151 research outputs found
Hurst's Rescaled Range Statistical Analysis for Pseudorandom Number Generators used in Physical Simulations
The rescaled range statistical analysis (R/S) is proposed as a new method to
detect correlations in pseudorandom number generators used in Monte Carlo
simulations. In an extensive test it is demonstrated that the RS analysis
provides a very sensitive method to reveal hidden long run and short run
correlations. Several widely used and also some recently proposed pseudorandom
number generators are subjected to this test. In many generators correlations
are detected and quantified.Comment: 12 pages, 12 figures, 6 tables. Replaces previous version to correct
citation [19
El enfoque teleológico de la educación Montessori y sus implicaciones
La teleologÃa es un elemento central de la educación Montessori. Entender las implicaciones del enfoque teleológico en Montessori ayuda a entender sus diferencias con el movimiento de la Educación Nueva, inspirado en Jean-Jacques Rousseau, asà como su profunda afinidad con el pensamiento aristotélico. El enfoque teleológico tiene varias implicaciones en la educación, como, por ejemplo, en lo que se refiere a los conceptos de aprendizaje significativo, de aprendizaje activo, de estÃmulos para el aprendizaje y de progreso. Para entender el enfoque teleológico en Montessori, hablaremos de algunos de los pilares fundamentales de esa pedagogÃa, como, por ejemplo, el ambiente preparado, el control del error, la mente absorbente, la atención sostenida, el desarrollo de la personalidad, la repetición con propósito, la actividad perfectiva, el placer de aprender y la inclinación de la naturaleza racional hacia su fin.
Para Montessori, la actividad humana está naturalmente orientada hacia un fin y ordenada por la razón. El fin de la educación es el niño mismo, ya que esta consiste en perfeccionar al agente, llevando al acto en el niño lo que en él solo está en potencia. El afán del niño por edificar su personalidad ocurre a través de la actividad espontánea de su mente absorbente y de la repetición con propósito, que genera hábitos positivos. El carácter absorbente de la mente del niño le urge a conocer, empapándose de su entorno. De ahà que el ambiente preparado y el control del error resulten cruciales. La actividad perfectiva, realizada con la cantidad justa y necesaria de estÃmulos, hace que el niño encuentre descanso en los actos voluntarios realizados con sentido y sin trabas. El placer que resulta no se entiende como mera experiencia, sino en relación con una actividad natural encaminada hacia su fin
Fast Monte Carlo Simulation for Patient-specific CT/CBCT Imaging Dose Calculation
Recently, X-ray imaging dose from computed tomography (CT) or cone beam CT
(CBCT) scans has become a serious concern. Patient-specific imaging dose
calculation has been proposed for the purpose of dose management. While Monte
Carlo (MC) dose calculation can be quite accurate for this purpose, it suffers
from low computational efficiency. In response to this problem, we have
successfully developed a MC dose calculation package, gCTD, on GPU architecture
under the NVIDIA CUDA platform for fast and accurate estimation of the x-ray
imaging dose received by a patient during a CT or CBCT scan. Techniques have
been developed particularly for the GPU architecture to achieve high
computational efficiency. Dose calculations using CBCT scanning geometry in a
homogeneous water phantom and a heterogeneous Zubal head phantom have shown
good agreement between gCTD and EGSnrc, indicating the accuracy of our code. In
terms of improved efficiency, it is found that gCTD attains a speed-up of ~400
times in the homogeneous water phantom and ~76.6 times in the Zubal phantom
compared to EGSnrc. As for absolute computation time, imaging dose calculation
for the Zubal phantom can be accomplished in ~17 sec with the average relative
standard deviation of 0.4%. Though our gCTD code has been developed and tested
in the context of CBCT scans, with simple modification of geometry it can be
used for assessing imaging dose in CT scans as well.Comment: 18 pages, 7 figures, and 1 tabl
Algorithm for normal random numbers
We propose a simple algorithm for generating normally distributed pseudo
random numbers. The algorithm simulates N molecules that exchange energy among
themselves following a simple stochastic rule. We prove that the system is
ergodic, and that a Maxwell like distribution that may be used as a source of
normally distributed random deviates follows when N tends to infinity. The
algorithm passes various performance tests, including Monte Carlo simulation of
a finite 2D Ising model using Wolff's algorithm. It only requires four simple
lines of computer code, and is approximately ten times faster than the
Box-Muller algorithm.Comment: 5 pages, 3 encapsulated Postscript Figures. Submitted to
Phys.Rev.Letters. For related work, see http://pipe.unizar.es/~jf
Cluster Hybrid Monte Carlo Simulation Algorithms
We show that addition of Metropolis single spin-flips to the Wolff cluster
flipping Monte Carlo procedure leads to a dramatic {\bf increase} in
performance for the spin-1/2 Ising model. We also show that adding Wolff
cluster flipping to the Metropolis or heat bath algorithms in systems where
just cluster flipping is not immediately obvious (such as the spin-3/2 Ising
model) can substantially {\bf reduce} the statistical errors of the
simulations. A further advantage of these methods is that systematic errors
introduced by the use of imperfect random number generation may be largely
healed by hybridizing single spin-flips with cluster flipping.Comment: 16 pages, 10 figure
GPU-based fast Monte Carlo simulation for radiotherapy dose calculation
Monte Carlo (MC) simulation is commonly considered to be the most accurate
dose calculation method in radiotherapy. However, its efficiency still requires
improvement for many routine clinical applications. In this paper, we present
our recent progress towards the development a GPU-based MC dose calculation
package, gDPM v2.0. It utilizes the parallel computation ability of a GPU to
achieve high efficiency, while maintaining the same particle transport physics
as in the original DPM code and hence the same level of simulation accuracy. In
GPU computing, divergence of execution paths between threads can considerably
reduce the efficiency. Since photons and electrons undergo different physics
and hence attain different execution paths, we use a simulation scheme where
photon transport and electron transport are separated to partially relieve the
thread divergence issue. High performance random number generator and hardware
linear interpolation are also utilized. We have also developed various
components to handle fluence map and linac geometry, so that gDPM can be used
to compute dose distributions for realistic IMRT or VMAT treatment plans. Our
gDPM package is tested for its accuracy and efficiency in both phantoms and
realistic patient cases. In all cases, the average relative uncertainties are
less than 1%. A statistical t-test is performed and the dose difference between
the CPU and the GPU results is found not statistically significant in over 96%
of the high dose region and over 97% of the entire region. Speed up factors of
69.1 ~ 87.2 have been observed using an NVIDIA Tesla C2050 GPU card against a
2.27GHz Intel Xeon CPU processor. For realistic IMRT and VMAT plans, MC dose
calculation can be completed with less than 1% standard deviation in 36.1~39.6
sec using gDPM.Comment: 18 pages, 5 figures, and 3 table
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Large contribution of supercooled liquid clouds to the solar radiation budget of the Southern Ocean
The Southern Ocean is a critical region for global climate, yet large cloud and solar radiation biases over the Southern Ocean are a long-standing problem in climate models and are poorly understood, leading to biases in simulated sea surface temperatures. This study shows that supercooled liquid clouds are central to understanding and simulating the Southern Ocean environment. A combination of satellite observational data and detailed radiative transfer calculations is used to quantify the impact of cloud phase and cloud vertical structure on the reflected solar radiation in the Southern Hemisphere summer. It is found that clouds with supercooled liquid tops dominate the population of liquid clouds. The observations show that clouds with supercooled liquid tops contribute between 27% and 38% to the total reflected solar radiation between 40° and 70°S, and climate models are found to poorly simulate these clouds. The results quantify the importance of supercooled liquid clouds in the Southern Ocean environment and highlight the need to improve understanding of the physical processes that control these clouds in order to improve their simulation in numerical models. This is not only important for improving the simulation of present-day climate and climate variability, but also relevant for increasing confidence in climate feedback processes and future climate projections
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The observed state of the water cycle in the early twenty-first century
Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 28 (2015): 8289–8318, doi:10.1175/JCLI-D-14-00555.1.This study quantifies mean annual and monthly fluxes of Earth’s water cycle over continents and ocean basins during the first decade of the millennium. To the extent possible, the flux estimates are based on satellite measurements first and data-integrating models second. A careful accounting of uncertainty in the estimates is included. It is applied within a routine that enforces multiple water and energy budget constraints simultaneously in a variational framework in order to produce objectively determined optimized flux estimates. In the majority of cases, the observed annual surface and atmospheric water budgets over the continents and oceans close with much less than 10% residual. Observed residuals and optimized uncertainty estimates are considerably larger for monthly surface and atmospheric water budget closure, often nearing or exceeding 20% in North America, Eurasia, Australia and neighboring islands, and the Arctic and South Atlantic Oceans. The residuals in South America and Africa tend to be smaller, possibly because cold land processes are negligible. Fluxes were poorly observed over the Arctic Ocean, certain seas, Antarctica, and the Australasian and Indonesian islands, leading to reliance on atmospheric analysis estimates. Many of the satellite systems that contributed data have been or will soon be lost or replaced. Models that integrate ground-based and remote observations will be critical for ameliorating gaps and discontinuities in the data records caused by these transitions. Continued development of such models is essential for maximizing the value of the observations. Next-generation observing systems are the best hope for significantly improving global water budget accounting.This research was funded by multiple
grants from NASA’s Energy and Water Cycle
Study (NEWS) program.2016-05-0
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