27,989 research outputs found
Mapping the train model for earthquakes onto the stochastic sandpile model
We perform a computational study of a variant of the ``train'' model for
earthquakes [PRA 46, 6288 (1992)], where we assume a static friction that is a
stochastic function of position rather than being velocity dependent. The model
consists of an array of blocks coupled by springs, with the forces between
neighbouring blocks balanced by static friction. We calculate the probability,
P(s), of the occurrence of avalanches with a size s or greater, finding that
our results are consistent with the phenomenology and also with previous models
which exhibit a power law over a wide range. We show that the train model may
be mapped onto a stochastic sandpile model and study a variant of the latter
for non-spherical grains. We show that, in this case, the model has critical
behaviour only for grains with large aspect ratio, as was already shown in
experiments with real ricepiles. We also demonstrate a way to introduce
randomness in a physically motivated manner into the model.Comment: 14 pages and 6 figures. Accepted in European Physical Journal
Estimativa do custo de produção de cafĂ© agroecolĂłgico no MunicĂpio de Alto ParaĂso - RondĂ´nia.
Considerando a necessidade de se gerar informações socioeconĂ´micas que contribuam para o delineamento de agroecossistemas sustentáveis, objetivo deste trabalho Ă© a determinação do custo de produção do cafĂ© robusta em sistema agroecolĂłgico no MunicĂpio de Alto ParaĂso, RondĂ´nia. Pretende-se ainda determinar a renda auferida por uma unidade de produção tĂpica do sistema cafĂ©-mel-madeira.bitstream/item/24715/1/cot317-cafeagroecologico.pd
The contact process in disordered and periodic binary two-dimensional lattices
The critical behavior of the contact process in disordered and periodic
binary 2d-lattices is investigated numerically by means of Monte Carlo
simulations as well as via an analytical approximation and standard mean field
theory. Phase-separation lines calculated numerically are found to agree well
with analytical predictions around the homogeneous point. For the disordered
case, values of static scaling exponents obtained via quasi-stationary
simulations are found to change with disorder strength. In particular, the
finite-size scaling exponent of the density of infected sites approaches a
value consistent with the existence of an infinite-randomness fixed point as
conjectured before for the 2d disordered CP. At the same time, both dynamical
and static scaling exponents are found to coincide with the values established
for the homogeneous case thus confirming that the contact process in a
heterogeneous environment belongs to the directed percolation universality
class.Comment: submitted to Physical Review
Modeling and simulation of micro direct methanol Fuel Cells
Fuel cells have unique technological attributes: efficiency, absence of moving parts and low emissions. The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. With the advance of micromachining technologies, miniaturization of power sources became one of the trends of evolution of research in this area. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices, so, MicroDMFC is an emergent technology. Models play an important role in fuel cell development since they facilitate a better understanding of parameters affecting the performance of fuel cells. In this work, a steady state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in a fuel cell, already developed and validated for DMFC in [1-3], is used to predict Micro DMFC performance. The model takes in account all relevant phenomena occurring in a DMFC. Polarization curves predicted by the model are compared with experimental data existing in literature and the model shows good agreement, mainly for lower current densities. The model is used to predict some important parameters to analyze fuel cell performance, such as water transport coefficient and leakage current density. This easily to implement simplified model is suitable for use in real-time MicroDMFC simulations
Experimental determination of the non-extensive entropic parameter
We show how to extract the parameter from experimental data, considering
an inhomogeneous magnetic system composed by many Maxwell-Boltzmann homogeneous
parts, which after integration over the whole system recover the Tsallis
non-extensivity. Analyzing the cluster distribution of
LaSrMnO manganite, obtained through scanning tunnelling
spectroscopy, we measure the parameter and predict the bulk magnetization
with good accuracy. The connection between the Griffiths phase and
non-extensivity is also considered. We conclude that the entropic parameter
embodies information about the dynamics, the key role to describe complex
systems.Comment: Submitted to Phys. Rev. Let
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