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Articles 9061 - 9090 of 11510
Full-Text Articles in Engineering
Efficient Electromagnetic Material Characterization Via 2-D Rectangular Waveguide Reduction, Derek R. Dwyer
Efficient Electromagnetic Material Characterization Via 2-D Rectangular Waveguide Reduction, Derek R. Dwyer
Theses and Dissertations
A new, low-frequency, rectangular waveguide-based electromagnetic material characterization technique is developed that will reduce the test sample size in two dimensions realizing up to 75 percent reduction in sample cross-sectional area. To achieve this, custom made, reduced aperture, sample holder flanges were used that reduce the waveguide’s excessive cross-sectional dimensions, resulting in reduced sample fabrication costs. Additionally, MatLab® code was developed to implement the rigorously derived modal-analysis solution that accommodates induced, higher-order transverse electric and transverse magnetic modes and accurately extract the reduced test sample’s constitutive parameters. Experimental results using various test samples are compared to known, full-aperture measurements for …
Reconstruction Of Chromotomographic Imaging System Infrared Hyperspectral Scenes, Malcolm G. Gould
Reconstruction Of Chromotomographic Imaging System Infrared Hyperspectral Scenes, Malcolm G. Gould
Theses and Dissertations
Hyperspectral imagery providing both spatial and spectral information has diverse applications in remote sensing and scientific imaging scenarios. The development of the Chromotomographic Imaging System (CTIS) allows simultaneous collection of both spatial and spectral data by a two-dimensional (2D) focal plane detector array. Post-processing of the 2D detector data reconstructs the three-dimensional (3D) hyperspectral content of the imaged scene. This thesis develops Estimation Theory based algorithms for reconstructing the hyperspectral scene data. The initial algorithm developed reconstructs the 3D hyperspectral scene data cube. An additional algorithm reconstructs a matrix comprised of one spectral dimension and one compound spatial dimension. This …
Modification Of A Modulation Recognition Algorithm To Enable Multi-Carrier Recognition, Angela M. Waters
Modification Of A Modulation Recognition Algorithm To Enable Multi-Carrier Recognition, Angela M. Waters
Theses and Dissertations
Modulation recognition is important for both military and commercial communication applications, particularly in cases where enhanced situation awareness and/or channel assessment is required to mitigate intentional or collateral interference. Modulation recognition via template matching or statistical analysis is a key aspect of non-cooperative (nonmatched filtering) signal interception, classification, and exploitation. This research concerns the evaluation and modification of a conventional Digitally Modulated Signal Recognition Algorithm (DMRA) to enable multi-carrier, Orthogonal Frequency Division Multiplexing (OFDM), waveform recognition. The original DMRA architecture was developed to classify communication signals for three fundamental data modulations, i.e., Amplitude Shift Keying (ASK), Phase Shift Keying (PSK), …
An Examination Of Range And Doppler Mismatch And Their Effects On Radar Modeling, Gregory L. Izdepski
An Examination Of Range And Doppler Mismatch And Their Effects On Radar Modeling, Gregory L. Izdepski
Theses and Dissertations
A commonly accepted airborne phased array radar model simplifies the analytical derivation by assuming a waveform is perfectly matched in range and Doppler shift. This assumption means the matched filter output is effectively constant for all possible received scatterer Doppler and range mismatches, greatly simplifying the analytical development from that point forward. This research removes the matched Doppler and range assumption and examines the effects of several common waveforms on the model's fidelity along with the associated impact on radar performance, both non-adaptive and adaptive. Analysis is completed using power spectral density comparisons and the fully adaptive output signal to …
Multiple Model Adaptive Estimator Target Tracker For Maneuvering Targets In Clutter, Brian D. Smith
Multiple Model Adaptive Estimator Target Tracker For Maneuvering Targets In Clutter, Brian D. Smith
Theses and Dissertations
The task of tracking a target in the presence of measurement clutter is a two-fold problem: one of handling measurement association uncertainty (due to clutter), and poorly known or significantly varying target dynamics. Measurement association uncertainty does not allow conventional tracking algorithms (such as Kalman filters) to be implemented directly. Poorly known or varying target dynamics complicate the design of any tracking filter, and filters using only a single dynamics model can rarely handle anything beyond the most benign target maneuvers. In recent years, the Multiple Hypothesis Tracker (MHT) has gained acceptance as a means of handling targets in a …
Accelerating Missile Threat Engagement Simulations Using Personal Computer Graphics Cards, Sean E. Jeffers
Accelerating Missile Threat Engagement Simulations Using Personal Computer Graphics Cards, Sean E. Jeffers
Theses and Dissertations
The 453rd Electronic Warfare Squadron supports on-going military operations by providing battlefield commanders with aircraft ingress and egress routes that minimize the risk of shoulder or ground-fired missile attacks on our aircraft. To determine these routes, the 453rd simulates engagements between ground-to-air missiles and allied aircraft to determine the probability of a successful attack. The simulations are computationally expensive, often requiring two-hours for a single 10-second missile engagement. Hundreds of simulations are needed to perform a complete risk assessment which includes evaluating the effectiveness of countermeasures such as flares, chaff, jammers, and missile warning systems. Thus, the need for faster …
Mixing Effects Of Pylon-Aided Fuel Injection Located Upstream Of A Flameholding Cavity In Supersonic Flow, Daniel R. Montes
Mixing Effects Of Pylon-Aided Fuel Injection Located Upstream Of A Flameholding Cavity In Supersonic Flow, Daniel R. Montes
Theses and Dissertations
The Air Force Research Lab (AFRL), Propulsion Directorate, Wright-Patterson Air Force Base, Ohio is conducting ongoing research into propulsive efficiency in supersonic ramjet (scramjet) technology. One current focus of this research is the usefulness of flameholding cavities implemented in the supersonic hydrocarbon-fueled combustion chamber. Because good mixing and proper cavity-core interaction lead to more efficient combustion, methods of optimizing fuel/air mixing both within and upstream the cavity are investigated. In a cooperative effort with the Air Force Institute of Technology (AFIT), AFRL provided a supersonic wind tunnel outfitted with an existing cavity design. A circular injection port was placed upstream …
Experimental Study Of The Subsonic Aerodynamics Of A Blended Wing Body Air Vehicle With A Focus On Rapid Technology Assessment, David A. Gebbie
Experimental Study Of The Subsonic Aerodynamics Of A Blended Wing Body Air Vehicle With A Focus On Rapid Technology Assessment, David A. Gebbie
Theses and Dissertations
The subsonic aerodynamic performance of a blended wing body aircraft constructed using selective laser sintering was assessed in the AFIT low-speed wind tunnel. The scaled-down model of a strike tanker aircraft consisted of a shaped fuselage and sweptback wings. The Reynolds number, based on mean wing chord, during testing was on the order of 105 while the Mach number ranged from 0.10 to 0.20. The model evaluation and analysis process included force and moment measurements acquired from a wind tunnel balance, pressure data measured with 8 taps located on the model’s upper surface, a comparison to computational fluid dynamics (CFD) …
Evaluation Techniques For Determining Damping Mechanisms On Titanium Plates, Kyle S. Allen
Evaluation Techniques For Determining Damping Mechanisms On Titanium Plates, Kyle S. Allen
Theses and Dissertations
High cycle fatigue (HCF) is the single largest cause of component failure for all modern military gas turbine engines. Hard coatings, such as magnesium aluminate spinel, have been found to provide significant damping properties. Past studies have had difficulties isolating the contributions of these hard coating damping layers from other damping mechanisms. This study explored techniques for assessing the contribution of different damping mechanisms on titanium plates during vibration testing. The study investigated 2nd bend and 2-stripe modes. Two different specimen sizes were tested in both a clamped-free-free-free and free-free-free-free condition. Specimens were tested at varying pressures. Increases in pressure …
Numerical Analysis And Optimization Of The Ultra Compact Combustors, Roger T. Greenwood
Numerical Analysis And Optimization Of The Ultra Compact Combustors, Roger T. Greenwood
Theses and Dissertations
In an effort to increase thrust per weight ratio and decrease pollutant emissions of aero-turbine jet engines, a circumferentially burning Ultra Compact Combustor (UCC) with a Cavity-in-a-Cavity design has been developed. A numerical analysis of this design has been conducted and compared with experimental results. The CFD model has been validated through a wide range of conditions and four alternative physical configurations of the UCC have been modeled. Emissions, combustor efficiencies, temperature and velocity profiles, and pressure drop values were used as comparison parameters. Numerical results indicate that increasing the outflow area will increase the pressure drop over the combustor …
Classification Of Battlespace Detonations From Temporally Resolved Mutli-Band Imagery And Mid-Infrared Spectra, Anthony N. Dills
Classification Of Battlespace Detonations From Temporally Resolved Mutli-Band Imagery And Mid-Infrared Spectra, Anthony N. Dills
Theses and Dissertations
The classification of battlespace detonations, specifically the determination of munitions type and size using temporal and spectral features, has been studied using near-infrared (NIR) and multi-color visible wavelength imagers. Key features from the time dependence of fireball size are identified for discriminating various types and sizes of detonation flashes. The five classes include three weights of trinitrotoluene (TNT) and two weights of an enhanced mixture, all of which are uncased and detonated with 10% C4. Using Fisher linear discriminant techniques, features are projected onto a line such that the projected points are maximally clustered for the different classes of detonations. …
Fuel-Air Injection Effects On Combustion In Cavity-Based Flameholders In A Supersonic Flow, William H. Allen Jr.
Fuel-Air Injection Effects On Combustion In Cavity-Based Flameholders In A Supersonic Flow, William H. Allen Jr.
Theses and Dissertations
The Air Force Research Lab, Propulsion Directorate, Wright-Patterson Air Force Base, Ohio has studied several designs regarding cavity flameholding for supersonic RAMJET (SCRAMJET) applications. The most recent of these studies have concluded that direct injection of ethylene fuel into the aft cavity ramp produced an efficient, robust flameholder given specific freestream condition and fuel flow rate. The main goals of this experiment are: 1) study the effect on combustion of direct fuel and air injection in the main flameholding cavity and 2) characterization of the operational limits (i.e., sustained combustion limits) over a variety of fuel and air flow rates. …
Boundary Layer Flow Control Using Plasma Induced Velocity, Brian E. Balcer
Boundary Layer Flow Control Using Plasma Induced Velocity, Brian E. Balcer
Theses and Dissertations
An examination of the effects of plasma induced velocity on boundary layer flow was conducted. A pair of thin copper film electrodes spanned the test section, oriented at thirty degrees from normal to the free stream flow. An adverse pressure gradient was imposed over the electrode configuration using a pressure coefficient profile similar to that associated with suction side of a Pac-B low pressure turbine blade. In addition, suction was applied to keep flow attached on the upper wall, inducing separation over the electrode. The electrode is supplied by an AC source at three different power levels with the free …
Application Of Functionally Graded Materials In Aircraft Structures, William G. Cooley
Application Of Functionally Graded Materials In Aircraft Structures, William G. Cooley
Theses and Dissertations
A Functionally Graded Material (FGM) is a material which has a continuous variation of material properties through-the-thickness. This thesis will explore analysis of FGM flat plates and shell panels, and use FGM's unique characteristics to retrofit a cracked exhaust wash structure. FGMs are first characterized as flat plates under pressure and thermal loading. Next, FGM shell panels are characterized under thermal loading. Conclusions drawn from FGM characterization are used to develop a patch to retrofit a cracked aircraft exhaust wash structure. The exhaust wash structure is first analyzed to provide a baseline and then several FGM patches are evaluated on …
Entropy Generation As A Means Of Examining Continuum Breakdown, Christopher R. Schrock
Entropy Generation As A Means Of Examining Continuum Breakdown, Christopher R. Schrock
Theses and Dissertations
To quantify the validity and breakdown of the continuum equations of fluid flow, the concept of entropy generation is examined. This parameter is formulated utilizing statistical mechanics and kinetic theory to avoid the use of equilibrium assumptions. This analysis leads to expressions in terms of energy distribution functions. These results are applied to monatomic and diatomic molecules. A numerical procedure for computing these values using the Direct Simulation Monte Carlo Method (DSMC) is presented. Normal shock waves in argon and nitrogen were simulated at Mach numbers ranging from 1.2 to 10. Results are compared to Navier-Stokes predictions. The Navier-Stokes equations …
Analysis Of Computational Methods For The Treatment Of Material Interfaces, Minh C. Nguyen
Analysis Of Computational Methods For The Treatment Of Material Interfaces, Minh C. Nguyen
Theses and Dissertations
Rocket sled tests at the Air Force Research Laboratory's Holloman High Speed Test Track frequently approach velocities where gouging development becomes the limiting factor to achieving higher operating velocities. Direct observation of the gouging process is not possible so computational modeling is necessary to study the phenomenon. Since gouging development is dependent on the impact surface conditions, the method used to model material interfaces directly affects the accuracy of the solution. Three methods are available in the hydrocode CTH to handle material interfaces: 1) materials are joined at the interface, 2) a frictionless slide line is inserted, and 3) a …
Structures Of Angled Aerated-Liquid Jets In March 1.94 Supersonic Cross Flow, Dion R. Dixon
Structures Of Angled Aerated-Liquid Jets In March 1.94 Supersonic Cross Flow, Dion R. Dixon
Theses and Dissertations
The structures of aerated-liquid jets injected into a supersonic crossflow have been studied experimentally. Aerated-liquid injectors with diameters of 0.020", 0.040" and 0.060" were flush mounted on the bottom plate of a supersonic wind tunnel to provide normal and angled injection. Free stream Mach number and temperature were held constant at 1.94 and 500°F, respectively. Water at room temperature was used as the liquid injectant and nitrogen gas used as the aerating gas. Wide ranges of test conditions for injection angle (θ), injector orifice diameter (d0), jet-to-air momentum flux ratios (q0), and gas-to-liquid ratios (GLR), were …
Creep Behavior Of An Oxide/Oxide Composite With Monazite Coating At Elevated Temperatures, Sean S. Musil
Creep Behavior Of An Oxide/Oxide Composite With Monazite Coating At Elevated Temperatures, Sean S. Musil
Theses and Dissertations
This study focuses on experimental investigation of stress-rupture behavior (creep response) of an oxide/oxide composite in a cross-ply (0/90) lay-up at elevated temperature. The test material, Nextel 610/monazite/alumina composite, employs monazite, an oxidation-resistant interfacial coating designed to improve performance at elevated temperatures. The experimental program included monotonic tensile tests to failure and creep-rupture tests at elevated temperatures. Tensile tests served to establish an ultimate tensile strength (UTS) for the material. The ensuing creep-rupture tests involved stress levels at varying percentages of the UTS. Stress-rupture curves at 900 and 1100 degrees C were established. A family of creep curves for various …
Computational Aeroelastic Analysis Of Micro Air Vehicle With Experimentally Determined Modes, Joshua A. Stults
Computational Aeroelastic Analysis Of Micro Air Vehicle With Experimentally Determined Modes, Joshua A. Stults
Theses and Dissertations
A computational aeroelastic analysis of a micro air vehicle (MAV) is conducted. This MAV has a 24 inch wing span, and is designed for local area reconnaissance. Wind tunnel data for the MAV with a rigid carbon fiber wing and a flexible carbon fiber ribbed nylon wing are compared to CFD results incorporating static and dynamic deformations. We use laser vibrometry to determine the mode shapes of the flexible wing. From these shapes, CFD grid deformations are calculated as part of a closely-coupled aeroelastic solution method. The accuracy of MAV performance predictions using CFD with and without aeroelastic modeling is …
Fatigue Behavior Of A Functionally-Graded Titanium Matrix Composite, Scott R. Cunningham
Fatigue Behavior Of A Functionally-Graded Titanium Matrix Composite, Scott R. Cunningham
Theses and Dissertations
Functionally-graded Titanium Matrix Composites are an attempt to utilize the high-strength properties of a titanium matrix composite with a monolithic alloy having the more practical machining qualities. This work studied the mechanical characteristics of the joint region as a first step toward future evaluation of this material. The scope of this effort involved testing under monotonic tension and fatigue loading conditions. Mechanical properties and cyclic behavior were evaluated for the joint area and then compared to those of the parent materials. The results of this study found that the strength of the transition region was slightly higher than the unreinforced …
Dynamic Characterization Of Thin Deformable Pvdf Mirror, Eric M. Trad
Dynamic Characterization Of Thin Deformable Pvdf Mirror, Eric M. Trad
Theses and Dissertations
Methodology for the design and fabrication of 5 inch diameter membrane-like mirror structures is explored, resulting in test articles with areal densities as low as 1.57 kg/m2 and a thickness of 1.5 mm. Each optical structure has 7 control patches etched into the PVDF layer, which can be used as actuators. These test articles are utilized to demonstrate the capabilities of an acquisition system to make measurements of static and dynamic actuation of the mirror surface. Static deformations are analyzed statistically and determined to yield maximum deformations in the actuated regions of -2.1 micro with a standard deviation of 0.33 …
Effects Of Temperature And Shot-Peening Intensity On Fretting Fatigue Behavior Of Titanium Alloy Ti-6al-4a, Salman Albinali
Effects Of Temperature And Shot-Peening Intensity On Fretting Fatigue Behavior Of Titanium Alloy Ti-6al-4a, Salman Albinali
Theses and Dissertations
Effects of temperature and shot-peening intensity on fretting fatigue behavior of Ti-6Al-4A were investigated in this study. S-N curves were obtained for both room and elevated temperatures (260 C) for two different shot-peened intensities (4A and 10A). Stress relaxation behavior under both fretting fatigue at elevated temperature and temperature exposure only were also investigated after their measurements were calculated using X-ray diffraction method. The crack initiation location and the crack angle orientation along the surface were determined using optical and scanning electron microscopy (SEM). Cracks initiated near the trailing edge of the tested specimens. Cracks initiated on the contact surface …
Validation Of Turbulence Models In The Beggar Code For Unsteady Flow, Kevin G. Storm
Validation Of Turbulence Models In The Beggar Code For Unsteady Flow, Kevin G. Storm
Theses and Dissertations
Validation of the turbulence models in the CFD code Beggar for unsteady turbulent flow are discussed. Six validation cases of the code are considered, three cases with the intent of validating the code without the turbulence model and three cases to validate the turbulence model itself. Finally, a more realistic simulation of a simplified store is examined. The turbulence models considered are the Baldwin-Lomax, Spalart-Allmaras, and a DES model. The conclusions made deal with necessary prerequisites to properly simulating unsteady turbulent flow and model selection. The prerequisites necessary in the Beggar code are a second order temporal discretization and the …
Characterization Of Upstream Mixing Cavities And A Downstream Combustion Cavity In Supersonic Flow, Adam P. Quick
Characterization Of Upstream Mixing Cavities And A Downstream Combustion Cavity In Supersonic Flow, Adam P. Quick
Theses and Dissertations
A key area of study in air-breathing hydrocarbon-fueled scramjet combustors is the characterization of cavity-based fuel injection and flame holding. One issue concerns oscillatory disturbances caused by trapped vortices in the main flame holder cavity as a fuel-air mixing enhancement technique. Previous research demonstrates that oscillatory disturbances can be carried downstream via the shear layer and alter the oscillatory characteristics of a downstream cavity. This study investigates the mixing effectiveness of three upstream direct-fueled mixing cavities as well as the effect on the oscillatory behavior of the downstream combustion cavity by the upstream mixing cavity. The three upstream mixing cavities …
The Use Of X-Ray Pulsars For Aiding Gps Satellite Orbit Determination, Dennis W. Woodfork Ii
The Use Of X-Ray Pulsars For Aiding Gps Satellite Orbit Determination, Dennis W. Woodfork Ii
Theses and Dissertations
This research proposes the use of an existing "signal of opportunity" - namely x-ray pulsars - to improve the accuracy and robustness of the GPS satellite and clock estimation algorithm. Improvement in satellite and clock accuracy results in a direct benefit to the user. A simulation has been developed to determine the effects of using x-ray pulsar measurements on the GPS Operational Control Segment. The epoch-specific position, velocity, and clock errors of all GPS satellites in the constellation were estimated using both pseudoranges and time-difference-of-arrival (TDOA) measurements from pulsars. The primary measure of accuracy is a constellation Signal-In-Space Range Error …
Maneuver Estimation Model For Relative Orbit Determination, Tara R. Storch
Maneuver Estimation Model For Relative Orbit Determination, Tara R. Storch
Theses and Dissertations
While the use of relative orbit determination has reduced the difficulties inherent in tracking geosynchronous satellites that are in close proximity, the problem is often compounded by stationkeeping operations or unexpected maneuvers. If a maneuver occurs, observations will no longer fit predicted data, increasing the risk of misidentification and cross-tagging. The goal of this research was to develop a model that will estimate the magnitude, direction, and time of a suspected maneuver performed by a collocated satellite in geosynchronous orbit. Relative motion was modelled using Hill's equations, and least squares estimation was employed to create both a linear non-maneuver model …
Weight Analysis Of Two-Stage-To-Orbit Reusable Launch Vehicles For Military Applications, Richard A. Caldwell
Weight Analysis Of Two-Stage-To-Orbit Reusable Launch Vehicles For Military Applications, Richard A. Caldwell
Theses and Dissertations
In response to Department of Defense (DoD) requirements for responsive and low-cost space access, this design study provides an objective empty weight analysis of potential reusable launch vehicle (RLV) configurations. Each two-stage-to-orbit (TSTO) RLV has a fixed payload requirement of 20,000 lbf to low Earth orbit. The propulsion systems considered in this study include pure rocket, pure turbine, rocket-based-combined-cycle (RBCC), and turbine-based-combined-cycle (TBCC). The hydrocarbon dual-mode scramjet (DMSJ) engines used in the RBCC and TBCC propulsion systems represent possible applications of the current research being performed in the U.S. Air Force HyTech program. Two sensitivity analyses were then performed on …
Biaxial Fatigue Behavior Of Niti Shape Memory Alloy, Daniel M. Jensen
Biaxial Fatigue Behavior Of Niti Shape Memory Alloy, Daniel M. Jensen
Theses and Dissertations
Nitinol is a shape memory alloy (SMA) capable of martensite-to-austenite phase transformations enabling shape-memory behavior. Shape-memory properties make Nitinol a strong candidate material for use in aircraft applications such as actuators. Structural integrity and reliability of torque tube actuators must be assured before this material can be used in flight-critical components. Thorough understanding of the fatigue response of the material is essential for a structurally-sound SMA actuator design. The present effort investigates pure torsion and combined tension-torsion fatigue behavior of Nitinol at room temperature. Monotonic tests in tension and torsion were conducted to typify uni-directional stress-strain behavior. Fully-reversed torsion fatigue …
A Numerical Analysis For Passive Attitude Stabilization Using A Tethered Balloon On A Gravity Gradient Satellite, Ernest Matias Maramba
A Numerical Analysis For Passive Attitude Stabilization Using A Tethered Balloon On A Gravity Gradient Satellite, Ernest Matias Maramba
Theses and Dissertations
This research effort analyzes the fundamental dynamics governing a satellite with a gravity gradient boom and a tethered balloon. Satellites that use gravity gradient booms for passive attitude control are characterized by undamped pitch oscillations and no roll control. The tethered balloon acts as a high drag device that accounts for the most drag on the satellite system. By attaching a drag device, the system resists rolling movements while also damping oscillations. This could potentially be a cost effective method for increasing satellite stabilization. The goal of this research is to model the dynamics and determine the feasibility of a …
Performance Study Of Staging Variable On Two-Stage-To-Orbit Reusable Launch Vehicles, James K. Nilsen
Performance Study Of Staging Variable On Two-Stage-To-Orbit Reusable Launch Vehicles, James K. Nilsen
Theses and Dissertations
The purpose of this research is to investigate the effects of staging variables on Two-Stage-To-Orbit reusable launch vehicles, specifically, the question of what measurable factors play important roles in staging performance. Three different configurations (Rocket-Rocket, Turbojet-Rocket and Turbine Based Combined Cycle-Rocket) were considered. The software, Program to Optimize Simulated Trajectories (POST), was used to analyze these configurations. Vehicle coasting time, staging dynamic pressure and staging Mach number were all varied to determine their influence on the final payload.