Publications

Results 78401–78500 of 99,299

Search results

Jump to search filters

ITS Version 6 : the integrated TIGER series of coupled electron/photon Monte Carlo transport codes

Franke, Brian C.; Kensek, Ronald P.; Laub, Thomas W.

ITS is a powerful and user-friendly software package permitting state-of-the-art Monte Carlo solution of lineartime-independent coupled electron/photon radiation transport problems, with or without the presence of macroscopic electric and magnetic fields of arbitrary spatial dependence. Our goal has been to simultaneously maximize operational simplicity and physical accuracy. Through a set of preprocessor directives, the user selects one of the many ITS codes. The ease with which the makefile system is applied combines with an input scheme based on order-independent descriptive keywords that makes maximum use of defaults and internal error checking to provide experimentalists and theorists alike with a method for the routine but rigorous solution of sophisticated radiation transport problems. Physical rigor is provided by employing accurate cross sections, sampling distributions, and physical models for describing the production and transport of the electron/photon cascade from 1.0 GeV down to 1.0 keV. The availability of source code permits the more sophisticated user to tailor the codes to specific applications and to extend the capabilities of the codes to more complex applications. Version 6, the latest version of ITS, contains (1) improvements to the ITS 5.0 codes, and (2) conversion to Fortran 90. The general user friendliness of the software has been enhanced through memory allocation to reduce the need for users to modify and recompile the code.

More Details
TYPE SAND Report YEAR 2008
DOIOSTI

Control of photonic package alignment with asynchronous laser spot welds

IEEE Transactions on Electronics Packaging Manufacturing

Fuerschbach, Phillip W.

The precise alignment of an optical fiber to a laser diode for maximum optical coupling is often accomplished with synchronous laser spot welds in three symmetric locations. To improve precision and reduce operational complexity, the utility of single-beam spot welds made in an asynchronous manner has been investigated. Independent measurements of fiber tip post weld shift have been made using eddy current sensors and CCD camera imaging analysis. For the cylindrical radially aligned Kovar ferrules examined, post weld shift has been found to be independent of both the location and number of prior spot welds. Post weld shift direction has been shown to be relatively consistent and predictable when the fiber containing ferrule is properly restrained. It has been demonstrated that through the application of an axial restraining force on radially aligned ferrules, post weld shift can be reduced to less than 2 μm. Analytical equations have been presented that predict the magnitude of the measured post weld shift and also serve to guide engineers in optimal design geometries and preferred welding conditions. © 2008 IEEE.

More Details
TYPE Journal Article YEAR 2008
OSTIScopus

Gradient effects on two-color soot optical pyrometry in a heavy-duty DI diesel engine

Combustion and Flame

Musculus, Mark P.B.; Singh, Satbir; Reitz, Rolf D.

Two-color soot optical pyrometry is a widely used technique for measuring soot temperature and volume fraction in many practical combustion devices, but line-of-sight soot temperature and volume fraction gradients can introduce significant uncertainties in the measurements. For diesel engines, these uncertainties usually can only be estimated based on assumptions about the soot property gradients along the line of sight, because full three-dimensional transient diesel soot distribution data are not available. Such information is available, however, from multidimensional computer model simulations, which are phenomenologically based, and have been validated against available in-cylinder soot measurements and diesel engine exhaust soot emissions. Using the model-predicted in-cylinder soot distributions, uncertainties in diesel two-color pyrometry data are assessed, both for a conventional high-sooting, high-temperature combustion (HTC) operating condition, and for a low-sooting, low-temperature combustion (LTC) condition. The simulation results confirm that the two-color soot measurements are strongly biased toward the properties of the hot soot. For the HTC condition, line-of-sight gradients in soot temperature span 600 K, causing relatively large errors. The two-color temperature is 200 K higher than the soot-mass-averaged value, while the two-color volume fraction is 50% lower. For the LTC condition, the two-color measurement errors are half as large as for the HTC condition, because the model-predicted soot temperature gradients along the line of sight are half as large. By contrast, soot temperature and volume fraction gradients across the field of view introduce much smaller errors of less than 50 K in temperature and 20% in volume fraction.

More Details
TYPE Journal Article YEAR 2008
ScopusOSTI

Force flux and the peridynamic stress tensor

Journal of the Mechanics and Physics of Solids

Lehoucq, Rich; Silling, Stewart

The peridynamic model is a framework for continuum mechanics based on the idea that pairs of particles exert forces on each other across a finite distance. The equation of motion in the peridynamic model is an integro-differential equation. In this paper, a notion of a peridynamic stress tensor derived from nonlocal interactions is defined. At any point in the body, this stress tensor is obtained from the forces within peridynamic bonds that geometrically go through the point. The peridynamic equation of motion can be expressed in terms of this stress tensor, and the result is formally identical to the Cauchy equation of motion in the classical model, even though the classical model is a local theory. We also establish that this stress tensor field is unique in a certain function space compatible with finite element approximations. © 2007 Elsevier Ltd. All rights reserved.

More Details
TYPE Journal Article YEAR 2008
OSTIScopus

Fabrication techniques for creating a thermally isolated TM-FPA (thermal microphotonic focal plane array)

Proceedings of SPIE - The International Society for Optical Engineering

Watts, Michael W.; Watts, Michael W.; Nielson, Gregory N.

A novel fabrication strategy has produced optical microring-resonator-based thermal detectors. The detectors are based on the thermo-optic effect and are thermally isolated from a silicon wafer substrate so as to maximize the temperature excursion for a given amount of incident radiation and minimize the impact of thermal phonon noise. The combination of high-Q, thermal isolation, and lack of Johnson noise offers thermal microphotonic detectors the potential to achieve significantly greater room temperature sensitivity than standard bolometric techniques. Several batch fabrication strategies were investigated for producing thermal microphotonic detectors using waveguide materials such as LPCVD Silicon Nitride (Si3N4) on Oxide and Silicon on Insulator (SOI). Fabrication challenges and loss reduction strategies will be presented along with some initial infrared detection results.

More Details
TYPE Conference YEAR 2008
ScopusOSTI

Challenges of designing and processing extreme low-G Micro Electrical-Mechanical System (MEMS) accelerometers

Proceedings of SPIE - The International Society for Optical Engineering

Swiler, Thomas P.; Krishnamoorthy, Uma; Baker, Michael S.; Clews, Peggy J.; Tanner, Danelle M.

There is an increasing demand to build highly sensitive, low-G, microscale acceleration sensors with the ability to sense accelerations in the nano-G (10-8 m/s2) regime. To achieve such sensitivities, these sensors require compliant mechanical springs attached to large masses. The high sensitivities and the difficulty in integrating robust mechanical stops into these designs make these parts inherently weak, lacking the robustness to survive even the low level accelerations encountered in standard handling, from release processing, where supporting interlayers present during fabrication are etched away, through packaging. Thus, the process of transforming a MEMS-based acceleration sensor from an unreleased state to a protected functional state poses significant challenges. We summarize prior experiences with packaging such devices and report on recent work in packaging and protecting a highly sensitive acceleration sensor that optically senses displacement through the use of sub-wavelength nanogratings. We find that successful implementation of such sensors requires starting with a clean and robust MEMS design, performing careful and controlled release processing, and designing and executing a robust handling and packaging solution that keeps a fragile MEMS device protected at all times.

More Details
TYPE Conference YEAR 2008
OSTIScopus

A methodology for quadrilateral finite element mesh coarsening

Engineering with Computers

Staten, Matthew L.

High fidelity finite element modeling of continuum mechanics problems often requires using all quadrilateral or all hexahedral meshes. The efficiency of such models is often dependent upon the ability to adapt a mesh to the physics of the phenomena. Adapting a mesh requires the ability to both refine and/or coarsen the mesh. The algorithms available to refine and coarsen triangular and tetrahedral meshes are very robust and efficient. However, the ability to locally and conformally refine or coarsen all quadrilateral and all hexahedral meshes presents many difficulties. Some research has been done on localized conformal refinement of quadrilateral and hexahedral meshes. However, little work has been done on localized conformal coarsening of quadrilateral and hexahedral meshes. A general method which provides both localized conformal coarsening and refinement for quadrilateral meshes is presented in this paper. This method is based on restructuring the mesh with simplex manipulations to the dual of the mesh. Finally, this method appears to be extensible to hexahedral meshes in three dimensions.

More Details
TYPE Journal Article YEAR 2008
OSTIDOI

A MEMS light modulator based on diffractive nanohole gratings

Optics Express

Skinner, Jack L.; Talin, Albert A.; Horsley, David A.

We present the design, fabrication, and testing of a microelectromechanical systems (MEMS) light modulator based on pixels patterned with periodic nanohole arrays. Flexure-suspended silicon pixels are patterned with a two dimensional array of 150 nm diameter nanoholes using nanoimprint lithography. A top glass plate assembled above the pixel array is used to provide a counter electrode for electrostatic actuation. The nanohole pattern is designed so that normally-incident light is coupled into an in-plane grating resonance, resulting in an optical stop-band at a desired wavelength. When the pixel is switched into contact with the top plate, the pixel becomes highly reflective. A 3:1 contrast ratio at the resonant wavelength is demonstrated for gratings patterned on bulk Si substrates. The switching time is 0.08 ms and the switching voltage is less than 15V. © 2008 Optical Society of America.

More Details
TYPE Journal Article YEAR 2008
ScopusOSTIDOI

Thought leadership: A new indicator for national and institutional comparison

Scientometrics

Boyack, Kevin W.

This article introduces a new method for evaluating national publication activities. This new indicator, thought leadership, captures whether the nation is a thought leader (building on the more recently cited literature for that field) or follower (building on the older cited literature for that field). Publication data for 2003 are used to illustrate which nations tend to build on the more recent discoveries in chemistry and clinical medicine. Finally, implications for national and laboratory policy are discussed.

More Details
TYPE Journal Article YEAR 2008
OSTIDOI

Components for integral evaluation in quantum chemistry

Journal of Computational Chemistry

Kenny, Joseph; Janssen, Curtis L.; Valeev, Edward F.; Windus, Theresa L.

Sharing low-level functionality between software packages enables more rapid development of new capabilities and reduces the duplication of work among development groups. Using the component approach advocated by the Common Component Architecture Forum, we have designed a flexible interface for sharing integrals between quantum chemistry codes. Implementation of these interfaces has been undertaken within the Massively Parallel Quantum Chemistry package, exposing both the IntV3 and Cints/Libint integrals packages to component applications. Benchmark timings for Hartree-Fock calculations demonstrate that the overhead due to the added interface code varies significantly, from less than 1% for small molecules with large basis sets to nearly 10% for larger molecules with smaller basis sets. Correlated calculations and density functional approaches encounter less severe performance overheads of less than 5%. While these overheads are acceptable, additional performance losses occur when arbitrary implementation details, such as integral ordering within buffers, must be handled. Integral reordering is observed to add an additional overhead as large as 12%; hence, a common standard for such implementation details is desired for optimal performance. © 2007 Wiley Periodicals, Inc.

More Details
TYPE Journal Article YEAR 2008
ScopusOSTIDOI

Verification and validation benchmarks

Nuclear Engineering and Design

Oberkampf, William L.; Trucano, Timothy G.

Verification and validation (V&V) are the primary means to assess the accuracy and reliability of computational simulations. V&V methods and procedures have fundamentally improved the credibility of simulations in several high-consequence fields, such as nuclear reactor safety, underground nuclear waste storage, and nuclear weapon safety. Although the terminology is not uniform across engineering disciplines, code verification deals with assessing the reliability of the software coding, and solution verification deals with assessing the numerical accuracy of the solution to a computational model. Validation addresses the physics modeling accuracy of a computational simulation by comparing the computational results with experimental data. Code verification benchmarks and validation benchmarks have been constructed for a number of years in every field of computational simulation. However, no comprehensive guidelines have been proposed for the construction and use of V&V benchmarks. For example, the field of nuclear reactor safety has not focused on code verification benchmarks, but it has placed great emphasis on developing validation benchmarks. Many of these validation benchmarks are closely related to the operations of actual reactors at near-safety-critical conditions, as opposed to being more fundamental-physics benchmarks. This paper presents recommendations for the effective design and use of code verification benchmarks based on manufactured solutions, classical analytical solutions, and highly accurate numerical solutions. In addition, this paper presents recommendations for the design and use of validation benchmarks, highlighting the careful design of building-block experiments, the estimation of experimental measurement uncertainty for both inputs and outputs to the code, validation metrics, and the role of model calibration in validation. It is argued that the understanding of predictive capability of a computational model is built on the level of achievement in V&V activities, how closely related the V&V benchmarks are to the actual application of interest, and the quantification of uncertainties related to the application of interest. © 2007 Elsevier B.V. All rights reserved.

More Details
TYPE Conference YEAR 2008
OSTIScopus

Towards Coulomb drag in vertically coupled quantum wires with independent contacts

Physica E: Low-Dimensional Systems and Nanostructures

Laroche, D.; Bielejec, E.S.; Reno, John L.; Gervais, G.; Lilly, M.P.

We report the details of design and fabrication of independently contacted, vertically coupled quantum wires using the epoxy-bond-and-stop-etch (EBASE) technique. These nanostructures are fabricated in high quality GaAs/AlGaAs parallel double quantum well heterostructures and are intended for Coulomb drag measurements of quantum wires. They will allow us to explore Coulomb drag in one-dimensional structures in a regime of small interlayer separation where the drag signal is expected to be stronger and less affected by phonon drag. © 2007 Elsevier B.V. All rights reserved.

More Details
TYPE Conference YEAR 2008
ScopusOSTI

Graph analysis with high-performance computing

Computing in Science and Engineering

Hendrickson, Bruce A.; Berry, Jonathan W.

Large, complex graphs arise in many settings including the Internet, social networks, and communication networks. To study such data sets, the authors explored the use of highperformance computing (HPC) for graph algorithms. They found that the challenges in these applications are quite different from those arising in traditional HPC applications and that massively multithreaded machines are well suited for graph problems. © 2008 IEEE.

More Details
TYPE Journal Article YEAR 2008
ScopusOSTI
Results 78401–78500 of 99,299
Results 78401–78500 of 99,299