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| 2. |
- Eliasson, Bengt
(författare)
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Domain decomposition of the Padé scheme and pseudo-spectral method, used in Vlasov simulations
- 2002
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In order to evaluate parallel algorithms for solving the Vlasov equation numerically in multiple dimensions, the algorithm for solving the one-dimensional Vlasov equation numerically has been parallelised. The one-dimensional Vlasov equation leads to a problem in the two-dimensional phase space (x,v), plus time. The parallelisation is performed by domain decomposition to a rectangular processor grid. Derivatives in x space are calculated by a pseudo-spectral method, where FFTs are used to perform discrete Fourier transforms. In velocity v space a Fourier method is used, together with the compact Padé scheme for calculating derivatives, leading to a large number of tri-diagonal linear systems to be solved. The parallelisation of the tri-diagonal systems in the Fourier transformed velocity space can be performed efficiently by the method of domain decomposition. The domain decomposition gives rise to Schur complement systems, which are tri-diagonal, symmetric and strongly diagonally dominant, making it possible to solve these systems with a few Jacobi iterations. Therefore, the parallel efficiency of the semi-implicit Padé scheme is comparable to the parallel efficiency of explicit difference schemes. The parallelisation in x space is less effective due to the FFTs used. The code has been tested on shared memory computers, on clusters of computers, and with the help of the Globus toolkit for communication over the Internet.
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| 3. |
- Eliasson, Bo, et al.
(författare)
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New Technology for Safe Critical Applications in Real Time
- 2001
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Several techniques are available for constraint satisfaction problems. Most of them lack real time capability and distributed computational properties. Configuration of complex systems has today become a competitive edge, e.g. within e-business. The other side of the same coin can be used for diagnosis and simulation of safe critical applications. This paper gives a scientific background and comparison of methods for constraint satisfaction problems implemented in real time environment. Especially, the technique and its features of the array-based logic will be illustrated by three examples. The examples are: 1.Modelling and simulation of system wide protection applications of power systems 2.Modelling, simulation and implementation of safe critical switching in power systems 3.Modelling, simulation and implementation of safe critical systems of automatic train control Array-based logic is a novel technology, which is founded on a geometrical representation of logic in terms of nested data arrays. In this representation, logical inferences are executed on finite domain systems by a few standardised array operations. In computational practice, array-based logic makes it possible to solve large-scale constraint problems with combinations very efficiently. The major advantages of the technology are completeness, compactness, and speed of simulation (real-time capability), all of which derive from intrinsic properties of the underlying geometrical model. A system in array-based logic is considered analogously to electrical networks and is thus described by the constraints of the isolated elements as well as the constraints of the interconnected elements (the system topology). All system constraints are eliminated in accordance with the methods known from electrical network theory and other engineering disciplines rooted in classical physics. The configuration space is represented explicitly in terms of nested data arrays and the system can therefore be simulated in real-time by co-ordinate indexing on these arrays. This data driven approach makes it suitable for parallel processing, which is an important quality for distributed systems.
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| 4. |
- Eliasson, Bengt
(författare)
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Numerical modelling of the two-dimensional Vlasov–Maxwell system
- 2002
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The two-dimensional Vlasov-Maxwell system for a plasma with mobile, magnetised electrons and ions, is investigated numerically. Special attention is paid to the conservation of the divergences of the electric and magnetic fields in the Maxwell equations. The Maxwell equations are rewritten, by means of the Lorentz potentials, in a form which conserves these divergences. Linear phenomena are investigated numerically and compared with theory and with previous numerical results.
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| 5. |
- Eliasson, Bengt
(författare)
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Numerical simulation of kinetic effects in ionospheric plasma
- 2001
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, we study numerically the one-dimensional Vlasov equation for a plasma consisting of electrons and infinitely heavy ions. This partial differential equation describes the evolution of the distribution function of particles in the two-dimensional phase space (x,v). The Vlasov equation describes, in statistical mechanics terms, the collective dynamics of particles interacting with long-range forces, but neglects the short-range "collisional" forces. A space plasma consists of electrically charged particles, and therefore the most important long-range forces acting on a plasma are the Lorentz forces created by electromagnetic fields.What makes the numerical solution of the Vlasov equation to a challenging task is firstly that the fully three-dimensional problem leads to a partial differential equation in the six-dimensional phase space, plus time, making it even hard to store a discretized solution in the computer's memory. Secondly, the Vlasov equation has a tendency of structuring in velocity space (due to free streaming terms), in which steep gradients are created and problems of calculating the v (velocity) derivative of the function accurately increase with time.The method used in this thesis is based on the technique of Fourier transforming the Vlasov equation in velocity space and then solving the resulting equation. We have developed a method where the small-scale information in velocity space is removed through an outgoing wave boundary condition in the Fourier transformed velocity space. The position of the boundary in the Fourier transformed variable determines the amount of small-scale information saved in velocity space.The numerical method is used to investigate a phenomenon of tunnelling of information through an ionospheric layer, discovered in experiments, and to assess the accuracy of approximate analytic formulæ describing plasma wave dispersion. The numerical results are compared with theoretical predictions, and further physical experiments are proposed.
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| 6. |
- Eliasson, Bengt, 1965-
(författare)
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Numerical Vlasov–Maxwell Modelling of Space Plasma
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Vlasov equation describes the evolution of the distribution function of particles in phase space (x,v), where the particles interact with long-range forces, but where shortrange "collisional" forces are neglected. A space plasma consists of low-mass electrically charged particles, and therefore the most important long-range forces acting in the plasma are the Lorentz forces created by electromagnetic fields. What makes the numerical solution of the Vlasov equation a challenging task is that the fully three-dimensional problem leads to a partial differential equation in the six-dimensional phase space, plus time, making it hard even to store a discretised solution in a computer’s memory. Solutions to the Vlasov equation have also a tendency of becoming oscillatory in velocity space, due to free streaming terms (ballistic particles), in which steep gradients are created and problems of calculating the v (velocity) derivative of the function accurately increase with time. In the present thesis, the numerical treatment is limited to one- and two-dimensional systems, leading to solutions in two- and four-dimensional phase space, respectively, plus time. The numerical method developed is based on the technique of Fourier transforming the Vlasov equation in velocity space and then solving the resulting equation, in which the small-scale information in velocity space is removed through outgoing wave boundary conditions in the Fourier transformed velocity space. The Maxwell equations are rewritten in a form which conserves the divergences of the electric and magnetic fields, by means of the Lorentz potentials. The resulting equations are solved numerically by high order methods, reducing the need for numerical over-sampling of the problem. The algorithm has been implemented in Fortran 90, and the code for solving the one-dimensional Vlasov equation has been parallelised by the method of domain decomposition, and has been implemented using the Message Passing Interface (MPI) method. The code has been used to investigate linear and non-linear interaction between electromagnetic fields, plasma waves, and particles.
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| 9. |
- Forsberg, Bertil, et al.
(författare)
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Luftföroreningshalters relation till fibrinogenhalten i blod
- 2002
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Som ett specialprojekt om hälsorelaterad miljöövervakning har studerats om hälsoundersökningsdata avseende fibrinogenhalt i blod kan användas som en indikator på luftföroreningspåverkan. Bakgrunden är att det under senare år har publicerats flera studier i vilka samband konstaterats mellan dygnets eller de senaste dygnens luftföroreningshalt (främst partiklar eller avgaskomponenter) och det dygnsvisa antalet dödsfall eller sjukhusinläggningar p g a hjärtsjukdom. Mekanismerna bakom effekterna är inte helt klarlagda, men mycket talar för att en mekanism kan vara kroppens inflammationssvar som inducerar förändringar i blodets koaguleringsbenägenhet. Fibrinogen är en av de viktigaste faktorerna bakom blodets viskositet. Ökad fibrinogenhalt i blodplasma har tidigare konstaterats dels i samband med en luftföroreningsepisod i Tyskland och när hälsoundersökningsdata från London korrelerats till kvävedioxidhalten dygnet innan blodprovet togs. I denna undersökning har vi använt data om fibrinogenhalten (g/l) i blodprover som tagits i den befolkningsbaserade MONICA-undersökningen i Norr- och Västerbotten under januari-april 1990 och 1994, och utifrån undersökningsdatum påfört individerna uppgifter om halten av ozon respektive sot samma och intilliggande dygn. För närmare 3400 personer vars blodprov kunnat sammankopplas med aktuell ozonhalt, konstaterar vi att fibrinogenhalten ökar med stigande ozonvärde. Sambandet är statistiskt säkerställt vid en analys med hänsyn till individfaktorer av betydelse för fibrinogenhalten. En ökning av ozonhalten med 50 μg/m3 (från 10:e till 90:e percentilen) leder till en i genomsnitt 3 % ökad fibrinogenhalt.
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| 10. |
- Kalani, Majid, et al.
(författare)
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Effect of dalteparin on healing of chronic foot ulcers in diabetic patients with peripheral arterial occlusive disease: a prospective, randomized, double-blind, placebo-controlled study.
- 2003
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Ingår i: Diabetes care. - : American Diabetes Association. - 0149-5992 .- 1935-5548. ; 26:9, s. 2575-80
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVE: Chronic foot ulcers are a common, severe, and expensive complication threatening life and limb in patients with diabetes. The aim of the present study was to investigate the effect of dalteparin on ulcer outcome in patients with diabetes, peripheral arterial occlusive disease, and chronic foot ulcers. RESEARCH DESIGN AND METHODS: A total of 87 patients were investigated in a prospective, randomized, double-blind, placebo-controlled trial. Participants were randomized to treatment with subcutaneous injection of 5000 units dalteparin (Fragmin, Pharmacia Corporation; n = 44) or an equivalent volume of physiological saline (n = 43) once daily until ulcer healing or for a maximum of 6 months. Ulcer outcome was investigated by evaluating the number of patients 1). who healed with intact skin; 2). in whom the study ulcer was improved, unchanged, or impaired; or 3). who were amputated above or below the ankle level, as compared with control subjects. RESULTS: Two patients, one on dalteparin and one on placebo, dropped out of the study. Ulcer outcome was significantly better (P = 0.042, two-sided chi(2) test for trend) in the dalteparin group (n = 43) compared with the placebo group (n = 42). A total of 29 patients healed with intact skin (n = 14) or decreased the ulcer area >or=50% (n = 15) in the dalteparin group compared with 20 (n = 9 and 11, respectively) in the placebo group. Five patients in each group showed impaired ulcer healing, i.e., the ulcer area increased >or=50%. Two patients in the dalteparin group were amputated compared with eight in the placebo group. Time to healing with intact skin was 17 +/- 8 weeks in the dalteparin group compared with 16 +/- 7 weeks in placebo group (NS). CONCLUSIONS: The results of the present study indicate that dalteparin improves the outcome of chronic foot ulcers in diabetic patients with peripheral arterial occlusive disease.
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