1,399 research outputs found
Self-similarity of single-channel transmission for electron transport in nanowires
We demonstrate that the single-channel transmission in the resonance
tunneling regime exhibits self-similarity as a function of the nanowire length
and the energy of incident electrons. The self-similarity is used to design the
nonlinear transformation of the nanowire length and energy which, on the basis
of known values of transmission for a certain region on the energy-length
plane, yields transmissions for other regions on this plane. Test calculations
with a one-dimensional tight-binding model illustrate the described
transformations. Density function theory based transport calculations of Na
atomic wires confirm the existence of the self-similarity in the transmission
Analysis of the convergence of the 1/t and Wang-Landau algorithms in the calculation of multidimensional integrals
In this communication, the convergence of the 1/t and Wang - Landau
algorithms in the calculation of multidimensional numerical integrals is
analyzed. Both simulation methods are applied to a wide variety of integrals
without restrictions in one, two and higher dimensions. The errors between the
exact and the calculated values of the integral are obtained and the efficiency
and accuracy of the methods are determined by their dynamical behavior. The
comparison between both methods and the simple sampling Monte Carlo method is
also reported. It is observed that the time dependence of the errors calculated
with 1/t algorithm goes as N^{-1/2} (with N the MC trials) in quantitative
agreement with the simple sampling Monte Carlo method. It is also showed that
the error for the Wang - Landau algorithm saturates in time evidencing the
non-convergence of the methods. The sources for the error are also determined.Comment: 8 pages, 5 figure
Learning strategies in modelling economic growth
Cornerstone economic growth models as the Solow-Swan model and their modern extensions normally assume the rate of population growth as exogenous without any explanation of the links between economic growth and most important demographic variables. Recently, some articles have presented models to explain many phenomena of population dynamics, including evolution and ageing. This paper is a first exercise to include endogenous population dynamics and learning strategies as ingredients of an economic growth model. The model includes two ways of learning that determinate economic growth: individual and social learning. We study the dynamics through computer simulations and we show that the model reflects some features of real economies.Economic Growth, Learning Strategies, Human Capital, Penna model
Numerical modeling of FSW welding process incorporating a parameter estimation technique
Los modelos numéricos de transferencia de calor y de flujo de fluido empleados en la simulación del proceso de soldadura por fricción-agitación (FSW) han contribuido a la comprensión del proceso. Sin embargo, existen ciertos parámetros de entrada a los modelos que no pueden determinarse fácilmente a partir de principios fundamentales o de las condiciones de soldadura. Como resultado, las predicciones de estos modelos no siempre están de acuerdo con los resultados experimentales. En este trabajo se usa el método de Levenberg-Marquardt (LM) para estimar de manera no lineal los parámetros desconocidos presentes en los modelos de transferencia de calor y de flujo de fluido, ajustando las temperaturas obtenidas con los modelos a mediciones experimentales de temperatura. Estos modelos se implementan en un programa de propósito general que emplea una formulación numérica desarrollada a partir del método de los elementos finitos (MEF). Los parámetros que se desconocen son: el coeficiente de fricción y la cantidad de adherencia del material a la superficie de la herramienta, el coeficiente de transferencia de calor en la superficie inferior y la cantidad de disipación viscosa convertida en calor. Los resultados obtenidos revelan una mejora de las predicciones de los modelos numéricos a partir de la incorporación de técnicas de estimación de parámetros.Numerical models of heat transfer and fluid flow used in the simulation of the friction-stir welding (FSW) process have contributed to the understanding of the process. However, there are some input model parameters that cannot be easily determined from fundamental principles or the welding conditions. As a result, the model predictions are not always in agreement with experimental results. In this work, the Levenberg-Marquardt (LM) method is used in order to perform a non-linear estimation of the unknown parameters present in the heat transfer and fluid flow models, by adjusting the temperatures results obtained with the models to temperature experimental measurements. These models are implemented in a general-purpose software that uses a numerical formulation developed from the finite element method (FEM). The unknown parameters are: the friction coefficient and the amount of adhesion of material to the surface of the tool, the heat transfer coefficient on the bottom surface and the amount of viscous dissipation converted into heat. The obtained results show an improvement in the numerical model predictions from the incorporation of parameter estimation techniques.Peer Reviewe
Fast Mid-IR Flashes Detected During Small Solar X-Ray Bursts
Solar observations in the mid-infrared 8-14 \mu\m band continuum were carried
out with cadence of 5 frames per second, in December 2007. Rapid small heated
sources, with typical duration of the order of seconds, were found on the
bright plage-like areas around sunspots, in association with relatively weak
GOES soft X-ray bursts. This work presents the analysis of fast mid-infrared
flashes detected during a GOES B2.0-class event on 10 December 2007, beginning
at about 10:40 UT. Rapid brightness temperature enhancements of 0.5 to 2.0 K
were detected at the Earth by a microbolometer array, using a telescope with
10.5 cm diameter aperture producing a diffraction limited field-of-view of 25
arcsec. Minimum detectable temperature change was of 0.1 K. The corresponding
fluxes are 30-130 solar flux units. At the solar surface the estimated rapid
brightenings were of 50-150 KComment: 12 pages including 6 figures. Accepted by Solar Physics, April 201
Discursos pronunciados en el acto de investidura de Doctor "Honoris Causa" del ExcelentÃsimo Señor D. Thomas Popkewitz
Discurso presentado por el Doctor Miguel A. Pereyra-GarcÃa Castr
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