138 research outputs found
Diffusion-Reorganized Aggregates: Attractors in Diffusion Processes?
A process based on particle evaporation, diffusion and redeposition is
applied iteratively to a two-dimensional object of arbitrary shape. The
evolution spontaneously transforms the object morphology, converging to
branched structures. Independently of initial geometry, the structures found
after long time present fractal geometry with a fractal dimension around 1.75.
The final morphology, which constantly evolves in time, can be considered as
the dynamic attractor of this evaporation-diffusion-redeposition operator. The
ensemble of these fractal shapes can be considered to be the {\em dynamical
equilibrium} geometry of a diffusion controlled self-transformation process.Comment: 4 pages, 5 figure
Optimal villi density for maximal oxygen uptake in the human placenta
We present a stream-tube model of oxygen exchange inside a human placenta
functional unit (a placentone). The effect of villi density on oxygen transfer
efficiency is assessed by numerically solving the diffusion-convection equation
in a 2D+1D geometry for a wide range of villi densities. For each set of
physiological parameters, we observe the existence of an optimal villi density
providing a maximal oxygen uptake as a trade-off between the incoming oxygen
flow and the absorbing villus surface. The predicted optimal villi density
is compatible to previous experimental measurements. Several
other ways to experimentally validate the model are also proposed. The proposed
stream-tube model can serve as a basis for analyzing the efficiency of human
placentas, detecting possible pathologies and diagnosing placental health risks
for newborns by using routine histology sections collected after birth
Screening effects in flow through rough channels
A surprising similarity is found between the distribution of hydrodynamic
stress on the wall of an irregular channel and the distribution of flux from a
purely Laplacian field on the same geometry. This finding is a direct outcome
from numerical simulations of the Navier-Stokes equations for flow at low
Reynolds numbers in two-dimensional channels with rough walls presenting either
deterministic or random self-similar geometries. For high Reynolds numbers,
when inertial effects become relevant, the distribution of wall stresses on
deterministic and random fractal rough channels becomes substantially dependent
on the microscopic details of the walls geometry. In addition, we find that,
while the permeability of the random channel follows the usual decrease with
Reynolds, our results indicate an unexpected permeability increase for the
deterministic case, i.e., ``the rougher the better''. We show that this complex
behavior is closely related with the presence and relative intensity of
recirculation zones in the reentrant regions of the rough channel.Comment: 4 pages, 5 figure
Localization landscape theory of disorder in semiconductors. III. Application to carrier transport and recombination in light emitting diodes
This paper introduces a novel method to account for quantum disorder effects
into the classical drift-diffusion model of semiconductor transport through the
localization landscape theory. Quantum confinement and quantum tunneling in the
disordered system change dramatically the energy barriers acting on the
perpendicular transport of heterostructures. In addition they lead to
percolative transport through paths of minimal energy in the 2D landscape of
disordered energies of multiple 2D quantum wells. This model solves the carrier
dynamics with quantum effects self-consistently and provides a computationally
much faster solver when compared with the Schr\"odinger equation resolution.
The theory also provides a good approximation to the density of states for the
disordered system over the full range of energies required to account for
transport at room-temperature. The current-voltage characteristics modeled by
3-D simulation of a full nitride-based light-emitting diode (LED) structure
with compositional material fluctuations closely match the experimental
behavior of high quality blue LEDs. The model allows also a fine analysis of
the quantum effects involved in carrier transport through such complex
heterostructures. Finally, details of carrier population and recombination in
the different quantum wells are given.Comment: 14 pages, 16 figures, 6 table
Interplay between geometry and flow distribution in an airway tree
Uniform fluid flow distribution in a symmetric volume can be realized through
a symmetric branched tree. It is shown here, however, that the flow
partitioning can be highly sensitive to deviations from exact symmetry if
inertial effects are present. This is found by direct numerical simulation of
the Navier-Stokes equations in a 3D tree geometry. The flow asymmetry is
quantified and found to depend on the Reynolds number. Moreover, for a given
Reynolds number, we show that the flow distribution depends on the aspect ratio
of the branching elements as well as their angular arrangement. Our results
indicate that physiological variability should be severely restricted in order
to ensure uniform fluid distribution in a tree. This study suggests that any
non-uniformity in the air flow distribution in human lungs should be influenced
by the respiratory conditions, rest or hard exercise
Exponential decay of Laplacian eigenfunctions in domains with branches
The behavior of Laplacian eigenfunctions in domains with branches is
investigated. If an eigenvalue is below a threshold which is determined by the
shape of the branch, the associated eigenfunction is proved to exponentially
decay inside the branch. The decay rate is twice the square root of the
difference between the threshold and the eigenvalue. The derived exponential
estimate is applicable for arbitrary domains in any spatial dimension.
Numerical simulations illustrate and further extend the theoretical estimate
Value of Urinary Albumin-to-Creatinine Ratio as a Predictor of Type 2 Diabetes in Pre-Diabetic Individuals
OBJECTIVE—The albumin-to-creatinine ratio (ACR) reflects urinary albumin excretion and is increasingly being accepted as an important clinical outcome predictor. Because of the great public health need for a simple and inexpensive test to identify individuals at high risk for developing type 2 diabetes, it has been suggested that the ACR might serve this purpose. We therefore determined whether the ACR could predict incident diabetes in a well-characterized cohort of pre-diabetic Americans
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