the hierarchy and division between theoretical and practical knowledge, as well as between masculinity and femininity. The students appear as knowledgeable actors who are aware of the generally low status of industrial work, while simultaneously expressing a great deal of pride in relation to their intended vocations. In the discussion, a model of the students’ vocational identity formation is proposed to capture the interplay between collective and structural dimensions (e.g. social background, class and status hierarchies at school) and students’ agency and strategies in becoming industrial workers. From the findings of this thesis, three main conclusions are drawn: (1) The students form vocational identities through using vocational agency in actively developing strategies for becoming accepted in the workplace community; (2) Workplace-based learning is central for the students’ vocational identity formation, in spite of the relatively short time that the students spend there, compared to the time spent at school; (3) The students’ vocational image awareness, expressed through awareness of, and reactions to, other people’s images of their vocation, constitutes an important part of their vocational identity formation. 
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6.
  • Lundesjö Kvart, Susanne, 1969- (författare)
  • Konsten att undervisa ryttare : En studie om ridlärares pedagogiska praktik
  • 2020
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis concerns how riding teachers’ reflect upon their teaching and how they teach. The overall aim of the thesis is to contribute with knowledge about how riding lessons are organized, regarding both the accomplishment of lessons as activity systems and the interactional organization of instructional work. Horseback riding can be understood as an embodied and practical knowledge, and includes communication and collaboration between horse and rider, sometimes referred to as equestrian feel (Dashper, 2016). Two different theoretical perspectives were applied to explore how such knowledge is taught. Activity theory (Engeström, 1987) was used to analyze interviews with and observations of ten riding teachers about their understanding and implementation of their pedagogical practice. Teachers’ and students’ interactions during riding lessons were examined using an ethnomethodological and conversation analytic approach (Goodwin, 2000; Schegloff, 1996). The analyses were based on video-recordings of ten group lessons and 40 one-on-one riding lessons.The results show that riding lessons can be conceptualized as activity systems where the focus of teaching constantly changes; from horse to student to routine. Traditions and safety regulations are shown to generate contradictions that may hinder the teachers from developing their teaching. However, the teachers express a wish to use more student collaborative methods, and display an intention to communicate with students about equestrian feel. Another result unveils how the teacher and the individual students, within the mobile context of riding lessons, make instructional sequences possible by co-creating instructional spaces. A third result illuminates the participants’ collaborative work to make equestrian feel available for instruction. The teacher molds equestrian feel through online instructions, i.e., instructions produced during the students’ active riding. These instructions shift focus between the students’ seat and influence, the horse’s actions and the student’s embodied feel. Moreover, the teachers are shown to use visual, verbal and embodied resources as they interpret equestrian feel for the student. In sum, the studies shed light on the complex art of teaching practical and embodied knowledge of riding.
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7.
  • Lövheim, Daniel, 1975- (författare)
  • Att inteckna framtiden : Läroplansdebatter gällande naturvetenskap, matematik och teknik i svenska allmänna läroverk 1900-1965
  • 2006
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This dissertation deals with curriculum debates concerning science, mathematics and technology within the Swedish secondary schools between 1900 and 1965. The aim of the study is to analyze different conceptions of these school subjects. How were they looked upon and presented during the debates? What kind of values and functions were they said to promote? I also discuss more principal questions regarding why the school subjects became objects for debate. Why did different actors engage in these discussions? A leading perspective of the study is that schools and their curricula often are used as arenas for larger debates concerning the role of science and technology in society. One of the reasons for this, I argue, is that a curriculum often is seen as representing a standpoint in these broader societal discussions. In the study, this is shown through a number of different debates concerning, for example, the amount of hours devoted to science, student conducted experiments, eugenics and environmental aspects. All of these issues demonstrate how the shape of curriculum was seen as having wider significance for society. An important reason for this is the connection between curriculum and future perspectives. Situations where a curriculum was to be rewritten were apprehended as opportunities to influence and secure the future. Much previous work on curriculum matters has been occupied with the question of what curriculum “does” in schools; it includes and excludes certain ideas, it regulates and governs students to understand and reason about themselves in specific ways. In relation to this, my own research deals with what curriculum – or rather the image of curriculum – does to other groups in society. Through analyzing the engagement of teachers, politicians, authors and editors, I conclude that a curriculum can threaten or promise future perspectives and values that are important to these groups.
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8.
  • Persson, Daniel, 1978 (författare)
  • Arithmetic and Hyperbolic Structures in String Theory
  • 2009
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis consists of an introductory text followed by two separate parts which may be read independently of each other. In Part I we analyze certain hyperbolic structures arising when studying gravity in the vicinity of spacelike singularities (the BKL-limit). In this limit, spatial points decouple and the dynamics exhibits ultralocal behaviour which may be mapped to an auxiliary problem given in terms of a (possibly chaotic) hyperbolic billiard. In all supergravities arising as low-energy limits of string theory or M-theory, the billiard dynamics takes place within the fundamental Weyl chambers of certain hyperbolic Kac-Moody algebras, suggesting that these algebras generate hidden infinite-dimensional symmetries of gravity. We investigate the modification of the billiard dynamics when the original gravitational theory is formulated on a compact spatial manifold of arbitrary topology, revealing fascinating mathematical structures known as galleries. We further use the conjectured hyperbolic symmetry E10 to generate and classify certain cosmological (S-brane) solutions in eleven-dimensional supergravity. Finally, we show in detail that eleven-dimensional supergravity and massive type IIA supergravity are dynamically unified within the framework of a geodesic sigma model for a particle moving on the infinite-dimensional coset space E10/K(E10). Part II of the thesis is devoted to a study of how (U-)dualities in string theory provide powerful constraints on perturbative and non-perturbative quantum corrections. These dualities are typically given by certain arithmetic groups G(Z) which are conjectured to be preserved in the effective action. The exact couplings are given by moduli-dependent functions which are manifestly invariant under G(Z), known as automorphic forms. We discuss in detail various methods of constructing automorphic forms, with particular emphasis on a special class of functions known as (non-holomorphic) Eisenstein series. We provide detailed examples for the physically relevant cases of SL(2,Z) and SL(3,Z), for which we construct their respective Eisenstein series and compute their (non-abelian) Fourier expansions. We also discuss the possibility that certain generalized Eisenstein series, which are covariant under the maximal compact subgroup K(G), could play a role in determining the exact effective action for toroidally compactified higher derivative corrections. Finally, we propose that in the case of rigid Calabi-Yau compactifications in type IIA string theory, the exact universal hypermultiplet moduli space exhibits a quantum duality group given by the Picard modular group SU(2,1;Z[i]). To verify this proposal we construct an SU(2,1;Z[i])-invariant Eisenstein series, and we present preliminary results for its Fourier expansion which reveals the expected contributions from D2-brane and NS5-brane instantons.
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10.
  • Persson, Daniel H. E., 1974- (författare)
  • On the Mechanisms behind the Tribological Performance of Stellites
  • 2005
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis reveals the tribological mechanisms behind the intrinsic low friction potential of the Co-based family of alloys called Stellites. Although being an established and important group of materials, a satisfactory explanation to why they exhibit low-friction properties under severe sliding conditions has not previously been found in the literature. The main part of this thesis is dedicated to the clarification of the tribological performance of Stellites in highly loaded sliding contact. The results should assist the development of Co-free alternatives, suitable for replacing Stellites in nuclear applications. Owing to their beneficial properties they are today the most commonly used material in the sealing surfaces on gate valves in the primary circuits of boiling water reactors (BWR). The underlying reason for the replacement in the nuclear applications is an undesired contribution to the background radiation level, originating from the Co in the Stellite surfaces. The Stellites mainly consist of Cr-rich carbides in a solid solution dominated by Co. The commonly used Stellite 6 and Stellite 21 were chosen as primary test materials and applied by laser cladding, providing a metallically bonded clad layer with a fine dendritic microstructure. By combining information from a series of dedicated tribological tests and modern high-resolution analysis instruments (e.g. SEM, XRD and TEM) available at the Ångström Laboratory at Uppsala University, the following conclusions can be made regarding the tribological performance of Stellites under high load sliding. Mechanisms. The (tested) Stellites form a thick deformation hardened layer, topped with a superficial easily sheared layer of hcp basal planes aligned parallel to the worn surface. The easy-shear layer is continually regenerated, replacing worn off material. Technical benefits. The Stellites offer low-friction properties thanks to their easily sheared surface layers. The risk of severe galling is also avoided by restricting shear and adhesive transfer to very thin superficial layers. In closed sliding contacts, self-generated protective layers formed by re-deposition of wear fragments are also offered.
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