40 research outputs found
Exponential Krylov time integration for modeling multi-frequency optical response with monochromatic sources
Light incident on a layer of scattering material such as a piece of sugar or
white paper forms a characteristic speckle pattern in transmission and
reflection. The information hidden in the correlations of the speckle pattern
with varying frequency, polarization and angle of the incident light can be
exploited for applications such as biomedical imaging and high-resolution
microscopy. Conventional computational models for multi-frequency optical
response involve multiple solution runs of Maxwell's equations with
monochromatic sources. Exponential Krylov subspace time solvers are promising
candidates for improving efficiency of such models, as single monochromatic
solution can be reused for the other frequencies without performing full
time-domain computations at each frequency. However, we show that the
straightforward implementation appears to have serious limitations. We further
propose alternative ways for efficient solution through Krylov subspace
methods. Our methods are based on two different splittings of the unknown
solution into different parts, each of which can be computed efficiently.
Experiments demonstrate a significant gain in computation time with respect to
the standard solvers.Comment: 22 pages, 4 figure
Exponential Euler and backward Euler methods for nonlinear heat conduction problems
In this paper a variant of nonlinear exponential Euler scheme is proposed for
solving nonlinear heat conduction problems. The method is based on nonlinear
iterations where at each iteration a linear initial-value problem has to be
solved. We compare this method to the backward Euler method combined with
nonlinear iterations. For both methods we show monotonicity and boundedness of
the solutions and give sufficient conditions for convergence of the nonlinear
iterations. Numerical tests are presented to examine performance of the two
schemes. The presented exponential Euler scheme is implemented based on
restarted Krylov subspace methods and, hence, is essentially explicit (involves
only matrix-vector products).Comment: 11 pages, 2 figures. This is a preprint of the work accepted for
publication in Lobachevskii Journal of Mathematic
Comparison of computational codes for direct numerical simulations of turbulent Rayleigh-B\'enard convection
Computational codes for direct numerical simulations of Rayleigh-B\'enard
(RB) convection are compared in terms of computational cost and quality of the
solution. As a benchmark case, RB convection at and in a
periodic domain, in cubic and cylindrical containers is considered. A dedicated
second-order finite-difference code (AFID/RBflow) and a specialized
fourth-order finite-volume code (Goldfish) are compared with a general purpose
finite-volume approach (OpenFOAM) and a general purpose spectral-element code
(Nek5000). Reassuringly, all codes provide predictions of the average heat
transfer that converge to the same values. The computational costs, however,
are found to differ considerably. The specialized codes AFID/RBflow and
Goldfish are found to excel in efficiency, outperforming the general purpose
flow solvers Nek5000 and OpenFOAM by an order of magnitude with an error on the
Nusselt number below . However, we find that alone is not
sufficient to assess the quality of the numerical results: in fact,
instantaneous snapshots of the temperature field from a near wall region
obtained for deliberately under-resolved simulations using Nek5000 clearly
indicate inadequate flow resolution even when is converged. Overall,
dedicated special purpose codes for RB convection are found to be more
efficient than general purpose codes.Comment: 12 pages, 5 figure
The High Voltage Feedthroughs for the ATLAS Liquid Argon Calorimeters
The purpose, design specifications, construction techniques, and testing
methods are described for the high voltage feedthrough ports and filters of the
ATLAS Liquid Argon calorimeters. These feedthroughs carry about 5000 high
voltage wires from a room-temperature environment (300 K) through the cryostat
walls to the calorimeters cells (89 K) while maintaining the electrical and
cryogenic integrity of the system. The feedthrough wiring and filters operate
at a maximum high voltage of 2.5 kV without danger of degradation by corona
discharges or radiation at the Large Hadron Collider
Construction, assembly and tests of the ATLAS electromagnetic barrel calorimeter
The construction and assembly of the two half barrels of the ATLAS central electromagnetic calorimeter and their insertion into the barrel cryostat are described. The results of the qualification tests of the calorimeter before installation in the LHC ATLAS pit are given
Stochastic modelling of air pollution impacts on respiratory infection risk
The impact of air pollution on people’s health and daily activities in China has recently aroused much attention. By using stochastic differential equations, variation in a 6 year long time series of air quality index (AQI) data, gathered from air quality monitoring sites in Xi’an from 15 November 2010 to 14 November 2016 was studied. Every year the extent of air pollution shifts from being serious to not so serious due to alterations in heat production systems. The distribution of such changes can be predicted by a Bayesian approach and the Gibbs sampler algorithm. The intervals between changes in a sequence indicate when the air pollution becomes increasingly serious. Also, the inflow rate of pollutants during the main pollution periods each year has an increasing trend. This study used a stochastic SEIS model associated with the AQI to explore the impact of air pollution on respiratory infections. Good fits to both the AQI data and the numbers of influenza-like illness cases were obtained by stochastic numerical simulation of the model. Based on the model’s dynamics, the AQI time series and the daily number of respiratory infection cases under various government intervention measures and human protection strategies were forecasted. The AQI data in the last 15 months verified that government interventions on vehicles are effective in controlling air pollution, thus providing numerical support for policy formulation to address the haze crisis