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Fractals, critical phenomena


1/f Noise in Sea-level Elevations truecm

We have analyzed a 101 year record of sea-level elevations. The data consist of hourly recordings of sea-level elevations from 1889 to 1990 for the port of Esbjerg in Denmark. Power spectra analysis shows three distinct regimes. In the frequency range from 1/2 Hour to 1/3 Days the spectrum shows 1/f behavior. This is then followed by a region with clear 1/f noise, followed by a crossover to white noise at the frequency 1/3 Years. This record is one of the rare examples where a lower cutoff to 1/f noise is found. In addition intermittent behavior is suggested by stretched exponential probability density distributions. By using methods based on the correlation integral both the 1/f noise and the high frequency part of the spectrum have been checked for low-dimensionality. truecm

(H. Svensmark), (J.D. Pietrzak) and P. Dimon truecm

Recurrent Epidemics in a Cellular Automaton truecm

We investigate an extended version of the Forest-Fire model conceived as a model of disease spreading. In contrast to previous claims, we show that the model self-organizes spatially and temporally from random initial conditions into a well-defined non-trivial state with a characteristic length and time scale. The time scale has a power law dependence on the correlation length , i.e., with in d=2. We show that stochastic recovery is essential for this behaviour. The aim of the investigation is primarily to investigate sufficient conditions for obtaining recurrent epidemic behaviour and propagation of fronts. truecm

A. Johansen truecm

Studies of Granular Flow truecm

When small beads flow through a narrow channel, fluctuations in the flow rate and density are observed. We can measure the local density anywhere in the flow with light transmission. In particular, it has been observed that as the angle of the channel walls is decreased so they become nearly parallel, there appears to be a critical angle at which the flow rate is a maximum and the fluctuations are strongly intermittent. truecm

C. Veje and P. Dimon truecm

Distributions of Self-Interactions and Voids in (1+1)-d Directed Percolation truecm

We investigate the scaling of self-interactions and voids in -d directed percolation clusters and backbones. We verify that the meandering of the backbone scales like the directed cluster. A geometric relation between the size distribution and the fractal dimensions of a set of objects is applied to find the scaling properties of self-interactions in directed percolation. Lastly we connect the geometric properties of the backbone with the avalanche distribution generated by interface dynamics at the depinning transition. truecm

(G. Huber), M.H. Jensen and K. Sneppen truecm

Intermittent Activity, Multiscaling and Self-Organized Criticality truecm

We characterize the spatio-temporal behavior of all self-organized critical phenomena by means of an activity function. The activity function is equivalent to a roughening front and shows intermittent behavior in space and time. Due to the intermittency, the moments of the activity function exhibits multiscaling in time with a continuous spectrum of exponents. This spectrum of exponents can for SOC dynamics be estimated analytically in terms of the roughness exponent for the activity function whereas for dynamics generated by Levy flights we have not been able to express the spectrum analytically. truecm

M.H. Jensen, K. Sneppen and (M. Sellitto) truecm

Self-Organized Criticality, Stochastic Cellular Automata, and the Game of Life truecm

The question of self-organized criticality in Conway's game of life is studied in the broader context of stochastic cellular automata. We are particularly interested in the directed percolation type of phase transitions observed. truecm

P. AlstrÝm, J. Nordfalk and C. Veje

next up previous contents
Next: Biology and physiology Up: RESEARCH PROJECTS Previous: Turbulence

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Mon Mar 6 19:42:06 MET 1995