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Articles 1 - 30 of 37
Full-Text Articles in Engineering
Modeling And Simulation Of High-Field, High-Temperature Sic Devices, Sanjay Pathak
Modeling And Simulation Of High-Field, High-Temperature Sic Devices, Sanjay Pathak
Electrical & Computer Engineering Theses & Dissertations
With superior properties such as a large band gap, high thermal conductivity, and large electron drift velocity, SiC is expected to give a new dimension to high power, high temperature electronic devices used in power applications, microwave circuits, and for the space and automobile industries. SiC possesses an inherent advantage over its other large bandgap competitors in terms of inherited processing and device technology from Si. However, there is inadequate understanding of SiC device and its parameters. Simple extrapolations from Si are known to be inadequate. The aim of this dissertation, therefore. is to produce better understanding of SiC devices …
Skin-Stiffener Separation In Buckled Composite Plates And Shallow Shells, Mark Edward Robeson
Skin-Stiffener Separation In Buckled Composite Plates And Shallow Shells, Mark Edward Robeson
Mechanical & Aerospace Engineering Theses & Dissertations
The development of an approximate analysis for predicting stresses in the skin-stiffener interface region of composite plates and shallow shells is presented. The analysis determines interlaminar normal stress and transverse shear stresses in the direction of the stiffener axial coordinate and the direction of the inplane coordinate perpendicular to the stiffener axis. The analysis accounts for skin-stiffener interfaces with nonzero thickness and for curvature in the skin and stiffener. The interlaminar normal stress and transverse shear stresses at the interface are applied to the lower face of the skin and the upper face of the stiffener flange as unknown functions, …
Architectural Optimization Of Digital Libraries, Aileen O. Biser
Architectural Optimization Of Digital Libraries, Aileen O. Biser
Computer Science Theses & Dissertations
This work investigates performance and scaling issues relevant to large scale distributed digital libraries. Presently, performance and scaling studies focus on specific implementations of production or prototype digital libraries. Although useful information is gained to aid these designers and other researchers with insights to performance and scaling issues, the broader issues relevant to very large scale distributed libraries are not addressed. Specifically, no current studies look at the extreme or worst case possibilities in digital library implementations. A survey of digital library research issues is presented. Scaling and performance issues are mentioned frequently in the digital library literature but are …
The Solution Of Hypersingular Integral Equations With Applications In Acoustics And Fracture Mechanics, Richard S. St. John
The Solution Of Hypersingular Integral Equations With Applications In Acoustics And Fracture Mechanics, Richard S. St. John
Mathematics & Statistics Theses & Dissertations
The numerical solution of two classes of hypersingular integral equations is addressed. Both classes are integral equations of the first kind, and are hypersingular due to a kernel containing a Hadamard singularity. The convergence of a Galerkin method and a collocation method is discussed and computationally efficient algorithms are developed for each class of hypersingular integral equation.
Interest in these classes of hypersingular integral equations is due to their occurrence in many physical applications. In particular, investigations into the scattering of acoustic waves by moving objects and the study of dynamic Griffith crack problems has necessitated a computationally efficient technique …
Microhollow Cathode Discharge Excimer Lamps, Ahmed El-Habachi
Microhollow Cathode Discharge Excimer Lamps, Ahmed El-Habachi
Electrical & Computer Engineering Theses & Dissertations
Microhollow cathode discharges are non-thermal discharges of such small sizes that thermalization of the electrons is prevented. By reducing the diameter of the cathode opening in these discharge geometries to values on the order of 100 $\mu$m we were able to operate discharges in argon and xenon in a direct current mode up to atmospheric pressure. The large concentration of high electrons, in combination with high pressures favors three body processes such as excimer formation. This was confirmed by experiments in xenon and argon where emission of excimer radiation at 172 nm and 130 nm, respectively, was observed when the …
Nonlinear Stability And Control Of Three-Dimensional Boundary Layers, Erik Janke
Nonlinear Stability And Control Of Three-Dimensional Boundary Layers, Erik Janke
Mechanical & Aerospace Engineering Theses & Dissertations
The linear and nonlinear evolution of steady and traveling disturbances in three-dimensional incompressible boundary layer flows is studied using Parabolized Stability Equations (PSE). Extensive primary stability analyses for the model problems of Swept Hiemenz flow and the DLR Transition experiment on a swept flat plate are performed first. Second, and building upon these results, detailed secondary instability studies based on both the classical Floquet Theory and a novel approach that uses the nonlinear PSE are conducted. The investigations reveal a connection of unstable secondary eigenvalues to both the linear eigenvalue spectrum of the undisturbed mean flow and the continuous spectrum, …
Consortia As Technology Innovation Management Vehicles: Toward A Framework For Success In Venture Based Public-Private Partnerships, Ralph B. Saunders Ii
Consortia As Technology Innovation Management Vehicles: Toward A Framework For Success In Venture Based Public-Private Partnerships, Ralph B. Saunders Ii
Engineering Management & Systems Engineering Theses & Dissertations
The purpose of this research was to explore the approach by federal/state agencies, university, and private sector consortia to develop and manage commercialization of innovation technologies. The evaluation, support, and management of technologically based consortia has traditionally been held in the private sector. There is a somewhat mature literature guiding innovation management (Utterback 1996; Rosenberg et al. 1994; Quinn 1997, 1992) in the private sector. However, there is an increasing emergence of consortia consisting of universities, industrial/private sector entities, and government agencies joining in collaborative efforts to launch technology based initiatives. These consortia are non-traditional and the applicability of traditional …
Effect Of Pulsed Electric Fields On Aquatic Nuisance Species, Amr Abou-Ghazala
Effect Of Pulsed Electric Fields On Aquatic Nuisance Species, Amr Abou-Ghazala
Electrical & Computer Engineering Theses & Dissertations
Clinical and theoretical evidence indicate that electric fields have biological effects ranging from recoverable disturbance to mortality induction depending on the field parameters and time of exposure. In this thesis, the effect of electrical field in pulsed form on aquatic nuisance species is investigated. Pulse parameters in terms of amplitude, width, shape, and repetition rate that stimulate the species to desirable levels are to be defined. Applying the electrical pulses with appropriate parameters to stun aquatic nuisance species entering marine cooling systems will prevent it from attaching to pipe walls. The use of electrical fields to seek efficient, environmentally friendly, …
Meta-Ethnographic Development Of A Dialogue Methodology Applied To Organization Discourse, Shelley P. Gallup
Meta-Ethnographic Development Of A Dialogue Methodology Applied To Organization Discourse, Shelley P. Gallup
Engineering Management & Systems Engineering Theses & Dissertations
A gap exists between theoretical stances that acknowledge the importance of dialogue as a dynamic within socially constructed structures, and "steersmanship" of those constructs--e.g., directing, intervening or transforming organizations. A "mechanism" which links theory with practice is missing, leaving practitioners with an acknowledgment of dialogue's central position, but without tools to enact this centrality in practice or research. This research constructs a conceptual model of dialogue, derived from the literature. Using this model as a base, the research seeks to generate a dialogue methodology bridging theory and practice with respect to organizational dialogue. The model, methodology, and research results are …
The Cluster Multipole Algorithm For Far-Field Computations, Rakesh R. Patel
The Cluster Multipole Algorithm For Far-Field Computations, Rakesh R. Patel
Electrical & Computer Engineering Theses & Dissertations
Computer simulations of N-body systems are beneficial to study the overall behavior of a number of physical systems in fields such as astrophysics, molecular dynamics, and computational fluid dynamics. A new approach for computer simulations of N-body systems is proposed in this research. The new algorithm is called the Cluster Multipole Algorithm (CMA). The goals of the new algorithm are to improve the applicability to non-point sources and to provide more control on the accuracy over current algorithms. The algorithm is targeted to applications that do not require rebuilding the data structure about the system every time step due to …
Vortex Wake And Exhaust Plume Interaction, Including Ground Effect, Ihab Gaber Adam
Vortex Wake And Exhaust Plume Interaction, Including Ground Effect, Ihab Gaber Adam
Mechanical & Aerospace Engineering Theses & Dissertations
Computational modeling and studies of the near-field wake-vortex turbulent flows, far-field turbulent wake-vortex/exhaust-plume interaction for subsonic and High Speed Civil Transport (HSCT) airplane, and wake-vortex/exhaust-plume interaction with the ground are carried out. The three-dimensional, compressible Reynolds-Averaged Navier-Stokes (RANS) equations are solved using the implicit, upwind, Roe-flux-differencing, finite-volume scheme. The turbulence models of Baldwin and Lomax, one-equation model of Spalart and Allmaras and two-equation shear stress transport model of Menter are implemented with the RANS solver for turbulent-flow modeling.
For the near-field study, computations are carried out on a fine grid for a rectangular wing with a NACA-0012 airfoil section and …
Multiple Streams Synchronization In Collaborative Multimedia Systems, Emilia Stoica
Multiple Streams Synchronization In Collaborative Multimedia Systems, Emilia Stoica
Computer Science Theses & Dissertations
With the recent increase of the communication bandwidth and processor power, new types of applications have emerged. Among them, there are multimedia application, in which users are able to control, combine, and manipulate different types of media, such as text, sound, video, computer graphics, and animation. A key requirement in any multimedia application is to synchronize the delivery of various media streams to the user. To achieve this, the sender has to provide the temporal relations between the streams as they are captured. Since the receiver uses this information in streams presentation, its accuracy is very important.
Our main contribution …
Approximate Analytical Relationships For Linear Optimal Aeroelastic Flight Control Laws, Ayman Hamdy Kassem
Approximate Analytical Relationships For Linear Optimal Aeroelastic Flight Control Laws, Ayman Hamdy Kassem
Mechanical & Aerospace Engineering Theses & Dissertations
This dissertation introduces new methods to uncover functional relationships between design parameters of a contemporary control design technique and the resulting closed-loop properties. Three new methods are developed for generating such relationships through analytical expressions: the Direct Eigen-Based Technique, the Order of Magnitude Technique, and the Cost Function Imbedding Technique. Efforts concentrated on the linear-quadratic state-feedback control-design technique applied to an aeroelastic flight control task. For this specific application, simple and accurate analytical expressions for the closed-loop eigenvalues and zeros in terms of basic parameters such as stability and control derivatives, structural vibration damping and natural frequency, and cost function …
Thermal Lattice Boltzmann Simulations Of Variable Prandtl Number Turbulent Flows, Min Soe, George Vahala, Pavol Pavlo, Linda L. Vahala, Hudong Chen
Thermal Lattice Boltzmann Simulations Of Variable Prandtl Number Turbulent Flows, Min Soe, George Vahala, Pavol Pavlo, Linda L. Vahala, Hudong Chen
Electrical & Computer Engineering Faculty Publications
Thermal lattice Boltzmann (TLBE) models that utilize the single relaxation time scalar Bhatnagar, Gross, and Krook collision operator have an invariant Prandtl number. For flows with arbitrary Prandtl number, a matrix collision operator is introduced. The relaxation parameters are generalized so that the transport coefficients become density independent. TLBE simulations are presented for two-dimensional free decaying turbulence induced by a strongly perturbed double velocity shear layer for various Prandtl numbers.
Sparse Equation-Eigen Solvers For Symmetric/Unsymmetric Positive-Negative-Indefinite Matrices With Finite Element And Linear Programming Applications, Hakakizumwami Birali Runesha
Sparse Equation-Eigen Solvers For Symmetric/Unsymmetric Positive-Negative-Indefinite Matrices With Finite Element And Linear Programming Applications, Hakakizumwami Birali Runesha
Civil & Environmental Engineering Theses & Dissertations
Vectorized sparse solvers for direct solutions of positive-negative-indefinite symmetric systems of linear equations and eigen-equations are developed. Sparse storage schemes, re-ordering, symbolic factorization and numerical factorization algorithms are discussed. Loop unrolling techniques are also incorporated in the coding to enhance the vector speed. In the indefinite solver, which employs various pivoting strategies, a simple rotation matrix is introduced to simplify the computer implementation. Efficient usage of the incore memory is accomplished by the proposed "restart memory management" schemes. A sparse version of the Interior Point Method, IPM, has also been implemented that incorporates the developed indefinite sparse solver for linear …
Inversion And Analysis Of Remotely Sensed Atmospheric Water Vapor Measurements At 940nm, Jack Cutler Larsen
Inversion And Analysis Of Remotely Sensed Atmospheric Water Vapor Measurements At 940nm, Jack Cutler Larsen
Mechanical & Aerospace Engineering Theses & Dissertations
The understanding and acceptance of remotely sensed atmospheric data depends strongly on the steps taken to characterize experiment error and validate observations through comparisons to other independent measurements. A formal error analysis of the Stratospheric Aerosol and Gas Experiment II (SAGE II) water vapor operational inversion algorithm is performed and compared to previous results. Random measurement errors were characterized by segmented least-squares profile fitting of the slant path absorption which found the error to be uncorrelated in the stratosphere with estimated variances significantly smaller than expected from 18-30 km. Estimates of null space error were developed from radiosonde hygrometers in …
Fiber-Reinforced Plastic Composite Beam Stability And Load And Resistance Factor Design Approach, Mojtaba B. Sirjani
Fiber-Reinforced Plastic Composite Beam Stability And Load And Resistance Factor Design Approach, Mojtaba B. Sirjani
Civil & Environmental Engineering Theses & Dissertations
Fiber Reinforced Plastic (FRP) structural members are rapidly gaining impetus in civil engineering applications. Also, thin-walled open beams with I-shaped, channel, and other types of sections are of practical importance to both structural analysts and designers. This dissertation presents the outcome of a detailed experimental and theoretical investigation of the strength and stability of FRP composite beams. Three- and four-point loading tests are conducted on FRP I- and channel section beams. The behavior of these beams is studied under gradually increasing static loads up to the maximum load-carrying capacity corresponding to either material cracking or flexural-torsional instability. First, the theoretical …
Evaluation Of Airport Operating Efficiencies Using Data Envelopment Analysis, Scott Andrew Cummings
Evaluation Of Airport Operating Efficiencies Using Data Envelopment Analysis, Scott Andrew Cummings
Engineering Management & Systems Engineering Theses & Dissertations
Airport congestion represents a serious obstacle to the future growth of the air transportation industry in the United States. Future development of new airports, and expansion of existing ones is required to meet the growing demands in domestic and commercial air travel. Globalization of economies and international travel has placed additional burdens on airports nationwide. This growth is requiring airports to operate at peak efficiencies. Economic growth of surrounding industries is dependent on the airport that services that geographical region. The variations in functions that airports offer, and therefore the differences in operations, do not lend themselves to traditional efficiency …
Experimental Geometry Optimization Techniques For Multi-Element Airfoils, Drew Landman
Experimental Geometry Optimization Techniques For Multi-Element Airfoils, Drew Landman
Mechanical & Aerospace Engineering Theses & Dissertations
A study is reported on geometry optimization techniques for high-lift airfoils. A modern three-element airfoil model with a remotely actuated flap was designed, tested, and used in wind tunnel experiments to investigate optimum flap positioning based on lift. All the results presented were obtained in the Old Dominion University low-speed wind tunnel. Detailed results for lift coefficient versus flap vertical and horizontal position are presented for two airfoil angles-of-attack: 8 and 14 degrees. Three automated optimization simulations, the method of steepest ascent and two variants of the sequential simplex method, were demonstrated using experimental data. An on-line optimizer was demonstrated …
Study Of Human Factors Variables In Battle Outcome Prediction Models, David Andrew Glovier
Study Of Human Factors Variables In Battle Outcome Prediction Models, David Andrew Glovier
Engineering Management & Systems Engineering Theses & Dissertations
Over time there have been many improvements in models that are used to predict the outcome of battles. Currently there is much supposition and speculation surrounding the use of human performance related factors as additional inputs to battle simulation models to improve their accuracy. However there is no conclusive scientific evidence which shows that these factors do make a significant difference. This study investigates the use of factors that may impact on the human performance directly or indirectly in battle prediction models. These factors consist of traditional human factors and external factors that may influence the human performance. The research …
Field Emission And Breakdown Processes In Vacuum Gaps With Sio(X)-Coated Cathodes, Raymond Jack Allen Iii
Field Emission And Breakdown Processes In Vacuum Gaps With Sio(X)-Coated Cathodes, Raymond Jack Allen Iii
Electrical & Computer Engineering Theses & Dissertations
Field emission of electrons is the major cause of electrical breakdown in high voltage systems in vacuum. The highest hold-off electric field of the carefully polished and cleaned stainless steel cathodes was increased to 70MV/m. Thin silicon monoxide, SiOx, cathode coatings reduced field emission and increased the hold-off field further. Coating the stainless steel cathodes with 2μ SiOx reduced the field emission current by at least two orders of magnitude at field of 50MV/m and increased the breakdown field to 140MV/m, doubling the breakdown voltage.
The increase in hold-off voltage with SiOx coatings is discussed in …
Thin-Layer Prestressed Composite Ferroelectric Driver And Sensor Characterization With Application To Separation Flow Control, Karla M. Mossi
Thin-Layer Prestressed Composite Ferroelectric Driver And Sensor Characterization With Application To Separation Flow Control, Karla M. Mossi
Mechanical & Aerospace Engineering Theses & Dissertations
Experiments were conducted in two different stages--general piezoelectric actuator characterization and flow separation control applications. The characterization of the piezoelectric devices was performed in several stages, due to the many variables that affect performance. The first stage of the characterization consisted of tests conducted on 13 different THUNDERTM (thin-layer composite unimorph ferroelectric driver and sensor) configurations. These configurations consisted of a combination of 1, 3, 5, 7, and 9 layers of 25μ thick aluminum as backing material, with and without a top layer of 25μ aluminum. All of these configurations used the same piezoelectric ceramic wafer (PZT-5A) with dimensions …
System Identification And Control Of Cavity Noise Reduction, Chien-Hsun Kuo
System Identification And Control Of Cavity Noise Reduction, Chien-Hsun Kuo
Mechanical & Aerospace Engineering Theses & Dissertations
This dissertation first presents indirect closed-loop system identification through residual whitening, then identifies the cavity noise system and applies controllers to reduce the noise. High speed air flow over the cavity produces a complex oscillatory flow-field and induces pressure oscillations within the cavity. The existence of cavities induces large pressure fluctuations which generate undesirable and loud noise. This may have an adverse effect on the objects, such as reducing the stability and performance of overall system, or damaging the sensitive instruments within the cavity.
System identification is the process of building mathematical models of dynamical systems based on the available …
Reduction Of Thermal Deflection And Random Response Of Composite Structures With Embedded Shape Memory Alloy At Elevated Temperature, Zhiwei Zhong
Mechanical & Aerospace Engineering Theses & Dissertations
A feasibility study on the reduction of thermal deflection and random response of the composite structures using shape memory alloys (SMA) at elevated temperatures is presented in this dissertation. The characteristics of SMA are introduced and the structural problems, static and dynamic, of SMA fiber reinforced composites are investigated. The stress-strain relations are developed for a composite lamina with embedded SMA fibers. The finite element system equations including shape memory effect are derived. A consistent two-step solution procedure is developed for solving the static and dynamic problems of composite structures with embedded SMA fibers subjected to combined acoustic and thermal …
Generation Of Intense Excimer Radiation From High-Pressure Hollow Cathode Discharges, Ahmed El-Habachi, Karl H. Schoenbach
Generation Of Intense Excimer Radiation From High-Pressure Hollow Cathode Discharges, Ahmed El-Habachi, Karl H. Schoenbach
Bioelectrics Publications
By reducing the diameter of the cathode opening in a hollow cathode discharge geometry to values on the order of 100 μm, we were able to operate these discharges in noble gases in a direct current mode up to atmospheric pressure. High-pressure discharges in xenon were found to be strong sources of excimer radiation. Highest intensities at a wavelength of 172 nm were obtained at a pressure of 400 Torr. At this pressure, the vacuum ultraviolet (VUV) radiant power of a single discharge operating at a forward voltage of 220 V and currents exceeding 2 mA reaches values between 6% …
Emission Of Excimer Radiation From Direct Current, High-Pressure Hollow Cathode Discharge, Ahmed El-Habachi, Karl H. Schoenbach
Emission Of Excimer Radiation From Direct Current, High-Pressure Hollow Cathode Discharge, Ahmed El-Habachi, Karl H. Schoenbach
Bioelectrics Publications
A novel, nonequilibrium, high-pressure, direct current discharge, the microhollow cathode discharge, has been found to be an intense source of xenon and argon excimer radiation peaking at wavelengths of 170 and 130 nm, respectively. In argon discharges with a 100 μm diam hollow cathode, the intensity of the excimer radiation increased by a factor of 5 over the pressure range from 100 to 800 mbar. In xenon discharges, the intensity at 170 nm increased by two orders of magnitude when the pressure was raised from 250 mbar to 1 bar. Sustaining voltages were 200 V for argon and 400 V …
Surface Morphology Of Laser-Superheated Pb(111) And Pb(100), Z. H. Zhang, Bo Lin, X. L. Zeng, H. E. Elsayed-Ali
Surface Morphology Of Laser-Superheated Pb(111) And Pb(100), Z. H. Zhang, Bo Lin, X. L. Zeng, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The surface step density on the vicinal Pb(111) and the surface vacancy density on Pb(100) after laser superheating and melting are investigated using reflection high-energy electron diffraction. With ∼100-ps laser pulses, Pb(111) surface superheating does not significantly change the density of the steps and step-edge roughness. However, after laser surface melting, the average terrace width and the string length at the step edge become as large as those at room temperature. The average terrace width at 573 K changes from 38±15 to 64±19 Å after laser surface melting, while the average string length at the step edge changes from 90±14 …
Femtosecond Photoemission Study Of Ultrafast Electron Dynamics In Single-Crystal Au(111) Films, J. Cao, Y. Gao, H. E. Elsayed-Ali, R. J. D. Miller, D. A. Mantell
Femtosecond Photoemission Study Of Ultrafast Electron Dynamics In Single-Crystal Au(111) Films, J. Cao, Y. Gao, H. E. Elsayed-Ali, R. J. D. Miller, D. A. Mantell
Electrical & Computer Engineering Faculty Publications
The energy-dependent relaxation of photoexcited electrons has been measured by time-resolved two-photon photoemission spectroscopy on single-crystal Au(111) films with thickness ranging from 150 to 3000 Å. It is found that the energy-dependent relaxation does not show any significant thickness dependence, which indicates that electron transport is a much slower dynamical process in the near-surface region than expected from bulk properties. Furthermore, lifetimes of the photoexcited electrons can be fitted well by the Fermi-liquid theory with a scaling factor plus an effective upper lifetime. This observation enables separation of electron-electron scattering, and to a lesser extent electron-phonon scattering, processes from electron-transport …
Atomic Hydrogen Cleaning Of Inp(100) For Preparation Of A Negative Electron Affinity Photocathode, K. A. Elamrawi, M. A. Hafez, H. E. Elsayed-Ali
Atomic Hydrogen Cleaning Of Inp(100) For Preparation Of A Negative Electron Affinity Photocathode, K. A. Elamrawi, M. A. Hafez, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
Atomic hydrogen cleaning is used to clean InP(100) negative electron affinity photocathodes. Reflection high-energy electron diffraction patterns of reconstructed, phosphorus-stabilized, InP(100) surfaces are obtained after cleaning at ∼400 °C. These surfaces produce high quantum efficiency photocathodes (∼8.5%), in response to 632.8 nm light. Without atomic hydrogen cleaning, activation of InP to negative electron affinity requires heating to ∼530 °C. At this high temperature, phosphorus evaporates preferentially and a rough surface is obtained. These surfaces produce low quantum efficiency photocathodes (∼0.1%). The use of reflection high-energy electron diffraction to measure the thickness of the deposited cesium layer during activation by correlating …
Temperature Dependence Of Step Density On Vicinal Pb(111), Z. H. Zhang, H. E. Elsayed-Ali
Temperature Dependence Of Step Density On Vicinal Pb(111), Z. H. Zhang, H. E. Elsayed-Ali
Electrical & Computer Engineering Faculty Publications
The temperature dependence of step density on the vicinal Pb(111) surface is investigated using reflection high-energy electron diffraction. When the temperature is increased from 323 to 590 K. the average terrace width and the average string length at the step edge decrease from 85±25 to 37±16 Å and from 220±33 to 25±8 Å, respectively. Thermal step collapse on the Pb(111) surface near its bulk melting temperature is not observed. Above 530±7 K, the change in the string length at the step edge with temperature becomes small, and the intensity of the (00) beam is significantly decreased. We conclude that partial …