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  • 1.
    Costa, Pedro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A FFT-based finite-difference solver for massively-parallel direct numerical simulations of turbulent flows2018In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 76, no 8, p. 1853-1862Article in journal (Refereed)
    Abstract [en]

    We present an efficient solver for massively-parallel direct numerical simulations of incompressible turbulent flows. The method uses a second-order, finite-volume pressure-correction scheme, where the pressure Poisson equation is solved with the method of eigenfunction expansions. This approach allows for very efficient FFT-based solvers in problems with different combinations of homogeneous pressure boundary conditions. Our algorithm explores all combinations of pressure boundary conditions valid for such a solver, in a single, general framework. The method is implemented in a 2D pencil-like domain decomposition, which enables efficient massively-parallel simulations. The implementation was validated against different canonical flows, and its computational performance was examined. Excellent strong scaling performance up to 104 cores is demonstrated for a domain with 109 spatial degrees of freedom, corresponding to a very small wall-clock time/time step. The resulting tool, CaNS, has been made freely available and open-source.

  • 2.
    Eriksson, Robert
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Coordinated control design of multiple HVDC links based on model identification2010In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 60, no 4, p. 944-953Article in journal (Refereed)
    Abstract [en]

    This paper presents a method for designing a centralized coordinated controller for several HVDC links. The controller increases the damping of the power oscillations by modulating the current through the HVDC links in a coordinated fashion. To design a centralized coordinated controller a reduced order open system model is estimated. The open system model of the power system is estimated using the Numerical Algorithms for Subspace State-Space System Identification (N4SID) algorithm which is a black-box system identification technique. The current set-point change through the HVDC links is the set of input signals and the speeds of the generators are the set of outputs. This controller design method increases the damping significantly, which is shown for a small power system.

  • 3. Kourdis, P. D.
    et al.
    Palasantza, Athanasia G.
    Department of Mechanics, School of Applied Mathematics and Physical Sciences, National Technical University of Athens, Greece.
    Goussis, D. A.
    Algorithmic asymptotic analysis of the NF-κ B signaling system2013In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 65, no 10, p. 1516-1534Article in journal (Refereed)
    Abstract [en]

    The results of a detailed, step-by-step, asymptotic analysis of the NF-κB signaling system are reported, in the case where the system exhibits limit cycle behavior. The analysis is based on the dimensional form of the model and exploits the various fast/slow time scale gaps that develop as the solution evolves along the limit cycle. It is shown that under the action of fast time scales of dissipative character, the limit cycle is confined on a low-dimensional surface in the phase space. The cycle can be divided in three parts, each one related to a different characteristic time scale. The first part refers to the slow rate of NF-κB release in the cytoplasm. The second part refers to the even slower rate by which the free NF-κB enters into the nucleus. The last part refers to the fast rate by which the nuclear NF-κB is first produced and then depleted. It is demonstrated that these and many other findings (regarding the fast variables, the equilibrated fast processes, the rate limiting and/or driving steps, etc.) can be acquired by simple algorithmic tools.

  • 4. M, Telek
    et al.
    A, Pfening
    Fodor, Gabor
    Department of Telecommunications and Telematics Technical University of Budapest, 1521 Budapest, Hungary.
    An effective numerical method to compute the moments of the completion time of Markov reward models1998In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 36, no 8, p. 59-65Article in journal (Refereed)
    Abstract [en]

    Analysis of Markov Reward Models (MRM) with preemptive resume (prs) policy results in a double transform expression, whose solution is based on the inverse transformations both in time and reward variable domain. We present a symbolic expression of moments of the completion time, from which a computationally effective recursive numerical method can be obtained. (C) 1998 Elsevier Science Ltd. All rights reserved.

  • 5.
    Maaranen, Heikki
    et al.
    Department of Mathematical Information Technology, University of Jyväskylä.
    Miettinen, Kaisa
    Helsinki School of Economics.
    Mäkelä, Marko M.
    Department of Mathematical Information Technology, University of Jyväskylä.
    Quasi-random initial population for genetic algorithms2004In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 47, no 12, p. 1885-1895Article in journal (Refereed)
    Abstract [en]

    The selection of the initial population in a population-based heuristic optimizationmethod is important, since it affects the search for several iterations and often has an influence on the final solution. If no a priori information about the optima is available, the initial population is often selected randomly using pseudorandom numbers. Usually, however, it is more important that the points are as evenly distributed as possible than that they imitate random points. In this paper, we study the use of quasi-random sequences in the initial population of a genetic algorithm. Sample points in a quasi-random sequence are designed to have good distribution properties. Here a modified genetic algorithm using quasi-random sequences in the initial population is tested by solving a large number of continuous benchmark problems from the literature. The numerical results of two implementations of genetic algorithms using different quasi-random sequences are compared to those of a traditional implementation using pseudorandom numbers. The results obtained are promising.

  • 6. Motamed, M.
    et al.
    Runborg, Olof
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    A wavefront-based Gaussian beam method for computing high frequency wave propagation problems2015In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 69, no 9, p. 949-963Article in journal (Refereed)
    Abstract [en]

    We present a novel wavefront method based on Gaussian beams for computing high frequency wave propagation problems. Unlike standard geometrical optics, Gaussian beams compute the correct solution of the wave field also at caustics. The method tracks a front of two canonical beams with two particular initial values for width and curvature. In a fast post-processing step, from the canonical solutions we recreate any other Gaussian beam with arbitrary initial data on the initial front. This provides a simple mechanism to include a variety of optimization processes, including error minimization and beam width minimization, for a posteriori selection of optimal beam initial parameters. The performance of the method is illustrated with two numerical examples.

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