检索结果-内蒙古大学图书馆

IEEE International Conference on Image Processing

作者： P.J. Bones N.D. Blakeley R.P. Millane Department Electrical and Electronic Engineering University of Canterbury Christchurch New Zealand Computational Science and Engineering Program Purdue University West Lafayette IN USA

Recovery of magnetic resonance images from irregular sampling sets is investigated from the point of view of moment discretization of the Fredholm equation of the first kind. The limited spatial extent of the object is known a priori and the sampling schemes considered each have mean density lower than that imposed by the Nyquist limit. The recovery formula obtained has the same form as a standard irregular sampling technique. A practical means of performing the recovery for very large data sets by utilising the block Toeplitz structure of the matrix involved is presented.

关键词： Image sampling Image reconstruction Magnetic resonance Magnetic resonance imaging Spirals Bones Equations Discrete Fourier transforms Earth Antenna measurements

来源：评论

学校读者我要写书评

暂无评论

引用

Physical Review E 2001年第5期64卷 051503-051503页

作者： Y. Gebremichael T. B. Schrøder F. W. Starr S. C. Glotzer Center for Theoretical and Computational Materials Science and Polymers Division National Institute of Standards and Technology Gaithersburg Maryland 20899 Chemical Physics Program Institute for Physical Sciences and Technology University of Maryland College Park Maryland 20742 Departments of Chemical Engineering and Materials Science and Engineering University of Michigan Ann Arbor Michigan 48109

In recent years, experimental and computational studies have demonstrated that the dynamics of glass-forming liquids are spatially heterogeneous, exhibiting regions of temporarily enhanced or diminished mobility. Here we present a detailed analysis of dynamical heterogeneity in a simulated “bead-spring” model of a low-molecular-weight polymer melt. We investigate the transient nature and size distribution of clusters of “mobile” chain segments (monomers) as the polymer melt is cooled toward its glass transition. We also explore the dependence of this clustering on the way in which the mobile subset is defined. We show that the mean cluster size is time dependent with a peak at intermediate time, and that the mean cluster size at the peak time grows with decreasing temperature T. We show that for each T a particular fraction of particles maximizes the mean cluster size at some characteristic time, and this fraction depends on T. The growing size of the clusters demonstrates the growing range of correlated motion, previously reported for this same system [C. Beneman et al. Nature (London) 399, 246 (1999)]. The distribution of cluster sizes approaches a power law near the mode-coupling temperature, similar to behavior reported for a simulated binary mixture and a dense colloidal suspension, but with a different exponent. We calculate the correlation length of the clusters, and show that it exhibits similar temperature- and time-dependent behavior as the mean cluster size, with a maximum at intermediate time. We show that the characteristic time of the maximum cluster size follows the scaling predicted by mode-coupling theory (MCT) for the β time scale, revealing a possible connection between spatially heterogeneous dynamics and MCT.

关键词： .size distribution clustering intermediate segments glass transition investigate toward glass-forming monomers Polymer Melt Dynamical Heterogeneity spatially heterogeneous mean cluster correlated dynamics mobile subset peak time

来源：评论

学校读者我要写书评

暂无评论

Diffraction by one-dimensional paracrystals and perturbed lattices

引用

Acta Crystallographica Section A: Foundations of Crystallography 2000年第5期56卷 497-506页

作者： Millane, R.P. Eads, J.L. Whistler Center for Carbohydrate Research Computational Science and Engineering Program Purdue University West Lafayette IN 47907-1160 United States

A detailed comparison of the one-dimensional paracrystalline and perturbed lattice models of disordered crystals, and their diffraction, is made. If noncumulative (thermal) disorder is added to the paracrystalline model, then the two models and their diffraction patterns are similar but not identical because of their different intersite statistics. However, they are identical in the limiting cases of either strong or weak correlations. Calculation of diffraction patterns shows that, with an appropriate choice of parameters, the two models give essentially identical diffraction for small crystallites and very similar diffraction for larger crystallites. An empirical relationship is derived between parameters of the two models such that they give similar diffraction patterns.

关键词： paracrystals perturbed lattices disorder diffraction.

来源：评论

学校读者我要写书评

暂无评论

The crystallite size-disorder relationship based on the spiral paracrystal

引用

Acta Crystallographica Section A: Foundations of Crystallography 2000年第6期56卷 549-553页

作者： Eads, J.L. Millane, R.P. Whistler Center for Carbohydrate Research Computational Science and Engineering Program Purdue University West Lafayette IN 47907-1160 United States

A simple physical model of the relationship between crystallite size and disorder for paracrystalline materials is presented, based on the spiral paracrystal and distortion of the lattice cells. Simulations show that the model leads to relationships similar to the α* rule. Average crystallite sizes predicted by the model are in agreement with experimental data and also allow crystallite-size distributions to be proposed. The model provides a more satisfactory and complete explanation of this relationship than do current descriptions.

关键词： paracrystal crystallite size disorder α* rule distortion.

来源：评论

学校读者我要写书评

暂无评论

Sub- And supercritical bifurcations and turning points in a simple BWR model

Sub- And supercritical bifurcations and turning points in a ...

引用

2000 ANS International Topical Meeting on Advances in Reactor Physics and Mathematics and Computation into the Next Millennium, PHYSOR 2000

作者： Rizwan-Uddin Department of Nuclear Plasma and Radiological Engineering University of Illinois at Urbana-Champaign UrbanaIL61801 United States Computational Science and Engineering Program University of Illinois at Urbana-Champaign United States

ISBN: (纸本)0894486551

Stability and bifurcation analyses of BWRs have been performed using a reduced order, nuclear-coupled thermal hydraulics model. This has been carried out using a bifurcation analysis code, BIFDD. A large segment of the parameter space has been investigated using this very efficient tool. Stability boundaries are obtained in several two-dimensional parameter spaces. In addition, the nature of bifurcation along these stability boundaries has also been determined. Results indicate that both subcritical as well as supercritical Poincaré-Andronov-Hopf bifurcations are likely to occur in regions of interest in parameter space. In addition to the semi-analytical bifurcation studies, the governing equations have also been integrated numerically. Results confirm the findings of the bifurcation analysis. Numerical integrations, carried out for parameter values away from the stability boundary, further show that the bifurcation curves, in many cases of subcritical bifurcations, have a turning point. The bifurcation curve in these cases extends back into the unstable region. These results show that it is possible to experience large amplitude stable oscillations in the unstable region infinitesimally close to the stability boundary. Moreover, large amplitude stable oscillations are also possible in the stable region of the parameter space near the stability boundary, following large but finite perturbations. These findings provide alternate explanation for the experimental and operational observations in BWRs that indicate the existence of stable limit cycle oscillations and the possibility of growing amplitude oscillations. Results obtained here using a simple model suggest that further work along these lines, with more detailed models, is needed to identify operating conditions and perturbation amplitudes that might lead to stable limit cycles or growing amplitude oscillations in current and next generation of BWRs. © 2000 Proceedings of the PHYSOR 2000 - ANS International To

关键词： Curve fitting

来源：评论

学校读者我要写书评

暂无评论

A MODULAR FINITE ELEMENT PARALLEL FLUID APPLICATIONS SIMULATOR

A MODULAR FINITE ELEMENT PARALLEL FLUID APPLICATIONS SIMULAT...

引用

ASME 1999 International Mechanical engineering Congress and Exposition, IMECE 1999

作者： Wong, K.L. Baker, A.J. Joint Institute for Computational Science University of Tennessee KnoxvilleTN37996 United States Engineering Science Program University of Tennessee KnoxvilleTN37996 United States

ISBN: (纸本)9780791816615

With the rapid advances in high-performance computing technology in recent years, researchers at the University of Tennessee have initiated a collaborative project to address the theoretical, algorithmic, and numerical implementation issues of computational mechanics simulations. To meet the challenge, a Parallel Interoperable computational Mechanics Simulator System (PICMSS) has been estabhshed. Such effort entails the development of a modular finite element platform capable of admitting diverse CFD formulations, as well as closure models for physics phenomena, as required/derived by the users operating on supercomputers and networks of PC clusters. Incompressible Navier Stokes Benchmark solutions via a velocity-vorticity formulation are proposed to examine the integrity emd scalability of PICMSS. © 1999 ASME International Mechanical engineering Congress and Exposition, Proceedings (IMECE). All Rights Reserved.

关键词： Navier Stokes equations

来源：评论

学校读者我要写书评

暂无评论

Modular finite element parallel fluid applications simulator

American Society of Mechanical Engineers, Fluids Engineering...

引用

American Society of Mechanical Engineers, Fluids engineering Division (Publication) FED 1999年 250卷 251-256页

作者： Wong, K.L. Baker, A.J. Jt. Inst. for Computational Science University of Tennessee Knoxville TN 37996 United States Engineering Science Program University of Tennessee Knoxville TN 37996 United States

ISBN: (纸本)0791816613

With the rapid advances in high-performance computing technology in recent years, researchers at the University of Tennessee have initiated a collaborative project to address the theoretical, algorithmic, and numerical implementation issues of computational mechanics simulations. To meet the challenge, a Parallel Interoperable computational Mechanics Simulator System (PICMSS) has been established. Such effort entails the development of a modular finite element platform capable of admitting diverse CFD formulations, as well as closure models for physics phenomena, as required/derived by the users operating on supercomputers and networks of PC clusters. Incompressible Navier Stokes Benchmark solutions via a velocity-vorticity formulation are proposed to examine the integrity and scalability of PICMSS.

关键词： Parallel processing systems

来源：评论

学校读者我要写书评

暂无评论

Simulation of the transient, compressible, gas-dynamic behavior of catalytic-combustion ignition in stagnation flows

Simulation of the transient, compressible, gas-dynamic behav...

引用

27th International Symposium on Combustion

作者： Raja, Laxminarayan L. Kee, Robert J. Petzold, Linda R. Engineering Division Colorado School of Mines Golden CO 80401 United States Computational Science and Engineering Program Department of Mechanical and Environmental Engineering University of California Santa Barbara Santa Barbara CA 93106 United States

This paper develops and uses a computation model to explore the transient ignition dynamics of catalytic combustion in a stagnation-flow configuration. The analysis considers the elementary heterogeneous chemistry associated with catalytic behavior at the surface. It also considers gas-dynamic effects in the boundary layer, including temporal and spatial pressure variations. The gas-dynamic effects are included through the axial momentum equation, which has been neglected in previous analyses of unsteady stagnation flows. In addition to the physical interpretation of ignition transients, the paper presents a mathematical and computational analysis and comparison of the constant-pressure and compressible stagnation-flow equations. The constant-pressure equations, as commonly formulated and used, are a system of differential-algebraic equations (DAE) that have an index greater than two. This high-index behavior is responsible for severe numerical difficulties in regions of fast transients or stringent numerical error control. This paper relaxes the constant-pressure assumption using a compressible-flow formulation, which extends the range of physical validity and reduces the index of the transient stagnation-flow problem while preserving stagnationflow "similarity.".

关键词： Ignition

来源：评论

学校读者我要写书评

暂无评论

An improved prior for image reconstruction in X-ray fiber diffraction

An improved prior for image reconstruction in X-ray fiber di...

引用

IEEE International Conference on Image Processing

作者： S. Baskaran R.P. Millane Computational Science and Engineering Program Purdue University West Lafayette IN USA

The structure completion problem in fiber diffraction is addressed from a Bayesian perspective. The experimental data are sums of the squares of the amplitudes of particular sets of Fourier coefficients of the electron density. In addition, a part of the electron density is known. The image reconstruction problem is to estimate the missing part of the electron density. A Bayesian approach is taken in which the prior model for the image is based on the fact that it consists of atoms, i.e. the unknown electron density consists of separated sharp peaks. The conventional prior assumes that the positions of the unknown atoms are uniformly distributed. We improve this prior by treating the positions of the known atoms as containing normally distributed coordinate errors. Currently used heuristic methods are shown to correspond to certain maximum a posteriori estimates of the Fourier coefficients. An analytical solution for the Bayesian minimum mean-square-error estimate is derived. Simulations show that the minimum mean-square-error estimate gives better results when the new prior is used.

关键词： Image reconstruction X-ray imaging Electrons Bayesian methods X-ray diffraction Maximum a posteriori estimation

来源：评论

学校读者我要写书评

暂无评论

Bayesian estimation in an image restoration problem in X-ray fiber diffraction

Bayesian estimation in an image restoration problem in X-ray...

引用

International Conference on Acoustics, Speech, and Signal Processing (ICASSP)

作者： S. Baskaran R.P. Millane Computational Science and Engineering Program Whistler Center for Carbohydrate Research Purdue University West Lafayette IN USA

The restoration of an incomplete image from a known part and experimental data in the form of the Fourier amplitude squared sums is formulated as a Bayesian estimation problem. This problem is motivated by the structure completion problem in X-ray fiber diffraction analysis. An appropriate prior of uniformly distributed impulses is used. The Bayesian MMSE and MAP estimates are obtained. Simulations are used to compare the performance of the estimates. The results show that the MMSE estimate significantly outperforms the other estimates. The restored images exhibit some bias towards the known part of the image. This can be partly reduced by an unbiasing procedure.

关键词： Bayesian methods Image restoration X-ray imaging X-ray diffraction Lattices Fourier transforms Electrons Crystallization Crystallography X-ray scattering

来源：评论

学校读者我要写书评

暂无评论

建议与咨询留下您的常用邮箱和电话号码，以便我们向您反馈解决方案和替代方法

时间限定

文献类型

馆藏选择

核心期刊

语言

文献类型

帮助

文字说明：

检索规则说明：

检索范例：

分类表

所选分类

限定检索结果

文献类型

馆藏范围

日期分布

学科分类号

主题

机构

作者

语言

请选择保存的检索档案：

请选择收藏分类：

通借通还

建议与咨询 留下您的常用邮箱和电话号码，以便我们向您反馈解决方案和替代方法

时间限定

文献类型

馆藏选择

核心期刊

语言

文献类型

帮助

文字说明：

检索规则说明：

检索范例：

分类表

所选分类

限定检索结果

文献类型

馆藏范围

日期分布

学科分类号

主题

机构

作者

语言

请选择保存的检索档案： 新增检索档案 确定 取消

请选择收藏分类： 新增自定义分类 确定 取消

通借通还

建议与咨询留下您的常用邮箱和电话号码，以便我们向您反馈解决方案和替代方法

请选择保存的检索档案：

请选择收藏分类：