| 1. |
- Anselm, Jonas, et al.
(författare)
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Bannlys alla politiska beslut som ger mer klimatutsläpp
- 2014
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Ingår i: Dagens Nyheter.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Torftig valdebatt. Dagspolitiken klarar inte att hantera ödesfrågan om klimatet, vilket oroar oss. Vi föreslår därför ett ”utsläppsmoratorium”: inga beslut får tas som ökar utsläppen av växthusgaser. Principen måste kopplas till mål om exempelvis förnybar energi och grön infrastruktur, skriver 23 forskare och debattörer.
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| 2. |
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| 3. |
- Eckerberg, Katarina, et al.
(författare)
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Varför brister politikerna när det gäller miljömålen?
- 2012
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Ingår i: Dagens Nyheter. - 1101-2447.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Vad ska vi svara? Politikernas ambitiösa miljömål uppfylls sällan eller aldrig. När våra elever frågar oss varför blir vi ofta svaret skyldiga. Om politikerna inte vill att väljarna ska dra slutsatsen att de har misslyckats, måste de ange en realistisk väg att nå de uppsatta målen. Det skulle väcka respekt, skriver ledande forskare och samhällsplanerare.
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| 4. |
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| 5. |
- Abdallah, J, et al.
(författare)
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Rapidity-alignment and p(T) compensation of particle pairs in hadronic Z(0) decays
- 2002
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 533:3-4, s. 243-252
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Tidskriftsartikel (refereegranskat)abstract
- Observation is made of rapidity-alignment of K+K- and (p) over barp pairs which results from their asymmetric orientation in rapidity, with respect to the direction from primary quark to antiquark. The K+K- and (p) over barp data are consistent with predictions from the fragmentation string model. However, the (p) over barp data strongly disagree with the conventional implementation of the cluster model. The non-perturbative process of 'gluon splitting to diquarks' has to be incorporated into the cluster model for it to agree with the data. Local conservation of PT between particles nearby in rapidity (i.e., p(T) compensation) is analysed with respect to the thrust direction for pi(+)pi(-), K+K-, and (p) over barp pairs. In this case, the string model provides fair agreement with the data. The cluster model is incompatible with the data for all three particle pairs. The model with its central premiss of isotropically-decaying clusters predicts a p(T) correlation not seen in the data. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 6. |
- Abdallah, J, et al.
(författare)
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Search for charged Higgs bosons in e(+)e(-) collisions root s=189-202 GeV
- 2002
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 525:1-2, s. 17-28
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Tidskriftsartikel (refereegranskat)abstract
- A search for pair-produced charged Higgs bosons was performed in the high energy data collected by the DELPHI detector at LEP II at centre-of-mass energies from 189 GeV to 202 GeV. The three different final states, taunutaunu, c (s) over bar(c) over bars and c (s) over bar taunu were considered. New methods were applied to reject wrong hadronic jet pairings and for the tau identification, where a discriminator based on tau polarisation and polar angles was used. No excess of data compared to the expected Standard Model processes was observed and the existence of a charged Higgs boson with mass lower than 71.5 GeV/c(2) is excluded at the 95% confidence level. (C) 2002 Published by Elsevier Science B.V.
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| 7. |
- Abdallah, J, et al.
(författare)
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Study of inclusive J/psi production in two-photon collisions at LEP II with the DELPHI detector
- 2003
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 565:1-4, s. 76-86
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Tidskriftsartikel (refereegranskat)abstract
- Inclusive J/psi production in photon-photon collisions has been observed at LEP II beam energies. A clear signal from the reaction gammagamma --> J/psi + X is seen. The number of observed N(J/psi --> mu(+)mu(-)) events is 36+/-7 for an integrated luminosity of 617 pb(-1), yielding a cross-section of sigma(J/psi + X) = 45+/-9(stat) +/- 17(syst) pb. Based on a study of the event shapes of different types of gammagamma processes in the PYTHIA program, we conclude that (74+/-22) % of the observed J/psi events are due to 'resolved' photons, the dominant contribution of which is most probably due to the gluon content of the photon. (C) 2003 Published by Elsevier B.V.
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| 8. |
- Abreu, P, et al.
(författare)
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b-tagging in DELPHI at LEP
- 2004
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 32:2, s. 185-208
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Tidskriftsartikel (refereegranskat)abstract
- The standard method used for tagging b-hadrons in the DELPHI experiment at the CERN LEP Collider is discussed in detail. The main ingredient of b-tagging is the impact parameters of tracks, which relies mostly on the vertex detector. Additional information, such as the mass of particles associated to a secondary vertex, significantly improves the selection efficiency and the background suppression. The paper describes various discriminating variables used for the tagging and the procedure of their combination. In addition, applications of b-tagging to some physics analyses, which depend crucially on the performance and reliability of b-tagging, are described briefly.
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| 9. |
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| 10. |
- Brandén, Henrik, 1970-
(författare)
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Convergence Acceleration for Flow Problems
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Convergence acceleration techniques for the iterative solution of system of equations arising in the discretisations of compressible flow problems governed by the steady state Euler or Navier-Stokes equations is considered. The system of PDE is discretised using a finite difference or finite volume method yielding a large sparse system of equations. A solution is computed by integrating the corresponding time dependent problem in time until steady state is reached. A convergence acceleration technique based on semicirculant approximations is applied. For scalar model problems, it is proved that the preconditioned coefficient matrix has a bounded spectrum well separated from the origin. A very simple time marching scheme such as the forward Euler method can be used, and the time step is not limited by a CFL-type criterion. Instead, the time step can asymptotically be chosen as a constant, independent of the number of grid points and the Reynolds number. Numerical experiments show that grid and parameter independent convergence is achieved also in more complicated problem settings. A comparison with a multigrid method shows that the semicirculant convergence acceleration technique is more efficient in terms of arithmetic complexity. Another convergence acceleration technique based on fundamental solutions is proposed. An algorithm based on Fourier technique is provided for the fast application. Scalar model problems are considered and a theory, where the preconditioner is represented as an integral operator is derived. Theory and numerical experiments show that for first order partial differential equations, grid independent convergence is achieved.
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| 11. |
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| 12. |
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| 13. |
- Brandén, Henrik, et al.
(författare)
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Convergence acceleration for the steady state Euler equations
- 2000
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We consider the iterative solution of systems of equations arising from discretizations of the non-linear Euler equations governing compressible flow. The differential equation is discretized on a structured grid, and the steady state solution is computed by a time-marching method.A convergence acceleration technique based on semicirculant approximations of the difference operator or the Jacobian is used. Implementation issues and variants of the scheme allowing for a reduction of the arithmetic complexity and memory requirement are discussed. The technique can be combined with a variety of iterative solvers, but we focus on non-linear explicit Runge-Kutta time-integration schemes. The results show that the single-stage forward Euler method can be used, and the time step is not limited by a CFL-criterion. Instead it can be chosen as a constant, independent of the number of grid points. This results in that the arithmetic work required for computing the solution is equivalent to the work required for a fixed number of residual evaluations.Two major advantages of the semicirculant convergence acceleration technique is that it contains few tunable parameters, and that it is robust with respect to the amount of artificial viscosity used in the discretization.
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| 14. |
- Brandén, Henrik, et al.
(författare)
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Discrete Fundamental Solution Preconditioning for Hyperbolic Systems of PDE
- 2003
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We present a new preconditioner for the iterative solution of linear systems of equations arising from discretizations of systems of first order partial differential equations (PDEs) on structured grids. Such systems occur in many important applications, including compressible fluid flow and electormagnetic wave propagation. The preconditioner is a truncated convolution operator, with a kernel that is a fundamental solution of a difference operator closely related to the original discretization.Analysis of a relevant scalar model problem in two spatial dimensions shows that grid independent convergence is obtained using a simple one-stage iterative method. As an example of a more involved problem, we consider the steady state solution of the non-linear Euler equations in a two dimensional, non-axisymmetric duct. We present results from numerical experiments, verifying that the preconditioning technique again achieves grid independent convergence, both for an upwind discretization and for a centered second order discretization with fourth order artificial viscosity.
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| 15. |
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| 16. |
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| 17. |
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| 18. |
- Grandin, Magnus, 1981-
(författare)
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Adaptive Solvers for High-Dimensional PDE Problems on Clusters of Multicore Processors
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Accurate numerical solution of time-dependent, high-dimensional partial differential equations (PDEs) usually requires efficient numerical techniques and massive-scale parallel computing. In this thesis, we implement and evaluate discretization schemes suited for PDEs of higher dimensionality, focusing on high order of accuracy and low computational cost.Spatial discretization is particularly challenging in higher dimensions. The memory requirements for uniform grids quickly grow out of reach even on large-scale parallel computers. We utilize high-order discretization schemes and implement adaptive mesh refinement on structured hyperrectangular domains in order to reduce the required number of grid points and computational work. We allow for anisotropic (non-uniform) refinement by recursive bisection and show how to construct, manage and load balance such grids efficiently. In our numerical examples, we use finite difference schemes to discretize the PDEs. In the adaptive case we show how a stable discretization can be constructed using SBP-SAT operators. However, our adaptive mesh framework is general and other methods of discretization are viable.For integration in time, we implement exponential integrators based on the Lanczos/Arnoldi iterative schemes for eigenvalue approximations. Using adaptive time stepping and a truncated Magnus expansion, we attain high levels of accuracy in the solution at low computational cost. We further investigate alternative implementations of the Lanczos algorithm with reduced communication costs.As an example application problem, we have considered the time-dependent Schrödinger equation (TDSE). We present solvers and results for the solution of the TDSE on equidistant as well as adaptively refined Cartesian grids.
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| 19. |
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| 20. |
- Gustafsson, G, et al.
(författare)
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The electric field and wave experiment for the Cluster mission
- 1997
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Ingår i: Space Science Reviews. - 0038-6308 .- 1572-9672. ; 79, s. 137-156
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Tidskriftsartikel (refereegranskat)abstract
- The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36 000 samples s(-1). Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (1) measurements of quasi-static electric fields of amplitudes lip to 700 mV m(-1) with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large- and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.
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| 21. |
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| 22. |
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| 23. |
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| 24. |
- Gustafsson, Magnus, et al.
(författare)
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Numerical evaluation of the Communication-Avoiding Lanczos algorithm
- 2012
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The Lanczos algorithm is widely used for solving large sparse symmetric eigenvalue problems when only a few eigenvalues from the spectrum are needed. Due to sparse matrix-vector multiplications and frequent synchronization, the algorithm is communication intensive leading to poor performance on parallel computers and modern cache-based processors. The Communication-Avoiding Lanczos algorithm [Hoemmen; 2010] attempts to improve performance by taking the equivalence of s steps of the original algorithm at a time. The scheme is equivalent to the original algorithm in exact arithmetic but as the value of s grows larger, numerical roundoff errors are expected to have a greater impact. In this paper, we investigate the numerical properties of the Communication-Avoiding Lanczos (CA-Lanczos) algorithm and how well it works in practical computations. Apart from the algorithm itself, we have implemented techniques that are commonly used with the Lanczos algorithm to improve its numerical performance, such as semi-orthogonal schemes and restarting. We present results that show that CA-Lanczos is often as accurate as the original algorithm. In many cases, if the parameters of the s-step basis are chosen appropriately, the numerical behaviour of CA-Lanczos is close to the standard algorithm even though it is somewhat more sensitive to loosing mutual orthogonality among the basis vectors.
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| 25. |
- Gustafsson, Magnus
(författare)
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Towards an adaptive solver for high-dimensional PDE problems on clusters of multicore processors
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Accurate numerical simulation of time-dependent phenomena in many spatial dimensions is a challenging computational task apparent in a vast range of application areas, for instance quantum dynamics, financial mathematics, systems biology and plasma physics. Particularly problematic is that the number of unknowns in the governing equations (the number of grid points) grows exponentially with the number of spatial dimensions introduced, often referred to as the curse of dimensionality. This limits the range of problems that we can solve, since the computational effort and requirements on memory storage directly depend on the number of unknowns for which to solve the equations.In order to push the limit of tractable problems, we are developing an implementation framework, HAParaNDA, for high-dimensional PDE-problems. By using high-order accurate schemes and adaptive mesh refinement (AMR) in space, we aim at reducing the number of grid points used in the discretization, thereby enabling the solution of larger and higher-dimensional problems. Within the framework, we use structured grids for spatial discretization and a block-decomposition of the spatial domain for parallelization and load balancing. For integration in time, we use exponential integration, although the framework allows the flexibility of other integrators to be implemented as well. Exponential integrators using the Lanzcos or the Arnoldi algorithm has proven a succesful and efficient approach for large problems. Using a truncation of the Magnus expansion, we can attain high levels of accuracy in the solution.As an example application, we have implemented a solver for the time-dependent Schrödinger equation using this framework. We provide scaling results for small and medium sized clusters of multicore nodes, and show that the solver fulfills the expected rate of convergence.
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| 26. |
- Hellander, Andreas, 1982-
(författare)
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Multiscale Stochastic Simulation of Reaction-Transport Processes : Applications in Molecular Systems Biology
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Quantitative descriptions of reaction kinetics formulated at the stochastic mesoscopic level are frequently used to study various aspects of regulation and control in models of cellular control systems. For this type of systems, numerical simulation offers a variety of challenges caused by the high dimensionality of the problem and the multiscale properties often displayed by the biochemical model. In this thesis I have studied several aspects of stochastic simulation of both well-stirred and spatially heterogenous systems. In the well-stirred case, a hybrid method is proposed that reduces the dimension and stiffness of a model. We also demonstrate how both a high performance implementation and a variance reduction technique based on quasi-Monte Carlo can reduce the computational cost to estimate the probability density of the system. In the spatially dependent case, the use of unstructured, tetrahedral meshes to sample realizations of the stochastic process is proposed. Using such meshes, we then extend the reaction-diffusion framework to incorporate active transport of cellular cargo in a seamless manner. Finally, two multilevel methods for spatial stochastic simulation are considered. One of them is a space-time adaptive method combining exact stochastic, approximate stochastic and macroscopic modeling levels to reduce the simualation cost. The other method blends together mesoscale and microscale simulation methods to locally increase modeling resolution.
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| 27. |
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| 28. |
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| 29. |
- Holm, Marcus
(författare)
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Scientific computing on hybrid architectures
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modern computer architectures, with multicore CPUs and GPUs or other accelerators, make stronger demands than ever on writers of scientific code. As a rule of thumb, the fastest, most efficient program consists of labor-intensive code written by expert programmers for a certain application on a particular computer. This thesis deals with several algorithmic and technical approaches towards effectively satisfying the demand for high-performance parallel programming without incurring such a high cost in expert programmer time. Effective programming is accomplished by writing performance-portable code where performance-critical functionality is provided either by external software or at least a balance between maintainability/generality and efficiency.
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| 30. |
- Holmgren, Fredrik, et al.
(författare)
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Interactive Visual Analysis of Networked Systems: Workflows for Two Industrial Domains
- 2008. - 1
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We report on a first study of interactive visual analysis of networked systems. Working with ABB Corporate Research and Ericsson Research, we have created workflows which demonstrate the potential of visualization in the domains of industrial automation and telecommunications. By a workflow in this context, we mean a sequence of visualizations and the actions for generating them. Visualizations can be any images that represent properties of the data sets analyzed, and actions typically either change the selection of data visualized or change the visualization by choice of technique or change of parameters.
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| 31. |
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| 32. |
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| 33. |
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| 34. |
- Holmgren, Sverker, et al.
(författare)
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Can one hear the composition of a drum?
- 2006
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Ingår i: Mediterranean Journal of Mathematics. - : Springer Science and Business Media LLC. - 1660-5446 .- 1660-5454. ; 3, s. 227-249
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Tidskriftsartikel (refereegranskat)
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| 35. |
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| 36. |
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| 37. |
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| 38. |
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| 39. |
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| 40. |
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| 41. |
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| 42. |
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| 43. |
- Holmgren, Sverker, et al.
(författare)
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The Swedish National Graduate School in Scientific Computing (NGSSC)
- 2003
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Ingår i: Future generations computer systems. - 0167-739X .- 1872-7115. ; 19, s. 1275-1283
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Swedish National Graduate School in Scientific Computing (NGSSC) was founded in 1996, and has so far admitted 75 graduate students in five nationally announced admission rounds. The aim of NGSSC is to produce PhDs with skills in strategically important disciplines together with a broad knowledge of numerical and computational techniques, and to spread the use of computations into new areas of research. NGSSC consists of both a geographical and a disciplinary network and focuses on providing core knowledge on advanced computational techniques to graduate students in application fields. This presents some special challenges to the program, such as developing its own identity and dealing with the very different backgrounds of the students. The NGSSC activities focus on a package of specially developed courses, tailored for student groups with very mixed backgrounds. The courses are given in intensive format, and are compulsory for the students participating in the program. © 2003 Elsevier B.V. All rights reserved.
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| 44. |
- Holmgren, Sverker, et al.
(författare)
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Time-marching methods for the time-dependent Schrödinger equation
- 2004
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Ingår i: Proc. Int. Conference on Computational and Mathematical Methods in Science and Engineering. - Uppsala, Sweden : Uppsala University. - 8460911543 ; , s. 53-56
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 45. |
- Jansson, Niclas
(författare)
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High performance adaptive finite element methods for turbulent fluid flow
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the mechanics of turbulent fluid flow is of key importance for industry and society as for example in aerodynamics and aero-acoustics. The massive computational cost for resolving all turbulent scales in a realistic problem makes direct numerical simulation of the underlying Navier-Stokes equations impossible. Recent advances in adaptive finite element methods offer a new powerful tool in Computational Fluid Dynamics (CFD). The computational cost for simulating turbulent flow can be minimized where the mesh is adaptively resolved, based on a posteriori error control. These adaptive methods have been implemented for efficient serial computations, but the extension to an efficient parallel solver is a challenging task. This work concerns the development of an adaptive finite element method for modern parallel computer architectures. We present efficient data structures and data decomposition methods for distributed unstructured tetrahedral meshes. Our work also concerns an efficient parallellization of local mesh refinement methods such as recursive longest edge bisection. We also address the load balance problem with the development of an a priori predictive dynamic load balancing method. Current results are encouraging with almost linear strong scaling to thousands of cores on several modern architectures.
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| 46. |
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| 47. |
- Jayawardena, Mahen, et al.
(författare)
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A grid portal for genetic analysis of complex traits
- 2009
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Ingår i: Proc. 32nd International Convention on Information and Communication Technology, Electronics and Microelectronics. - Rijeka, Croatia : MIPRO. - 9789532330441 ; , s. 281-284
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Konferensbidrag (refereegranskat)
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| 48. |
- Jayawardena, Mahen, 1977-
(författare)
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An e-Science Approach to Genetic Analysis of Quantitative Traits
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many important traits in plants, animals and humans are quantitative, and most such traits are generally believed to be affected by multiple genetic loci. Standard computational tools for mapping of quantitative traits (i.e. for finding Quantitative Trait Loci, QTL, in the genome) use linear regression models for relating the observed phenotypes to the genetic composition of individuals in an experimental population. Using these tools to simultaneously search for multiple QTL is computationally demanding. The main reason for this is the complex optimization landscape for the multidimensional global optimization problems that must be solved. This thesis describes parallel algorithms, implementations and tools for simultaneous mapping of several QTL. These new computational tools enable genetic analysis exploiting new classes of multidimensional statistical models, potentially resulting in interesting results in genetics. We first describe how the standard, brute-force algorithm for global optimization in QTL analysis is parallelized and implemented on a grid system. Then, we also present a parallelized version of the more elaborate global optimization algorithm DIRECT and show how this can be efficiently deployed and used on grid systems and other loosely-coupled architectures. The parallel DIRECT scheme is further developed to exploit both coarse-grained parallelism in grid systems or clusters as well as fine-grained, tightly-coupled parallelism in multi-core nodes. The results show that excellent speedup and performance can be archived on grid systems and clusters, even when using a tightly-coupled algorithm such as DIRECT. Finally, we provide two distinctly different front-ends for our code. One is a grid portal providing a graphical front-end suitable for novice users and standard forms of QTL analysis. The other is a prototype of an R-based grid-enabled problem solving environment. Both of these front-ends can, after some further refinement, be utilized by geneticists for performing multidimensional genetic analysis of quantitative traits on a regular basis.
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| 49. |
- Jayawardena, Mahen, et al.
(författare)
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Computational and visualization tools for genetic analysis of complex traits
- 2010
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We present grid based tools for simultaneous mapping of multiple locations (QTL) in the genome that affect quantitative traits (e.g. body weight, blood pressure) in experimental populations. The corresponding computational problem is very computationally intensive. We have earlier shown that, using appropriate parallelization schemes, this type of application is suitable for deployment on grid systems.A grid portal interface is often an ideal tool for biologists performing advanced genetic analysis. We describe an implementation of such a portal system and how it is used for performing multidimensional QTL searches efficiently.
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| 50. |
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