207 research outputs found
Dynamics of Fractures in Quenched Disordered Media
We introduce a model for fractures in quenched disordered media. This model
has a deterministic extremal dynamics, driven by the energy function of a
network of springs (Born Hamiltonian). The breakdown is the result of the
cooperation between the external field and the quenched disorder. This model
can be considered as describing the low temperature limit for crack propagation
in solids. To describe the memory effects in this dynamics, and then to study
the resistance properties of the system we realized some numerical simulations
of the model. The model exhibits interesting geometric and dynamical
properties, with a strong reduction of the fractal dimension of the clusters
and of their backbone, with respect to the case in which thermal fluctuations
dominate. This result can be explained by a recently introduced theoretical
tool as a screening enhancement due to memory effects induced by the quenched
disorder.Comment: 7 pages, 9 Postscript figures, uses revtex psfig.sty, to be published
on Phys. Rev.
Generalized Dielectric Breakdown Model
We propose a generalized version of the Dielectric Breakdown Model (DBM) for
generic breakdown processes. It interpolates between the standard DBM and its
analog with quenched disorder, as a temperature like parameter is varied. The
physics of other well known fractal growth phenomena as Invasion Percolation
and the Eden model are also recovered for some particular parameter values. The
competition between different growing mechanisms leads to new non-trivial
effects and allows us to better describe real growth phenomena.
Detailed numerical and theoretical analysis are performed to study the
interplay between the elementary mechanisms. In particular, we observe a
continuously changing fractal dimension as temperature is varied, and report an
evidence of a novel phase transition at zero temperature in absence of an
external driving field; the temperature acts as a relevant parameter for the
``self-organized'' invasion percolation fixed point. This permits us to obtain
new insight into the connections between self-organization and standard phase
transitions.Comment: Submitted to PR
Laplacian Fractal Growth in Media with Quenched Disorder
We analyze the combined effect of a Laplacian field and quenched disorder for
the generation of fractal structures with a study, both numerical and
theoretical, of the quenched dielectric breakdown model (QDBM). The growth
dynamics is shown to evolve from the avalanches of invasion percolation (IP) to
the smooth growth of Laplacian fractals, i. e. diffusion limited aggregation
(DLA) and the dielectric breakdown model (DBM). The fractal dimension is
strongly reduced with respect to both DBM and IP, due to the combined effect of
memory and field screening. This implies a specific relation between the
fractal dimension of the breakdown structures (dielectric or mechanical) and
the microscopic properties of disordered materials.Comment: 11 pages Latex (revtex), 3 postscript figures included. Submitted to
PR
Statistical properties of fractures in damaged materials
We introduce a model for the dynamics of mud cracking in the limit of of
extremely thin layers. In this model the growth of fracture proceeds by
selecting the part of the material with the smallest (quenched) breaking
threshold. In addition, weakening affects the area of the sample neighbour to
the crack. Due to the simplicity of the model, it is possible to derive some
analytical results. In particular, we find that the total time to break down
the sample grows with the dimension L of the lattice as L^2 even though the
percolating cluster has a non trivial fractal dimension. Furthermore, we obtain
a formula for the mean weakening with time of the whole sample.Comment: 5 pages, 4 figures, to be published in Europhysics Letter
Experimental analysis of the performance of fractal stirrers for impinging jets heat transfer enhancement
Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.A new passive method for the heat transfer enhancement of circular impinging jets is proposed and tested. The method is based on enhancing the mainstream turbulence of impinging jets using square fractal grids, i.e. a grid with a square pattern repeated at increasingly smaller scales. Fractal grids can generate much higher turbulence intensity than regular grids under the same inflow conditions and with similar blockage ratio, at the expense of a slightly larger pressure drop. An experimental investigation on the heat transfer enhancement achieved by impinging jets with fractal turbulence promoters is carried out. The heated-thin foil technique is implemented to measure the spatial distribution of the Nusselt number on the target plate. The heat transfer rates of impinging jets with a regular grid and a fractal grid insert are compared to that of a jet without any turbulator under the same condition of power input. A parametric study on the effect of the Reynolds number, the nozzle-to-plate distance and the position of the insert within the nozzle is carried out. The results show that a fractal turbulence promoter can provide a significant heat transfer enhancement for relatively small nozzle-to-plate separation (at distance equal to 2 diameters 63% increase with respect to the circular jet at the stagnation point, and 25% if averaged over an area of radius equal to 1 nozzle diameter; respectively, against 9% and 6% of the regular grid in the same conditions of power input).dc201
Non-perturbative renormalization of the KPZ growth dynamics
We introduce a non-perturbative renormalization approach which identifies
stable fixed points in any dimension for the Kardar-Parisi-Zhang dynamics of
rough surfaces. The usual limitations of real space methods to deal with
anisotropic (self-affine) scaling are overcome with an indirect functional
renormalization. The roughness exponent is computed for dimensions
to 8 and it results to be in very good agreement with the available
simulations. No evidence is found for an upper critical dimension. We discuss
how the present approach can be extended to other self-affine problems.Comment: 4 pages, 2 figures. To appear in Phys. Rev. Let
Levy-Nearest-Neighbors Bak-Sneppen Model
We study a random neighbor version of the Bak-Sneppen model, where "nearest
neighbors" are chosen according to a probability distribution decaying as a
power-law of the distance from the active site, P(x) \sim |x-x_{ac
}|^{-\omega}. All the exponents characterizing the self-organized critical
state of this model depend on the exponent \omega. As \omega tends to 1 we
recover the usual random nearest neighbor version of the model. The pattern of
results obtained for a range of values of \omega is also compatible with the
results of simulations of the original BS model in high dimensions. Moreover,
our results suggest a critical dimension d_c=6 for the Bak-Sneppen model, in
contrast with previous claims.Comment: To appear on Phys. Rev. E, Rapid Communication
Diffusion limited aggregation as a Markovian process. Part I: bond-sticking conditions
Cylindrical lattice Diffusion Limited Aggregation (DLA), with a narrow width
N, is solved using a Markovian matrix method. This matrix contains the
probabilities that the front moves from one configuration to another at each
growth step, calculated exactly by solving the Laplace equation and using the
proper normalization. The method is applied for a series of approximations,
which include only a finite number of rows near the front. The matrix is then
used to find the weights of the steady state growing configurations and the
rate of approaching this steady state stage. The former are then used to find
the average upward growth probability, the average steady-state density and the
fractal dimensionality of the aggregate, which is extrapolated to a value near
1.64.Comment: 24 pages, 20 figure
Two-dimensional Granular Gas of Inelastic Spheres with Multiplicative Driving
We study a two-dimensional granular gas of inelastic spheres subject to
multiplicative driving proportional to a power of the
local particle velocity . The steady state properties of the model
are examined for different values of , and compared with the
homogeneous case . A driving linearly proportional to
seems to reproduce some experimental observations which could not be reproduced
by a homogeneous driving. Furthermore, we obtain that the system can be
homogenized even for strong dissipation, if a driving inversely proportional toComment: 4 pages, 5 figures (accepted as Phys. Rev. Lett.
Discretized Diffusion Processes
We study the properties of the ``Rigid Laplacian'' operator, that is we
consider solutions of the Laplacian equation in the presence of fixed
truncation errors. The dynamics of convergence to the correct analytical
solution displays the presence of a metastable set of numerical solutions,
whose presence can be related to granularity. We provide some scaling analysis
in order to determine the value of the exponents characterizing the process. We
believe that this prototype model is also suitable to provide an explanation of
the widespread presence of power-law in social and economic system where
information and decision diffuse, with errors and delay from agent to agent.Comment: 4 pages 5 figure, to be published in PR
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