1,175 research outputs found
Epidemic spreading with immunization and mutations
The spreading of infectious diseases with and without immunization of
individuals can be modeled by stochastic processes that exhibit a transition
between an active phase of epidemic spreading and an absorbing phase, where the
disease dies out. In nature, however, the transmitted pathogen may also mutate,
weakening the effect of immunization. In order to study the influence of
mutations, we introduce a model that mimics epidemic spreading with
immunization and mutations. The model exhibits a line of continuous phase
transitions and includes the general epidemic process (GEP) and directed
percolation (DP) as special cases. Restricting to perfect immunization in two
spatial dimensions we analyze the phase diagram and study the scaling behavior
along the phase transition line as well as in the vicinity of the GEP point. We
show that mutations lead generically to a crossover from the GEP to DP. Using
standard scaling arguments we also predict the form of the phase transition
line close to the GEP point. It turns out that the protection gained by
immunization is vitally decreased by the occurrence of mutations.Comment: 9 pages, 13 figure
Dynamics and stability of wind turbine generators
Synchronous and induction generators are considered. A comparison is made between wind turbines, steam, and hydro units. The unusual phenomena associated with wind turbines are emphasized. The general control requirements are discussed, as well as various schemes for torsional damping such as speed sensitive stabilizer and blade pitch control. Integration between adjacent wind turbines in a wind farm is also considered
Absorbing Phase Transitions of Branching-Annihilating Random Walks
The phase transitions to absorbing states of the branching-annihilating
reaction-diffusion processes mA --> (m+k)A, nA --> (n-l)A are studied
systematically in one space dimension within a new family of models. Four
universality classes of non-trivial critical behavior are found. This provides,
in particular, the first evidence of universal scaling laws for pair and
triplet processes.Comment: 4 pages, 4 figure
First order phase transition with a logarithmic singularity in a model with absorbing states
Recently, Lipowski [cond-mat/0002378] investigated a stochastic lattice model
which exhibits a discontinuous transition from an active phase into infinitely
many absorbing states. Since the transition is accompanied by an apparent
power-law singularity, it was conjectured that the model may combine features
of first- and second-order phase transitions. In the present work it is shown
that this singularity emerges as an artifact of the definition of the model in
terms of products. Instead of a power law, we find a logarithmic singularity at
the transition. Moreover, we generalize the model in such a way that the
second-order phase transition becomes accessible. As expected, this transition
belongs to the universality class of directed percolation.Comment: revtex, 4 pages, 5 eps figure
Limited genetic diversity among clones of red wine cultivar 'CarmenĂšre' as revealed by microsatellite and AFLP markers
'CarmenĂšre' is a fine red wine cultivar (Vitis vinifera L.) that has spread, unrecorded from France to other countries. It probably arrived in Chile before the Phylloxera crisis in Europe where it remained confused with Merlot and other red wine cultivars until the mid 1990s. In this study, genetic diversity among 26 accessions from Chile, France and Italy was analysed using microsatellite (SSR) and AFLP markers. Using 20 SSR markers, a âstandard genotypeâ was established and three different haplotypes were found, presumably arising by a mutation at the VVMD7 and VMC5g7 loci. In the case of AFLP, using 11 primer combinations five groups were identified, with one main cluster of 22 accessions not differentiated. Combining both techniques it was possible to identify five out of the 26 accessions analysed. Together, these results suggest that 'CarmenĂšre' exhibits a lower genetic diversity in comparison with other French red wine cultivars. This is a factor to consider when managing a clonal selection assay. Possible causes are discussed.
Epidemic processes with immunization
We study a model of directed percolation (DP) with immunization, i.e. with
different probabilities for the first infection and subsequent infections. The
immunization effect leads to an additional non-Markovian term in the
corresponding field theoretical action. We consider immunization as a small
perturbation around the DP fixed point in d<6, where the non-Markovian term is
relevant. The immunization causes the system to be driven away from the
neighbourhood of the DP critical point. In order to investigate the dynamical
critical behaviour of the model, we consider the limits of low and high first
infection rate, while the second infection rate remains constant at the DP
critical value. Scaling arguments are applied to obtain an expression for the
survival probability in both limits. The corresponding exponents are written in
terms of the critical exponents for ordinary DP and DP with a wall. We find
that the survival probability does not obey a power law behaviour, decaying
instead as a stretched exponential in the low first infection probability limit
and to a constant in the high first infection probability limit. The
theoretical predictions are confirmed by optimized numerical simulations in 1+1
dimensions.Comment: 12 pages, 11 figures. v.2: minor correction
Multifractal current distribution in random diode networks
Recently it has been shown analytically that electric currents in a random
diode network are distributed in a multifractal manner [O. Stenull and H. K.
Janssen, Europhys. Lett. 55, 691 (2001)]. In the present work we investigate
the multifractal properties of a random diode network at the critical point by
numerical simulations. We analyze the currents running on a directed
percolation cluster and confirm the field-theoretic predictions for the scaling
behavior of moments of the current distribution. It is pointed out that a
random diode network is a particularly good candidate for a possible
experimental realization of directed percolation.Comment: RevTeX, 4 pages, 5 eps figure
Explosive Ising
We study a two-dimensional kinetic Ising model with Swendsen-Wang dynamics,
replacing the usual percolation on top of Ising clusters by explosive
percolation. The model exhibits a reversible first-order phase transition with
hysteresis. Surprisingly, at the transition flanks the global bond density
seems to be equal to the percolation thresholds.Comment: 7 pages, 5 figure
Phase transition of the one-dimensional coagulation-production process
Recently an exact solution has been found (M.Henkel and H.Hinrichsen,
cond-mat/0010062) for the 1d coagulation production process: 2A ->A, A0A->3A
with equal diffusion and coagulation rates. This model evolves into the
inactive phase independently of the production rate with density
decay law. Here I show that cluster mean-field approximations and Monte Carlo
simulations predict a continuous phase transition for higher
diffusion/coagulation rates as considered in cond-mat/0010062. Numerical
evidence is given that the phase transition universality agrees with that of
the annihilation-fission model with low diffusions.Comment: 4 pages, 4 figures include
A precise approximation for directed percolation in d=1+1
We introduce an approximation specific to a continuous model for directed
percolation, which is strictly equivalent to 1+1 dimensional directed bond
percolation. We find that the critical exponent associated to the order
parameter (percolation probability) is beta=(1-1/\sqrt{5})/2=0.276393202..., in
remarkable agreement with the best current numerical estimate beta=0.276486(8).Comment: 4 pages, 3 EPS figures; Submitted to Physical Review Letters v2:
minor typos + 1 major typo in Eq. (30) correcte
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