Spelling suggestions: "subject:"radiative transfer amathematical models"" "subject:"radiative transfer dmathematical models""
1 |
Monte Carlo simulation of radiation heat transfer in a three-dimensional enclosure containing a circular cylinderHong, Seung-Ho 14 April 1994 (has links)
Graduation date: 1994
|
2 |
Modeled and observed longwave radiances at the top of the atmosphereStone, Kenneth A. 11 July 1990 (has links)
One method of estimating the longwave radiative heating of the atmosphere is to
combine satellite observations of emitted radiances with those computed from synoptic
temperature and humidity profiles. Modeled and observed radiances are brought into
agreement by altering cloud properties or even by adjusting the temperature and water
vapor profiles.
Here this strategy is examined in an exploratory study using global meteorological
data sets and a radiative transfer model typical of those found in general circulation
models. Calculated radiances are compared to those observed by the Earth Radiation
Budget Satellite (ERBS). Input for the model is obtained from the National Meteorological
Center (NMC) in the form of vertical profiles of temperature and relative
humidity. The comparisons are limited to clear sky as deduced by ERBE algorithms,
and additional filtering which requires homogeneous surface type for a 3 x 3 array
of ERBS scanner fields of view. Observations are obtained from 60° N to 60° S that
lie within 30 minutes of the NMC analysis time. Following the work of Ramanathan
and Downey (1986), comparisons are separated into climatologically distinct groups as
well as by satellite viewing angle. This separation is an attempt to distinguish between
biases in the radiation model and those in the NMC data set. Results are presented for
the months of July 1985, and January 1986.
A comparison of the present radiation model's output with that obtained from a
Geophysical Fluid Dynamics Laboratory (GFDL) model shows a bias of nearly 3% in
the present model for a standard mid-latitude summer profile.
Global results show a negative bias in the modeled values for nearly all scenes,
except for nighttime desert. The nighttime desert bias may be a result of a skin-air
temperature difference not resolved by the NMC analyses. The overall negative bias
may be a result of an overestimation of water vapor for regions with low relative
humidity. / Graduation date: 1991
|
3 |
Modeling and application of multispectral oceanic sun glint observationsLuderer, Gunnar 02 October 2003 (has links)
The atmospheric radiative transfer model MOCARAT was developed and is
presented in this thesis. MOCARAT employs a Monte Carlo Technique for the
accurate modeling of band radiances and reflectances in an atmospheric system
with a ruffled ocean surface as a lower boundary. The atmospheric radiative
transfer is modeled with consideration of molecular Rayleigh scattering, Mie
Scattering and absorption on particulate matter, as well as band absorption by
molecules in the wavelength channels of interest. The bidirectional reflection
of downwelling light at the ocean surface is computed using the empirical relationship
between surface wind field and the slope distribution of wave facets
derived by Cox and Munk (1954a).
A method is proposed to use the oceanic sun glint for remote sensing applications.
The sensitivity of channel correlations to aerosol burden and type as well
as other atmospheric and observational parameters is assessed. Comparisons
of observed correlations with model results are used to check the consistency
of the calibration of the airborne Multichannel Cloud Radiometer (MCR) that
was employed during the Indian Ocean Experiment (INDOEX). The MCR calibration
exhibited large variability from flight to flight. The method was applied
to MODIS observations. Unlike the MCR, MODIS was stable where expected,
although numerical values for some of the wavelengths appear to depart from
theory. / Graduation date: 2004
|
4 |
The iterative thermal emission Monte Carlo method for thermal radiative transferLong, Alex R. 01 June 2012 (has links)
For over 30 years, the Implicit Monte Carlo (IMC) method has been used to
solve challenging problems in thermal radiative transfer. These problems are typically optically thick and di ffusive, as a consequence of the high degree of "pseudo-scattering" introduced to model the absorption and reemission of photons from a tightly-coupled, radiating material. IMC has several well-known features which could be improved: a) it can be prohibitively computationally expensive, b) it introduces statistical noise into the material and radiation temperatures, which
may be problematic in multiphysics simulations, and c) under certain conditions,
solutions can be unphysical and numerically unstable, in that they violate a maximum principle - IMC calculated temperatures can be greater than the maximum
temperature used to drive the problem.
We have developed a variant of IMC called "iterative thermal emission" IMC,
which is designed to be more stable than IMC and have a reduced parameter
space in which the maximum principle is violated. ITE IMC is a more implicit
method version of the IMC in that it uses the information obtained from a series
of IMC photon histories to improve the estimate for the end of time-step material
temperature during a time step. A better estimate of the end of time-step material
temperature allows for a more implicit estimate of other temperature dependent
quantities: opacity, heat capacity, Fleck Factor (probability that a photon absorbed during a time step is not reemitted) and the Planckian emission source.
The ITE IMC method is developed by using Taylor series expansions in material
temperature in a similar manner as the IMC method. It can be implemented in a
Monte Carlo computer code by running photon histories for several sub-steps in a
given time-step and combining the resulting data in a thoughtful way. The ITE IMC
method is then validated against 0-D and 1-D analytic solutions and compared
with traditional IMC. We perform an in finite medium stability analysis of ITE
IMC and show that it is slightly more numerically stable than traditional IMC.
We find that significantly larger time-steps can be used with ITE IMC without
violating the maximum principle, especially in problems with non-linear material
properties. We also compare ITE IMC to IMC on a two-dimensional, orthogonal
mesh, x-y geometry problem called the "crooked pipe" and show that our new
method reproduces the IMC solution. The ITE IMC method yields results with
larger variances; however, the accuracy of the solution is improved in comparison
with IMC, for a given choice of spatial and temporal grid. / Graduation date: 2013
|
5 |
Radiation from an infinite plane to parallel rows of infinitely long tubes - hottel extendedQualey, Douglas L. 10 May 1994 (has links)
A two-dimensional model for predicting the rate of radiation
heat transfer for the interior of an industrial furnace is described.
The model is two-dimensional due to the assumptions of the heat
source as an infinite radiating plane and the heat sink as rows of
parallel tubes that are both infinite in length and in number. A
refractory back wall, located behind the tube rows, is also included
in some of the model configurations.
The optical properties for the heat source, heat sink, and
refractory back wall are simplified by assuming the "black-body"
case: all are treated as perfect absorbers and emitters of radiation.
This assumption allows three different solution techniques-a
graphical, crossed-string, and numerical method-to be used in
solving for the radiant transfer rate. The numerical method, an
innovative Monte Carlo technique, is the one employed in this study.
Hottel used a graphical technique to solve the furnace model
for a two row configuration in which the tubes are arranged on
equilateral triangular centers. His results, along with those
produced by the crossed-string method, are used in this work to
validate the numerical technique. Having been validated, the
numerical method was then employed to extend Hottel's work by
adding more tube rows to the original equilateral triangular
configuration and by generalizing the results to isosceles
arrangements.
Findings of this investigation are summarized in a table that
lists the direct view factors for a ten tube row configuration
arranged in an equilateral triangular array. Values from this table
can be used to solve the transfer rate problem for twenty different
cases by assuming a nonconducting refractory back wall. Results for
twelve cases are represented graphically in this document The
results are used to demonstrate the importance of a refractory back
wall on overall radiation absorption. Examinations of the two row
and five row cases for an isosceles triangular array indicate that
the tabular values can be applied to any isosceles arrangement if the
ratio of row separation distance to tube center-to-center distance
is 0.7 or greater. / Graduation date: 1995
|
6 |
Parallel Performance Analysis of The Finite Element-Spherical Harmonics Radiation Transport MethodPattnaik, Aliva 21 November 2006 (has links)
In this thesis, the parallel performance of the finite element-spherical harmonics (FE-PN) method implemented in the general-purpose radiation transport code EVENT is studied both analytically and empirically. EVENT solves the coupled set of space-angle discretized FE-PN equations using a parallel block-Jacobi domain decomposition method. As part of the analytical study, the thesis presents complexity results for EVENT when solving for a 3D criticality benchmark radiation transport problem in parallel. The empirical analysis is concerned with the impact of the main algorithmic factors affecting performance. Firstly, EVENT supports two solution strategies, namely MOD (Moments Over Domains) and DOM (Domains Over Moments), to solve the transport equation in parallel. The two strategies differ in the way they solve the multi-level space-angle coupled systems of equations. The thesis presents empirical evidence of which of the two solution strategies is more efficient. Secondly, different preconditioners are used in the Preconditioned Conjugate Gradient (PCG) inside EVENT. Performance of EVENT is compared when using three preconditioners, namely diagonal, SSOR(Symmetric Successive Over-Relaxation) and ILU. The other two factors, angular and spatial resolutions of the problem affect both the performance and precision of EVENT. The thesis presents comparative results on EVENTs performance as these two resolutions are increased.
From the empirical performance study of EVENT, a bottleneck is identified that limits the improvement in performance as number of processors used by EVENT is increased. In some experiments, it is observed that uneven assignment of computational load among processors causes a significant portion of the total time being spent in synchronization among processors. The thesis presents two indicators that identify when such inefficiency occur; and in such a case, a load rebalancing strategy is applied that computes a new partition of the problem so that each partition corresponds to equal amount of computational load.
|
7 |
A Time-Dependent Slice Balance Method for High-Fidelity Radiation Transport ComputationsHamilton, Steven 09 April 2007 (has links)
A general finite difference discretization of the time-dependent radiation transport equation is developed around the framework of an existing steady-state three dimensional radiation transport solver based on the slice-balance approach. Three related algorithms are outlined within the general finite difference scheme: an explicit, an implicit, and a semi-implicit approach. The three algorithms are analyzed with respect to the discretizations of each element of the phase space in the transport solver. The explicit method, despite its small computational cost per time step, is found to be unsuitable for many purposes due to its inability to accurately handle rapidly varying solutions. The semi-implicit method is shown to produce results nearly as reliable as the fully implicit solver, while requiring significantly less computational effort.
|
8 |
Silicon wafer surface temperature measurement using light-pipe radiation thermometers in rapid thermal processing systemsQu, Yan 28 August 2008 (has links)
Not available / text
|
9 |
Monte Carlo radiation transfer studies of protoplanetary environmentsWalker, Christina H. January 2007 (has links)
Monte Carlo radiation transfer provides an efficient modelling tool for probing the dusty local environment of young stars. Within this thesis, such theoretical models are used to study the disk structure of objects across the mass spectrum - young low mass Brown Dwarfs, solar mass T-Tauri stars, intermediate mass Herbig Ae stars, and candidate B-stars with massive disks. A Monte Carlo radiation transfer code is used to model images and photometric data in the UV - mm wavelength range. These models demonstrate how modelling techniques have been updated in an attempt to reduce the number of unknown parameters and extend the diversity of objects that can be studied.
|
10 |
Hawking radiation in dispersive mediaRobertson, Scott James January 2011 (has links)
Hawking radiation, despite its presence in theoretical physics for over thirty years, remains elusive and undetected. It also suffers, in its original context of gravitational black holes, from conceptual difficulties. Of particular note is the trans-Planckian problem, which is concerned with the apparent origin of the radiation in absurdly high frequencies. In order to gain better theoretical understanding and, it is hoped, experimental verification of Hawking radiation, much study is being devoted to systems which model the spacetime geometry of black holes, and which, by analogy, are also thought to emit Hawking radiation. These analogue systems typically exhibit dispersion, which regularizes the wave behaviour at the horizon but does not lend itself well to analytic treatment, thus rendering Hawking’s prediction less secure. A general analytic method for dealing with Hawking radiation in dispersive systems has proved difficult to find. This thesis presents new numerical and analytic results for Hawking emission spectra in dispersive systems. It examines two black-hole analogue systems: it begins by introducing the well-known acoustic model, presenting some original results in that context; then, through analogy with the acoustic model, goes on to develop the lesser-known fibre-optical model. The following original results are presented in the context of both of these models: • an analytic expression for the low-frequency temperature is found for a hyperbolic tangent background profile, valid in the entire parameter space; it is well-known that the spectrum is approximately thermal at low frequencies, but a universally valid expression for the corresponding temperature is an original development; • an analytic expression for the spectrum, valid over almost the entire frequency range, when the velocity profile parameters lie in the regime where the low-frequency temperature is given by the Hawking prediction; previous work has focused on the low-frequency thermal spectrum and the characterization of the deviations from thermality, rather than a single analytic expression; and • a new unexplored regime where no group-velocity horizon exists is examined; the Hawking spectra are found to be non-zero here, but also highly non-thermal, and are found, in the limit of small deviations, to vary with the square of the maximum deviation; the analytic expression for the case with a horizon is found to carry over to this new regime, with appropriate modifications. Furthermore, the thesis examines the results of a classical frequency-shifting experiment in the context of fibre-optical horizons. The theory of this process is presented for both a constant-velocity and a constantly-decelerating pulse, the latter case taking account of the Raman effect. The resulting spectra are at least qualititively explained, but there is a discrepancy between theory and experiment that has not yet been accounted for.
|
Page generated in 0.1339 seconds