3,228 Matching Results

Search Results

Advanced search parameters have been applied.

Simulating relativistic beam and plasma systems using an optimal boosted frame

Description: It was shown recently that it may be computationally advantageous to perform computer simulations in a Lorentz boosted frame for a certain class of systems. However, even if the computer model relies on a covariant set of equations, it was pointed out that algorithmic difficulties related to discretization errors may have to be overcome in order to take full advantage of the potential speedup. In this paper, we summarize the findings, the difficulties and their solutions, and review the applications of the technique that have been performed to date.
Date: May 1, 2009
Creator: Vay, J.-L.; Bruhwiler, D. L.; Geddes, C. G. R.; Fawley, W. M.; Martins, S. F.; Cary, J. R. et al.
Partner: UNT Libraries Government Documents Department

Development of an advanced system identification technique for comparing ADAMS analytical results with modal test data for a MICON 65/13 wind turbine

Description: This work uses the theory developed in NREL/TP--442-7110 to analyze simulated data from an ADAMS (Automated Dynamic Analysis of Mechanical Systems) model of the MICON 65/13 wind turbine. The Observer/Kalman Filter identification approach is expanded to use input-output time histories from ADAMS simulations or structural test data. A step by step outline is offered on how the tools developed in this research, can be used for validation of the ADAMS model.
Date: July 1, 1995
Creator: Bialasiewicz, J.T.
Partner: UNT Libraries Government Documents Department

Using EnergyPlus for California Title-24 compliancecalculations

Description: For the past decade, the non-residential portion of California's Title-24 building energy standard has relied on DOE-2.1E as the reference computer simulation program for development as well as compliance. However, starting in 2004, the California Energy Commission has been evaluating the possible use of Energy Plus as the reference program in future revisions of Title-24. As part of this evaluation, the authors converted the Alternate Compliance Method (ACM) certification test suite of 150 DOE-2 files to Energy Plus, and made parallel DOE-2 and Energy Plus runs for this extensive set of test cases. A customized version of DOE-2.1E named doe2ep was developed to automate the conversion process. This paper describes this conversion process, including the difficulties in establishing an apples-to-apples comparison between the two programs, and summarizes how the DOE-2 and Energy Plus results compare for the ACM test cases.
Date: August 26, 2006
Creator: Huang, Joe; Bourassa, Norman; Buhl, Fred; Erdem, Ender & Hitchcock, Rob
Partner: UNT Libraries Government Documents Department

Manifold methods for methane combustion

Description: Objective is to develop a new method for studying realistic chemistry in turbulent methane combustion with NO{sub x} mechanism. The realistic chemistry is a simplification to a more detailed chemistry based on the manifold method; accuracy is determined by interaction between the transport process and the chemical reaction. In this new (tree) method, probability density function or partially stirred reactor calculations are performed. Compared with the reduced mechanism, manifold, and tabulation methods, the new method overcomes drawbacks of the reduced mechanism method and preserves the advantages of the manifold method. Accuracy is achieved by specifying the size of the cell.
Date: December 31, 1995
Creator: Yang, B. & Pope, S.B.
Partner: UNT Libraries Government Documents Department

Nonlinear simulation of a tetrode vacuum tube

Description: We have developed a variety of circuit models that may be used to simulate vacuum triodes and tetrodes in the extremes of nonlinear operation - from cutoff to saturation. These models, which run on Spectrum Software`s Microcap-IV{reg_sign} (MC4) electronic circuit analysis program, have been used to analyze radiofrequency (rf) amplifiers and high- voltage pulse modulators. In this preliminary report, we provide a single example of a high-power tetrode model and its use in a simple radio-frequency amplifier circuit. Within the next few months, we intend to produce a more exhaustive report that will provide a detailed explanation of the purpose of specific model elements, simulation of a variety of vacuum triodes and tetrodes, and additional circuit applications for these models.
Date: August 1, 1997
Creator: Doss, J.D.
Partner: UNT Libraries Government Documents Department

Meteorological Simulations of Ozone Episode Case Days during the 1996 Paso del Norte Ozone Study

Description: Meteorological simulations centered around the border cities of El Paso and Ciudad Juarez have been performed during an ozone episode that occurred on Aug. 13,1996 during the 1996 Paso del Norte Ozone Study field campaign. Simulations were petiormed using the HOTMAC mesoscale meteorological model using a 1,2,4, and 8 km horizontal grid size nested mesh system. Investigation of the vertical structure and evolution of the atmospheric boundary layer for the Aug. 11-13 time period is emphasized in this paper. Comparison of model-produced wind speed profiles to rawirisonde and radar profiler measurements shows reasonable agreement. A persistent upper-level jet was captured in the model simulations through data assimilation. In the evening hours, the model was not able to produce the strong wind direction shear seen in the radar wind profiles. Based on virtual potential temperature profile comparisons, the model appears to correctly simulate the daytime growth of the convective mixed layer. However, the model underestimates the cooling of the surface layer at night. We found that the upper-level jet significantly impacted the turbulence structure of the boundary layer, leading to relatively high turbulent kinetic energy (tke) values aloft at night. The model indicates that these high tke values aloft enhance the mid-morning growth of the boundary layer. No upper-level turbulence measurements were available to verify this finding, however. Radar profiler-derived mixing heights do indicate relatively rapid morning growth of the mixed layer.
Date: February 1, 1999
Creator: Brown, M.J.; Costigan, K.; Muller, C. & Wang, G.
Partner: UNT Libraries Government Documents Department

Magnetic domain structure and magnetization reversal in submicron-scale Co dots

Description: We present a magnetic force microscopy (MFM) analysis of arrays of submicron-scale Co dots fabricated by interference lithography. The dots are thin (180-300 A) and are elliptical in shape. MFM of these structures reveals that they relax into highly ordered remanent states whose symmetry and configuration are governed by their shape anisotropy. In particular, when the dots are saturated along the easy-axis, a uniformly magnetized state persists at remanence. However, when the dots are saturated in hard-axis, they relax into a single-vortex state in which the circulation can have either sign. Both remanent states are characterized by smoothly varying magnetization patterns and a high degree of uniformity across the array. We attribute the ordered behavior of these structures to the film microstructure, which allows the shape anisotropy to dominate over magnetocrystalline anisotropy. By imaging a series of minor-loop remanent states, we show that magnetization reversal in these structures occurs via the nucleation and annihilation of a single vortex. Magnetic hysteresis loop measurements are consistent with these observations and provide additional details. Furthermore, we present the results of micromagnetic simulations, which are in excellent agreement with both the MFM images and the hysteresis loop measurements.
Date: February 17, 1998
Creator: Fernandez, A.
Partner: UNT Libraries Government Documents Department

Molecular dynamics modeling of ultrathin amorphous carbon films

Description: Amorphous carbon films about 20 mn thick are used by the computer industry as protective coatings on magnetic disks. The structure and function of this family of materials at the atomic level is poorly understood. The growth and properties of a:C and a:CH films 1 to 5 nm thick has been simulated using classical molecular dynamics and a bond-order potential with torsional terms. Studies of quenched melts that verify the ability of this potential to reproduce known features of extended structures of carbon in two and three dimensions are briefly described. In molecular dynamics calculations the incident species were neutral atoms C, or C and H with energies up to 100 eV. The stoichiometry, chemical bonding and distribution functions within the films were analyzed using IBM`s Power Visualization System for different incident gas energies. Microscopic features such as multiple ring structures, including planar graphitic structures, were easily identified. Some preliminary studies of the nanotribology of the a:C films are described, including nano-indentation and sliding in contact with a rigid probe.
Date: May 1, 1995
Creator: Glosli, J.N.; Belak, J. & Philpott, M.R.
Partner: UNT Libraries Government Documents Department

Longitudinal impedance measurement of an RK-TBA induction accelerating gap

Description: Induction accelerating gap designs are being studied for Relativistic Klystron Two-Beam Accelerator (RK-TBA) applications. The accelerating gap has to satisfy the following major requirements: hold-off of the applied accelerating voltage pulse, low transverse impedance to limit beam breakup, low longitudinal impedance at the beam-modulation frequency to minimize power loss. Various gap geometries, materials and novel insulating techniques were explored to optimize the gap design. We report on the experimental effort to evaluate the rf properties of the accelerating gaps in a simple pillbox cavity structure. The experimental cavity setup was designed using the AMOS, MAFIA and URMEL numerical codes. Longitudinal impedance measurements above beam-tube cut-off frequency using a single-wire measuring system are presented.
Date: May 1, 1997
Creator: Eylon, S.; Henestroza, E.; Kim, J.-S.; Houck, T.L.; Westenskow, G.A. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Alignment tolerance of accelerating structures and corrections for future linear colliders

Description: The alignment tolerance of accelerating structures is estimated by tracking simulations. Both single-bunch and multi-bunch effects are taken into account. Correction schemes for controlling the single and multi-bunch emittance growth in the case of large misalignment are also tested by simulations.
Date: June 1, 1995
Creator: Kubo, K.; Adolphsen, C.; Bane, K.L.F.; Raubenheimer, T.O. & Thompson, K.A.
Partner: UNT Libraries Government Documents Department

Survey and alignment report on the primary control network for the APS

Description: During November 1992 the survey and alignment team measured the entire primary control network for the APS. This task had to be finished before the enclosure of the EAA and the RF buildings were put in place, inhibiting several lines of sight necessary for the determination of the monument locations.
Date: February 1, 1993
Creator: Friedsam, H.; Penicka, M. & Zhao, S.
Partner: UNT Libraries Government Documents Department

Phase transitions: An overview with a view

Description: The dynamics of phase transitions plays a crucial role in the so- called interface between high energy particle physics and cosmology. Many of the interesting results generated during the last fifteen years or so rely on simplified assumptions concerning the complex mechanisms typical of nonequilibrium field theories. After reviewing well-known results concerning the dynamics of first and second order phase transitions, I argue that much is yet to be understood, in particular in situations where homogeneous nucleation theory does not apply. I present a method to deal with departures from homogeneous nucleation, and compare its efficacy with numerical simulations. Finally, I discuss the interesting problem of matching numerical simulations of stochastic field theories with continuum models.
Date: October 1, 1997
Creator: Gleiser, M.
Partner: UNT Libraries Government Documents Department

List mode reconstruction for PET with motion compensation: A simulation study

Description: Motion artifacts can be a significant factor that limits the image quality in high-resolution PET. Surveillance systems have been developed to track the movements of the subject during a scan. Development of reconstruction algorithms that are able to compensate for the subject motion will increase the potential of PET. In this paper we present a list mode likelihood reconstruction algorithm with the ability of motion compensation. The subject motion is explicitly modeled in the likelihood function. The detections of each detector pair are modeled as a Poisson process with time-varying rate function. The proposed method has several advantages over the existing methods. It uses all detected events and does not introduce any interpolation error. Computer simulations show that the proposed method can compensate simulated subject movements and that the reconstructed images have no visible motion artifacts.
Date: July 1, 2002
Creator: Qi, Jinyi & Huesman, Ronald H.
Partner: UNT Libraries Government Documents Department

List mode reconstruction for PET with motion compensation: A simulation study

Description: Motion artifacts can be a significant factor that limits the image quality in high-resolution PET. Surveillance systems have been developed to track the movements of the subject during a scan. Development of reconstruction algorithms that are able to compensate for the subject motion will increase the potential of PET. In this paper we present a list mode likelihood reconstruction algorithm with the ability of motion compensation. The subject moti is explicitly modeled in the likelihood function. The detections of each detector pair are modeled as a Poisson process with time vary ingrate function. The proposed method has several advantages over the existing methods. It uses all detected events and does not introduce any interpolation error. Computer simulations show that the proposed method can compensate simulated subject movements and that the reconstructed images have no visible motion artifacts.
Date: July 3, 2002
Creator: Qi, Jinyi & Huesman, Ronald H.
Partner: UNT Libraries Government Documents Department

Advanced Techniques for Reservoir Simulation and Modeling of Non-Conventional Wells

Description: Research results for the second year of this project on the development of improved modeling techniques for non-conventional (e.g., horizontal, deviated or multilateral) wells were presented. The overall program entails the development of enhanced well modeling and general simulation capabilities. A general formulation for black-oil and compositional reservoir simulation was presented.
Date: August 23, 2001
Creator: Durlofsky, Louis J. & Aziz, Khalid
Partner: UNT Libraries Government Documents Department

Optimal dynamic performance for high-precision actuators/stages.

Description: System dynamic performance of actuator/stage groups, such as those found in optical instrument positioning systems and other high-precision applications, is dependent upon both individual component behavior and the system configuration. Experimental modal analysis techniques were implemented to determine the six degree of freedom stiffnesses and damping for individual actuator components. These experimental data were then used in a multibody dynamic computer model to investigate the effect of stage group configuration. Running the computer model through the possible stage configurations and observing the predicted vibratory response determined the optimal stage group configuration. Configuration optimization can be performed for any group of stages, provided there is stiffness and damping data available for the constituent pieces.
Date: July 3, 2002
Creator: Preissner, C.; Lee, S.-H.; Royston, T. J. & Shu, D.
Partner: UNT Libraries Government Documents Department

Numerical simulations of magnetic reversal in layered spring magnets.

Description: This report summarizes the results of numerical investigations of magnetic reversal in layered spring magnets. A one-dimensional model is used of a film consisting of several atomic layers of soft material on top of several atomic layers of hard material. Each atomic layer is taken to be uniformly magnetized, and spatial inhomogeneities within an atomic layer are neglected. The state of such a system is described by a chain of magnetic spin vectors. Each spin vector behaves like a spinning top driven locally by the effective magnetic field and subject to damping (Landau-Lifshitz-Gilbert equation). A numerical integration scheme for the LLG equation is presented that is unconditionally stable and preserves the magnitude of the magnetization vector at all times. The results of numerical investigations for a bilayer in a rotating in-plane magnetic field show hysteresis with a basic period of 2{pi} at moderate fields and hysteresis with a basic period of {pi} (or any multiple thereof) at strong fields.
Date: January 24, 2001
Creator: Jiang, J.S.; Kaper, H.G. & Leaf, G.K.
Partner: UNT Libraries Government Documents Department