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Articles 5641 - 5670 of 5695
Full-Text Articles in Engineering
A Comparison Of Molecular Vibration Modeling For Thermal Nonequilibrium Airflow, Kenneth J. Moran
A Comparison Of Molecular Vibration Modeling For Thermal Nonequilibrium Airflow, Kenneth J. Moran
Theses and Dissertations
The effects of thermal nonequilibrium on flows about blunt-bodies have not been studied numerically in isolation from chemical reactions. Typically, air is modeled as a perfect gas or as a chemically reacting mixture. In the former case, significant errors result at Mach numbers exceeding about 5. However, below Mach numbers around 8, the effects of chemical reactions are negligible. This study analyzes flow about a simple axisymmetric blunt-body at moderate hypersonic speeds (Mach numbers between 5-8). A first-order-accurate Roe scheme was used to compute solutions of the Euler equations assuming different gas models: perfect gas, thermal equilibrium, and thermal nonequilibrium. …
Investigation Of Damage Mechanisms In A Cross-Ply Metal Matrix Composite Under Thermo-Mechanical Loading, Joel J. Schubbe
Investigation Of Damage Mechanisms In A Cross-Ply Metal Matrix Composite Under Thermo-Mechanical Loading, Joel J. Schubbe
Theses and Dissertations
Metal matrix composites (MMCs) are rapidly becoming strong candidates for high temperature and high stiffness structural applications such as the Advanced Tactical Fighter (ATF). This study systematically investigated the failure modes and associated damage in a cross-ply, [0/90]2s SCS6/Ti-15-3 metal matrix composite under in-phase and out-of-phase thermomechanic fatigue. Initiation and progression of fatigue damage were recorded and correlated to changes in Young's Modulus of the composite material. Experimental results show an internal stabilization of reaction zone size but degradation and separation from constituent materials under extended cyclic thermal loading. Critical to damage were transverse cracks initiating in the 90° …
A Numerical Investigation Of The Series Reversion/Inversion And Series Reversion Of Lambert's Time Function, Michael P. Ward
A Numerical Investigation Of The Series Reversion/Inversion And Series Reversion Of Lambert's Time Function, Michael P. Ward
Theses and Dissertations
An expression for the semi-major axis as a function of time may be determined by performing a series reversion and inversion of Lambert's Time Function. Since the resulting series contains a singularity, it is desirable to perform only a reversion on the original series to obtain an expression for the inverse of the semi-major axis. Using a Lagrange expansion to obtain the coefficients for this series is very computer intensive. Therefore, alternative methods are presented. Also, each series was expanded into a continued fraction which provided greater accuracy than the series using the same number of coefficients. The accuracy was …
Use Of Grasp Force Focus Positioning To Enhance The Torque Resistance Capability Of Robotic Grasps, Stephen G. Edwards
Use Of Grasp Force Focus Positioning To Enhance The Torque Resistance Capability Of Robotic Grasps, Stephen G. Edwards
Theses and Dissertations
Three-point-contact grasps are unique in that the homogeneous solution for the contact forces always produces a grasp force focus. Careful positioning of this focus point in the grasp plane can help avoid two things; slipping at the contact points, and violation of joint torque limits. The focus placement method is explored theoretically by examining two types of grasps on cylinders; 1) fingertip grasps using three independently operated fingers, and 2) single-finger power grasps with one contact point on each of three links. Constraint maps are generated for various fingertip grasps in order to show how proper placement of the grasp …
Material Properties Of Silicon Carbide Fibers With Continuously Applied Sol-Gel Alumina Coatings, Christian C. Daehnick
Material Properties Of Silicon Carbide Fibers With Continuously Applied Sol-Gel Alumina Coatings, Christian C. Daehnick
Theses and Dissertations
An investigation was conducted to determine the effect of non-alpha- alumina coatings applied under varying conditions on some of the properties of silicon carbide fibers. Limited characterization of the coating by optical, scanning electron, and transmitted electron microscopy as well as energy- dispersive x-ray analysis was performed. Tensile tests were used to determine changes in elastic modulus, ultimate strength, and strain to failure of the coated fibers relative to uncoated fibers. The coatings were found to lower the mean value of the measured properties as their thickness increased, but the measurements were not accurate enough to determine if this corresponded …
A Low-Cost Part-Task Flight Training System: An Application Of A Head Mounted Display, David A. Dahn
A Low-Cost Part-Task Flight Training System: An Application Of A Head Mounted Display, David A. Dahn
Theses and Dissertations
To investigate the feasibility of using head-mounted display(HMD) and Computer Image Generators(CIG) in a low cost part-task trainer, we created a prototype system. Our approach was to build a virtual world interface using a HMD to an existing flight simulator application. This allowed a CIG with only one graphics channel to drive the display. To investigate which class of computing platform was suitable for use as the CIG, a cooperative thesis effort was launched to host the simulator on a mini-computer and micro-computer platform. The appropriate CIG could then be determined through demonstration and comparison between the systems. A software …
Multi-Input Multi-Output Flight Control System Design For The Yf-16 Using Nonlinear Qft And Pilot Compensation, Russell B. Miller
Multi-Input Multi-Output Flight Control System Design For The Yf-16 Using Nonlinear Qft And Pilot Compensation, Russell B. Miller
Theses and Dissertations
Nonlinear Quantitative Feedback Theory (QFT) and pilot compensation techniques are used to design a 2x2 flight control system for the YF-16 aircraft over a large range of plant uncertainty. The design is based on numerical input- output time histories generated with a FORTRAN implemented nonlinear simulation of the YF-16. The first step of the design process is the generation of a set of equivalent linear time-invariant (LTI) plant models to represent the actual nonlinear plant. It has been proven that the solution to the equivalent plant problem is guaranteed to solve the original nonlinear problem. Standard QFT techniques are then …
An Experimental Study Of A Sting-Mounted Single-Slot Circulation Control Wing, Michael E. Pelletier
An Experimental Study Of A Sting-Mounted Single-Slot Circulation Control Wing, Michael E. Pelletier
Theses and Dissertations
This wind tunnel study investigated the feasibility of using a sting and force balance to measure the aerodynamic forces and moments on a circulation control wing. A 20% this, 8.5% camber, single blowing slot, rectangular wing was designed, built, and tested in the AFIT 5-ft wind tunnel. Lift and drag coefficients were referenced to the stability axis. The Reynolds number for all tests was 950,000; angle of attack was varied from -6 to +6 degrees. Trends in the data were similar to two dimensional data, with the exception of high drag coefficients with increased blowing. Results show it is feasible …
Effect Of Riblets On Pressure Recovery In A Straight-Walled Diffuser, Michael K. Reagan
Effect Of Riblets On Pressure Recovery In A Straight-Walled Diffuser, Michael K. Reagan
Theses and Dissertations
The objective of this thesis was to investigate the effect of riblets on the pressure recovery in a straight-walled diffuser. Previous work has shown that riblets were effective in reducing the viscous drag over surfaces subjected to a turbulent boundary layer. More recently, riblets were shown to delay the flow separation within a subsonic, straight-walled diffuser by as much as 200%. The purpose of this investigation was to determine the effect on the pressure distribution within a diffuser that has had the flow separation point favorably altered by the application of riblets. Investigation results revealed that riblets not only delayed …
Failure Mechanisms In A Quasi-Isotropic Ceramic Composite Laminate Under Tensile Fatigue Loading, Gregory D. Tracy
Failure Mechanisms In A Quasi-Isotropic Ceramic Composite Laminate Under Tensile Fatigue Loading, Gregory D. Tracy
Theses and Dissertations
A study was conducted to experimentally investigate the tensile fatigue damage mechanisms in a quasi-isotropic ceramic matrix composite (Corning's Nicalon/CASIII). The goals of this study were to (1) develop a grip fixture for a servo-hydraulic MTS test machine and (2) conduct a series of tests with (0/±45/90)s specimens subjected to tensile fatigue loading to study their fatigue damage growth mechanisms. Results showed that when the material was subjected to stresses below 90 MPa, it lasted over 1000000 cycles but when stressed at or above 90 MPa, it failed in less than 1000 cycles.
Investigation Of Direct And Indirect Optimization Algorithms For Aerospace Structures, Harry Hopkins Iii
Investigation Of Direct And Indirect Optimization Algorithms For Aerospace Structures, Harry Hopkins Iii
Theses and Dissertations
This thesis investigated the performance of two direct and two indirect methods of structural optimization. A gradient calculator and overall design system was created with the finite element code FRAME. Three of the methods were employed using this system with the fourth being a commercial available code. The algorithms were applied to two different sized trusses using static constraints and were measured for accuracy, reliability, and efficiency. Results for the problems tested showed the Direct methods were sensitive to starting designs and their performance depended on the proper selection of internal parameters. They were also shown to have a high …
Brightness Invariant Port Recognition For Robotic Aircraft Refueling, Richard A. Bennett
Brightness Invariant Port Recognition For Robotic Aircraft Refueling, Richard A. Bennett
Theses and Dissertations
The purpose of this thesis effort was to develop and test a brightness invariant recognition algorithm that would locate and identify the half-scale mock up of a Universal Aerial Refueling Receptacle Slipway Installation (UARRSI) aerial refueling port in different lightening conditions. This was accomplished by developing a brightness invariant port recognition of identifiable attributes of the refueling port to locate and identify the UARRSI port. Based on this approach, a brightness invariant port recognition system (BIPRS) was demonstrated which identified the refueling receptacle in different lightening conditions. The BIPRS was also invariant to orientation and size of the UARRSI port. …
Evaluation Of An Interdigitated Gate Electrode Field-Effect Transistor For Detecting Organophosphorus Compounds, Charles P. Brothers Jr.
Evaluation Of An Interdigitated Gate Electrode Field-Effect Transistor For Detecting Organophosphorus Compounds, Charles P. Brothers Jr.
Theses and Dissertations
This study used integrated circuit microsensors to detect organophosphorus compounds. Chemically-sensitive thin films, copper phthalocyanine, DFPase, succinyl chloride, succinylcholine chloride, 2- naphthol(B), and L-histidine dihydrochloride,were deposited on interdigitated gate electrode (IGE) structures, with an average thickness of 2000A. Thin film electrical performance characteristics were measured for several parameters, including: DC resistance, AC impedance, time-domain, and spectral responses from 10 Hz to 1 MHz. Each microsensor contained nine IGEs; each IGE possessed an in situ field-effect transistor amplifier. After purging each sensor with filtered air, it was exposed to one or two of the following gases: diisopropyl fluorophosphate (DFP), diisopropyl methylphosphonate, …
Fluorescent Lifetime Measurements Of Rare Earth Elements In Gallium Arsenide, Danny J. Topp
Fluorescent Lifetime Measurements Of Rare Earth Elements In Gallium Arsenide, Danny J. Topp
Theses and Dissertations
Lifetime measurements of the excited states of three GaAs semiconductors doped with the rare earth elements Erbium (ER), Praseodymium (Pr) , and Thulium (Tm) has been studied using a pulsed nitrogen laser and germanium detector. The measurements were made with an experimental set up with a system response time of 0.34 micro seconds. A 330 milliwatt nitrogen laser with a wavelength of 3370 angstrums was used to excite transitions of the rare earth elements.
An Analysis Of The Effects Of Phenytoin In Treating Motion Sickness And The Effects Of Motion Sickness On The Human Electroencephalogram, Todd M. Banducci
An Analysis Of The Effects Of Phenytoin In Treating Motion Sickness And The Effects Of Motion Sickness On The Human Electroencephalogram, Todd M. Banducci
Theses and Dissertations
This research had two goals: to continue the ongoing research at AFIT to determine the efficacy of the anticonvulsant drug phenytoin in combating the onset and progression of motion sickness; and to note whether or not there is a discernable similarity in the brains of the test subjects to indicate a common point of origin (epicenter) and propagation pattern of the effects of motion sickness in the brain. Eight male DOD personnel were used as subjects to complete twelve trials. Four subjects completed the phenytoin verses placebo double-blind crossover experiment. These four subjects experienced a 99% mean increase in their …
Application Of Influence Diagrams In Identifying Soviet Satellite Missions, Cary C. Chun
Application Of Influence Diagrams In Identifying Soviet Satellite Missions, Cary C. Chun
Theses and Dissertations
This thesis investigates the feasibility of applying influence diagrams to capture expert knowledge and incorporate decision theory to construct a Soviet satellite mission prediction model. Before a Soviet launch occurs, formulations of the model's prior probability distribution allows the input of expert knowledge and any available information. As the predictive variables of the model become known, the information is used to successively refine the estimate of the probable mission, thereby, reducing the uncertainty in the model. Discretizing is used to approximate continuous variables as discrete variables which successfully allows the combination of both variable types in a single influence diagram …
Bifurcation Analysis Of The Longitudinal Dynamics Of A Simple Powered Lifting Hypersonic Vehicle, Eric E. Fox
Bifurcation Analysis Of The Longitudinal Dynamics Of A Simple Powered Lifting Hypersonic Vehicle, Eric E. Fox
Theses and Dissertations
Bifurcation analysis was used to investigate the nonlinear behavior of a simple powered lifting hypersonic vehicle in circular orbit about a spherical nonrotating Earth with gradients in atmospheric density and pressure and an inverse square law for gravity. Vehicle motion is constrained to a vertical plane so only longitudinal dynamics were modeled. Bifurcation analysis was conducted using the AUTO software package. A simple five-state model with three different thrust laws was derived to describe an unaugmented vehicle whose geometric and aerodynamic characteristics follow those of the literature. A parameter representing a body flap deflection was used to conduct one set …
Investigation Of The High Angle Of Attack Dynamics Of The F-15b Using Bifurcation Analysis, Robert J. Mcdonnell
Investigation Of The High Angle Of Attack Dynamics Of The F-15b Using Bifurcation Analysis, Robert J. Mcdonnell
Theses and Dissertations
Previous studies predicted the F-15B high angle of attack and flat spin behavior using bifurcation analysis. These studies varied control surface deflections to find equilibrium and periodic solutions. The purpose of this research was to use bifurcation analysis to predict the F-15B high angle attack and flat spin behavior as a result of variable thrust, asymmetric thrust, and thrust vectoring. Using a previously developed model of the F-15, bifurcation analysis and continuation methods were used to map out the equilibrium and periodic solutions of the model as a function of the thrust parameters. A baseline bifurcation diagram, as a function …
Modelling And Analysis Of Kernel Function And Development Of Equivalent Theodorsen Function For Three-Dimensional Aeroelastic Analysis, John K. Ryder
Modelling And Analysis Of Kernel Function And Development Of Equivalent Theodorsen Function For Three-Dimensional Aeroelastic Analysis, John K. Ryder
Theses and Dissertations
A fractional calculus model is developed for the kernal function under incompressible subsonic flow conditions for rectangular planform airfoils with small aspect ratio. A model valid for restricted regions of the kernel function for compressible subsonic flow conditions is also developed. Additionally, a method for numerically solving the pressure-downwash integral equation for rectangular planform wings of aspect ratio two through ten in incompressible flow is developed. An equivalent Theodorsen function for three- dimensional unsteady flow is developed, enabling the use of simpler two- dimensional aeroelastic equations of motion to fully capture the more complicated three-dimensional effects.
Effects Of Specific Heat Ratio On A Simulated Chemical Laser Cavity Flow, Curt Dewayne Botts
Effects Of Specific Heat Ratio On A Simulated Chemical Laser Cavity Flow, Curt Dewayne Botts
Theses and Dissertations
Mixing of primary cold flow air and secondary helium to control the ratio of specific heats for the medium flowing through a simulated chemical laser nozzle/lasing cavity was accomplished. The effects of a range of mixture specific heat ratios on flowfield behavior were examined using static pressure ports in the test cavity. Schlieren photography and high speed filming aided description of the flow dynamics. Results indicated that boundary layer effects became evident in the nozzles as specific heat ratios increased. Large pressure fluctuations were observed in the cavity when helium was introduced into the flow to raise the specific heat …
Damping Of Layered Beams With Mixed Boundary Conditions, Eugene T. Cottle
Damping Of Layered Beams With Mixed Boundary Conditions, Eugene T. Cottle
Theses and Dissertations
For many years, viscoelastic materials have been used in damping treatments to control vibration. In order to optimize a damping treatment for maximum damping, a design engineer must be able to predict frequencies of vibration of damped normal modes and loss factors as a function of the design parameters. Theoretical models of layered beams have been developed, but to date, exact solutions have assumed boundary conditions to be the same for all layers. In this thesis, equations of motion for a damped, layered beam are solved for the longitudinal displacement of both elastic layers, as well as the lateral displacement …
Analysis Of Hypersonic Blunt-Body Flows Using A Total Variation Diminishing Scheme And The Maccormack Scheme, Montgomery C. Hughson
Analysis Of Hypersonic Blunt-Body Flows Using A Total Variation Diminishing Scheme And The Maccormack Scheme, Montgomery C. Hughson
Theses and Dissertations
An explicit second-order accurate, total variation diminishing (TVD) scheme and the MacCormack scheme are applied to the Euler equations in axisymmetric form to study hypersonic blunt body flows. The modified flux approach of Harten, with modification by Yee, for two-dimensional flows is extended to treat axisymmetric flows. Calculated flow properties for the steady state, blunt body problem such as shock standoff distance, bow shock shape, surface pressure distribution and entropy jump conditions are compared to theory, results from the MacCormack scheme, and experimental data for Mach numbers of 3.0, 4.03, 5.06, 6.03 and 8.1. Additionally, the TVD and MacCormack schemes …
Recursive Optimization Of Digital Circuits, John Knutson
Recursive Optimization Of Digital Circuits, John Knutson
Theses and Dissertations
The goal of this thesis is twofold: first, to identify the advantages and disadvantages of existing optimization systems and second, to develop an optimization system that uses Boolean principles to generate a recursive realization of combinational logic. Current multi-level optimization systems fall into two categories: local optimization which removes redundancy by pattern matching on a local scale and global optimization which works with the equations that specify a circuit rather than with the circuit implementation itself. While global systems are very flexible and can produce near-optimal solutions, they are inherently complex. This research effort demonstrates that an effective global optimization …
Model-Based Reasoning In Electronic Repair, Kenneth B. Cohen
Model-Based Reasoning In Electronic Repair, Kenneth B. Cohen
Theses and Dissertations
Diagnosis is the process of identifying the cause for an observed behavior of a system. Model-based reasoning uses knowledge of the behavior of the component and the interconnection of the components to diagnose a system. This thesis investigates the application of model-based reasoning to the problem of isolating faulty components of an analog electronic circuit. More specifically, this thesis describes an architecture for a model-based diagnostic system for electronic modules, implements an assumption-based truth maintenance system (one of the component parts of a model-based diagnostic system), and creates the interface between the truth maintenance system and the diagnostic system.
A Study Of The Effects Of Noise Jamming On Radar Receivers, Paul E. Bishop
A Study Of The Effects Of Noise Jamming On Radar Receivers, Paul E. Bishop
Theses and Dissertations
A comprehensive study of the effects of noise jamming on various pulsed radar receiver types is proposed. First, a literature review on the sources of electrical noise and various jamming waveforms is presented. The power spectra for three noise jamming waveforms are rigorously derived. These cases are shot noise in a parallel-plate diode, a series of pulses with random amplitudes and intervals, and for a series of pulses with random amplitudes, spacing and phases. Next, the probability density functions for the output of linear and quadratic detectors are developed. For each detector type, the probability density functions for the cases …
Nonlinear Analysis Of A Spinning Symmetric Satellite In An Elliptical Orbit, Michael Ulisse
Nonlinear Analysis Of A Spinning Symmetric Satellite In An Elliptical Orbit, Michael Ulisse
Theses and Dissertations
Controlled and uncontrolled equations of motion for a spinning symmetric satellite in an elliptical orbit are analysed using Floquet theory, Poincare maps, and continuation. The equations of motion for the uncontrolled system are derived via energy methods. A simple feedback controller design and one based on Floquet theory are presented. Parameters varied in the equations of motion include the spin about the symmetry axis of the satellite, the inertia ratio of the satellite, the eccentricity of the orbit, and the controller gains. Floquet analyses determine local stability and bifurcation points. Poincare maps exhibit some interesting and complex structure in the …
Classification Of Correlation Signatures Of Spread Spectrum Signals Using Neural Networks, Richard A. Chapman
Classification Of Correlation Signatures Of Spread Spectrum Signals Using Neural Networks, Richard A. Chapman
Theses and Dissertations
The major goals of this thesis were to determine if Artificial Neural Networks (ANNs) could be trained to classify the correlation signatures of two classes of spread spectrum signals and four classes of spread spectrum signals. Also, the possibility of training an ANN to classify features of the signatures other than signal class was investigated. Radial Basis Function Networks and Back-Propagation Networks were used for the classification problems. Correlation signatures of four types or classes were obtained from United States Army Harry Diamond Laboratories. The four types are as follows: direct sequence (DS), linearly-stepped frequency hopped (LSFH), randomly-driven frequency hopped …
Rule-Based Frequency Domain Speech Coding, Vance M. Mcmillan
Rule-Based Frequency Domain Speech Coding, Vance M. Mcmillan
Theses and Dissertations
A speech processing system is designed to simulate the transmission of speech signals using a speech coding scheme. The transmitter portion of the simulation extracts a minimized set of frequencies in Fourier space which represents the essence of each of the speech timeslices. These parameters are then adaptively quantized and transmitted to a receiver portion of the coding scheme. The receiver then generates an estimate of the original timeslice from the transmitted parameters using a sinusoidal speech model. After initial design, the thesis investigates how each of the design parameters affect the human perceived quality of speech. This is done …
Improving Reliability In A Stochastic Communication Network, William L. Gaught
Improving Reliability In A Stochastic Communication Network, William L. Gaught
Theses and Dissertations
This research investigated the behavior of increasing the reliability of components in stochastic communication networks. A stochastic communication network is one in which the components, links and nodes, are not 100 percent reliable. In addition the reliability value, each component also has a capacity value that limits the flow through the component. The throughput in a stochastic communication network can be improved by increasing the capacity or the reliability of the individual components. A set of linear and nonlinear models was developed to determine the best investment strategy for increasing the reliability of individual components. Once the models were developed, …
Skin Friction And Heat Transfer Predictions For Hypersonic Turbulent Flow Over An Ogive-Cylinder, Richard H. Byers
Skin Friction And Heat Transfer Predictions For Hypersonic Turbulent Flow Over An Ogive-Cylinder, Richard H. Byers
Theses and Dissertations
The WRDC/FIMG Parabolized Navier-Stokes (PNS) Code is used to numerically predict both laminar and turbulent hypersonic flows over an ogive- cylinder body at zero angle of attack. A parametric study is conducted to determine the effects of selected code input variables on heat transfer and skin friction predictions. Comparisons are made between the PNS predictions and experimental data from the WRDC/Mach 6 High Reynolds Number Test Facility. Various problems encountered while obtaining PNS starting and marching solutions and corrective steps taken are discussed. A mesh resolution study indicates the minimum radial grid density sufficient for obtaining reasonable heat transfer results. …