471 research outputs found
Viscous stabilization of 2D drainage displacements with trapping
We investigate the stabilization mechanisms due to viscous forces in the
invasion front during drainage displacement in two-dimensional porous media
using a network simulator. We find that in horizontal displacement the
capillary pressure difference between two different points along the front
varies almost linearly as function of height separation in the direction of the
displacement. The numerical result supports arguments taking into account the
loopless displacement pattern where nonwetting fluid flow in separate strands
(paths). As a consequence, we show that existing theories developed for viscous
stabilization, are not compatible with drainage when loopless strands dominate
the displacement process.Comment: The manuscript has been substantially revised. Accepted in Phys. Rev.
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Immigrant community integration in world cities
As a consequence of the accelerated globalization process, today major cities
all over the world are characterized by an increasing multiculturalism. The
integration of immigrant communities may be affected by social polarization and
spatial segregation. How are these dynamics evolving over time? To what extent
the different policies launched to tackle these problems are working? These are
critical questions traditionally addressed by studies based on surveys and
census data. Such sources are safe to avoid spurious biases, but the data
collection becomes an intensive and rather expensive work. Here, we conduct a
comprehensive study on immigrant integration in 53 world cities by introducing
an innovative approach: an analysis of the spatio-temporal communication
patterns of immigrant and local communities based on language detection in
Twitter and on novel metrics of spatial integration. We quantify the "Power of
Integration" of cities --their capacity to spatially integrate diverse
cultures-- and characterize the relations between different cultures when
acting as hosts or immigrants.Comment: 13 pages, 5 figures + Appendi
Linkage mapping reveals sex-dimorphic map distances in a passerine bird
Linkage maps are lacking for many highly influential model organisms in evolutionary research, including all passerine birds. Consequently, their full potential as research models is severely hampered. Here, we provide a partial linkage map and give novel estimates of sex-specific recombination rates in a passerine bird, the great reed warbler (Acrocephalus arundinaceus). Linkage analysis of genotypic data at 51 autosomal microsatellites and seven markers on the Z-chromosome (one of the sex chromosomes) from an extended pedigree resulted in 12 linkage groups with 2–8 loci. A striking feature of the map was the pronounced sex-dimorphism: males had a substantially lower recombination rate than females, which resulted in a suppressed autosomal map in males (sum of linkage groups: 110.2cM) compared to females (237.2cM; female/male map ratio: 2.15). The sex-specific recombination rates will facilitate the building of a denser linkage map and cast light on hypotheses about sex-specific recombination rates
Modelling of the radiative properties of an opaque porous ceramic layer
Solid Oxide Fuel Cells (SOFCs) operate at temperatures above 1,100 K where radiation effects can be significant. Therefore, an accurate thermal model of an SOFC requires the inclusion of the contribution of thermal radiation. This implies that the thermal radiative properties of the oxide ceramics used in the design of SOFCs must be known. However, little information can be found in the literature concerning their operating temperatures. On the other hand, several types of ceramics with different chemical compositions and microstructures for designing efficient cells are now being tested. This is a situation where the use of a numerical tool making possible the prediction of the thermal radiative properties of SOFC materials, whatever their chemical composition and microstructure are, may be a decisive help. Using this method, first attempts to predict the radiative properties of a lanthanum nickelate porous layer deposited onto an yttria stabilized zirconium substrate can be reported
Simulating temporal evolution of pressure in two-phase flow in porous media
We have simulated the temporal evolution of pressure due to capillary and
viscous forces in two-phase drainage in porous media. We analyze our result in
light of macroscopic flow equations for two-phase flow. We also investigate the
effect of the trapped clusters on the pressure evolution and on the effective
permeability of the system. We find that the capillary forces play an important
role during the displacements for both fast and slow injection rates and both
when the invading fluid is more or less viscous than the defending fluid. The
simulations are based on a network simulator modeling two-phase drainage
displacements on a two-dimensional lattice of tubes.Comment: 12 pages, LaTeX, 14 figures, Postscrip
Creep via dynamical functional renormalization group
We study a D-dimensional interface driven in a disordered medium. We derive
finite temperature and velocity functional renormalization group (FRG)
equations, valid in a 4-D expansion. These equations allow in principle for a
complete study of the the velocity versus applied force characteristics. We
focus here on the creep regime at finite temperature and small velocity. We
show how our FRG approach gives the form of the v-f characteristics in this
regime, and in particular the creep exponent, obtained previously only through
phenomenological scaling arguments.Comment: 4 pages, 3 figures, RevTe
Collective Particle Flow through Random Media
A simple model for the nonlinear collective transport of interacting
particles in a random medium with strong disorder is introduced and analyzed. A
finite threshold for the driving force divides the behavior into two regimes
characterized by the presence or absence of a steady-state particle current.
Below this threshold, transient motion is found in response to an increase in
the force, while above threshold the flow approaches a steady state with motion
only on a network of channels which is sparse near threshold. Some of the
critical behavior near threshold is analyzed via mean field theory, and
analytic results on the statistics of the moving phase are derived. Many of the
results should apply, at least qualitatively, to the motion of magnetic bubble
arrays and to the driven motion of vortices in thin film superconductors when
the randomness is strong enough to destroy the tendencies to lattice order even
on short length scales. Various history dependent phenomena are also discussed.Comment: 63 preprint pages plus 6 figures. Submitted to Phys Rev
Core-scale description of porous media dissolution during acid injection - Part I: theoretical development
Fluctuation spectrum of fluid membranes coupled to an elastic meshwork: jump of the effective surface tension at the mesh size
We identify a class of composite membranes: fluid bilayers coupled to an
elastic meshwork, that are such that the meshwork's energy is a function
\textit{not} of the real microscopic membrane area ,
but of a \textit{smoothed} membrane's area , which corresponds to the
area of the membrane coarse-grained at the mesh size . We show that the
meshwork modifies the membrane tension both below and above the scale
, inducing a tension-jump . The
predictions of our model account for the fluctuation spectrum of red blood
cells membranes coupled to their cytoskeleton. Our results indicate that the
cytoskeleton might be under extensional stress, which would provide a means to
regulate available membrane area. We also predict an observable tension jump
for membranes decorated with polymer "brushes"
Pattern Formation in Interface Depinning and Other Models: Erratically Moving Spatial Structures
We study erratically moving spatial structures that are found in a driven
interface in a random medium at the depinning threshold. We introduce a
bond-disordered variant of the Sneppen model and study the effect of extremal
dynamics on the morphology of the interface. We find evidence for the formation
of a structure which moves along with the growth site. The time average of the
structure, which is defined with respect to the active spot of growth, defines
an activity-centered pattern. Extensive Monte Carlo simulations show that the
pattern has a tail which decays slowly, as a power law. To understand this sort
of pattern formation, we write down an approximate integral equation involving
the local interface dynamics and long-ranged jumps of the growth spot. We
clarify the nature of the approximation by considering a model for which the
integral equation is exactly derivable from an extended master equation.
Improvements to the equation are considered by adding a second coupled equation
which provides a self-consistent description. The pattern, which defines a
one-point correlation function, is shown to have a strong effect on ordinary
space-fixed two-point correlation functions. Finally we present evidence that
this sort of pattern formation is not confined to the interface problem, but is
generic to situations in which the activity at succesive time steps is
correlated, as for instance in several other extremal models. We present
numerical results for activity-centered patterns in the Bak-Sneppen model of
evolution and the Zaitsev model of low-temperature creep.Comment: RevTeX, 18 pages, 19 eps-figures, To appear in Phys. Rev.
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