PD Dr. Jan Freund

Function

Postdoctoral

Phone

researcher

Info

Institut für Chemie und Biologie des Meeres (ICBM), Carl von Ossietzky Universität Oldenburg, Germany

Publications

Markovsche Analyse nasser Gemeinschaften

Irreversible Prozesse und Selbstorganisation (Pöschel, T. and Malchow, H. and Schimansky-Geier, L.), 281 - 292, Logos, Berlin

(2006)

Research Projects
Abstract

In ökologischen Systemen ist die saisonal variierende Präsenz von Arten sowohl durch die jahreszeitlich schwankenden Umgebungsbedingungen, als auch durch die Wechselwirkungen zwischen den Arten bedingt. Letztere erfahren ihre Ausprägung in Sukzession und Koexistenz. Wir wollen die zyklische Wiederkehr der Arten im Jahresgang wahrscheinlichkeitstheoretisch beschreiben, wobei der Wechsel von Jahr zu Jahr im Rahmen einer Markovschen Kette modelliert wird. Neben allgemeinen Ausführungen zu dieser modellhaften Art der Analyse werden wir als eine konkrete Anwendung die Zeitreihen (Zellzahlen) dreier prominenter Algenarten der südlichen Nordsee analysieren. Über den Aspekt einer quantitativen Beschreibung der Verhältnisse im marinen Habitat hinaus zielt eine derartige Untersuchung insbesondere auf einen Nachweis möglicher Veränderungen des ökologischen Systems der „nassen Gemeinschaft“ vor dem Hintergrund einer bereits beobachteten Temperaturerhöhung des Meerwassers bei Helgoland.

Materials
 

How to decide whether small samples comply with an equidistribution

BioSystems, 69, 63-72

(2003)

Research Projects
Abstract

The decision whether a measured distribution complies withan equidistribution is a central element of many biostatistical methods. High through put differential expression measurements, for instance, necessitate to judge possible over-representation of genes. The reliability of this judgement, however, is strongly affected when rarely expressed genes are pooled. We propose a method that can be applied to frequency ranked distributions and that yields a simple but efficient criterion to assess the hypothesis of equiprobable expression levels. By applying our technique to surrogate data we exemplify how the decision criterion can differentiate between a true equidistribution and a triangular distribution. The distinction succeeds even for small sample sizes where standard tests of significance (e.g. chi²) fail. Our method will have a major impact on several problems of computational biology where rare events baffle a reliable assessment of frequency distributions.

Materials
 

Finite-sample frequency distributions originating from an equiprobability distribution

Physical Review E, 66, 026103

(2002)

Research Projects
Abstract

Given an equidistribution for probabilities p(i)=1/N, i=1..N. What is the expected corresponding rank ordered frequency distribution f(i), i=1..N, if an ensemble of M events is drawn?

Materials
 

Molecular Dynamics of comminution in ball mills

Bulk Solids Handling, 20, 159-172

(2000)

Research Projects
Abstract

We 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]. For animated sequences of our simulations see url http://www.mss.cbi.uni-erlangen.de/index.php?p1=research&p2=articles&r=granular/RotatingCylinder/Comminution

Materials
 

Molekulardynamische Untersuchung von Zerkleinerungsprozessen in Kugelmühlen

Schüttgut, 6, 11-24

(2000)

Research Projects
Abstract

Molekulardynamische 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 Kraftketten liefert eine Erklärung des Experiments von Rothkegel [1] und Rolf. Für animierte Sequenzen der Simulationen s. url: http://www.mss.cbi.uni-erlangen.de/index.php?p1=research&p2=articles&r=granular/RotatingCylinder/Comminution

Materials
 

Molecular Dynamics of comminution in ball mills

European Physical Journal, 16, 169-182

(2000)

Research Projects
Abstract

We 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]. For animated sequences of our simulations see url http://www.mss.cbi.uni-erlangen.de/php/research/granular/RotatingCylinder/Comminution/

Materials
 

Stochastic Processes in Physics, Chemistry, and Biology

Lecture Notes in Physics, 557, Springer, Berlin, Heidelberg, New York

(2000)

Research Projects
 

Cluster Statistics and Traffic on a Lattice

Stochastic Dynamics (Lecture Notes in Physics) (Schimansky-Geier, L. and Pöschel, T.), 484, 220-231, Springer, Berlin, Heidelberg, New York

(1997)

Research Projects
Abstract

We describe traffic on a two dimensional lattice modelled using a cellular automaton. The theoretical approach valid in the low density region employs cluster statistics. The derived central formula for the velocity vs. density relation nicely agrees with simulation results. In our approach the explicit traffic rules solely enter through combinatorics accounting for average crossing traffics.

Materials
 

A statistical approach to vehicular traffic

Physica A, 219, 95-113

(1995)

Research Projects
Abstract

A two-dimensional cellular automaton is introduced to model the flow and jamming of vehicular traffic in cities. Each site of the automaton represents a crossing where a finite number of cars can wait approaching the crossing from each of the four directions. The flow of cars obeys realistic traffic rules. We investigate the dependence of the average velocity of cars on the global traffic density. At a critical threshold for the density the average velocity reduces drastically caused by jamming. For the low density regime we provide analytical results which agree with the numerical results.

Materials