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Sökning: WFRF:(Andersson Michael) > Doktorsavhandling

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1.
  • Andersson, Anders, 1957- (författare)
  • Numerical Conformal Mappings for Waveguides
  • 2009
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Acoustic or electro-magnetic scattering in a waveguide with varying direction and cross-section can be re-formulated as a two-dimensional scattering problem, provided that the variations take place in only one dimension at a time. By using the so-called Building Block Method, it is possible to construct the scattering properties of a combination of scatterers when the properties of each scatterer are known. Hence, variations in the waveguide geometry or in the boundary conditions can be treated one at a time. Using the Building Block Method, the problem takes the form of the Helmholtz equation for stationary waves in a waveguide of infinite length and with smoothly varying geometry and boundary conditions. A conformal mapping is used to transform the problem into a corresponding problem in a straight horizontal waveguide, and by expanding the field in Fourier trigonometric series, the problem can be reformulated as an infinite-dimensional ordinary differential equation. From this, numerically solvable differential equations for the reflection and transmission operators are derived. To be applicable in the Building Block Method, the numerical conformal mapping must be constructed such that the direction of the boundary curve can be controlled. At the channel ends ,it is an indispensable requirement, that the two boundary curves are (at least) asymptotically parallel and straight. Furthermore, to achieve bounded operators in the differential equations, the boundary curves must satisfy different regularity conditions, depending on the boundary conditions. In this work, several methods to accomplish such conformal mappings are presented. The Schwarz–Christoffel mapping, which is a natural starting point and for which also efficient numerical software exists, can be modified in different ways in order to achieve polygons with rounded corners. We present algorithms by which the parameters in the mappings can be determined after such modifications. We show also how the unmodified Schwarz–Christoffel mapping can be used for regions with a smooth boundary. This is done by constructing an appropriate outer polygon to the considered region.Finally, we introduce one method that is not Schwarz–Christoffel-related, by showing how one of the so-called zipper algorithms can be used for waveguides. Keywords: waveguides, building block method, numerical conformalmappings, Schwarz–Christoffel mapping, rounded corners method, approximate curve factors, outer polygon method, boundary curvature, zipper method, geodesic algorithm, acoustic wave scattering, electro-magnetic wave scattering
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2.
  • Andersson, Gunnar, 1972- (författare)
  • Differentiation and Pathogenicity within the Saprolegniaceae : Studies on Physiology and Gene Expression Patterns in Saprolegnia parasitica and Aphanomyces astaci
  • 2001
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Saprolegnia parasitica and Aphanomyces astaci are parasitic water moulds belonging to the Oomycetes. Despite their importance as parasites they are very little studied at the molecular level and the work described in this thesis was aimed at increasing the molecular knowledge of these organisms by cloning and characterising genes of potential importance for reproduction and pathogenicity.Stage-specific transcripts from Saprolegnia parasitica were isolated by differential display RT-PCR. One of the markers, puf1 encodes a putative mRNA binding protein which may be involved in post-transcriptional regulation of gene expression. S. parasitica puf1 is expressed exclusively in spore cysts that have not been determined for germination or repeated zoospore emergence indicating that the cyst stage has two phases, of about equal duration, which are physiologically and transcriptionally distinct. A similar expression pattern is observed in Aphanomyces spp. with different regulation of spore development and in the transcript is detected in both primary and secondary cysts.A putative chitinase AaChi1, was cloned from the crayfish plague fungus, Aphanomyces astaci. Analysis of chitinase activity and AaChi1 expression showed that chitinase in A. astaci is constitutively expressed in growing and sporulating mycelia, but absent in zoospores, a pattern which reflects the infectious life cycle of A. astaci. This expression pattern is conserved between the four known genotypes of A. astaci, in contrast to saprophytic and fish-pathogenic Aphanomyces spp. Genetic and physiological analysis were conducted on five strains of Aphanomyces, isolated from suspected outbreaks of crayfish plague in Spain and Italy. The strains are not virulent against freshwater crayfish, and RAPD PCR and ITS sequence analysis show that they are unrelated to the crayfish plague fungus, A. astaci.
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3.
  • Andersson, Josefine, 1988- (författare)
  • Insurances against job loss and disability : Private and public interventions and their effects on job search and labor supply
  • 2017
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Essay I: Employment Security Agreements, which are elements of Swedish collective agreements, offer a unique opportunity to study very early job search counselling of displaced workers. These agreements provide individual job search assistance to workers who are dismissed due to redundancy, often as early as during the period of notice. Compared to traditional labor market policies, the assistance provided is earlier and more responsive to the needs of the individual worker. In this study, I investigate the effects of the individual counseling and job search assistance provided through the Employment Security Agreement for Swedish blue-collar workers on job finding and subsequent job quality. The empirical strategy is based on the rules of eligibility in a regression discontinuity framework. I estimate the effect for workers with short tenure, who are dismissed through mass-layoffs. My results do not suggest that the program has an effect on the probability of becoming unemployed, the duration of unemployment, or income. However, the results indicate that the program has a positive effect on the duration of the next job.Essay II: The well-known positive relationship between the unemployment benefit level and unemployment duration can be separated into two potential sources; a moral hazard effect, and a liquidity effect pertaining to the increased ability to smooth consumption. The latter is a socially optimal response due to credit and insurance market failures. These two effects are difficult to separate empirically, but the social optimality of an unemployment insurance policy can be evaluated by studying the effect of a non-distortionary lump-sum severance grant on unemployment durations. In this study, I evaluate the effects on unemployment duration and subsequent job quality of a lump-sum severance grant provided to displaced workers, by means of a Swedish collective agreement. I use a regression discontinuity design, based on the strict age requirement to be eligible for the grant. I find that the lump-sum grant has a positive effect on the probability of becoming unemployed and the length of the completed unemployment duration, but no effect on subsequent job quality. My analysis also indicates that spousal income is important for the consumption smoothing abilities of displaced workers, and that the grant may have a greater effect in times of more favorable labor market conditions.Essay III: Evidence from around the world suggest that individuals who are awarded disability benefits in some cases still have residual working capacity, while disability insurance systems typically involve strong disincentives for benefit recipients to work. Some countries have introduced policies to incentivize disability insurance recipients to use their residual working capacities on the labor market. One such policy is the continuous deduction program in Sweden, introduced in 2009. In this study, I investigate whether the financial incentives provided by this program induce disability insurance recipients to increase their labor supply or education level. Retroactively determined eligibility to the program with respect to time of benefit award provides a setting resembling a natural experiment, which could be used to estimate the effects of the program using a regression discontinuity design. However, a simultaneous regime change of disability insurance eligibility causes covariate differences between treated and controls, which I adjust for using a matching strategy. My results suggest that the financial incentives provided by the program have not had any effect on labor supply or educational attainment.
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4.
  • Andersson, Linus, 1985- (författare)
  • Essays on Family Dynamics : Partnering, Fertility and Divorce in Sweden
  • 2019
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Diversity in household and family structures poses interesting questions for scientific inquiry. What accounts for patterns of reproduction, partnering, household formation and household dissolution? This dissertation investigates facets of this question in the context of modern Sweden from a longitudinal and individual level perspective. It consists of three empirical studies using data from administrative registers and panel survey data. The first study begins with noting a rapid expansion in online education and analyzes whether this development leads to higher fertility in student populations. The second study asks whether individuals’ predispositions towards divorce change after exposure to the experience of parenthood, union formation and union dissolution. The third study builds on the literature on assortative mating and investigates what drives underlying preferences for this behavior. 
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5.
  • ANDERSSON, MICHAEL, 1988 (författare)
  • Characterisation and Modelling of Graphene FETs for Terahertz Mixers and Detectors
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Graphene is a two-dimensional sheet of carbon atoms with numerous envisaged applications owing to its exciting properties. In particular, ultrahigh-speed graphene field effect transistors (GFETs) are possible due to the unprecedented carrier velocities in ideal graphene. Thus, GFETs may potentially advance the current upper operation frequency limit of RF electronics. In this thesis, the practical viability of high-frequency GFETs based on large-area graphene from chemical vapour deposition (CVD) is investigated. Device-level GFET model parameters are extracted to identify performance bottlenecks. Passive mixer and power detector terahertz circuits operating above the present active GFET transit time limit are demonstrated. The first device-level microwave noise characterisation of a CVD GFET is presented. This allows for the de-embedding of the noise parameters and construction of noise models for the intrinsic device. The correlation of the gate and drain noise in the PRC model is comparable to that of Si MOSFETs. This indicates higher long-term GFET noise relative to HEMTs. An analytical power detector model derived using Volterra analysis on the FET large-signal model is verified at frequencies up to 67 GHz. The drain current derivatives, intrinsic capacitors and parasitic resistors of the closed-form expressions for the noise equivalent power (NEP) are extracted from DC and S-parameter measurements. The model shows that a short gate length and a bandgap in the channel are required for optimal FET sensitivity. A power detector integrated with a split bow-tie antenna on a Si substrate demonstrates an optical NEP of 500 pW/Hz^1/2 at 600 GHz. This represents a state-of-the-art result for quasi-optically coupled, rectifying direct detectors based on GFETs operating at room temperature. The subharmonic GFET mixer utilising the electron-hole symmetry in graphene is scaled to operate with a centre frequency of 200 GHz, the highest frequency reported so far for graphene integrated circuits. The down-converter circuit is implemented in a coplanar waveguide (CPW) on Si and exhibits a conversion loss (CL) of 29 ± 2 dB in the 185-210 GHz band. In conclusion, the CVD GFETs in this thesis are unlikely to reach the performance required for high-end RF applications. Instead, they currently appear more likely to compete in niche applications such as flexible electronics.
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6.
  • Andersson, Michael (författare)
  • High Strain Fatigue Crack Growth and Crack Closure
  • 2005
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Understanding of the growth of fatigue cracks is of utmost importance since such growth often has a profound influence on the life of components subjected to cyclic loading. Thus, reliable fatigue life models enable a more efficient use of materials and improve the performance and efficiency in many applications. This thesis deals with the growth of fatigue cracks subjected to high load amplitudes. One mechanism that is known to have a strong influence on fatigue crack growth is crack closure, i.e. premature contact between the crack surfaces, caused by for instance residual plastic deformation, crack surface asperities or oxidation of the crack surfaces. Closure can reduce the effective load driving the crack, thus influencing the crack growth rate. In this thesis the use of the electrical potential drop (PD) technique for crack closure measurements has been investigated by a combination of numerical simulations and experiments. It has been shown that crack closure has a strong influence on the variations of the PD-value during a load cycle. Also, crack opening measured with the PDtechnique is consistent with closure measurements made from in situ fatigue crack growth observations using a scanning electron microscope. Furthermore, the growth of fatigue cracks subjected to high load amplitudes in Ti-6Al-4V at room temperature and Inconel 718 at 650°C has also been studied. It has been shown that crack growth under high strain amplitudes can be analysed using a strain intensity approach or by using the cyclic J-integral. In both cases crack closure plays an important role and must be accounted for by using an effective strain intensity range or an effective cyclic J-integral. For Inconel 718 at 650°C, crack growth occurs by a combination of cyclic and time dependent growth, and thus the load frequency is of importance for the crack growth rate. A crack growth law based on the product between the effective cyclic J-integral and a function compensating for the frequency was proposed. Finally an in situ SEM study of crack growth in an aluminium alloy was performed. It was shown that on a micro scale crack growth is a highly irregular process that is strongly influenced by the local microstructure at the crack tip. Also, there is no correlation between the local crack opening and the crack growth rate. Thus, it is difficult to predict the crack growth during an individual load cycle, but for crack growth rate on a macro scale the stress intensity range, compensated for crack closure, is a proper measure.
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7.
  • Andersson, Michael K. (författare)
  • On testing and forecasting in fractionally integrated time series models
  • 1998
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This volume contains five essays in the field of time series econometrics. All five discuss properties of fractionally integrated processes and models. The first essay, entitled Do Long-Memory Models have Long Memory?, demonstrates that fractional integration can enhance the memory of ARMA processes enormously. This is however not true for all combinations of diffe-rencing, autoregressive and moving average parameters. The second essay, with the title On the Effects of Imposing or Ignoring Long-Memory when Forecasting, investigates how the choice between mo-delling stationary time series as ARMA or ARFIMA processes affect the accu-racy of forecasts. The results suggest that ignoring long-memory is worse than imposing it and that the maximum likelihood estimator for the ARFIMA model is to prefer. The third essay, Power and Bias of Likelihood Based Inference in the Cointegration Model under Fractional Cointegration, investigates the performance of the usual cointegration approach when the processes are fractionally cointegrated. Under these circumstances, it is shown that the maximum likelihood estimates of the long-run relationship are severely biased. The fourth and fifth essay, entitled respectively Bootstrap Testing for Fractional Integration andRobust Testing for Fractional Integration using the Bootstrap, propose and investigate the performance of some bootstrap testing procedures for fractional integration. The results suggest that the empirical size of a bootstrap test is (almost) always close to the nominal, and that a well-designed bootstrap test is quite robust to deviations from standard assumptions.
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8.
  • Andersson, Michael, 1980 (författare)
  • Muscarinic Receptors in the Urinary Bladder - The role of the urothelium regarding cholinergic and nitrergic effects in inflammation
  • 2010
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Inflammation alters the functional properties of the urinary bladder. Interstitial cystitis (IC) is a chronic inflammatory syndrome in man that is characterized by urgency, frequency and visceral pain. The overall aim of this study was to investigate how the rat urinary bladder is affected by inflammation, and what specific part the urothelium plays in this. Methods: Cystitis was induced in rats by a single injection of cyclophosphamide (CYP; 100 mg/kg). This treatment causes a disease state which is highly comparable to IC. Data comparing the properties of the healthy and inflamed bladder were gathered from (1) contraction experiments in vitro in an organ bath setup, (2) cystometrical studies in vivo in anaesthetized rats and (3) wake, freely moving rats in a metabolism cage. Cell cultures were also cultivated in order to investigate if proliferation of urothelial cells is influenced by receptor activation. Key findings: Induction of cystitis by CYP altered the cholinergic response of the urinary bladder. In vitro studies showed a significantly lower response to carbachol in the inflamed bladder. Both in vitro and in vivo, the altered cholinergic response could be normalized by either removal of the urothelium, blockade of nitric oxide (NO) synthase or blockade of muscarinic M1/M3/M5 receptors. These findings indicate that during CYP-induced cystitis, NO is released from the urothelium upon muscarinic receptor activation. Further characterization in vitro revealed the M5 receptor as the most likely candidate for mediating this release. In vivo experiments in the metabolism cage showed that micturition parameters are affected by CYP-induced cystitis. Increasing doses of a muscarinic antagonist eliminated these differences, and a connection between the effects of antimuscarinic and antinitrergic drugs was indicated. These findings underline the importance of muscarinic receptors and NO in the alterations seen during cystitis. Proliferation experiments indicated that adrenergic, but not muscarinic, nicotinic or EGF receptors, are involved in the regulation of urothelial cell proliferation. Conclusions: In CYP-induced cystitis in the rat, the urothelium exerts an inhibitory influence on the cholinergic response of the urinary bladder. We conclude that this is caused by the release of NO upon activation of urothelial muscarinic M5 receptors.
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9.
  • Andersson, Michael R., 1974- (författare)
  • Functional aspects of inorganic phosphate transport
  • 2012
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Inorganic phosphate is an essential nutrient for all organisms. It is required for many cellular components as nucleic acids and phospholipids, and as energy-carrying compounds such as ATP. Thus, a regulated uptake of this pivotal nutrient is of outermost importance. Depending of the availability of phosphate in the surroundings the yeast Saccharomyces cerevisiae make use of two different systems for transporting phosphate into the interior of the cell: a low-affinity system that is active during surplus phosphate conditions and a high-affinity system that is active when the availability becomes limited. This thesis focuses on the high-affinity system, which is comprised of the Pho84 and Pho89 transporters. Of the two transporters, Pho84 is the predominant one, responsible for almost all phosphate uptake during low phosphate conditions, and the contribution of Pho89 is of minor importance. Hence Pho84 is by far the most well characterized phosphate transporter. Even though much is known about phosphate transporters in yeast little in known about how phosphate is transported. The work in this thesis aims to broaden the knowledge about the transport mechanism by the means of site-directed mutagenesis and functional characterization. Also the similarity of Pho84 to glucose sensors and the potential role of conserved residues in phosphate signaling are investigated. By the use of a high-affinity system deletion strain (∆Pho84 ∆Pho89), we also managed to investigate the functional importance of well conserved residues in Pho89. In summary: the work presented in this thesis has contributed to increase the knowledge about transport mechanisms in phosphate transporters.
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10.
  • Andersson, Peter, 1981- (författare)
  • Fast-Neutron Tomography using a Mobile Neutron Generator for Assessment of Steam-Water Distributions in Two-Phase Flows
  • 2014
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis describes the measurement technique of fast-neutron tomography for assessing spatial distributions of steam and water in two-phase flows. This so-called void distribution is of importance both for safe operation and for efficient use of the fuel in light water reactors, which compose the majority of the world’s commercial nuclear reactors. The technique is aimed for usage at thermal-hydraulic test loops, where heated two-phase flows are being investigated under reactor-relevant conditions.By deploying portable neutron generators in transmission tomography, the technique becomes applicable to stationary objects, such as thermal-hydraulic test loops. Fast neutrons have the advantage of high transmission through metallic structures while simultaneously being relatively sensitive to the water/void content. However, there are also challenges, such as the relatively low yield of commercially available fast-neutron generators, the tendency of fast neutrons to scatter in the interactions with materials and the relatively low efficiency encountered in fast-neutron detection.The thesis describes the design of a prototype instrument, FANTOM, which has been assembled and demonstrated. The main design parameters have been optimized to achieve maximal signal count rate in the detector elements, while simultaneously reaching an image unsharpness of ≤0.5 mm. Radiographic projections recorded with the assembled instrument are presented, and the performance parameters of FANTOM are deduced.Furthermore, tomographic reconstruction methods for axially symmetric objects, which is relevant for some test loops, have been developed and demonstrated on measured data from three test objects. The attenuation distribution was reconstructed with a radial resolution of 0.5 mm and an RMS error of 0.02 cm-1, based on data recorded using an effective measurement time of 3.5 hours per object. For a thermal-hydraulic test loop, this can give a useful indication of the flow mode, but further development is desired to improve the precision of the measurements.Instrument upgrades are foreseen by introducing a more powerful neutron generator and by adding detector elements, speeding up the data collection by several orders of magnitude and allowing for higher precision data. The requirements and performance of an instrument for assessment of arbitrary non-symmetric test loops is discussed, based on simulations.
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