# Fakultäten / Faculties

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- Book partOpen AccessSimulation of Network Machines(Akademie-Verlag, 1989) Pöschel, Thorsten
- ArticleOpen AccessPrimärprozesse der Photosynthese - kinetische Modellierung(Humboldt-Universität zu Berlin, 1989-05-01) Voigt, Jochen; Maciy, Katina; Pöschel, Thorsten; Schimansky-Geier, Lutz
- ArticleOpen AccessModellierung Neuronaler Netzwerke(Humboldt-Universität zu Berlin, 1989-05-01) Boseniuk, Thorsten; Pöschel, Thorsten
- ArticleOpen AccessA Multiprocessor System for High Speed Simulation of Neural Networks(Akademie-Verlag, 1990) Boseniuk, Thorsten; van der Meer, Marius; Pöschel, ThorstenA special purpose low-cost multiprocessor system for the high-speed simulation of error back-propagation (and related) algorithms will be proposed. It will be shown that the processing time rises with 0(LN) (L-number of layers in the network, N-number of elements per layer), while universal serial machines need 0(LN^2). A detailed hardware description will be given.
- Book partOpen AccessSelf-organization of Nonlinear Networks and Applications to Information Processing(Manchester University Press, 1991) Pöschel, Thorsten; Ebeling, Werner; Boseniuk, ThorstenThe Hopfield Associative Memory (AM) can be understood as an extremely simple Error Backpropagation Network (EBN). This feature we use for the initialisation of the synaptic matrix. Motivated by findings of neural physiology we further consider networks of artificial neurons with multiplicative couplings. Two proposals for the self-organisation of the network architecture are given in the third section.
- ArticleOpen AccessComputer simulieren verstopfte Rohre(Forschungszentrum Jülich, 1993-03-01) Pöschel, Thorsten; Puhl, HaraldComputersimulationen gewinnen in Forschung und Industrie immer mehr an Bedeutung; sie ersetzen und ergänzen experimentelle Untersuchungen oder erleichtern das Verständnis von Ergebnissen. Solche Simulationen werden bekanntlich auch im Höchstleistungsrechenzentrum (HLRZ) durchgeführt. Mitarbeiter des HLRZ haben in der Vergangenheit unter anderem den Fluß von granularen Medien durch Rohre berechnet. Dabei konnte die Frage beantwortet werden, warum z. B. beim Ausfluß von Sand oder Körnern aus Silos Verstopfungen auftreten. Der Grund sind unregelmäßige Dichtewellen; ihre Berechnung kann zur Lösung zahlreicher industrieller Probleme beitragen.
- Conference objectOpen AccessOn the Hypercube Structure of the Genetic Code(World Scientific, 1995-09-01) Jiménez-Montaño, Miguel A.; de la Mora-Basáñez, Carlos R.; Pöschel, ThorstenA representation of the genetic code as a six–dimensional Boolean hypercube is proposed. It is assumed here that this structure is the result of the hierarchical order of the interaction energies of the bases in codon–anticodon recognition. The proposed structure demonstrates that in the genetic code there is a balance between conservatism and innovation. Comparing aligned positions in homologous protein sequences two different behaviors are found: a)There are sites in which the different amino acids present may be explained by one or two “attractor nodes” (coding for the dominating amino acid(s)) and their one–bit neighbors in the codon hypercube, and b) There are sites in which the amino acids present correspond to codons located in closed paths in the hypercube. The structure of the code facilitates evolution: the variation found at the variable positions of proteins do not corresponds to random jumps at the codon level, but to well defined regions of the hypercube.
- Conference objectOpen AccessDynamic Entropies, Long-range Correlations and Fluctuations in Complex Linear Structures(World Scientific, 1995-12-01) Ebeling, Werner; Neiman, Alexander; Pöschel, ThorstenWe investigate symbolic sequences and in particular information carriers as e.g. books and DNA{strings. First the higher order Shannon entropies are calculated, a characteristic root law is detected. Then the algorithmic entropy is estimated by using Lempel-Ziv compression algorithms. In the third section the correlation function for distant letters, the low frequency Fourier spectrum and the characteristic scaling exponents are calculated. We show that all these measures are able to detect long{range correlations. However, as demonstrated by shuffling experiments, different measures operate on different length scales. The longest correlations found in our analysis comprise a few hundreds or thousands of letters and may be understood as long-wave fluctuations of the composition.
- Book partOpen AccessDie Kühlrate eines granularen Gases(Köster, 1996) Schwager, Thomas; Pöschel, Thorsten
- Book partOpen AccessKann die Entropie von Sequenzen vermittelst der Kompressibilität gemessen werden?(Köster, 1996) Pöschel, Thorsten
- Conference objectOpen AccessEntropy and Compressibility of Symbol Sequences(Complex Systems Institute, 1996-11-22) Ebeling, Werner; Pöschel, Thorsten; Neiman, AlexanderThe purpose of this paper is to investigate long-range correlations in symbol sequences using methods of statistical physics and nonlinear dynamics. Beside the principal interest in the analysis of correlations and fluctuations comprising many letters, our main aim is related here to the problem of sequence compression. In spite of the great progress in this field achieved in the work of Shannon, Fano, Huffman, Lempel, Ziv and others [1] many questions still remain open. In particular one must note that since the basic work by Lempel and Ziv the improvement of the standard compression algorithms is rather slow not exceeding a few percent per decade. One the other hand several experts expressed the idee that the long range correlations, which clearly exist in texts, computer programs etc. are not sufficiently taken into account by the standard algorithms [1]. Thus, our interest in compressibility is twofold: (i) We would like to explore how far compressibility is able to measure correlations. In particular we apply the standard algorithms to model sequences with known correlations such as, for instance, repeat sequences and symbolic sequences generated by maps. (ii) We aim to detect correlations which are not yet exploited in the standard compression algorithms and belong therefore to potential reservoirs for compression algorithms. First, the higher-order Shannon entropies are calculated. For repeat sequences analytic estimates are derived, which apply in some approximation to DNA sequences too. For symbolic strings obtained from special nonlinear maps and for several long texts a characteristic root law for the entropy scaling is detected. Then the compressibilities are estimated by using grammar representations and several standard computing algorithms. In particular we use the Lempel-Ziv compression algorithm. Further the mean square fluctuations of the composition with respect to the letter content and several characteristic scaling exponents are calculated. We show finally that all these measuresare able to detect long-range correlations. However, as demonstrated by shuffling experiments, different measuring methods operate on different length scales. The algorithms based on entropy or compressibility operate mainly on the word and sentence level. The characteristic scaling exponents reflect mainly the long-wave fluctuations of the composition which may comprise a few hundreds or thousands of letters.
- Book partOpen AccessA measure of the information content of neural spike trains(DIRAC, 1997) Jiménez-Montaño, Miguel Angel; Pöschel, Thorsten; Rapp, Paul
- Conference objectOpen AccessHydrodynamic fluctuations and averaging problems in dense granular flows(Balkema, 1997) Saluneña, Clara; Esipov, Sergei E.; Pöschel, ThorstenWe analyze the properties of dense granular systems by assuming a hydrodynamical description based on conservation laws for the particle number density and linear momentum. We combine analytical methods and experimental and numerical results obtained by ensemble-averaging of data on creep during compaction and molecular dynamics simulations of convective flow.
- Book partOpen AccessContact of Viscoelastic Spheres(World Scientific, 1997-09-01) Schwager, Thomas; Pöschel, ThorstenIn a recent paper an implicit equation for contacting viscoelastic spheres was derived [1]. Integrating this equation it can be shown that the coefficient of normal restitution $\varepsilon$ depends on the impact velocity, $g$, as as $1 − \varepsilon \propto g^{1/5}$.
- Book partOpen AccessForce Distribution and Comminution in Ball Mills(World Scientific, 1997-09-01) Buchholtz, Volkhard; Pöschel, ThorstenThe motion of granular material in a ball mill is investigated using molecular dynamics simulations in two dimensions. In agreement with experimental observations by Rothkegel [1] we find that local stresses – and hence the comminution efficiency – are maximal close to the bottom of the container. This effect will be explained using analysis of statistics of force chains in the material.
- ArticleOpen AccessMolekulardynamische Untersuchung von Zerkleinerungsprozessen in Kugelmühlen(Vogel Business Media, 2000) Buchholtz, Volkhard; Freund, Jan Alfred; Pöschel, ThorstenMolekulardynamische Untersuchungen eignen sich zur Simulation des Verhaltens makroskopischer Mengen granularen Materials unter bestimmten, technologisch relevanten Beanspruchungen. Wir untersuchen das Zerkleinerungsverhalten von Mahlgut in einer Kugelmühle, insbesondere im Hinblick auf die Effizienz als Funktion der Drehzahl und auf die räumliche Verteilung von Beanspruchungen. Die Untersuchung der Verteilung von Kraflketten (force chains) liefert eine Erklärung des Experiments von Rothkegel und Rolf [3].
- ArticleOpen AccessMolecular Dynamics of Comminution in Ball Mills(Vogel Verlag, 2000) Buchholtz, Volkhard; Freund, Jan Alfred; Pöschel, ThorstenWe Investigate autogenous fragmentation of dry granular material in rotating cylinders using two-dimensional molecular dynamics. By evaluation of spatial force distributions achieved numerically for various rotation velocities we argue that comminution occurs mainly due to the existence of force chains. A statistical analysis of these force chains explains the spatial distribution of comminution efficiency in ball mills as measured experimentally by Rothkegel [1] and Rolf [2].
- Master thesisOpen AccessKritische Parameter der vertikalen Oszillation für das Einsetzen granularer KonvektionProceedings of the Institute for Multiscale Simulation (2000-05-24) Renard, Simon
- Conference objectOpen AccessCan we scale granular systems?(Swets & Zeitlinger, 2001-05-21) Pöschel, Thorsten; Salueña, Clara; Schwager, ThomasFor the experimental investigation of large scale phenomena in the laboratory such as in geophysical or industrial applications one has to scale down all length in the system, e.g. particle size, container size. We show that besides length scaling one as to scale the material properties too to achieve identical behavior of the scaled and the original systems. We provide the scaling laws for a system of viscoelastic spheres.
- Conference objectOpen AccessRolling as a “continuing collision” for viscoelastic spheres(Kluwer Academic Press, 2001-09-30) Brilliantov, Nikolay; Pöschel, ThorstenWe show that two basic mechanical processes, the collision of particles and rolling motion of a sphere on a plane, are intimately related. According to our recent findings, the restitution coefficient for colliding spherical particles $\varepsilon$, which characterizes the energy loss upon collision, is directly related to the rolling friction coefficient $\mu_\text{roll}$ for a viscous sphere on a hard plane. We quantify both coefficients in terms of material constants which allows to determine either of them provided the other is known. This relation between the coefficients may give rise to a novel experimental technique to determine alternatively the coefficient of restitution or the coefficient of rolling friction.