umu.sePublikationer
Ändra sökning
Avgränsa sökresultatet
23456 201 - 250 av 265
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Träffar per sida
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
Markera
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 201.
    Murtiyoso, Arnadi
    et al.
    INSA Strasbourg, France.
    Grussenmeyer, Pierre
    INSA Strasbourg, France.
    Börlin, Niclas
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Reprocessing Close Range Terrestrial and UAV Photogrammetric Projects with the DBAT Toolbox for Independent Verification and Quality Control2017Ingår i: LowCost 3D – Sensors, Algorithms, Applications: 5th International Workshop / [ed] T. P. Kersten, F. Remondino, 2017, Vol. XLII-2/W8, s. 171-177Konferensbidrag (Refereegranskat)
    Abstract [en]

    Photogrammetry has recently seen a rapid increase in many applications, thanks to developments in computing power and algorithms. Furthermore with the democratisation of UAVs (Unmanned Aerial Vehicles), close range photogrammetry has seen more and more use due to the easier capability to acquire aerial close range images. In terms of photogrammetric processing, many commercial software solutions exist in the market that offer results from user-friendly environments. However, in most commercial solutions, a black-box approach to photogrammetric calculations is often used. This is understandable in light of the proprietary nature of the algorithms, but it may pose a problem if the results need to be validated in an independent manner. In this paper, the Damped Bundle Adjustment Toolbox (DBAT) developed for Matlab was used to reprocess some photogrammetric projects that were processed using the commercial software Agisoft Photoscan. Several scenarios were experimented on in order to see the performance of DBAT in reprocessing terrestrial and UAV close range photogrammetric projects in several configurations of self-calibration setting. Results show that DBAT managed to reprocess PS projects and generate metrics which can be useful for project verification.

  • 202.
    Murtiyoso, Arnadi
    et al.
    INSA Strasbourg, France.
    Grussenmeyer, Pierre
    INSA Strasbourg, France.
    Börlin, Niclas
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Vandermeerschen, Julien
    Drone Alsace, 4 Rue Sainte-Catherine, 67000 Strasbourg, France.
    Freville, Tristan
    Drone Alsace, 4 Rue Sainte-Catherine, 67000 Strasbourg, France.
    Open Source and Independent Methods for Bundle Adjustment Assessment in Close-Range UAV Photogrammetry2018Ingår i: Drones, ISSN 2504-446X, Vol. 2, nr 1, artikel-id 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Close-range photogrammetry as a technique to acquire reality-based 3D data has, in recenttimes, seen a renewed interest due to developments in sensor technologies. Furthermore, the strong democratization of UAVs (Unmanned Aerial Vehicles) or drones means that close-rangephotogrammetry can now be used as a viable low-cost method for 3D mapping. In terms of software development, this led to the creation of many commercial black-box solutions (PhotoScan, Pix4D, etc.). This paper aims to demonstrate how the open source toolbox DBAT (Damped Bundle AdjustmentToolbox) can be used to generate detailed photogrammetric network diagnostics to help assess the quality of surveys processed by the commercial software, PhotoScan. In addition, the Apero module from the MicMac software suite was also used to provide an independent assessment of the results. The assessment is performed by the careful examination of some of the bundle adjustment metrics generated by both open source solutions. A UAV project was conducted on a historical church in the city center of Strasbourg, France, in order to provide a dataset with a millimetric level of precision. Results showed that DBAT can be used to reprocess PhotoScan projects under similar conditions and derive useful metrics from them, while Apero provides a completely independent verification of the results of commercial solutions. Overall, this paper shows that an objective assessment of photogrammetric results is important. In cases where problems are encountered in the project, this assessment method can be useful to detect errors that may not be explicitly presented by PhotoScan.

  • 203.
    Myllykoski, Mirko
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    A Task-Based Algorithm for Reordering the Eigenvalues of a Matrix in Real Schur Form2018Ingår i: Parallel Processing and Applied Mathematics: PPAM 2017 / [ed] Roman Wyrzykowski, Jack Dongarra, Ewa Deelman, Konrad Karczewski, Springer, 2018, s. 207-216Konferensbidrag (Refereegranskat)
    Abstract [en]

    A task-based parallel algorithm for reordering the eigenvalues of a matrix in real Schur form is presented.The algorithm is realized on top of the StarPU runtime system.Only the aspects which are relevant for shared memory machines are discussed here, but the implementation can be configured to run on distributed memory machines as well.Various techniques to reduce the overhead and the core idle time are discussed.Computational experiments indicate that the new algorithm is between 1.5 and 6.6 times faster than a state of the art MPI-based implementation found in ScaLAPACK.With medium to large matrices, strong scaling efficiencies above 60\% up to 28 CPU cores are reported.The overhead and the core idle time are shown to be negligible with the exception of the smallest matrices and highest core counts.

  • 204. Naik, Naren
    et al.
    Patil, Nishigandha
    Yadav, Yamini
    Eriksson, Jerry
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Pradhan, Asima
    Fully Nonlinear SP3 Approximation Based Fluorescence Optical Tomography2017Ingår i: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 36, nr 11, s. 2308-2318Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In fluorescence optical tomography, many works in the literature focus on the linear reconstruction problem to obtain the fluorescent yield or the linearized reconstruction problem to obtain the absorption coefficient. The nonlinear reconstruction problem, to reconstruct the fluorophore absorption coefficient, is of interest in imaging studies as it presents the possibility of better reconstructions owing to a more appropriate model. Accurate and computationally efficient forward models are also critical in the reconstruction process. The SPN approximation to the radiative transfer equation (RTE) is gaining importance for tomographic reconstructions owing to its computational advantages over the full RTE while being more accurate and applicable than the commonly used diffusion approximation. This paper presents Gauss-Newton-based fully nonlinear reconstruction for the SP3 approximated fluorescence optical tomography problem with respect to shape as well as the conventional finite-element method-based representations. The contribution of this paper is the Frechet derivative calculations for this problem and demonstration of reconstructions in both representations. For the shape reconstructions, radial-basis-function represented level-set-based shape representations are used. We present reconstructions for tumor-mimicking test objects in scattering and absorption dominant settings, respectively, for moderately noisy data sets in order to demonstrate the viability of the formulation. Comparisons are presented between the nonlinear and linearized reconstruction schemes in an element wise setting to illustrate the benefits of using the former especially for absorption dominant media.

  • 205.
    Nguyen, Quoc Khanh
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Serra-Capizzano, Stefano
    Wadbro, Eddie
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    On using a zero lower bound on the physical density in material distribution topology optimization2020Ingår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 359, artikel-id 112669Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The current paper studies the possibility of allowing a zero lower bound on the physical density in material distribution based topology optimization. We limit our attention to the standard test problem of minimizing the compliance of a linearly elastic structure subject to a constant forcing. First order tensor product Finite Elements discretize the problem. An elementwise constant material indicator function defines the discretized, elementwise constant, physical density by using filtering and penalization. To alleviate the ill-conditioning of the stiffness matrix, due to the variation of the elementwise constant physical density, we precondition the system. We provide a specific spectral analysis for large matrix sizes for the one-dimensional problem with Dirichlet-Neumann conditions in detail, even if most of the mathematical tools apply also in a d-dimensional setting, d >= 2. It is easy to find an elementwise constant material indicator function so that the resulting preconditioned system matrix is singular when allowing the vanishing physical densities. However, for a large class of material indicator functions, the corresponding preconditioned system matrix has a condition number of the same order as the system matrix for the case when the physical density is one in all elements. Finally, we critically report and illustrate results from numerical experiments: as a conclusion, it is indeed possible to solve large-scale topology optimization problems, allowing a vanishing physical density, without encountering ill-conditioned system matrices during the optimization.

  • 206.
    Nilsson, Gabriel
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Implementing and testing possible hedging strategies to minimise value fluctuations in a defaulted portfolio2019Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    A Central Counterparty (CCP) handles clearing between its members and can mutualise and reduce the counterparty and credit risk in a network. In the case of a clearing member defaulting on its obligations, the defaulted portfolio will be taken over by the CCP, which will attempt to close out the positions as quickly as possible. It is vital that the CCP minimises the losses they may suffer during the period between default and close out, the so called holding period.

    This thesis investigates and tests several potential hedging strategies to minimise value fluctuations during the holding period. These include neutralising the exposure to different risk factors, as well as finding the ideal hedging position using principal component analysis. The defaulted portfolio can contain different instruments, such as options, interest rate swaps and bonds, which requires different approaches to neutralise exposure. To determine the performance of the different strategies, backtesting was performed on historical data from the years 2001 to 2013, and the results were analysed in order to determine the effectiveness and potential costs of the hedging.

    The results show that significant reduction in value fluctuations can be achieved by employing these strategies, while not exceeding an affordable level of cost. Based on the findings, a function was created in Java that can recommend optimal hedging positions given a defaulted portfolio of any composition.

  • 207.
    Nilsson, Niklas
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Feature detection for geospatial referencing2019Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Då drönarindustrin växer så det knakar, har flygfoton blivit allt viktigare för en rad applikationer i vårt samhälle. Att flyga över ett svårnavigerat område med en drönare kan ge bättre översikt och är ofta snabbare, billigare och mer precist än skisser eller andra alternativa översiktsmetoder. Med denna ökade användning kommer också ett ökat behov av automatisk bildprocessering för att hjälpa till i analysen av dessa fotografier.

    Denna avhandling presenterar en metod för automatisk positionsbedömning av flygfoton, med hjälp av databaser med flygfoton och satellitfoton. Den presenterade metoden är baserad på inledande tester av existerande feature detection, feature description och feature matching algoritmer på ett något förenklat problem, där givna foton är väldigt grafiskt lika. Efter detta implementerades ytterligare modifikationer och förbättringar för att göra metoden mer robust även för bilder med en hög nivå av grafisk diskrepans, exempelvis skillnad i synvinkel, kamera- och linsparametrar, temporära objekt och vädereffekter.

    Den föreslagna metoden ger nöjaktiga resultat i geografiska regioner med en proportionellt stor mängd grafiska särdrag som enkelt kan särskiljas från varandra och där den grafiska diskrepansen inte är allt för stor. Särskilt goda resultat ses i bland annat städer och vissa typer av jordbruksområden, där metoden kan ge betydligt bättre resultat än metoder baserade på kända kameraparametrar och fotografens GPS-positionering, vilket har varit ett vanligt sätt att utföra denna typ av automatisk positionsbestämning tidigare. Dessutom är den presenterade metoden ofta enklare att applicera, då precisionen för diverse mätinstrument som annars måste användas när fotot tas inte spelar in alls i metodens beräkningar.

    Dessutom har metoden utökats för automatisk processering av videoströmmar. På grund av bristfälligt referensdata kan inga definitiva slutsatser dras angående metodens precision för detta användningsområde. Men det är ändå tydligt att beräkningstiden kan minskas drastiskt genom att använda faktumet att två påföljande ögonblicksbilder har ett stort grafiskt överlapp. Genom att använda en sorts extrapolering kan inverkan från grafiskt brus också minskas, brus som kan göra positionsbestämning omöjligt för en given ögonblicksbild.

  • 208.
    Niu, Bin
    et al.
    Dalian University of Technology.
    Wadbro, Eddie
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    On equal-width length-scale control in topology optimization2019Ingår i: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 59, nr 4, s. 1321-1334Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper deals with equal-width length-scale control in topology optimization. To realize this aim, we first review different notions of minimum and maximum length-scale control and highlight some perhaps counterintuitive consequences of the various definitions. Here, we implement equal-width control within the moving morphable components (MMC) framework by imposing the same upper and lower bounds on the width of the components. To avoid partially overlapping beams and nearly parallel beams, as well as beams crossing at small angles, we introduce penalty functions of the angle difference and the minimum distance between any two beams. A penalized optimization formulation of compliance minimization is established and studied in several numerical examples with different load cases and boundary conditions. The numerical results show that equal-width length-scale control can be obtained by using the proposed penalty function in combination with a continuation approach for the amount of penalization.

  • 209.
    Nordberg, John
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Servin, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Meshfree elastoplastic solid for non-smooth multi-domain dynamicsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    A method for simulation of elastoplastic solids in multibody systems with nonsmooth and multidomain dynamics is developed. The solid is discretised into pseudo-particles using the meshfree moving least squares method. The particles carry strain and stress tensor variables that are mapped to deformation constraints and constraint forces. The discretised solid model thus fit a unified framework for nonsmooth multidomain dynamics for realtime simulations including strong coupling of rigid multibodies with complex kinematic constraints such as articulation joints, unilateral contacts with dry friction, drivelines and hydraulics. The nonsmooth formulation allow for impulses, due to impacts for instance, to propagate instantly between the rigid multibody and the solid. Plasticity is introduced through an associative perfectly plastic modified Drucker-Prager model. The elastic and plastic dynamics is verified for simple test systems and the capability of simulating tracked terrain vehicles driving on a deformable terrain is demonstrated.

  • 210.
    Nyroos, Mikaela
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Umeå forskningscentrum för matematikdidaktik (UFM). Umeå universitet, Samhällsvetenskapliga fakulteten, Institutionen för tillämpad utbildningsvetenskap. Umeå universitet, Samhällsvetenskapliga fakulteten, Pedagogiska institutionen.
    Does standardized assessment in mathematics hamper or support young pupils’ learning?2010Ingår i: MAtematikDIdaktiskaForskningsseminariet-7, 2010Konferensbidrag (Refereegranskat)
  • 211.
    Nyroos, Mikaela
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Umeå forskningscentrum för matematikdidaktik (UFM). Umeå universitet, Samhällsvetenskapliga fakulteten, Institutionen för tillämpad utbildningsvetenskap.
    Testing or Not Testing Young Pupils: What are the consequences for learning in mathematics?2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    For several years Sweden has fall down in the international rankings on education, especially in mathematics. As a consequence different political initiatives have been made; however, none has had the wanted effect on pupils’ mathematical performance. The latest reform involves mandatory national tests in mathematics for grade-3-pupils. From having one of the most decentralised educational systems in the EU, the pendulum is now heading another course, i.e. re-centralisation. International comparisons reveal that different countries have various kinds of standardised school tests. Still, Sweden appears to protrude with this step. The downside of standardised testing is that it may lead to situation specific stress, i.e. test anxiety (TA) among pupils. Repeated of times research has proved that serve TA has a significant negative impact on performance, and high anxious pupils considerable under-achieve. Thus, the result is not only biased, it also adventures the individual’s future academic and occupational choices. Some groups tend to be more exposed to TA; girls, elderly, and low-achievers are suggested to experiencing more TA. Additionally, a subject mentioned to call forth more stress is mathematics. Contrary to impairing education, there is substantial, but very often neglected, support for repeated testing to improve pupils’ learning, i.e. the testing effect (TE). Tests could enhance later retention more than additional study of the material, even without feedback, and is applicable for related but nontested material. This effect is robust across many types of materials, has been observed in different age groups, and under most circumstances. Moreover, repeated testing has been shown to reduce TA, make pupils more positive to education, and low-achievers seem to benefit more from it. A key factor influencing a pupil’s ability to learn is the cognitive system of working memory (WM). The WM is also stressed to be the underlying component in TA, and TE. Thus, when anxiety interferes with WM in a negative way there will be a knock-on effect on the capacity. Pupils are preoccupied with worrisome thoughts which interrupt the maintenance of task-relevant information and disrupt performance. Yet, WM is a component that is modulated by repeated testing. Every time a memory is retrieved it enhances the memory traces which in turn decrease the load on WM. In understanding the contribution of TA and TE, a proposed theoretical model based on the characteristics of the WM, outlined by Baddeley, is here suggested for this two-sided possibility of testing. Research on the effects of testing in younger children is generally a neglected area, and from a multi-theoretical perspective yet to be explored. In comparison to other countries Sweden has few tests in mathematics but now moves another direction. Consequently, the aim of this paper is to clarify the influence of national test in mathematics on young pupils’ learning. The following question will be addressed: i) What are pupils’ experiences of TA, and how does this affect learning in mathematics?, ii) How does recurring testing affect pupils’ learning in mathematics?, iii) Are different groups of pupils (i.e. gender, achievement level) experiencing above aspects differently?

    Method

    Present study uses a refined and modernised standardised test measuring children’s TA, and a WM test for children tapping on the phonological loop (believed to be most sensitive to anxiety). Mathematical achievements are determined with the national tests. Statistical analyses (SPSS), supported by lengthy observations during the whole grade 3 year, are used. Forty grade-3-pupils (20 girls and 20 boys) in two classes are in focus.

    Expected Outcomes

    Preliminary results suggest that WM capacity could predict mathematics achievement. However, there is no evidence for experienced TA among the pupils, thus no tendency for under achievement in any group. It is therefore possible that the increased number of testing, by the national tests and the teachers’ additional preparation tests at the prospect of the national tests, have a TE, i.e. learning in mathematics is benefitted by more testing of relevant as well as non-tested material. Further, the teachers’ attitudes and active focus on the national tests probably have had positive effect on the pupils’ test realisation. However, it is not excluded that the young age is answerable for the absence of TA.

  • 212.
    Nyroos, Mikaela
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Umeå forskningscentrum för matematikdidaktik (UFM). Umeå universitet, Samhällsvetenskapliga fakulteten, Pedagogiska institutionen.
    Young pupils as a part of the local education agency: their experiences2009Ingår i: Third Nordic Research Network: Critical perspectives on Education and Agency, Halden Norway, Nordcrit , 2009Konferensbidrag (Refereegranskat)
  • 213.
    Nyroos, Mikaela
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Umeå forskningscentrum för matematikdidaktik (UFM). Umeå universitet, Samhällsvetenskapliga fakulteten, Institutionen för tillämpad utbildningsvetenskap. Umeå universitet, Samhällsvetenskapliga fakulteten, Pedagogiska institutionen.
    Wiklund-Hörnqvist, Carola
    Umeå universitet, Samhällsvetenskapliga fakulteten, Institutionen för psykologi.
    Hampering or supporting: young children’s’ experiences of national test2009Ingår i: Special Needs Education in Mathematics, 2009Konferensbidrag (Refereegranskat)
  • 214. Odsaeter, Lars H.
    et al.
    Wheeler, Mary F.
    Kvamsdal, Trond
    Larson, Mats G.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Postprocessing of non-conservative flux for compatibility with transport in heterogeneous media2017Ingår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 315, s. 799-830Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A conservative flux postprocessing algorithm is presented for both steady-state and dynamic flow models. The postprocessed flux is shown to have the same convergence order as the original flux. An arbitrary flux approximation is projected into a conservative subspace by adding a piecewise constant correction that is minimized in a weighted L-2 norm. The application of a weighted norm appears to yield better results for heterogeneous media than the standard L-2 norm which has been considered in earlier works. We also study the effect of different flux calculations on the domain boundary. In particular we consider the continuous Galerkin finite element method for solving Darcy flow and couple it with a discontinuous Galerkin finite element method for an advective transport problem.

  • 215. Odsæter, Lars H.
    et al.
    Kvamsdal, Trond
    Larson, Mats G.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    A simple embedded discrete fracture-matrix model for a coupled flow and transport problem in porous media2019Ingår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 343, s. 572-601Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Accurate simulation of fluid flow and transport in fractured porous media is a key challenge in subsurface reservoir engineering. Due to the high ratio between its length and width, fractures can be modeled as lower dimensional interfaces embedded in the porous rock. We apply a recently developed embedded finite element method (EFEM) for the Darcy problem. This method allows for general fracture geometry, and the fractures may cut the finite element mesh arbitrarily. We present here a velocity model for EFEM and couple the Darcy problem to a transport problem for a passive solute. The main novelties of this work are a locally conservative velocity approximation derived from the EFEM solution, and the development of a lowest order upwind finite volume method for the transport problem. This numerical model is compatible with EFEM in the sense that the same computational mesh may be applied, so that we retain the same flexibility with respect to fracture geometry and meshing. Hence, our coupled solution strategy represents a simple approach in terms of formulation, implementation and meshing. We demonstrate our model by some numerical examples on both synthetic and realistic problems, including a benchmark study for single-phase flow. Despite the simplicity of the method, the results are promising.

  • 216.
    Olofsson, Peter
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Produktionsplanering och vattenvärden: En studie av produktionsplanering för regleringsbar vattenkraft vid Skellefteå Kraft AB2010Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Syftet med detta examensarbete var att på uppdrag av Skellefteå Kraft AB studera produktionsplanering för regleringsbar vattenkraft och avgöra hur vatten i ett magasin kan värderas. Vidare skulle även en applikation skapas för att kunna beräkna dessa vattenvärden.

     

    Produktionsplanering för vattenkraft visade sig oftast delas upp i tre nivåer, i en lång-, säsong- och korttidsplaneringsdel, där detaljrikedomen i modellbeskrivningarna ökar med minskande tidshorisont. Anledningen till uppdelningen är en följd av att både noggrannhet och långsiktighet önskas, tanken är därmed att låta den högre nivån ge den lägre en långsiktighet den annars saknat. Möjliga kopplingar mellan nivåerna diskuterades, där en priskoppling genom vattenvärden argumenterades för att vara den bästa.

     

    Därefter studerades metoder för vattenvärdeberäkningar och matematiska villkor för optimal produktion, varvid ett eget program i Matlab kunde skrivas. Programmet beräknar vattenvärden vid en enmagasinmodell utifrån tillrinnings- och prisprognoser, där prognoserna tillåts att delas upp i ett valfritt antal scenarion.

     

    Testkörningar av programmet visar att vattenvärdena går ned inför en prognostiserad vårflod, för att på så vis skapa plats åt det inkommande vattnet. Det visas även att vattenvärdenas ISO-kurvor, som markerar var i tid och magasinsnivå ett visst vattenvärde gäller, blir flackare vid större magasin och djupare vid mindre inför den kommande vårfloden. Men även att ISO-kurvorna påverkas av produktionskapaciteten.

     

    Vidare visar testkörningarna att vattenvärdena är starkt kopplade till både tillrinningarnas- och prisprognosernas form och nivå, eftersom att spill vill undvikas samtidigt som produktionen önskas styras till perioder av höga priser.

     

    Innan vattenvärdena från programmet används i praktiken bör ytterligare testkörningar göras för att jämföra ekonomiskt utfall i förhållande till nuvarande arbetssätt. Därför är detta också ett förslag på vidare arbete, tillsammans med en eventuell utvidgning av programmet till en flermagasinsmodell.

  • 217.
    Pogulis, Markus
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Algebraic multigrid for a mass-consistent wind model, the Nordic Urban Dispersion model2015Självständigt arbete på avancerad nivå (masterexamen), 300 hpStudentuppsats (Examensarbete)
    Abstract [en]

    In preparation for, and for decision support during, CBRN (chemical, biological, radiological and nuclear) emergencies it is essential to know how such an event would turn out, so that one can prepare a possible evacuation. Afterwards it might be good to know how to backtrack and see what caused the emergency, and in the case of e.g. a gas leak, where did it begin? The Swedish Defence Research Agency (FOI) develops models for such scenarios.

    In this thesis FOI's model, "The Nordic Urban Dispersion model" (NUD), has been studied. The system of equations set up by this model was originally solved using Intel's PARDISO solver, which is a direct solver. An evaluation on how an iterative multigrid method would work to solve the system has been done in this thesis. The wind model is a mass-consistent model which sets up a diagnostic initial wind field. The final wind field is later minimized under the constraint of the continuity equation. The minimization problem is solved using Lagrange multipliers and the system turns into a Poisson-like problem.

    The iterative algebraic multigrid solver (AMG) which has been evaluated had difficulties solving the problem of an asymmetric system matrix generated by NUD. The AMG solver was then tried on a symmetric discrete Poisson problem instead, and the solution turns out to be the same as for the PARDISO solver. A comparison was made between the AMG and PARDISO solver, and for the discrete Poisson case the AMG solver turned out on top for both larger system size and less computational time.

    To try out the solvers for the original NUD case a modification of the boundary conditions was made to make the system matrix symmetric. This modification turns the problem into a mathematical problem rather than a physical one, as the wind fields generated are not physically correct. For this modified case both the solvers get the same solution in essentially the same computational time. A method of how to in the future solve the original (asymmetric) problem, by modifying the discretization of the boundary conditions, has been discussed.

  • 218.
    Rydén, Patrik
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för klinisk mikrobiologi, Klinisk bakteriologi. Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Andersson, Henrik
    Umeå universitet, Medicinska fakulteten, Institutionen för klinisk mikrobiologi, Klinisk bakteriologi.
    Landfors, Mattias
    Umeå universitet, Medicinska fakulteten, Institutionen för klinisk mikrobiologi, Klinisk bakteriologi.
    Näslund, Linda
    Umeå universitet, Medicinska fakulteten, Institutionen för klinisk mikrobiologi, Klinisk bakteriologi.
    Hartmanová, Blanka
    Noppa, Laila
    Umeå universitet, Medicinska fakulteten, Institutionen för klinisk mikrobiologi, Klinisk bakteriologi.
    Sjöstedt, Anders
    Umeå universitet, Medicinska fakulteten, Institutionen för klinisk mikrobiologi, Klinisk bakteriologi.
    Evaluation of microarray data normalization procedures using spike-in experiments2006Ingår i: BMC Bioinformatics, ISSN 1471-2105, E-ISSN 1471-2105, Vol. 7, nr 300, s. 17-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Background: Recently, a large number of methods for the analysis of microarray data have been proposed but there are few comparisons of their relative performances. By using so-called spike-in experiments, it is possible to characterize the analyzed data and thereby enable comparisons of different analysis methods.

    Results: A spike-in experiment using eight in-house produced arrays was used to evaluate established and novel methods for filtration, background adjustment, scanning, channel adjustment, and censoring. The S-plus package EDMA, a stand-alone tool providing characterization of analyzed cDNA-microarray data obtained from spike-in experiments, was developed and used to evaluate 252 normalization methods. For all analyses, the sensitivities at low false positive rates were observed together with estimates of the overall bias and the standard deviation. In general, there was a trade-off between the ability of the analyses to identify differentially expressed genes (i.e. the analyses' sensitivities) and their ability to provide unbiased estimators of the desired ratios. Virtually all analysis underestimated the magnitude of the regulations; often less than 50% of the true regulations were observed. Moreover, the bias depended on the underlying mRNA-concentration; low concentration resulted in high bias. Many of the analyses had relatively low sensitivities, but analyses that used either the constrained model (i.e. a procedure that combines data from several scans) or partial filtration (a novel method for treating data from so-called not-found spots) had with few exceptions high sensitivities. These methods gave considerable higher sensitivities than some commonly used analysis methods.

    Conclusion: The use of spike-in experiments is a powerful approach for evaluating microarray preprocessing procedures. Analyzed data are characterized by properties of the observed log-ratios and the analysis' ability to detect differentially expressed genes. If bias is not a major problem; we recommend the use of either the CM-procedure or partial filtration.

     

  • 219. Saglietti, Clio
    et al.
    Schlatter, Philipp
    Wadbro, Eddie
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Henningson, Dan
    Topology optimization of heat sinks in a square differentially heated cavity2018Ingår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 74, s. 36-52Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Innovative designs of heat sinks are generated in the present paper through numerical optimization, by applying a material distribution topology optimization approach. The potential of the method is demonstrated in a two-dimensional differentially heated cavity, in which the heat transfer is increased by means of introducing a solid structure that acts as a heat sink. We simulate the heat transfer in the whole system by performing direct numerical simulations of the conjugated problem, i.e. temperature diffusion and convection in the entire domain and momentum conservation in the fluid surrounding the solid. The flow is driven by the buoyancy force, under the Boussinesq approximation, and we describe the presence of solid material as the action of a Brinkman friction force in the Navier–Stokes equations. To obtain a design with a given length scale, we apply regularization techniques by filtering the material distribution. Two different types of filters are applied and compared for obtaining the most realistic solution. Given the large scale of the problem, the optimization is solved with a gradient based method that relies on adjoint sensitivity analysis. The results show the applicability of the method by presenting innovative geometries that are increasing the heat flux. Moreover, the effect of various factors is studied: We investigate the impact of boundary conditions, initial designs, and Rayleigh number. Complex tree-like structures are favored when a horizontal temperature gradient is imposed on the boundary and when we limit the amount of solid volume in the cavity. The choice of the initial design affects the final topology of the generated solid structures, but not their performance for the studied cases. Additionally, when the Rayleigh number increases, the topology of the heat exchanger is able to substantially enhance the convection contribution to the heat transfer.

  • 220.
    Schelin, Lina
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Sjöstedt-de Luna, Sara
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Kriging prediction intervals based on semiparametric bootstrap2010Ingår i: Mathematical Geosciences, ISSN 1874-8961, E-ISSN 1874-8953, Vol. 42, nr 8, s. 985-1000Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Kriging is a widely used method for prediction, which, given observations of a (spatial) process, yields the best linear unbiased predictor of the process at a new location. The construction of corresponding prediction intervals typically relies on Gaussian assumptions. Here we show that the distribution of kriging predictors for non-Gaussian processes may be far from Gaussian, even asymptotically. This emphasizes the need for other ways to construct prediction intervals. We propose a semiparametric bootstrap method with focus on the ordinary kriging predictor. No distributional assumptions about the data generating process are needed. A simulation study for Gaussian as well as lognormal processes shows that the semiparametric bootstrap method works well. For the lognormal process we see significant improvement in coverage probability compared to traditional methods relying on Gaussian assumptions.

  • 221.
    Schmidt, Stephan
    et al.
    Institut für Mathematik , Universität Würzburg.
    Wadbro, Eddie
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Large-Scale Three-Dimensional Acoustic Horn Optimization2016Ingår i: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 38, nr 6, s. B917-B940Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider techniques that enable large-scale gradient-based shape optimization of wave-guiding devices in the context of three-dimensional time-domain simulations. The approach relies on a memory efficient boundary representation of the shape gradient together with primal and adjoint solvers semiautomatically generated by the FEniCS framework. The hyperbolic character of the governing linear wave equation, written as a first-order system, is exploited through systematic use of the characteristic decomposition both to define the objective function and to obtain stable numerical fluxes in the discontinuous Galerkin spatial discretization. The methodology is successfully used to optimize the shape of a midrange acoustic horn, described by 1,762 design variables, for maximum transmission efficiency, where the parallel computations involve a total of $3.5\times10^9$ unknowns.

  • 222.
    Servin, Martin
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Berglund, Tomas
    Algoryx Simulation AB.
    Mickelsson, Kjell-Ove
    LKAB.
    Rönnbäck, Stefan
    Optimation AB.
    Wang, Da
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Modeling and simulation of a granulation system using a nonsmooth discrete element method2015Ingår i: ECCOMAS IV international conference on particle-based methods 2015, 2015Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Granulation is the process of forming macroscopic granules, e.g. tablets or pellets, from microscopic particles. A common system for continuous granulation consist of a rotary drum or disc, a sieve and chrusher connected in a circuit by a number of conveyors. The granulation occur in the drum (or disc) and the main mechanisms are nucleation, layering, coalescence and breakage [1]. The process can be controlled by drum velocity and feed rate of fine material, binding agencies and moisture. The geometric design of components affect the material flow and thus the sieving capacity and ultimately the production capacity. Many granulation plants operate well below their capacity and suffer from high recycle rates and dynamic instabilities [2]. The main challenge of modelling and simulation of granulation processes is the occurrence of multiple length and time scales. The traditional approaches are typically focused either the largescale level of processing units or on the microscale level of particles. A complete model of a granulation process need to include also the intermediate scale of granule dynamics [1]. This is, however, a very challenging computational task given the vast number of granules, typically many millions or more. We present a meso-scale approach to modeling and simulating iron ore granulation systems with granules modeled as nonsmooth discrete elements (NDEM) [3-6]. This extend a previous iron ore granule model [7] used for balling drum outlet design simulation [8]. Ore fines and moisture is modeled by a quasiparticle model for slurry [9]. Interaction models for nucleation, layering, coalescence and breakage are proposed and tested in a virtual balling circuit. The computational performance is analysed and different methods for accelerating the NDEM computations are tested.

  • 223.
    Servin, Martin
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Lacoursière, Claude
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Högpresterande beräkningscentrum norr (HPC2N).
    Nordfelth, Fredrik
    Bodin, Kenneth
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Högpresterande beräkningscentrum norr (HPC2N).
    Hybrid, multiresolution wires with massless frictional contacts2011Ingår i: IEEE Transactions on Visualization and Computer Graphics, ISSN 1077-2626, E-ISSN 1941-0506, Vol. 7, nr 7, s. 970-982Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We describe a method for the visual interactivesimulation of wires contacting with rigid multibodies. Thephysical model used is a hybrid combining lumped elementsand massless quasistatic representations. The latter is basedon a kinematic constraint preserving the total length of thewire along a segmented path which can involve multiple bodiessimultaneously and dry frictional contact nodes used for roping,lassoing and fastening. These nodes provide stick and slidefriction along edges of the contacting geometries. The lumpedelement resolution is adapted dynamically based on local stabilitycriteria, becoming coarser as the tension increases, and up to thepurely kinematic representation. Kinematic segments and contactnodes are added and deleted and propagated based on contactgeometries and dry friction configurations. The method givesdramatic increase on both performance and robustness becauseit quickly decimates superfluous nodes without loosing stability,yet adapts to complex configurations with many contacts andhigh curvature, keeping a fixed, large integration time step.Numerical results demonstrating the performance and stabilityof the adaptive multiresolution scheme are presented along withan array of representative simulation examples illustrating theversatility of the frictional contact model.

  • 224.
    Servin, Martin
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Wang, Da
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Adaptive model reduction for nonsmooth discrete element simulation2016Ingår i: Computational Particle Mechanics, ISSN 2196-4378, Vol. 3, nr 1, s. 107-121Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A method for adaptive model order reduction for nonsmooth discrete element simulation is developed and analysed in numerical experiments. Regions of the granular media that collectively move as rigid bodies are substituted with rigid bodies of the corresponding shape and mass distribution. The method also support particles merging with articulated multibody systems. A model approximation error is defined and used to derive conditions for when and where to apply reduction and refinement back into particles and smaller rigid bodies. Three methods for refinement are proposed and tested: prediction from contact events, trial solutions computed in the background and using split sensors. The computational performance can be increased by 5-50 times for model reduction level between 70-95 %.

  • 225.
    Shcherbak, Denys
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Enumerative approaches and structural results for selected combinatorial problems2019Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
  • 226.
    Shykula, Mykola
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Matematisk statistik.
    Seleznjev, Oleg
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Matematisk statistik.
    Stochastic structure of asymptotic quantization errors2004Rapport (Övrigt vetenskapligt)
  • 227.
    Sundberg, Johan
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Parallel projected Gauss-Seidelsolver for large-scale granular matter: Examining the physics of the parallel solver and development of amultigrid solver2014Självständigt arbete på avancerad nivå (yrkesexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    Granular matter is found everywhere in nature and some examples include sand, rice,coee beans and iron ore pellets. Many dierent methods exists for simulating granularmatters using computers. In the scope of this thesis a physics engine called AgX Dynamicsfrom Algoryx Simulation AB is used to investigate and further develop methodsinvolving the discrete element method. During the rst half of 2013 a parallel solverfor the projected Gauss-Seidel method was implemented in AgX in order to speed upthe simulation time of simulations involving granular materials. In this thesis projectit is shown that the behaviour of the physics of this parallel solver is identical to theserial solver. Secondly this thesis works on the development of a multigrid solver forthe Gauss-Seidel method. Multigrid in this context means that the particle systemis partitioned in space. Each partition is then merged into a rigid body and contactforces between these rigid bodies is solved to machine precision using a direct solver.The forces from this direct solve is then used when solving the internal part of thepartitions using an iterative projected Gauss-Seidel method. The motivation for developinga multigrid method is to achieve faster convergence and even more speed-upof the solver. Numerical experiments has been performed on a 1D column and a 3Dsilo. The results show high potential of the method and the one-dimensional columnbehaves closer to a direct solver than an iterative solver. The thesis was done for UMIT Research Lab, Umea University and Algoryx SimulationAB.

  • 228.
    Sundling, Emma
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Validation toolbox for a Physics Engine2016Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Fysikmotorer blir mer och mer vanliga på grund av den snabba utvecklingen och efterfrågan på simuleringar. I och med detta ökar också behovet av att testa motorerna och ett sätt att mäta prestandan, inte bara snabbheten utan också noggrannheten och stabiliteten. Syftet med detta examensarbete har varit att skapa ett set av prestandatester. De syftar till att testa de fysikaliska aspekterna av fysikmotorn, särskilt inom mekanik. En strategi och exportfunktioner för testresultaten för att automatisera testningen har också utvecklats.

    De resulterande testerna blev en balk på pålar som analyserar stabiliteten hos villkoren, ett överbestämt system bestående av en statisk dörr på flera gångjärn, ett fallande objekt som granskar precisionen hos integratorn, en låda på ett lutande plan som testar friktionsmodellen, en enkel pendel samt en flerkropppspendel som kontrollerar villkorsprecisionen och energikonservering, jordens bana runt solen som testar integratorns stabilitet och slutligen en utskjutande balk som är ett statiskt test av ett verkligt fall. När testerna är genomförda presenteras resultaten på en HTML-sida. En prototyp av en webb-applikation har också utvecklats samt ett set med skalära tester som kan utföras kontinuerligt för att följa upp trender och jämföra motorns prestanda över tid.

    Det här examensarbetet initierades av Algoryx Simulation AB som även tillhandahållit fysikmotorn, AgX Dynamics, med den numeriska metoden SPOOK. Motorn presterade överlag bra på testerna. För att bygga en allmän verktygslåda behövs fler tester så som interaktion mellan material, skalbara tester med tusentals kroppar samt mer komplexa simuleringar, t.ex. en saxlyft och robotar. Arbetet kan också utökas med mer utvecklade exportfunktioner, både mot webben och som dokument. Förhoppningsvis kan detta ses som ett komplement till de tidigare ansträgningar som gjorts för att skapa ett generellt set av prestandatester och ett automatiskt ramverk för kontinuerlig testning.

  • 229.
    Svebilius, Christian
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Rigid Body Simulation of MacroMolecules2008Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    A computer model for simulating experiments done on surface organelles(so called pili) on the Escherichia Coli bacteria have been developedand implemented. The objective of the computer simulation wasto mimic the results of experiments done with optical tweezers and to displaya graphical, three dimensional, representation of these experiments.The experiments measured the force response to elongation of pili.This force response can be divided into three regions of elongation, regionI, II and III, each with different properties. Region I is characterized by aconstant increase in force, in region II the pilus is unfolded under constantforce, and in region III the force versus elongation curve assumes a nontrivialshape with increasing force. The pili are also able to retract to itsoriginal length giving a similar force response curve. The computer modelshould be able to handle all these properties. The developed model couldhandle elongation in region I and II. In region III, the force response givenby the simulation differed from the one given by the experiments.

  • 230.
    Tobias, Brännvall
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Source Term Estimation in the Atmospheric Boundary Layer: Using the adjoint of the Reynolds Averaged Scalar Transport equation2015Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Detta arbete utvärderar hurvida Reynolds medelvärdesmodellering inom flödessimuleringar kan användas till att finna källan till en viss gas baserat på verkliga mätningar ute i fält. Metoden går ut på att använda den adjungerade ekvationen till Reynolds tidsmedlade skalära transportekvationen, beskriven och härledd häri. Då bakåtmodellen bygger på framåtmodellen, måste såleds framåtmodellen utvärderas först.

    Navier-Stokes ekvationer med en turbulensmodell löses i en domän, innehållandes 4 kuber i en 2x2 orientering, för vilken en hastighetsprofil erhålles. Turbulensmodellen som användes är en union av två olika k-ε modeller, där den ena fångar turbulens runt tröga objekt och den andra som modellerar atmosfäriska gränsskiktet. Detta fält används sedan i framåtmodellen av skalära transportekvationen, som sedan jämförs med körningar från EnFlo windtunneln i Surrey. Slutligen testkörs även den adjungerade ekvationen, både för syntetiskt data genererat i framåtkörningen men även för data från EnFlo tunneln.

    Då det visade sig att det turbulenta Schmidttalet spelar stor roll inom spridning i det atmosfäriska gränsskiktet, gjordes testkörningar med tre olika Schmidttal, det normala 0.7, det väldigt låga talet 0.3 samt ett höjdberoende Schmidttal. Det visade sig att det vanligtvis använda talet 0.7 inte alls lyckas fånga spridningen tillfredställande och gav ett stort modellfel. Därför löstes den adjungerade ekvationen för 0.3 samt för ett höjdberoende Schmidttal.

    Interaktionen mellan mätningar, den riktiga källstyrkan (som är okänd i den adjungerade ekvationen) samt källpositionen är onekligen intrikat. Över- samt underestimationer av framåtmodellen kan ta ut varandra i bakåtmodellen för att finna rätt källa, med rätt källstyrka. Det ter sig som Reynolds turbulensmodellering mycket möjligt kan användas inom källtermsuppskattning. 

  • 231.
    Verdier, Olivier
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Reductions of Operator Pencils2014Ingår i: Mathematics of Computation, ISSN 0025-5718, E-ISSN 1088-6842, Vol. 83, nr 285, s. 189-214Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study problems associated with an operator pencil, i.e., a pair of operators onBanach spaces. Two natural problems to consider are linear constrained dierentialequations and the description of the generalized spectrum. The main tool to tackleeither of those problems is the reduction of the pencil. There are two kinds of naturalreduction operations associated to a pencil, which are conjugate to each other.Our main result is that those two kinds of reductions commute, under some mildassumptions that we investigate thoroughly.Each reduction exhibits moreover a pivot operator. The invertibility of all thepivot operators of all possible successive reductions corresponds to the notion ofregular pencil in the nite dimensional case, and to the inf-sup condition for saddlepoint problems on Hilbert spaces.Finally, we show how to use the reduction and the pivot operators to describe thegeneralized spectrum of the pencil.

  • 232.
    Viklands, Thomas
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Datavetenskap.
    Algorithms for the Weighted Orthogonal Procrustes Problem and other Least Squares Problems2006Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In this thesis, we present algorithms for local and global minimization of some Procrustes type problems. Typically, these problems are about rotating and scaling a known set of data to fit another set with applications related to determination of rigid body movements, factor analysis and multidimensional scaling. The known sets of data are usually represented as matrices, and the rotation to be determined is commonly a matrix Q with orthonormal columns.

    The algorithms presented use Newton and Gauss-Newton search directions with optimal step lengths, which in most cases result in a fast computation of a solution.

    Some of these problems are known to have several minima, e.g., the weighted orthogonal Procrustes problem (WOPP). A study on the maximal amount of minima has been done for this problem. Theoretical results and empirical observations gives strong indications that there are not more than 2n minimizers, where n is the number of columns in Q. A global optimization method to compute all 2n minima is presented.

    Also considered in this thesis is a cubically convergent iteration method for solving nonlinear equations. The iteration method presented uses second order information (derivatives) when computing a search direction. Normally this is a computational heavy task, but if the second order derivatives are constant, which is the case for quadratic equations, a performance gain can be obtained. This is confirmed by a small numerical study.

    Finally, regularization of ill-posed nonlinear least squares problems is considered. The quite well known L-curve for linear least squares problems is put in context for nonlinear problems.

  • 233.
    Wadbro, Eddie
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Analysis and design of acoustic transition sections for impedance matching and mode conversion2014Ingår i: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 50, nr 3, s. 395-408Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This work considers the problem of designing passive transition sections to provide impedance matching and mode conversion for acoustic wave propagation. The base configuration consists of two waveguides connected by a transition section. The objective is to find a placement of material inside this section to make it function as an impedance matcher or a mode converter with minimal losses. A finite element approximation of the Helmholtz equation in a truncated domain together with Dirichlet-to-Neumann type non-reflecting boundary conditions models the wave propagation. Material distribution techniques solve the resulting topology optimization problem and the resulting interfacial devices show good transmission properties.

  • 234.
    Wadbro, Eddie
    et al.
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    Berggren, Martin
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    High contrast microwave tomography using topology optimization techniques2007Ingår i: Proceedings of Waves 2007: The 8th International Conference on Mathematical and Numerical Aspects of Waves / [ed] N. Biggs et al., Reading, U.K.: University of Reading , 2007, s. 448-450Konferensbidrag (Refereegranskat)
  • 235.
    Wadbro, Eddie
    et al.
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Megapixel topology optimization on a graphics processing unit2009Ingår i: SIAM Review, ISSN 0036-1445, E-ISSN 1095-7200, Vol. 51, nr 4, s. 707-721Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show how the computational power and programmability of modern graphics processing units (GPUs) can be used to efficiently solve large-scale pixel-based material distribution problems using a gradient-based optimality criterion method. To illustrate the principle, a so-called topology optimization problem that results in a constrained nonlinear programming problem with over 4 million decision variables is solved on a commodity GPU.

  • 236.
    Wadbro, Eddie
    et al.
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Microwave tomography using topology optimization techniques2008Ingår i: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 30, nr 3, s. 1613-1633Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Microwave tomography is a technique in which microwaves illuminate a specimen, and measurements of the scattered electrical field are used to determine and depict the specimen's dielectric and conductive properties. This article presents a new method to perform the reconstruction. The reconstruction method is illustrated by assuming time harmonic scattering in two space dimensions in a setup tailored for medical applications. We prove that the resulting constrained nonlinear least-squares problem admits a solution. The governing Helmholtz equation is discretized by using the finite-element method, and the dielectric properties are allowed to attain different values at each element within a given region. The reconstruction algorithm uses methodologies borrowed from topology optimization of linearly elastic structures. Numerical examples illustrate the reconstruction method in a parameter range typical for human tissue and for the challenging case where the size of the object is in the same order as the wavelength. A reasonable estimate of the dielectric properties is obtained by using one observation per 20 unknowns when the permittivity is allowed to vary continuously within a given interval. Using a priori information that the permittivity attains only certain values results in a good estimate and a sharp image. As opposed to topology optimization for structures, there is no indication of mesh dependency and checkerboarding when forcing the permittivity to attain discrete values.

  • 237.
    Wadbro, Eddie
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Niu, Bin
    Dalian University of Technology.
    Multiscale design for additive manufactured structures with solid coating and periodic infill pattern2019Ingår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 357, artikel-id UNSP 112605Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper considers multiscale topology optimization of an infill structure. The structure is composed of an optimized layout configuration at the macroscale with uniform optimized microstructures as infill lattices coated by a thin skin. The design optimization of the infill lattice is performed simultaneously with the topology optimization of macroscale structure, which also includes the coating. The classic Solid Isotropic Material with Penalization (SIMP) method is used to design the topology of the microstructure. The Novel Implementation of Asymptotic Homogenization (NIAH) evaluates the effective properties of the microstructure; the macroscale structural analysis uses these homogenized properties. The Porous Anisotropic Material with Penalization (PAMP) method realizes the two-scale topology optimization of the macrostructure and microstructure. Harmonic-mean based non-linear filters identify the coating layer and realize length scale control on the topologies of the macrostructure as well as the microstructure. These morphology mimicking non-linear filters also distinguish the skin and the infill lattice parts of the design at the macroscale. Several numerical examples demonstrate the effectiveness of the proposed optimization method.

  • 238.
    Wadbro, Eddie
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap. The Forestry Research Institute of Sweden (Skogforsk), Uppsala, Sweden.
    Noreland, Daniel
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap. The Forestry Research Institute of Sweden (Skogforsk), Uppsala, Sweden.
    Continuous transportation as a material distribution topology optimization problem2019Ingår i: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 59, s. 1471-1482Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The problem of moving a commodity with a given initial mass distribution to a pre-specified target mass distribution so that the total work is minimized can be traced back at least to Monge’s work from 1781. Here, we consider a version of this problem aiming to minimize a combination of road construction and transportation cost by determining, at each point, the local direction of transportation. This paper covers the modeling of the problem, highlights how it can be formulated as a material distribution topology optimization problem, and shows some results.

  • 239.
    Wadbro, Eddie
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Zahedi, Sara
    Department of Information Technology, Uppsala University, Uppsala.
    Kreiss, Gunilla
    Department of Information Technology, Uppsala University, Uppsala.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    A uniformly well-conditioned, unfitted Nitsche method for interface problems2013Ingår i: BIT Numerical Mathematics, ISSN 0006-3835, E-ISSN 1572-9125, Vol. 53, nr 3, s. 791-820Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A finite element method for elliptic partial differential equations that allows for discontinuities along an interface not aligned with the mesh is presented. The solution on each side of the interface is separately expanded in standard continuous, piecewise-linear functions, and jump conditions at the interface are weakly enforced using a variant of Nitsche’s method. In our method, the solutions on each side of the interface are extended to the entire domain which results in a fixed number of unknowns independent of the location of the interface. A stabilization procedure is included to ensure well-defined extensions. We prove that the method provides optimal convergence order in the energy and the L2 norms and a condition number of the system matrix that is independent of the position of the interface relative to the mesh. Numerical experiments confirm the theoretical results and demonstrate optimal convergence order also for the pointwise errors.

  • 240.
    Wahlström, Dennis
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Probabilistic Multidisciplinary Design Optimization on a high-pressure sandwich wall in a rocket engine application2017Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    A need to find better achievement has always been required in the space industrythrough time. Advanced technologies are provided to accomplish goals for humanityfor space explorer and space missions, to apprehend answers and widen knowledges. These are the goals of improvement, and in this thesis, is to strive and demandto understand and improve the mass of a space nozzle, utilized in an upperstage of space mission, with an expander cycle engine. The study is carried out by creating design of experiment using Latin HypercubeSampling (LHS) with a consideration to number of design and simulation expense.A surrogate model based optimization with Multidisciplinary Design Optimization(MDO) method for two different approaches, Analytical Target Cascading (ATC) and Multidisciplinary Feasible (MDF) are used for comparison and emend the conclusion.

    In the optimization, three different limitations are being investigated, designspace limit, industrial limit and industrial limit with tolerance. Optimized results have shown an incompatibility between two optimization approaches, ATC and MDF which are expected to be similar, but for the two limitations, design space limit and industrial limit appear to be less agreeable. The ATC formalist in this case dictates by the main objective, where the children/subproblems only focus to find a solution that satisfies the main objective and its constraint. For the MDF, the main objective function is described as a single function and solved subject to all the constraints. Furthermore, the problem is not divided into subproblems as in the ATC.

    Surrogate model based optimization, its solution influences by the accuracy ofthe model, and this is being investigated with another DoE. A DoE of the full factorial analysis is created and selected to study in a region near the optimal solution.In such region, the result has evidently shown to be quite accurate for almost allthe surrogate models, except for max temperature, damage and strain at the hottestregion, with the largest common impact on inner wall thickness of the space nozzle. Results of the new structure of the space nozzle have shown an improvement of mass by ≈ 50%, ≈ 15% and ≈ -4%, for the three different limitations, design spacelimit, industrial limit and industrial limit with tolerance, relative to a reference value,and ≈ 10%, ≈ 35% and ≈ 25% cheaper to manufacture accordingly to the defined producibility model.

  • 241.
    Wang, Da
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Accelerated granular matter simulation2015Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Modeling and simulation of granular matter has important applications in both natural science and industry. One widely used method is the discrete element method (DEM). It can be used for simulating granular matter in the gaseous, liquid as well as solid regime whereas alternative methods are in general applicable to only one. Discrete element analysis of large systems is, however, limited by long computational time. A number of solutions to radically improve the computational efficiency of DEM simulations are developed and analysed. These include treating the material as a nonsmooth dynamical system and methods for reducing the computational effort for solving the complementarity problem that arise from implicit treatment of the contact laws. This allow for large time-step integration and ultimately more and faster simulation studies or analysis of more complex systems. Acceleration methods that can reduce the computational complexity and degrees of freedom have been invented. These solutions are investigated in numerical experiments, validated using experimental data and applied for design exploration of iron ore pelletising systems.

  • 242.
    Wang, Da
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Servin, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Berglund, Tomas
    Algoryx Simulation AB.
    Warm starting the projected Gauss-Seidel algorithm for granular matter simulation2016Ingår i: Computational Particle Mechanics, ISSN 2196-4378, Vol. 3, nr 1, s. 43-52Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect on the convergence of warm start-ing the projected Gauss-Seidel solver for nonsmoothdiscrete element simulation of granular matter are in-vestigated. It is found that the computational perfor-mance can be increased by a factor 2 to 5.

  • 243.
    Wang, Da
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Servin, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Berglund, Tomas
    Algoryx Simulation AB, Umeå, Sweden.
    Mickelsson, Kjell-Ove
    LKAB R&D, Malmberget, Sweden.
    Rönnbäck, Stefan
    Optimation AB, Luleå, Sweden.
    Parametrization and validation of a nonsmooth discrete element method for simulating flows of iron ore green pellets2015Ingår i: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 283, s. 475-487Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The nonsmooth discrete element method (NDEM) has the potential of high computational efficiency for rapid exploration of large design space of systems for processing and transportation of mineral ore. We present parametrization, verification and validation of a simulation model based on NDEM for iron ore green pellet flow in balling circuits. Simulations are compared with camera based measurements of individual pellet motion as well as bulk behavior of pellets on conveyors and in rotating balling drum. It is shown that the NDEM simulation model is applicable for the purpose of analysis, design and control of iron ore pelletizing systems. The sensitivity to model and simulation parameters is investigated. It is found that: the errors associated with large time-step integration do not cause statistically significant errors to the bulk behavior; rolling resistance is a necessary model component; and the outlet flow from the drum is sensitive to fine material adhering to the outlet creating a thick coating that narrows the outlet gaps.

  • 244.
    Wang, Jianfeng
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Zhou, Zhiyong
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Garpebring, Anders
    Umeå universitet, Medicinska fakulteten, Institutionen för strålningsvetenskaper.
    Yu, Jun
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Sparsity estimation in compressive sensing with application to MR images2017Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    The theory of compressive sensing (CS) asserts that an unknown signal x in C^N canbe accurately recovered from m measurements with m << N provided that x is sparse. Most of the recovery algorithms need the sparsity s = ||x||_0 as an input. However,generally s is unknown, and directly estimating the sparsity has been an open problem.In this study, an estimator of sparsity is proposed by using Bayesian hierarchical model. Its statistical properties such as unbiasedness and asymptotic normality are proved. Inthe simulation study and real data study, magnetic resonance image data is used asinput signal, which becomes sparse after sparsified transformation. The results fromthe simulation study confirm the theoretical properties of the estimator. In practice, theestimate from a real MR image can be used for recovering future MR images under theframework of CS if they are believed to have the same sparsity level after sparsification.

  • 245.
    Wang, Jianfeng
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Zhou, Zhiyong
    Department of Statistics, Zhejiang University City College, China.
    Yu, Jun
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Error bounds of block sparse signal recovery based on q-ratio block constrained minimal singular values2019Ingår i: EURASIP Journal on Advances in Signal Processing, ISSN 1687-6172, E-ISSN 1687-6180, Vol. 2019, artikel-id 57Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, we introduce the q-ratio block constrained minimal singular values (BCMSV) as a new measure of measurement matrix in compressive sensing of block sparse/compressive signals and present an algorithm for computing this new measure. Both the mixed ℓ2/ℓq and the mixed ℓ2/ℓ1 norms of the reconstruction errors for stable and robust recovery using block basis pursuit (BBP), the block Dantzig selector (BDS), and the group lasso in terms of the q-ratio BCMSV are investigated. We establish a sufficient condition based on the q-ratio block sparsity for the exact recovery from the noise-free BBP and developed a convex-concave procedure to solve the corresponding non-convex problem in the condition. Furthermore, we prove that for sub-Gaussian random matrices, the q-ratio BCMSV is bounded away from zero with high probability when the number of measurements is reasonably large. Numerical experiments are implemented to illustrate the theoretical results. In addition, we demonstrate that the q-ratio BCMSV-based error bounds are tighter than the block-restricted isotropic constant-based bounds.

  • 246. Wang, Mengyu
    et al.
    Engström, Christian
    ETH Zurich, Electromagnetic Fields and Microwave Electronics Laboratory.
    Schmidt, Kersten
    Hafner, Christian
    On high-order FEM applied to canonical scattering problems in plasmonics2011Ingår i: Journal of Computational and Theoretical Nanoscience, ISSN 1546-1955, Vol. 8, nr 8, s. 1564-1572Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper a high-order finite element method with curvilinear elements is proposed for the simulation of plasmonic structures. Most finite element packages use low order basis functions and non-curved elements, which is very costly for demanding problems such as the simulation of nanoantennas. To enhance the performance of finite elements, we use curvilinear quadrilateral elements to calculate the near-field from an impinging plane wave with second order absorbing boundary conditions. The magnetic field amplitude on the surface of one object is compared with a computation based on a multiple multipole expansion. Moreover, the convergence behavior of p-FEM with absorbing boundary conditions motivate an adaptive strategy of polynomial degree enhancement and enlargement of the domain.

  • 247.
    Wendelius, Ludvig
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Development of a SimulationModel of an Automatic Gearbox2012Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    I det här examensarbetet har en simuleringsmodell för en automatisk växellåda med primär retarder utvecklats och implementerats. Tillsammans med andra modeller från fordonoch drivlina skulle denna simuleringsmodell kunna användas för att uppskatta ett fordonsbränsleförbrukning eller till att optimera olika fordonsparametrar.De olika mekaniska komponenterna samt kontrollsystemet i växellådan modellerades separat.Dessa modeller kunde sedan sammanfogas för att bygga den slutliga växellådsmodellen.Alla modeller implementerades i det objektorienterade programmeringsspråket Modelica,som tillåter en stor återanvändningsbarhet till vardera enskild komponent.Den implementerade modellen verierades genom ett antal provcykler, utformade för attfånga olika beteenden hos växellådan. Dessa cykler har återskapats i simuleringsmiljön ochmed det kunde resultat från simuleringar jämföras mot data från ett verkligt fordon somutförde samma prov.Från verieringen har slutsatsen dragits att modellen uppfyllde målen med projektet. Målen var, att den slutliga simuleringsmodellen visar ett liknande beteende som en växellåda i ett verkligen fordon. Växlingar och låsning/upplåsning hos momentomvandlareninträande vid ungefär samma situationer i simuleringarna som i provningen med det verkligafordonet.

  • 248.
    Westin, Lars
    et al.
    Umeå universitet, Samhällsvetenskapliga fakulteten, Centrum för regionalvetenskap (CERUM).
    Westin, Jonas
    Umeå universitet, Samhällsvetenskapliga fakulteten, Centrum för regionalvetenskap (CERUM).
    Planering av godstransporter på regional nivå: Modeller, verktyg och förvaltningspolitik2019Rapport (Övrigt vetenskapligt)
    Abstract [sv]

    Rapporten fokuserar på den kunskapsbrist som finns beträffande nuläget i det svenska gods­transport­nätverket på regional och lokal nivå. Det påverkar kommunikation och skapar osäkerhet i dialogen mellan aktörer inom transportplanering på regional och kommunal nivå. Som bakgrund diskuteras planeringsprocessen för gods­transporternas infrastruktur inom Trafikverket. Den relateras till planering på kommunal och regional nivå. Inom Trafikverket och dess föregångare har tyngdpunkten mellan nationell styrning och lokala förhandlingar varierat. Inom Trafikverket sker en förskjutning av fokus från nationell nivå till dialogprocesser med lokala planerare och näringsliv, från ”government” till ”governance”. Samtidigt är modell- och investeringskalkylsidan för åtgärder inom godstransporter mycket starkt nationellt fokuserad, med rötter i 1990-talets nationella produktivitetsproblematik. För godsplaneringen innebär det att representationen av de lokala och regionala godstransportnätverken är begränsad. Det skapar oklarheter i dialogprocesserna på regional och lokal nivå. Enskilda regioner har därför själva eller i samverkan med Trafikverket genomfört egna godsflödesenkäter och byggt regionala modeller. Några av dess presenteras i rapporten. Därpå diskuteras vilka verktyg och dataunderlag som behövs för att på ett bättre sätt stödja regional och lokal transportplanering för godstransporter.

  • 249.
    Westsson, David
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Simulation of torque caused by the lubrication fluid in a ball bearing2013Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
  • 250.
    Wickman, Jonas
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Evolution of Ecological Communities in Spatially Heterogeneous Environments2019Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [sv]

    Evolutionärt stabila samhällen är slutpunkten för evolution, och ekologiska samhällen vars egenskaper är under selektion kommer till slut att bli ett sådant. Således är egenskaperna hos sådana samhällen särskilt intressanta, då dessa kan bestå oförändrade under långa evolutionära tidsskalor. Idén om evolutionärt stabila strategier - strategier som är oslagbara när de väl evolverat - är sedan nästan 50 år tillbaka ett väl etablerat koncept inom teoretisk ekologi. Under dessa år har teorin för evolutionärt stabila strategier och samhällen blivit alltmer välutvecklad. Denna utveckling till trots saknas fortfarande effektiva analytiska och numeriska verktyg för att studera evolutionärt stabila samhällen i heterogena miljöer, där de ekologiska betingelserna - och således även krafterna från naturligt urval - varierar från punkt till punkt i rummet. På grund av detta har många frågor rörande hur evolutionärt stabil mångfald blir till och bevaras när ekologiska och evolutionära krafter varierar i rummet förblivit outforskade. I synnerhet är kännedomen låg om hur rumsligt genomsnittlig selektion och selektion som härstammar från rummets variabilitet samverkar för att antingen förstärka eller förhindra mångfald.

    I den här avhandlingen använder jag ett dubbelt grepp för att närma mig svar på dessa frågor, dels genom att utveckla de nödvändiga analytiska och numeriska verktygen för att sätta samman och analysera evolutionärt stabila samhällen i heterogena miljöer, och dels genom att använda dessa metoder för att studera samhällen av resurskonkurenter och näringsvävar. För att åstadkomma detta härleder jag uttryck för riktad och stabiliserande/disruptiv selektion när den rumsligt heterogena ekologiska dynamiken beskrivs av reaktion-diffusion ekvationer. Dessa uttryck möjliggör förståelse för selektion i en heterogen miljö i termer av hur selektion verkar lokalt i rummet, och möjliggör effektiva numeriska implementationer av metoder för att evolutionärt sätta samman samhällen.

    När dessa metoder appliceras på samhällena av resurskonkurenter och näringsvävar kan jag utröna att de selektiva krafterna som härstammar från rumsligt genomsnittlig selektion och de som härstammar från rumslig variabilitet både kan samverka och motverka varandra. Om de motverkar varandra, och graden av rumslig variabilitet är hög kan hög mångfald uppstå, även när genomsnittlig selektion är stabiliserande. I kontrast till detta kan disruptiv genomsnittlig selektion motverka mångfald genom att selektera för ett fåtal globalt dominanta strategier. Dessa krafter kan dock också samverka disruptivt och tillsammans bilda en stor mångfald. Tillsammans påvisar dessa resultat en stor variation av kvalitativt olika utfall när evolutionärt stabila samhällen sätts samman i rumsligt heterogena miljöer.

23456 201 - 250 av 265
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf