1,547 Matching Results

Search Results

Advanced search parameters have been applied.

Numerical simulations of flow in a three-dimensional cavity-channel geometry

Description: The computational fluid dynamics code FIDAP (Fluid Dynamics International) is used to perform simulations of the steady laminar flow of an incompressible fluid in a three-dimensional rectangular cavity. Although most previous studies have considered a ``lid- driven`` cavity, where a uniform horizontal velocity is imposed on the cavity lid, the flow in the channel above the cavity is explicitly included in the computational domain in these simulations. Simulations are performed for various Reynolds numbers in the range 0 {le} Re {le} 1000 and are compared to corresponding two-dimensional results. The three-dimensional flow are seen to exhibit a topological complexity not present in the two-dimensional results, including a change in topology around Re {approx} 35.
Date: November 1, 1993
Creator: Torczynski, J. R. & O`Hern, T. J.
Partner: UNT Libraries Government Documents Department

Magnetotelluric interpretations in a crustal environment. Final report

Description: Problems that arise in interpreting magnetotelluric measurements in complex geologic environments are approached by considering the low frequency approximation where the upper crust can be considered a thin layer of varying conductivity. The formulation of the thin layer problem, is modified to allow resistive coupling between the upper crust and the mantle. A new parameter arises, called the adjustment distance, which is the distance required for the current to readjust itself between the upper crust and the mantle when changes in crustal conductivity are encountered. This distance is given by the square root of the product of the integrated conductivity and the integrated resistivity across the crust (and some of the upper mantle). Layered media interpretations made on magnetotelluric data from areas that are closer to a conductivity boundary than the adjustment distance can be very misleading. Since the adjustment distance is often greater than 100 km, one has to consider more complex modelling in many situations. In order to simplify the modelling of the large region dictated by the adjustment distance the concept of a multiple scale analysis to the thin layer calculations was applied. A simple interpretation of actual field data from the Snake River Plains is given which illustrates the adjustment distance effect on magnetotelluric fields.
Date: January 1, 1980
Creator: Madden, T.R.
Partner: UNT Libraries Government Documents Department

Nuclear accident dosimetry: calculations and comparison with experimental data

Description: Calculated results, carried out by the method of discrete ordinates, of the absorbed dose received by personnel at various locations relative to the source in a simulated reactor criticality accident are presented and compared with experimental data. The geometry of the room in which the ''accident'' took place is included approximately in the calculations. The calculated and experimental data are in good agreement in all cases.
Date: July 1, 1978
Creator: Santoro, R.T.; Alsmiller, R.G. Jr. & Barnes, J.M.
Partner: UNT Libraries Government Documents Department

The D0 Monte Carlo

Description: The D0 detector at the Fermilab Tevatron began its first data taking run in May 1992. For analysis of the expected 25 pb{sup {minus}1} data sample, roughly half a million simulated events will be needed. The GEANT-based Monte Carlo program used to generate these events is described, together with comparisons to test beam data. Some novel techniques used to speed up execution and simplify geometrical input are described.
Date: October 1, 1992
Creator: Womersley, J.
Partner: UNT Libraries Government Documents Department

Resistive toroidal-field coils for tokamak reactors

Description: This paper analyzes the optimization of the geometry of resistive TF coils of rectangular bore for tokamak fusion test reactors and practical neutron generators. In examining the trade-offs between geometric parameters and magnetic field for reactors giving a specified neutron wall loading, either the resistive power loss or the lifetime coil cost can be minimized. Aspects of cooling, magnetic stress, and construction are addressed for several reference designs. Bending moment distributions in closed form have been derived for rectangular coils on the basis of the theory of rigid frames. Candidate methods of fabrication and of implementing demountable joints are summarized.
Date: November 1, 1980
Creator: Kalnavarns, J. & Jassby, D.L.
Partner: UNT Libraries Government Documents Department

The geometry of continuum regularization

Description: This lecture is primarily an introduction to coordinate-invariant regularization, a recent advance in the continuum regularization program. In this context, the program is seen as fundamentally geometric, with all regularization contained in regularized DeWitt superstructures on field deformations.
Date: March 1, 1987
Creator: Halpern, M.B.
Partner: UNT Libraries Government Documents Department

Lewis number effects on turbulent premixed flame structure

Description: The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H{sub 2}/Air and C{sub 3}H{sub 8}/Air mixtures with similar burning rates were investigated with Lewis numbers of 0.33 and 1.85 respectively. At the conditions studied the laminar H{sub 2}/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant is generated by a perforated plate and the turbulent length scale (3mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the burning velocity is the same for all the cases. Results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame front curvature distribution is not found to be very sensitive to the Lewis number. For the H{sub 2}/Air mixture the distribution is skewed toward the positive values indicating the presence of cusps while for the C{sub 3}H{sub 8}/Air mixture the distribution is more symmetrical. In both cases the average curvature is found to be zero, and if the local burning speed varies linearly with curvature, the local positive and negative burning velocity variations due to curvature will balance.
Date: September 1, 1992
Creator: Goix, P. J. & Shepherd, I. G.
Partner: UNT Libraries Government Documents Department

Density equalizing map projections: A new algorithm

Description: In the study of geographic disease clusters, an alternative to traditional methods based on rates is to analyze case locations on a transformed map in which population density is everywhere equal. Although the analyst's task is thereby simplified, the specification of the density equalizing map projection (DEMP) itself is not simple and continues to be the subject of considerable research. Here a new DEMP algorithm is described, which avoids some of the difficulties of earlier approaches. The new algorithm (a) avoids illegal overlapping of transformed polygons; (b) finds the unique solution that minimizes map distortion; (c) provides constant magnification over each map polygon; (d) defines a continuous transformation over the entire map domain; (e) defines an inverse transformation; (f) can accept optional constraints such as fixed boundaries; and (g) can use commercially supported minimization software. Work is continuing to improve computing efficiency and improve the algorithm.
Date: February 1, 1992
Creator: Merrill, D.W.; Selvin, S. & Mohr, M.S.
Partner: UNT Libraries Government Documents Department

The influence of the rib width on the performance of tubes with the separation and reattachment enhancement mechanism

Description: This paper presents numerically predicted turbulent heat-transfer and friction-factor results for tubes with transverse, rectangular ribs for different width-to-height ratios. The rib spacing was maintained at values where the separated flow over the rib reattached between adjacent ribs (i.e. the separation and reattachment enhancement mechanism). The mean Nusselt number was found to decrease slightly with an increase in the width to height ratio for low Prandtl number fluids (Pr = 0.71). However, the trend is more complex for higher Prandtl number fluids. The mean Nusselt number can either increase or decrease depending on the magnitude of the Prandtl number and rib spacing. The friction factors decreased with an increase in the width to height ratio and the magnitude of this decrease was somewhat Reynolds number dependent.
Date: January 1, 1992
Creator: Arman, B. & Rabas, T.J.
Partner: UNT Libraries Government Documents Department

Longitudinal coupling impedance of step discontinuities in a circular beam tube

Description: The longitudinal coupling impedance presented by a single wall discontinuity to the circulating beam in a circular accelerator or storage ring is usually analyzed by considering a developed periodic structure. However, the typical parameters are often such that it becomes adequate to treat the discontinuity as a nonperiodic problem. Using modal field matching methods, solutions were derived for the cases of a single as well as a double-step discontinuity in a circular beam tube. Numerical results are presented in this paper and the typical behavior at low frequency, at reasonance, and above cut-off is discussed.
Date: January 1, 1979
Creator: Hahn, H. & Zatz, S.
Partner: UNT Libraries Government Documents Department

Plasma-induced conversion of surface-adsorbed hydrocarbons

Description: Experimental results are reported for an electrical device for direct conversion of methane into higher hydrocarbons. A microchannel plate is excited with electrons from a photoemissive source, and electron impact ionization of methane on the inner surfaces of the microchannels creates an ion feedback process. The resulting low-density plasma creates higher hydrocarbons when charged particles impact the surfaces at grazing incidence. The production Of C{sub 2} to C{sub 8}-containing gases was noted, with a selectivity for C{sub 2} of 39% in one case. The proportions of converted products and the conversion rates depend upon the electrical voltage, the microchannel geometry, and the operating pressure. Conversion rates increase with operating pressure.
Date: July 1, 1992
Creator: Sackinger, W. M.
Partner: UNT Libraries Government Documents Department

Selection of regenerator geometry for magnetic refrigerator applications

Description: In our effort to develop magnetic regenerators of high efficiency we have looked at the following geometries: (1) tube channels in solid block; (2) stack of perforated plates normal to the fluid flow direction; (3) stack of solid plates parallel to fluid flow direction, and packed bed of spherical particles; (4) loose packed; and (5) sintered. Reported are computations of the overall efficiency of the regenerator, considering heat transfer, longitudinal conduction, and fluid pressure drop, for all the above arrangements as a function of geometrical variables, such as overall length and particle diameter or plate thickness. The results yield the optimum geometry for a given combination of other controlling parameters, such as frequency, porosity, and fluid properties. The different geometries are compared under the constraint that the mass of magnetic material is the same for all. This condition is peculiar to the magnetic refrigeration process because the net refrigeration and driving forces are proportional to the mass of magnetic material.
Date: January 1, 1984
Creator: Barclay, J.A. & Sarangi, S.
Partner: UNT Libraries Government Documents Department

Symmetry issues in a class of ion beam targets using sufficiently short direct drive pulses

Description: Controlling asymmetries in direct drive ion beam targets depends upon the ability to control the effects of residual target asymmetries after an appropriate illumination scheme has already been utilized. A class of modified ion beam targets where residual asymmetries are ameliorated is considered. The illumination scheme used is an axially symmetric one convenient for reactor designs. Residual asymmetries are controlled by limiting the radial motion of the radius R/sub dep/ of peak ion energy deposition. Limiting the motion of R/sub dep/ is achieved by lengthening the time scale t/sub s/ where changes in R/sub dep/ adversely affect asymmetries. In our example, t/sub s/ becomes longer than the duration ..delta..t/sub D/ of the entire direct drive pulse train (t/sub s/ > ..delta..t/sub D/).
Date: October 23, 1986
Creator: Mark, J.W.K. & Lindl, J.D.
Partner: UNT Libraries Government Documents Department

Additional evaluation of ``LDS Moldable`` for melter construction

Description: Use of ``LDS Moldable`` as a cushion material for future DWPF melters is deemed feasible, because it serves the purpose of reducing the stress in the steel shell to acceptable levels. The assessment is based on the design geometry of the DWPF melter currently undergoing testing.
Date: October 8, 1993
Creator: Yau, W. W. F.
Partner: UNT Libraries Government Documents Department

Thermal-hydraulic and structural considerations on the gap variation between fuel bundle and hexcan

Description: One of the safety concerns in reactor operation is the temperature of the fuel elements, which are contained in a hexagonal duct, referred to as the hexcan. There are gaps between the fuel bundle and the hexcan, and the gaps somewhat affect thermal-hydraulic behavior of the fuel element. This paper investigates the impact of gap variations on the thermal-hydraulic responses of the fuel element as well as the possibility of fuel bundle and hexcan interaction. The gap variation between hex duct and fuel bundle is caused by differential thermal expansion, creep strain, and irradiation-induced swelling of fuel-cladding and hex duct, and it is a function of fuel burnup. The effects of gap variations on the thermal-hydraulic responses of typical driver subassemblies in the Experimental Breeder Reactor (EBR-II) are investigated. A structural analysis was performed to predict the in-reactor deformation behavior of fuel bundle and hex duct, and followed by a thermal-hydraulic analysis to determine the flow and temperature response due to gap changes. The effects of geometry variation on the thermal-hydraulic response are discussed. The study indicates that the effect of gap variation should not be ignored for subassembly designs.
Date: May 1, 1992
Creator: Chang, L. K.; Lee, M. J. & Ku, J. Y.
Partner: UNT Libraries Government Documents Department

Development of self-absorption coefficients for the determination of gamma-emitting radionuclides in environmental and mixed waste samples

Description: As the need for rapid and more accurate determinations of gamma-emitting radionuclides in environmental and mixed waste samples grows, there is continued interest in the development of theoretical tools to eliminate the need for some laboratory analyses and to enhance the quality of information from necessary analyses. In gamma spectrometry the use of theoretical self-absorption coefficients (SACs) can eliminate the need to determine the SAC empirically by counting a known source through each sample. This empirical approach requires extra counting time and introduces another source of counting error, which must be included in the calculation of results. The empirical determination of SACs is routinely used when the nuclides of interest are specified; theoretical determination of the SAC can enhance the information for the analysis of true unknowns, where there may be no prior knowledge about radionuclides present in a sample. Determination of an exact SAC does require knowledge about the total composition of a sample. In support of the Department of Energy`s (DOE) Environmental Survey Program, the Analytical Chemistry Laboratory (ACL) at Argonne National Laboratory developed theoretical self-absorption models to estimate SACs for the determination of non-specified radionuclides in samples of unknown, widely-varying, compositions. Subsequently, another SAC model, in a different counting geometry and for specified nuclides, was developed for another application. These two models are now used routinely for the determination of gamma-emitting radionuclides in a wide variety of environmental and mixed waste samples.
Date: May 1, 1994
Creator: Streets, W. E.
Partner: UNT Libraries Government Documents Department

Simulation of the GEM silicon central tracker using GEANT

Description: The silicon central tracker of the GEM detector has been simulated using the high energy physics simulations code GEANT. This paper will describe the level of detail of the geometry of the tracker that is in the code, including the silicon detectors themselves as well as all non-sensitive volumes such as support structures; the digitization, or detector response to particles, of the silicon detectors; the coordinate reconstruction from the digitizations, and some of the results of the simulations regarding the detector performance.
Date: January 1, 1994
Creator: Brooks, M. L. & Kinnison, W. W.
Partner: UNT Libraries Government Documents Department

Three-dimensional visualization of reactive flows in complex geometries

Description: The visualization of fluid flows has become more challenging, as recent advancements in computational methods have increased the complexity and size of simulations. Our objective is to develop a flexible flow visualization tool for fluid simulations that include the full physics and geometrical complexities found in modeling practical combustion systems, such as internal combustion engines. The challenges to flow visualization come from: (1) the large simulation output, especially when using massively parallel computers; (2) the increasingly complex geometries that include moving surfaces such as pistons and valves; (3) the complex physical phenomena in realistic problems of fuel injection, combustion fronts, boundary flows, and large scale turbulence; and (4) the numerical complexity of indirect addressing of computational elements, variable mesh connectivity, distorted elements, and moving meshes. We have developed a visualization program that addresses these complexities. The program was developed as a post-processor to the KIVA family of codes for reactive flow simulations. Because of the complexity of the KIVA codes, the visualization program is very versatile and applicable to any code with meshes of arbitrary hexahedrons. No comparable commercial visualization package could be found. To visualize the fluid flow, we use mass-less tracer particles that follow the movement of the fluid as determined from output files from the simulation. The movement of the tracer particles is based on a second-order spatial and temporal interpolation of the fluid velocities from successive simulation output files. The visualization program runs on a Silicon Graphics IRIS GTX workstation.
Date: February 1, 1995
Creator: Fairfield, M. S.
Partner: UNT Libraries Government Documents Department

Structural design, analysis, and code evaluation of an odd-shaped pressure vessel

Description: This paper is the result of an effort to design, analyze and evaluate a rectangular pressure vessel. Normally pressure vessels are designed in circular or spherical shapes to prevent stress concentrations. In this case, because of operational limitations, the choice of vessels was limited to a rectangular pressure box with a removable cover plate. The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code is used as a guideline for pressure containments whose width or depth exceeds 15.24 cm (6.0 in.) and where pressures will exceed 103.4 KPa (15.0 lbf/in{sup 2}). This evaluation used Section VIII of this Code, hereafter referred to as the Code. The dimensions and working pressure of the subject vessel fall within the pressure vessel category of the Code. The Code design guidelines and rules do not directly apply to this vessel. Therefore, finite-element methodology was used to analyze the pressure vessel, and the Code then was used in qualifying the vessel to be stamped to the Code. Section VIII, Division 1 of the Code was used for evaluation. This action was justified by selecting a material for which fatigue damage would not be a concern. The stress analysis results were then chocked against the Code, and the thicknesses adjusted to satisfy Code requirements. Although not directly applicable, the Code design formulas for rectangular vessels were also considered and presented in this study.
Date: December 1, 1992
Creator: Rezvani, M. A. & Ziada, H. H.
Partner: UNT Libraries Government Documents Department

Electronic structure and spectroscopy of nucleic acid bases: Ionization energies, ionization-induced structural changes, and photoelectron spectra

Description: We report high-level ab initio calculations and single-photon ionization mass spectrometry study of ionization of adenine (A), thymine (T), cytosine (C) and guanine (G). For thymine and adenine, only the lowest-energy tautomers were considered, whereas for cytosine and guanine we characterized five lowest-energy tautomeric forms. The first adiabatic and several vertical ionization energies were computed using equation-of-motion coupled-cluster method for ionization potentials with single and double substitutions. Equilibrium structures of the cationic ground states were characterized by DFT with the {omega}B97X-D functional. The ionization-induced geometry changes of the bases are consistent with the shapes of the corresponding molecular orbitals. For the lowest-energy tautomers, the magnitude of the structural relaxation decreases in the following series G > C > A > T, the respective relaxation energies being 0.41, 0.32, 0.25 and 0.20 eV. The computed adiabatic ionization energies (8.13, 8.89, 8.51-8.67 and 7.75-7.87 eV for A,T,C and G, respectively) agree well with the onsets of the photoionization efficiency (PIE) curves (8.20 {+-} 0.05, 8.95 {+-} 0.05, 8.60 {+-} 0.05 and 7.75 {+-} 0.05 eV). Vibrational progressions for the S{sub 0}-D{sub 0} vibronic bands computed within double-harmonic approximation with Duschinsky rotations are compared with previously reported experimental photoelectron spectra.
Date: August 2, 2010
Creator: Bravaya, Ksenia B.; Kostko, Oleg; Dolgikh, Stanislav; Landau, Arie; Ahmed, Musahid & Krylov, Anna I.
Partner: UNT Libraries Government Documents Department

Two-photon Photoemission of Organic Semiconductor Molecules on Ag(111)

Description: Angle- and time-resolved two-photon photoemission (2PPE) was used to study systems of organic semiconductors on Ag(111). The 2PPE studies focused on electronic behavior specific to interfaces and ultrathin films. Electron time dynamics and band dispersions were characterized for ultrathin films of a prototypical n-type planar aromatic hydrocarbon, PTCDA, and representatives from a family of p-type oligothiophenes.In PTCDA, electronic behavior was correlated with film morphology and growth modes. Within a fewmonolayers of the interface, image potential states and a LUMO+1 state were detected. The degree to which the LUMO+1 state exhibited a band mass less than a free electron mass depended on the crystallinity of the layer. Similarly, image potential states were measured to have free electron-like effective masses on ordered surfaces, and the effective masses increased with disorder within the thin film. Electron lifetimes were correlated with film growth modes, such that the lifetimes of electrons excited into systems created by layer-by-layer, amorphous film growth increased by orders of magnitude by only a few monolayers from the surface. Conversely, the decay dynamics of electrons in Stranski-Krastanov systems were limited by interaction with the exposed wetting layer, which limited the barrier to decay back into the metal.Oligothiophenes including monothiophene, quaterthiophene, and sexithiophene were deposited on Ag(111), and their electronic energy levels and effective masses were studied as a function of oligothiophene length. The energy gap between HOMO and LUMO decreased with increasing chain length, but effective mass was found to depend on domains from high- or low-temperature growth conditions rather than chain length. In addition, the geometry of the molecule on the surface, e.g., tilted or planar, substantially affected the electronic structure.
Date: May 15, 2008
Creator: Yang, Aram & Yang, Aram
Partner: UNT Libraries Government Documents Department

Where Water is Oxidized to Dioxygen: Structure of the Photosynthetic Mn4Ca Cluster from X-ray Spectroscopy

Description: Light-driven oxidation of water to dioxygen in plants, algae and cyanobacteria iscatalyzed within photosystem II (PS II) by a Mn4Ca cluster. Although the cluster has been studied by many different methods, the structure and the mechanism have remained elusive. X-ray absorption and emission spectroscopy and EXAFS studies have been particularly useful in probing the electronic and geometric structure, and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn4Ca cluster geometry to a setof three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 Angstrom-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-rayspectroscopy and crystallography are predominantly because of damage to the Mn4Ca cluster by X-rays under the conditions used for structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn4Ca catalytic center as it cycles through the five intermediate states known as the Si-states (i=0-4). The electronic structure of the Mn4Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formaloxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms that includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structure of the Mn4Ca cluster in the S-states derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of the O-O bond formation during the photosynthetic water splitting process.
Date: October 24, 2007
Creator: Yano, Junko; Yano, Junko & Yachandra, Vittal K.
Partner: UNT Libraries Government Documents Department

Hydromechanical modeling of pulse tests that measure both fluidpressure and fracture-normal displacement of the Coaraze Laboratory site,France

Description: In situ fracture mechanical deformation and fluid flowinteractions are investigated through a series of hydraulic pulseinjection tests, using specialized borehole equipment that cansimultaneously measure fluid pressure and fracture displacements. Thetests were conducted in two horizontal boreholes spaced one meter apartvertically and intersecting a near-vertical highly permeable faultlocated within a shallow fractured carbonate rock. The field data wereevaluated by conducting a series of coupled hydromechanical numericalanalyses, using both distinct-element and finite-element modelingtechniques and both two- and three-dimensional model representations thatcan incorporate various complexities in fracture network geometry. Oneunique feature of these pulse injection experiments is that the entiretest cycle, both the initial pressure increase and subsequent pressurefall-off, is carefully monitored and used for the evaluation of the insitu hydromechanical behavior. Field test data are evaluated by plottingfracture normal displacement as a function of fluid pressure, measured atthe same borehole. The resulting normal displacement-versus-pressurecurves show a characteristic loop, in which the paths for loading(pressure increase) and unloading (pressure decrease) are different. Bymatching this characteristic loop behavior, the fracture normal stiffnessand an equivalent stiffness (Young's modulus) of the surrounding rockmass can be back-calculated. Evaluation of the field tests by couplednumerical hydromechanical modeling shows that initial fracture hydraulicaperture and normal stiffness vary by a factor of 2 to 3 for the twomonitoring points within the same fracture plane. Moreover, the analysesshow that hydraulic aperture and the normal stiffness of the pulse-testedfracture, the stiffness of surrounding rock matrix, and the propertiesand geometry of the surrounding fracture network significantly affectcoupled hydromechanical responses during the pulse injection test. Morespecifically, the pressure-increase path of the normaldisplacement-versus-pressure curve is highly dependent on thehydromechanical parameters of the tested fracture and the stiffness ofthe matrix near the injection point, whereas the pressure-decrease pathis highly influenced by mechanical processes within a larger portion ofthe surrounding fractured rock.
Date: April 22, 2006
Creator: Cappa, F.; Guglielmi, Y.; Rutqvist, J.; Tsang, C-F. & Thoraval, A.
Partner: UNT Libraries Government Documents Department