Showing papers by "Wright-Patterson Air Force Base published in 1995"

New polymer electrolyte nanocomposites: Melt intercalation of poly(ethylene oxide) in mica‐type silicates

Abstract: Polymer electrolyte nanocomposites have been synthesized by direct melt intercalation of poly(ethylene oxide) (PEO) in layered silicates. Intercalation of the polymer chains in the silicate galleries greatly suppresses their tendency to crystallize. The conductivity of PEO/Li + -montmorillonite nanocomposite containing 40 wt% PEO is 1.6x10 -6 S/cm at 30°C and exhibits a weak temperature dependence with an activation energy of 2.8 kcal/mol

Two-photon absorption and optical-limiting properties of novel organic compounds

Abstract: The optical-limiting behavior and two-photon absorption properties of four novel organic compound solutions in tetrahydrofuran have been investigated. An ultrashort laser source with 0.5-ps pulse width and 602-nm wavelength was employed. The transmissivities of the various 1-cm-thick solution samples have been measured as a function of the beam intensity as well as of the solute concentration. The measured results can be fitted on the assumption that two-photon absorption is the only predominant mechanism causing the observed opticallimiting behavior. Based on the intensity-dependent transmissivity measurements, the molecular two-photon absorption coefficients for the four compounds are presented.

Digital Techniques for Wideband Receivers

Abstract: From the Publisher: With the most comprehensive description available of digital processing applicable to wideband receivers, this book helps you better understand the requirements of wideband digital RF receivers, learn numerous solutions to receiver problems, and get an overview of future trends in receiver development.

System identification for adaptive and reconfigurable control

Abstract: In this work, an efficient static system identification method is developed that incorporates prior information. The prior information from flight mechanics enhances the performance of the identification algorithm, yielding more accurate parameter estimates. The proposed static system identification approach is superior to dynamic system identification for on-line identification of rapidly varying plant parameters, making it suitable for adaptive and reconfigurable control. The effectiveness of the static system identification scheme is illustrated in an example problem. A derivative F-16 aircraft control surface (elevator) failure is simulated and the abruptly changing pitch channel control system's stability and control derivatives are successfully identified on-line in the presence of unmodeled dynamics, process noise (clear air turbulence), and realistic measurement noise.

Three-photon-absorption-induced fluorescence and optical limiting effects in an organic compound

Abstract: Three-photon-absorption-induced frequency-upconversion f luorescence emission has been observed in a solution of 2,5-benzothiazole 3,4-didecyloxy thiophene (BBTDOT) in tetrahydrofuran, pumped with 10-ns Q-switched 1.06-microm laser pulses. The spectral peak of the observed fluorescence emission is located in the 450-480-nm range, and the intensity dependence of the visible emission on the IR excitation obeys the cubic law. At a higher solute concentration (0.18 M/L) and moderate IR excitation intensity levels (50-200 MW/cm(2)), obvious optical limiting behavior has been observed as a result of three-photon absorption. The measured nonlinear absorption coeff icient and the corresponding molecular three-photon-absorption cross section are gamma= 2.7 x 10(-18) cm(3)/W(2) and sigma'(3) = 8.8 x 10(-76) cm(6) s(2), respectively.

Behavior of linear reconstruction techniques on unstructured meshes

Abstract: : This report presents an assessment of a variety of reconstruction schemes on meshes with both quadrilateral and triangular tessellations. The investigations measure the order of accuracy, absolute error and convergence properties associated with each method. Linear reconstruction approaches using both Green-Gauss and least squares gradient estimation are evaluated against a structured MUSCL scheme wherever possible. In addition to examining the influence of polygon degree and reconstruction strategy, results with three limiters are examined and compared against unlimited results when feasible. The methods are applied on quadrilateral, right triangular, and equilateral triangular elements in order to facilitate an examination of the scheme behavior on a variety of element shapes. The numerical test cases include well known internal and external inviscid examples and also a supersonic vortex problem for which there exists a closed form solution to the 2-D compressible Euler equations. Such investigations indicate that the least squares gradient estimation provides significantly more reliable results on poor quality meshes. Furthermore, limiting only the face normal component of the gradient can significantly increase both accuracy and convergence while still preserving the integral cell average, and maintaining monoticity. The first order method performs poorly on stretched triangular meshes, and analysis shows that such meshes result in poorly aligned left and right states for the Riemann problem. The higher average valence of a vertex in the triangular tessellations does not appear to enhance the wave propagation, accuracy, or convergence properties of the method. Unstructured, Upwind, Inviscid, Reconstruction, Limiters, Riemann problems.

Volume holographic image storage and electro-optical readout in a polymer-dispersed liquid-crystal film.

Abstract: We report storage and electrical switching of holographic image data in an economical polymer-dispersed liquidcrystal material. The hologram is recorded in a fast, single-step process and can be reversibly erased and restored repeatedly by the application of fields of approximately 10-15 V/ microm, with a response time of 22 micros and a relaxation time of 42 micros. Simple (quasi-sinusoidal) holographic transmission gratings also are studied with switching fields of <5 V/ microm and with response and relaxation times of 25 and 44 micros, respectively.

Mobility assist for the paralyzed, amputeed and spastic person

Abstract: Apparatus to actively assist neuromotor disabled to have better leg functional control/use This system serves as a strength enhancer, support device, and attenuator of spastic motions The disclosed orthosis is worn on the outside of the leg (or legs) and serves as an active/dynamic brace to prevent leg spasms and other untoward leg motion It is made of lightweight material such as fiberglass or aluminum and is actuated by pneumatic gas sources which are small, portable, and carried with the device The device is totally self contained and has no external connection The system has applicability to patients such as: stroke (cerebral vascular accident), TBI (traumatic brain injury), Muscular Dystrophy, and some spinal cord injured

Reconfigurable tracking control with saturation

Abstract: On-line system identification and on-line optimization is used to maximize aircraft tracking performance before and after control surface failure, yet prevent instability and departure. The on-line optimization consists of receding horizon/model predictive optimal control with one-step-ahead actuator rate constraint enforcement. Such hard constraint enforcement is rigorous through use of a linear programming algorithm to include rate saturation in the design. The paper demonstrates that the full, infinity norm solution with one-step-ahead constraint enforcement is equivalent to a simple one-dimensional optimization whose solution is attainable in closed form. This equivalence allows for an efficient on-line implementation of reconfigurable control with hard actuator rate constraints. Good tracking performance after severe failure is demonstrated in realistic simulations.

Effect of microstructure on fatigue and tensile properties of the gamma TiAl alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W

Abstract: The relationships between the microstructure and tensile and axial load-controlled fatigue properties of the alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W (atomic per cent) have been studied. Two different microstructures, i.e. duplex (grain size, 20 μm) and fully lamellar (grain size, 300 μm), were produced, through two-step forging and subsequent heat treatments, giving similar yield strengths at room temperature. The fracture strains at room temperature were about 1.1% and 2.9% for the materials with the fully lamellar and the duplex microstructures respectively. At 600 °C, the duplex material shows a 15% higher fatigue strength than that of the fully lamellar material. At this temperature, the gamma alloy of both microstructures reaches high ratios of the fatigue strength at 10 7 cycles to the ultimate tensile strength (UTS), i.e. about 0.95. At 800 °C, the fully lamellar material exhibits higher fatigue strength values above 10 5 cycles, and both microstructures result in a two-stage behavior, in contrast to the test at 600 °C. The second stage features the characteristic conventional fatigue behavior, with a broad amplitude stress range, while the first stage is characterized by a narrow band of fatigue stress levels near the UTS. The fracture modes for the duplex material showed a general trend from transgranular to intergranular failure with increasing temperature. For the fully lamellar material, a change from predominantly translamellar failure to a mixture of inter lamellar and translamellar failure was observed, resulting in a microscopically and macroscopically rough fracture surface. The strain rate sensitivity of the fully lamellar material was negligible in the temperature range tested.

Computation of Delta-Wing Roll Maneuvers

Abstract: This article presents computations of delta-wing roll maneuvers for an 80-deg sweep delta-wing at 30-deg angle of attack. Three constant roll-rate maneuvers are considered. Two of the maneuvers consist of a roll from 0 to 45 deg at nondimensiona l roll rates of = 0.0233 and 0.0467. The third roll maneuver computed starts at a 45-deg roll angle and rolls back to a -45-deg roll angle at a roll rate 4> = -0.0467. The governing equations are the unsteady, three-dimensional Navier-Stokes equations. The equations are solved using the implicit, approximately-factored, diagonal form of the Beam-Warming algorithm. Subiterations are used to provide a more accurate means of implementing the diagonal form of the algorithm for unsteady flows. The effects of roll-rate and differing initial roll angles on the dynamical behavior of the vortices positions and strengths as well as their corresponding effect on surface pressure and roll moment coefficient are described.

Space-time correlation measurements in a hypersonic transitional boundary layer

Abstract: An experimental investigation of the spatial structure of second-mode instability waves was carried out in the boundary layer of a 7-deg half-angle, sharp-nosed cone at an edge Mach number of 6.8. Measurements were made at Reynolds numbers of 2.3 x 10 6 to 9.1 x 10 6 based on boundary edge conditions, spanning the range from unstable laminar flow to nearly turbulent flow. Simultaneous measurements with two hot-film probes in the boundary layer comprise the primary data set. The mean boundary-layer state was measured with pitot and total temperature surveys. Correlations were taken with circumferential, streamwise, and vertical probe separations. The second-mode waves preceding transition are relatively limited in their circumferential dimension, typically less than four boundary-layer thicknesses, based on a coherence level of 20%.

Friction behavior of pulsed laser deposited tungsten disulfide films

Abstract: This research describes the friction behavior of pulsed laser-deposited tungsten disulfide films. A ball-on-flat apparatus, in which a 440C stainless steel ball was held on a rotating disk coated with a WS2 film, was used as the lest configuration. Friction measurements were made in dry nitrogen and in laboratory air. Wear surfaces were characterized by scanning electron microscopy and Raman spectroscopy. The friction coefficient of the film in dry nitrogen was 0.04, and in laboratory air it rose to between 0.10 and 0.15. In the dry nitrogen case, friction induced some degree of crystallinity into the otherwise amorphous film, while rubbing in air mostly resulted in oxidation of the film. Transfer films formed in a dry environment were smooth, tenacious and firmly adherent to the steel counterface. By contrast, the films formed in air were patchy and powdery in nature. Presented at the 49th Annual Meeting in Pittsburgh, Pennsylvania May 1–5, 1994

Estimation of Thermal Properties and Surface Heat Flux in Carbon-Carbon Composite

Abstract: This article discusses a laboratory method of measurement of the thermal properties of a carbon matrix carbon fiber composite material. Another important objective is the determination of the surface heat flux from measured temperatures within the body. This article is unique because the thermal properties are being estimated with known heat flux and temperature histories; then the opposite problem of recovering the heat flux history is solved using the estimated values for the thermal properties and temperature history. This calculated heat flux history can then be compared with the measured input heat flux. Results were obtained for a temperature range of 30-600 C. The thermal properties demonstrate a presumed quadratic relationship with temperature, and a good agreement between the estimated and measured heat flux histories is demonstrated. 18 refs.

Derivation of a general expression for the momentum matrix elements within the envelope-function approximation.

Abstract: A general expression for the momentum matrix elements for both intrasubband and intersubband transitions in n- or p-type doped quantum wells, wires, and dots is derived within the envelope-function approximation, in the process unifying the description of optical absorption in n- and p-type heterostructures The derivation is nontrivially extended to the case of wave-function penetration into the barrier in a way that satisfies the principle of microscopic reversibility The contribution of the valence-band anisotropy is shown to contribute to normal-incidence absorption in p-type heterostructures

Creep deformation in near-γ TiAl: Part 1. the influence of microstructure on creep deformation in Ti-49Al-1V

Abstract: The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxedy constituent, that results from partitioning of oxygen from the γ phase to the α2 phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of γ/γ or γ/α2 interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed y microstructures.

Structure of a turbulent double-fin interaction at Mach 4

Abstract: This paper examines the three-dimensional shock-wave/turbulent boundary-layer interaction in an inletlike geometry consisting of symmetrically placed 15-deg fins on a plate in an oncoming Mach 4 flow. Two different implicit numerical methods are employed to solve the full three-dimensional mean compressible Navier-Stokes equations: 1) a high resolution upwind scheme with the Baldwin-Lomax algebraic turbulence model and 2) the traditional Beam-Warming approach with a k-e model. The comparison with experiment is very good for surface pressures, pitot pressure surveys, and surface shear patterns, although discrepancies with skin friction data are evident in the trailing portion of the interaction. Both computational schemes, despite major differences in modeling, predict similar flow structures in agreement with previous results at Mach 8. The flow is described in terms of a separated, nonreattaching boundary layer, a vortex interaction, longitudinal centerline vortices, and entrainment. The computed and experimental shock patterns are related to the streamline structure to provide a unified understanding of the flowfield.

Structure of a double-fin turbulent interaction at high speed

Abstract: This study examines the interaction of a Mach 8.3 turbulent boundary layer with intersecting oblique shock waves generated by 15-deg sharp fins mounted symmetrically on a flat plate. The full three-dimensional mass-averaged compressible Navier-Stokes equations are solved with a high-resolution implicit finite-volume scheme. Turbulence closure is achieved with variations of the Baldwin-Lomax algebraic model. Excellent agreement with experimental data is observed for plate surface pressure. However, accurate heat transfer rates are obtained only near the plane of symmetry. Some quantitative details are dependent on the manner in which the turbulence model is implemented. Within this limitation, the overall computed mean flow structure remains similar and mesh independent and compares well with available field surveys. The incoming flat plate boundary layer separates along the entire spanwise width and does not reattach in the domain of computation. Beneath the separated boundary layer are a vortex interaction with an off-surface stagnation point, a centerline longitudinal vortex, and an entrainment flow originating from the essentially inviscid stream near the fin leading edge.

Creep deformation in near-γ TiAl. II: Influence of carbon on creep deformation in Ti-48Al-1V-0.3C

Abstract: The influence of interstitial strengthening and microstructure on creep deformation has been examined in the near-γ TiAl alloy Ti-48Al-lV-0.3C. Creep studies were conducted under constant load in air at 815 °C in the stress range of 50 to 200 MPa. Significant improvement in creep resistance was observed in this alloy compared with a similar alloy (Ti-49Al-lV) containing low levels of carbon (0.07 at. pct). The degree of strengthening resulting from the addition of carbon was found to be dependent on microstructure. At 815 °C and 150 MPa, the addition of carbon reduced the minimum creep rate by a factor of approximately 20 in the equiaxedy and duplex microstructures and by a factor of 3 in the fully lamellar microstructures. Carbide precipitation occurred in this alloy when aged in the temperature range of 700 °C to 950 °C. The addition of carbon leads to a decrease in the stress exponent from 4 to 3 in the duplex and equiaxedy microstructures and the inhibition of sub-boundary formation in the duplex microstructure. This suggests that solute/dislocation interaction mechanisms, rather than a direct effect of carbide precipitates, are responsible for the significant increase in creep resistance observed in this alloy.

Characteristic-based, time-dependent Maxwell equation solvers on a general curvilinear frame

Abstract: Four cell-centered, finite-volume characteristic-based procedures for solving time-dependent, three-dimensional Maxwell equations on a general curvilinear frame are developed and implemented. On a spherical coordinate system, these methods generate simulations or an oscillating electric dipole without wave reflection from the truncated computational domain. Through a comparative study, it is established that an upwind-biased third-order scheme demonstrates less dispersive and dissipative errors than second-order schemes

Mode Tracking Issues in Structural Optimization

Abstract: Within the context of optimization of the structural dynamics properties of finite element models, methodology is developed for the tracking of eigenpairs through changes in the structural eigenvalue problem. The goal is to eliminate difficulties caused by "mode switching" (i.e., frequency crossing). Out of several candidate methods, two methods for mode tracking are successful. The first method, the higher order eigenpair perturbation algorithm, is based on a perturbation expansion of the eigenproblem. It iteratively computes changes in the eigenpairs due to parameter perturbations with the important feature of maintaining the correspondence between the baseline and perturbed eigenpairs. The second method is a cross-orthogonality check method, which uses mass orthogonality to reestablish correspondence after a standard reanalysis. Modified eigenpair extraction routines (Lanczos, subspace iteration, inverse power) were unsuccessful in tracking modes. Applications of mode tracking technology that are presented are frequency-constrained optimization and optimization with mode shape constraints. Each application procedure is outlined and examples are given. Recommendations are made based on method efficiency and robustness in the example problems.

The effects of JP-8 jet fuel on male Sprague-Dawley rats after a 90-day exposure by oral gavage.

Abstract: The U.S. Air Force is converting from JP-4 jet fuel to the less volatile JP-8 jet fuel, which is similar to commercial Jet Fuel A. Our previous 90-day inhalation study with JP-8 vapor, using F-344 rats and C57BL/6 mice, resulted in no treatment-related adverse effects other than alpha 2-microglobulin nephropathy in male rats (Mattie et al., 1991). In the present study, male rats were dosed with neat JP-8 (0, 750, 1500, 3000 mg/kg) daily by gavage for 90 days in an effort to characterize the kidney lesion and assess further any additional adverse effects associated with prolonged oral exposure to this fuel. Results of this study revealed a significant dose-dependent decrease in body weights of rats exposed to JP-8. Male rat-specific alpha 2-microglobulin nephropathy was observed by histopathologic examination. A number of significant changes were also seen in blood and urine that were not dose-dependent. Additional treatment-related effects were a gastritis and a perianal dermatitis. Although there were no histopathological or weight changes in the livers of exposed rats, there was an increase in the liver enzymes AST and ALT. The elevated enzymes did not increase with increasing dose of JP-8.

A pointing technique for rapidly collecting localization responses in auditory research

Abstract: A technique is described for rapidly collecting responses in auditory-localization experiments. Subjects indicate the perceived direction of the sound by pointing at a 20-cm-diam spherical model. In Experiment 1, the subjects judged the direction of a broadband signal, which could originate from any of 239 directions ranging through 360° of azimuth and between −45° and +90° of elevation. Using this technique, the subjects responded 2–8 times more rapidly than previous subjects who used either a verbal-reporting technique or a head-pointing technique. Localization judgments were as accurate as they had been with verbal reports, but were not as accurate as judgments collected using the head-pointing technique. In Experiment 2, the signal was turned off and the experimenter read the spherical coordinates of the signal location to the subjects. The subjects pointed to these coordinates more accurately than they had judged the direction of the sounds in Experiment 1, suggesting that the response technique had not been the limiting factor in that experiment. Circumstances relevant to the choice of response techniques for auditory-localization experiments are discussed.