| 1. |
- Flodén, Liselott, 1967-
(author)
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G-Convergence and Homogenization of some Sequences of Monotone Differential Operators
- 2009
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Doctoral thesis (other academic/artistic)abstract
- This thesis mainly deals with questions concerning the convergence of some sequences of elliptic and parabolic linear and non-linear operators by means of G-convergence and homogenization. In particular, we study operators with oscillations in several spatial and temporal scales. Our main tools are multiscale techniques, developed from the method of two-scale convergence and adapted to the problems studied. For certain classes of parabolic equations we distinguish different cases of homogenization for different relations between the frequencies of oscillations in space and time by means of different sets of local problems. The features and fundamental character of two-scale convergence are discussed and some of its key properties are investigated. Moreover, results are presented concerning cases when the G-limit can be identified for some linear elliptic and parabolic problems where no periodicity assumptions are made.
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| 2. |
- Lund Ohlsson, Marie
(author)
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New methods for movement technique development in cross-country skiing using mathematical models and simulation
- 2009
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Licentiate thesis (other academic/artistic)abstract
- This Licentiate Thesis is devoted to the presentation and discussion of some new contributions in applied mathematics directed towards scientific computing in sports engineering. It considers inverse problems of biomechanical simulations with rigid body musculoskeletal systems especially in cross-country skiing. This is a contrast to the main research on cross-country skiing biomechanics, which is based mainly on experimental testing alone. The thesis consists of an introduction and five papers. The introduction motivates the context of the papers and puts them into a more general framework. Two papers (D and E) consider studies of real questions in cross-country skiing, which are modelled and simulated. The results give some interesting indications, concerning these challenging questions, which can be used as a basis for further research. However, the measurements are not accurate enough to give the final answers. Paper C is a simulation study which is more extensive than paper D and E, and is compared to electromyography measurements in the literature. Validation in biomechanical simulations is difficult and reducing mathematical errors is one way of reaching closer to more realistic results. Paper A examines well-posedness for forward dynamics with full muscle dynamics. Moreover, paper B is a technical report which describes the problem formulation and mathematical models and simulation from paper A in more detail. Our new modelling together with the simulations enable new possibilities. This is similar to simulations of applications in other engineering fields, and need in the same way be handled with care in order to achieve reliable results. The results in this thesis indicate that it can be very useful to use mathematical modelling and numerical simulations when describing cross-country skiing biomechanics. Hence, this thesis contributes to the possibility of beginning to use and develop such modelling and simulation techniques also in this context.
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| 3. |
- Persson, Jens, 1978-
(author)
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Homogenization of Some Selected Elliptic and Parabolic Problems Employing Suitable Generalized Modes of Two-Scale Convergence
- 2010
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Licentiate thesis (other academic/artistic)abstract
- The present thesis is devoted to the homogenization of certain elliptic and parabolic partial differential equations by means of appropriate generalizations of the notion of two-scale convergence. Since homogenization is defined in terms of H-convergence, we desire to find the H-limits of sequences of periodic monotone parabolic operators with two spatial scales and an arbitrary number of temporal scales and the H-limits of sequences of two-dimensional possibly non-periodic linear elliptic operators by utilizing the theories for evolution-multiscale convergence and λ-scale convergence, respectively, which are generalizations of the classical two-scale convergence mode and custom-made to treat homogenization problems of the prescribed kinds. Concerning the multiscaled parabolic problems, we find that the result of the homogenization depends on the behavior of the temporal scale functions. The temporal scale functions considered in the thesis may, in the sense explained in the text, be slow or rapid and in resonance or not in resonance with respect to the spatial scale function. The homogenization for the possibly non-periodic elliptic problems gives the same result as for the corresponding periodic problems but with the exception that the local gradient operator is everywhere substituted by a differential operator consisting of a product of the local gradient operator and matrix describing the geometry and which depends, effectively, parametrically on the global variable.
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| 4. |
- Persson, Jens, 1978-
(author)
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Selected Topics in Homogenization
- 2012
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Doctoral thesis (other academic/artistic)abstract
- The main focus of the present thesis is on the homogenization of some selected elliptic and parabolic problems. More precisely, we homogenize: non-periodic linear elliptic problems in two dimensions exhibiting a homothetic scaling property; two types of evolution-multiscale linear parabolic problems, one having two spatial and two temporal microscopic scales where the latter ones are given in terms of a two-parameter family, and one having two spatial and three temporal microscopic scales that are fixed power functions; and, finally, evolution-multiscale monotone parabolic problems with one spatial and an arbitrary number of temporal microscopic scales that are not restricted to be given in terms of power functions. In order to achieve homogenization results for these problems we study and enrich the theory of two-scale convergence and its kins. In particular the concept of very weak two-scale convergence and generalizations is developed, and we study an application of this convergence mode where it is employed to detect scales of heterogeneity.
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| 5. |
- Wokiyi, Dennis, 1986-
(author)
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Non-linear inverse geothermal problems
- 2017
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Licentiate thesis (other academic/artistic)abstract
- The inverse geothermal problem consist of estimating the temperature distribution below the earth’s surface using temperature and heat-flux measurements on the earth’s surface. The problem is important since temperature governs a variety of the geological processes including formation of magmas, minerals, fosil fuels and also deformation of rocks. Mathematical this problem is formulated as a Cauchy problem for an non-linear elliptic equation and since the thermal properties of the rocks depend strongly on the temperature, the problem is non-linear. This problem is ill-posed in the sense that it does not satisfy atleast one of Hadamard’s definition of well-posedness.We formulated the problem as an ill-posed non-linear operator equation which is defined in terms of solving a well-posed boundary problem. We demonstrate existence of a unique solution to this well-posed problem and give stability estimates in appropriate function spaces. We show that the operator equation is well-defined in appropriate function spaces.Since the problem is ill-posed, regularization is needed to stabilize computations. We demostrate that Tikhonov regularization can be implemented efficiently for solving the operator equation. The algorithm is based on having a code for solving a well- posed problem related to the operator equation. In this study we demostrate that the algorithm works efficiently for 2D calculations but can also be modified to work for 3D calculations.
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| 6. |
- Bergström, Per
(author)
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Computational methods for shape verification of free-form surfaces
- 2011
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Doctoral thesis (other academic/artistic)abstract
- Beräkningsmetoder för formverifiering av friformsytor utgör huvudinnehållet i denna doktorsavhandling. En gemensam egenskap för dessa metoder är att de möjliggör formverifiering online direkt i produktionslinan. Av den anledningen måste metoderna vara snabba och robusta. Ett av problemen som uppkommer i formverifieringen av friformsytor är registrering. Det är problemet med att matcha datapunkter i 3D-rymden, som representerar den uppmätta ytan, med ett CAD-objekt genom att ansätta en stelkropps transformation. En metod för att utföra registreringen snabbt och robust är utvecklad. Metoden är en utveckling av ”the iterative closest point method, ICP”. Vi förprocessar CAD-objektet genom att skapa en datastruktur för att möjliggöra snabb närmsta-punkt sökning. Initialt läggs mycket tid på att skapa datastrukturen för att de enskilda registreringarna skall gå snabbt. Den robusta registreringen baserar sig på teorier från robust statistik genom att tillämpa ”iteratively re-weighted least squares” i kombination med ICP metoden. Detta resulterar i en snabb registreringsmetod som är okänslig för avvikande data. Metoden med registreringen används i en tillämpning för att hitta avvikelser mellan formen för ett objekt och dess ideala form. Den ideala formen är känd och ges av ett CAD-objekt. En optisk formmätningsmetod, projicerade fransar med en enda mönsterdetektering, används för att skapa datapunkter av den uppmätta ytan. Denna metod är snabb och okänslig för vibrationer men datapunkterna kan innehålla fel i vissa regioner, vilket hanteras av registreringen. Ett inversproblem som uppkommer i många optiska formmätningsmetoder är fasuppvikning. Vi introducerar en uppvikningsmetod med regularisering genom att använda information från ett CAD-objekt. Formmätningsmetoden som vi använder oss av här baserar sig på två-våglängds holografi. Vår fasuppvikningsmetod funkar oberoende av diskoninuiteter men mätobjektet får inte avvika alltför mycket i form jämfört med CAD-objektet. En metod för att snabbt få fram den behövda forminformationen från CAD-objektet är också utvecklad. För att få fram lämplig forminformation från datapunkter kan en parametrisk kurva eller yta, t.ex. NURBS, anpassas till dessa punkter. Ett delproblem som uppstår vid NURBS-anpassning vid användandet av Gauss-Newton metoden är studerad. Beräkningsaspekter för att få fram en sökriktning är diskuterade. Vi behandlar också metoder för NURBS anpassning som baserar sig på en teknik för separabla icke-linjära minstakvadratproblem. Denna teknik använder sig av variabelprojektioner för att separera beräkningarna av de linjära parametrarna från beräkningarna av de icke-linjära parametrarna.
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| 7. |
- Olsson, Marianne, 1973-
(author)
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G-Convergence and Homogenization of some Monotone Operators
- 2008
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Doctoral thesis (other academic/artistic)abstract
- In this thesis we investigate some partial differential equations with respect to G-convergence and homogenization. We study a few monotone parabolic equations that contain periodic oscillations on several scales, and also some linear elliptic and parabolic problems where there are no periodicity assumptions. To begin with, we examine parabolic equations with multiple scales regarding the existence and uniqueness of the solution, in view of the properties of some monotone operators. We then consider G-convergence for elliptic and parabolic operators and recall some results that guarantee the existence of a well-posed limit problem. Then we proceed with some classical homogenization techniques that allow an explicit characterization of the limit operator in periodic cases. In this context, we prove G-convergence and homogenization results for a monotone parabolic problem with oscillations on two scales in the space variable. Then we consider two-scale convergence and the homogenization method based on this notion, and also its generalization to multiple scales. This is further extended to the case that allows oscillations in space as well as in time. We prove homogenization results for a monotone parabolic problem with oscillations on two spatial scales and one temporal scale, and for a linear parabolic problem where oscillations occur on one scale in space and two scales in time. Finally, we study some linear elliptic and parabolic problems where no periodicity assumptions are made and where the coefficients are created by certain integral operators. Here we prove results concerning when the G-limit may be obtained immediately and is equal to a certain weak limit of the sequence of coefficients.
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