Showing papers by "Wright-Patterson Air Force Base published in 1998"

Neutral-donor–bound-exciton complexes in ZnO crystals

Abstract: Neutral-donor--bound-exciton transitions have been observed in ZnO. The isolated neutral donors are made up of defect pair complexes. The neutral-donor nature of these pair complexes was determined from magnetic-field measurements and from two-electron transitions. Excited states of the neutral-donor bound excitons were observed in the form of rotator states analogous to rotational states of the ${\mathrm{H}}_{2}$ molecule.

High-Resolution Image Reconstruction from a Sequence of Rotated and Translated Frames and its Application to an Infrared Imaging System

Abstract: Some imaging systems employ detector arrays which are not su‐ciently dense so as to meet the Nyquist criteria during image acquisition. This is particularly true for many staring infrared imagers. Thus, the full resolution afiorded by the optics is not being realized in such a system. This paper presents a technique for estimating a high resolution image, with reduced aliasing, from a sequence of undersampled rotated and translationally shifted frames. Such an image sequence can be obtained if an imager is mounted on a moving platform, such as an aircraft. Several approaches to this type of problem have been proposed in the literature. Here we extend some of this previous work. In particular, we deflne an observation model which incorporates knowledge of the optical system and detector array. The high resolution image estimate is formed by minimizing a new regularized cost function which is based on the observation model. We show that with the proper choice of a tuning parameter, our algorithm exhibits robustness in the presence of noise. We consider both gradient descent and conjugate gradient optimization procedures to minimize the cost function. Detailed experimental results are provided to illustrate the performance of the proposed algorithm using digital video from an infrared imager.

Viscoelastic and morphological behavior of hybrid styryl-based polyhedral oligomeric silsesquioxane (POSS) copolymers

Abstract: We report on the viscoelastic behavior of linear thermoplastic nonpolar hybrid inorganic-organic polymers. These materials have been synthesized through copolymerization of an oligomeric inorganic macromer with 4-methylstyrene where the inorganic portion of the material is a well-defined polyhedral oligosilsesquioxane (POSS), R7(Si8O12)(CH2CH2C6H4CH|CH2), with R A c-C6H11 or c-C5H9. A series of 4-methyl styrene copolymers with approximately 4, 8, and 16 mol % POSS macromer incorporation were investigated. Rheological measurements show that the polymer dynamics are profoundly affected as the percent of POSS increases. In particular, a high-temperature rubbery plateau develops (where a terminal zone is not observed), despite the fact that the parent poly 4-methylstyrene is unentangled. It is also observed that the thermal properties are influenced as the percent of POSS incorporation in- creases, with increases in the glass and decomposition temperatures. The results sug- gest that interchain interactions between the massive inorganic groups are responsible for the retardation of polymer chain motion, a mechanism similar to the ''sticky repta- tion'' model conceived for hydrogen-bonded elastomers and developed by Leibler et al. (Macromolecules, 24, 4701 (1991)).Control over the interchain interactions would also give rise to the observed increases in glass transition and the establishment of a rubbery plateau. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1857-1872, 1998

Characteristics of a Trapped-Vortex Combustor

Abstract: The characteristics of a Trapped-Vortex (TV) combustor are presented. A vortex is trapped in the cavity established between two disks mounted in tandem. Fuel and air are injected directly into the cavity in such a way as to increase the vortex strength. Some air from the annular flow is also entrained into the recirculation zone of the vortex. Lean blow-out limits of the combustor are determined for a wide range of annular air flow rates. These data indicate that the lean blow-out limits are considerably lower for the TV combustor than for flames stabilized using swirl or bluff-bodies. The pressure loss through the annular duct is also low, being less than 2% for the flow conditions in this study. The instantaneous shape of the recirculation zone of the trapped vortex is measured using a two-color PIV technique. Temperature profiles obtained with CARS indicate a well mixed recirculation zone and demonstrate the impact of primary air injection on the local equivalence ratio.

Load-adaptive crystalline–amorphous nanocomposites

Abstract: Advances in laser-assisted deposition have enabled the production of hard composites consisting of nanocrystalline and amorphous materials Deposition conditions were selected to produce super-tough coatings, where controlled formation of dislocations, nanocracks and microcracks was permitted as stresses exceeded the elastic limit This produced a self-adjustment in the composite deformation from hard elastic to quasiplastic, depending on the applied stress, which provided coating compliance and eliminated catastrophic failure typical of hard and brittle materials The load-adaptive concept was used to design super-tough coatings consisting of nanocrystalline (10–50 nm) TiC grains embedded in an amorphous carbon matrix (about 30 vol%) They were deposited at near room temperature on steel surfaces and studied using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, Raman spectroscopy, nanoindentation and scratch tests Design concepts were verified using composition–structure–property investigations in the TiC–amorphous carbon (a-C) system A fourfold increase in the toughness of hard (32 GPa) TiC–a-C composites was achieved in comparison with nanocrystalline single-phase TiC

Spray Structures of Liquid Jets Atomized in Subsonic Crossflows

Abstract: The structures of spray plumes from 0.5-mm waterjets injected into a subsonic crosse ow were experimentally investigated using phase Doppler particle anemometry. Droplet size, axial velocity, and volume e ux were measured across the spray plume at several axial distances downstream of the injector exit. Results indicate that large droplets can be found in the central portion of the spray plume for cases with small liquid/air momentum e ux ratios and in which the momentum exchange between column waves and the airstream is signie cant. For cases with large-momentum e ux ratios, the droplet size distribution exhibits a concave-layered structure, with the peak on the centerline and large droplets at the top. Droplets were found to concentrate in a small area within the spray plume, which indicates that the liquid mass distribution is not uniform. The height of the maximum volume e ux locations, an indicator of the location of the highest concentration of droplets, was measured and correlated with momentum e ux ratios and axial distances. It was found that more droplets are distributed toward the upper portion of the spray plume for larger momentum e ux ratios. Spray penetration, spray width, penetration-to-width ratio, and spray cross-sectional area were also found to increase with the momentum e ux ratio.

Accuracy and Coupling Issues of Aeroelastic Navier-Stokes Solutions on Deforming Meshes

Abstract: An implicit time-accurate approach to aeroelastic simulation was developed with particular attention paid to the issues of time accuracy, structural coupling, grid-deformation strategy, and geometric conservation. A Beam ‐Warming, approximate-factored algorithm, modie ed to include Newton-like subiterations was coupled with a structural model, also in subiteration form. With a sufe cient number of subiterations, this approach becomes a fully implicit, e rst- or second-order-accurate aeroelastic solver. The solver was used to compute time-accurate solutions of an elastically mounted cylinder. The fully implicit coupling allowed the overall scheme to become second-order accurate in time, signie cantly reducing the workload for a given accuracy. A new algebraic grid deformation strategy was developed that preserves grid orthogonality near the surface under large deformations. Finally, the oscillatory behavior of an elastically mounted cylinder was reproduced accurately by the present approach, and results compared favorably to previous experiments and simulations.

Plasma electrolytic fabrication of oxide ceramic surface layers for tribotechnical purposes on aluminium alloys

Abstract: The problem of technical and economical optimization of the process of micro-arc discharge oxidation of high-strength aluminium for the fabrication of oxide ceramic layers for tribotechnical purposes is considered in terms of experimental design. To estimate the effectiveness of the process, a generalized parameter is used which accounts for oxide mass yield as a principal parameter, and mechanical and geometrical characteristics of the layer as restricting parameters. The methods of chemical weight, scanning electron microscopy, optical and durometric analyses are used. The influence of the silicate–alkali electrolyte composition and the amount of electricity carried through the cell on the layer properties is discussed. The response surface of the generalized parameter is plotted with the aid of desirability functions. The area of regimes corresponding to 2–3 g l −1 KOH and 2–3 g l −1 Na 2 SiO 3 electrolyte composition and (2.50–3.33)×10 3 C m −2 of carried electricity is outlined for the most effective fabrication of uniform oxide layers with 165–190 μm thickness and 18–23 GPa hardness.

Comparison of Physical and Aerodynamic Ramps as Fuel Injectors in Supersonic Flow

Abstract: An experimental investigation was conducted to compare the supersonic mixing performance of a novel e ush-wall aerodynamic ramp injector with that of a physical ramp injector. The aerodynamic ramp injector consists of nine e ush-wall jets arranged to produce fuel‐ vortex interactions for mixing enhancement in a supersonic crosse ow. Test conditions included a Mach 2.0 crosse ow of air with a Reynolds number of 3.63 10 7 per meter and helium injection with jet-to-freestream momentum e ux ratios of 1.0 and 2.0. Conventional probing techniques, including species composition sampling, were employed to interrogate the e owe eld at several downstream locations. Results show that with increasing jet momentum the aeroramp exhibited a signie cant increase in fuel penetration, whereas the physical ramp showed no discernible change. The near-e eld mixing of the aeroramp was superior to that of the physical ramp; however, the physical ramp reaches a fully mixed condition at approximately half the distance of the aeroramp. As the jet momentum was increased, the far-e eld mixing performance of the aeroramp approached that of the physical ramp. In all cases the total pressure loss incurred with the aeroramp was less than that caused by the physical ramp. For both injectors the total pressure loss decreased with increasing jet momentum. It was concluded that, although physical ramps may provide better far-e eld mixing, properly designed e ush-wall injection can provide comparable mixing performance while avoiding the practical problems associated with an intrusive geometry in a scramjet combustor.

Refractive-Index Measurements of Undoped Yttrium Aluminum Garnet from 0.4 to 5.0 mum.

Abstract: We report measurements of the refractive index of undoped yttrium aluminum garnet from 0.4 to 5.0 mum and the calculation of Sellmeier coefficients based on our data. The data differ considerably from previously published reports. The effect of the new data on the design of optical devices such as intracavity etalons for lasers is discussed.

Green's function boundary conditions in two-dimensional and three-dimensional atomistic simulations of dislocations

Abstract: A method for dynamically updating the boundary conditions of atomistic simulations is presented. The lattice Green's function boundary relaxation method, originally introduced by Sinclair et al. in 1978, is extended to treat three-dimensional (3D) simulations. The boundary conditions for two-dimensional (2D) and 3D defect cells are evaluated using line and point force distributions respectively. The method is general and has been incorporated into several potential interaction schemes. Examples of the method using embeddedatom method potentials are presented for the following: simulation of a straight (a/2)[110] screw dislocation in Ni; an isolated a kink on an (a/2)[111] screw dislocation in bcc Fe; simulation of a periodic array of a kinks on an (a/2)[111] screw dislocation in bcc Fe. The first simulation is 2D in nature and the last two defects are 3D.

Investigation of reverse-saturable absorption in brominated porphyrins

Abstract: To develop more efficient materials to be used in optical limiters, we have synthesized a series of metal-containing brominated tetraphenylporphyrin (MOBP) reverse-saturable absorber (RSA) dyes. ZnOBP was the highest efficiency RSA chromophore. Moreover, ZnOBP had nonlinear absorption that compared well to state-of-the-art phthalocyanine RSA dyes. The results suggest that other factors (ring nonplanarity, halogenation) influence limiting behavior as well as insertion of heavy metal atoms into the ring.

Role of Blade Passage Flow Structures in Axial Compressor Rotating Stall Inception

Abstract: The influence of three-dimensional flow structures within a compressor blade passage has been examined computationally to determine their role in rotating stall inception. The computations displayed a short length-scale (or spike) type of stall inception similar to that seen in experiments; to the authors’ knowledge this is the first time such a feature has been simulated. A central feature observed during the rotating stall inception was the tip clearance vortex moving forward of the blade row leading edge. Vortex kinematic arguments are used to provide a physical explanation of this motion as well as to motivate the conditions for its occurrence. The resulting criterion for this type of stall inception (which appears generic for axial compressors with tip-critical flow fields) depends upon local flow phenomena related to the tip clearance and it is thus concluded that the flow structure within the blade passages must be addressed to explain the stability of an axial compression system which exhibits such short length-scale disturbances.© 1998 ASME

A hardware-efficient, multirate, digital channelized receiver architecture

Abstract: An approach is presented to realizing a digital channelized receiver for signal intercept applications that provides a hardware efficient implementation of a uniform filter bank in which the number of filters K is greater than the decimation factor M. The proposed architecture allows simple channel arbitration logic to be used and provides reliable instantaneous frequency measurements, even in adjacent channel crossover regions. In the proposed implementation of the filter bank, K is related to M by K=FM where F is an integer. It is shown that the optimum selection of F allows the instantaneous frequency measurement to be made in the channel crossover region and the arbitration function to be based solely on the instantaneous frequency measurement. The development of a filter bank structure which combines the flexibility of the short-time Fourier transform (STFT) with the implementation efficiency of the polyphase filter bank decomposition, meeting these requirements and leading to a hardware-efficient implementation, is presented.

Effects of localized auditory information on visual target detection performance using a helmet-mounted display.

Abstract: An experiment was conducted to evaluate the effects of localized auditory information on visual target detection performance. Visual targets were presented on either a wide field-of-view dome display or a helmet-mounted display and were accompanied by either localized, nonlocalized, or no auditory information. The addition of localized auditory information resulted in significant increases in target detection performance and significant reductions in workload ratings as compared with conditions in which auditory information was either nonlocalized or absent. Qualitative and quantitative analyses of participants′ head motions revealed that the addition of localized auditory information resulted in extremely efficient and consistent search strategies. Implications for the development and design of multisensory virtual environments are discussed. Actual or potential applications of this research include the use of spatial auditory displays to augment visual information presented in helmet-mounted displays, t...

Calculation of the structure and absorption spectra of phthalocyanines in the gas-phase and in solution

Abstract: Ab initio calculations have been carried out on both gas-phase and solvated phthalocyanines in order to determine equilibrium structures, vibrational spectra, and electronic spectra. Density functional theory (DFT) optimized geometries have the expected symmetries of D 2h for free-base phthalocyanine, D 4h for copper phthalocyanine, and C 4v for tin phthalocyanine, whereas Hartree–Fock optimized geometries, which also include lead phthalocyanine, are slightly displaced from the expected symmetries. Both sets of optimized geometries agree reasonably well with the available experimental structures. Vibrational spectra, calculated by the Hartree–Fock method, are in partial agreement with measured spectra, whereas visible absorption spectra calculated using molecular geometries obtained at three different levels of theory (DFT, HF, and PM3) showed good agreement with the measured vapor-phase spectra in the Q-band, and using the DFT optimized geometry resulted in very good agreement with measurements in the B-band. The self-consistent reaction field method for including bulk solvent effects in HF calculations had a small, nearly negligible, effect on the molecular structure of SnPc, whereas the DFT/ cosmo solvation model yielded the predictable result of increasing the Sn–N bond length by pulling the Sn atom further out of the molecular plane.

Study on Trapped-Vortex Combustor-Effect of Injection on Flow Dynamics

Abstract: Low-velocity  ows in the cavities of a combustor can aid in establishing stable  ames. However, unsteady  ows in and around cavities may destabilize these  ames. By proper cavity design it is possible to lock (trap) the vortices spatially and, thereby, stabilize the  ames. The spatially locked vortices restrict the entrainment of main air into the cavity. For obtaining good performance characteristics with a trapped-vortex combustor, a sufŽ cient amount of fuel and air must be injected directly into the cavity. This mass injection can alter the dynamic characteristics of the  ow inside and around the cavity. The present study employed a numerical simulation to investigate the vortex dynamics of a cavity into which  uid mass is directly injected through jets. A third-order-accurate, time-dependent, computational  uid dynamics with chemistry code was used for simulating the dynamic  ows associated with an axisymmetric, centerbody trapped-vortex combustor under nonreacting and reacting conditions. It was found that mass injection increases the optimum size (width-to-diameter ratio) of the cavity. Injection of small amounts of  uid into a nonoptimum cavity increases the unsteadiness of the  ow. Fluid injected into the optimumsize cavity is transported along the outer core of the vortex, providing more efŽ cient mixing and a longer residence time for the fuel/air mixture. It was also found that use of thinner afterbodies results in the cavity  ow being more dynamic. Calculations made with a global-chemistry model revealed that at higher annulus air velocities, combustion is limited to the cavity region. As in the case of cold  ows, the injection jets in reacting  ows are pushed outward from the center when the cavity size is small.

Energy transfer coupling of two-photon absorption and reverse saturable absorption for enhanced optical power limiting

Abstract: We introduce a new approach to enhancing the optical power-limiting function at near-IR wavelength ?800 nm by coupling effective two-photon absorption in one molecule with excited-state absorption in another molecule. We experimentally demonstrate this approach by using a strong two-photon absorbing dye, AF-380, and a strong reverse saturable absorber, C60. A nanosecond time-resolved experiment is used to show that energy transfer from AF-380 to C60 generates triplet excitation in C60 that further absorbs the pump beam to enhance the power-limiting function.

Stability Analysis of a Missile Control System with a Dynamic Inversion Controller

Abstract: Examines the closed-loop stability of a bank-to-turn, air-to-air missile with a dynamic inversion controller using a two time-scale separation assumption. A state-space formulation for the /spl alpha/, /spl beta/, and /spl phi/ dynamics of the missile, assuming the inner-loop dynamic inversion is performed exactly, is presented. It is then shown that, under certain assumptions, the exponential stability of the /spl alpha/, /spl beta/, and /spl phi/ dynamics about the commanded values can be guaranteed if the inner loop design frequency is large enough. An example calculation of the required inner-loop frequency to guarantee stability is done for a particular bank-to-turn missile. Finally, nonlinear six degree-of-freedom simulation results of a maneuver performed with the dynamic inversion controller are presented.

Investigation into three-dimensional laser processing of tribological coatings

Abstract: Tribological coating design has evolved from single layer/single phase to multilayer and composite architectures. The early single-layer coatings were not efficient for arresting cracks, distributing loads, relaxing stress, and preventing adhesive failures. Second-generation coatings with multilayer, gradient, and composite architectures added another dimension to the coating design and allowed much better accommodation of stresses and crack arresting. This paper considers the further evolution of coating design to three dimensions, where the lateral property variation was added to the cross-thickness property variation. The three-dimensional design considerably improved the tribological characteristics of hard coatings, by permitting solid lubricant replenishment inside the friction contacts. A functionally gradient Ti–TiC–TiC/diamond-like carbon coating with an upper layer of a tough nanocrystalline/amorphous composite was used for load support, crack prevention, and stress equalization. This coating was processed with laser irradiation to form grooved tracks along wear paths, which were then filled with MoS2. This provided a solid lubricant reservoir in the lateral dimension of the coating. The three-dimensional coating was tested in long-duration sliding tests at fixed and cycling humidity. The coating exhibited environmental adaptation with friction coefficients of 0.15 in humid air and 0.02 in dry nitrogen. The wear life was increased by at least one order of magnitude in comparison to that for a hard gradient coating with a top layer of MoS2 without any three-dimensional laser processing. Discussions of three-dimensional coating tribological properties, friction and wear mechanisms are provided. The environmental adaptation achieved with three-dimensional coating processing can be beneficial for aerospace applications.

Treatment of diencephalic syndrome with chemotherapy: growth, tumor response, and long term control.

Abstract: BACKGROUND The diencephalic syndrome (DS), which is manifested by progressive emaciation and failure to thrive in an apparently alert, cheerful infant, usually is due to a low grade hypothalamic glioma. Treatment with aggressive surgery and/or radiotherapy is variably successful in controlling disease and may result in severe neurologic sequelae. Chemotherapy recently has been shown to be effective in patients with low grade gliomas of childhood, but it is used infrequently in those with DS. METHODS The authors evaluated the efficacy of a regimen of carboplatin and vincristine on improving weight, causing tumor shrinkage, and delaying the need for alternative therapies in seven children (ages 9-20 months; median age, 11 months) with DS. Five patients weighed less than the 5th percentile for their age at the start of the study, one weighed within the 10th percentile, and one weighed within the 25th percentile. RESULTS At follow-up (range, 6-54 months; median, 28 months), the patients' weights had increased by 66-95% (median, 80%). On magnetic resonance imaging, four patients had a >50% reduction in tumor mass, one had a 25-50% reduction, and two had stable disease. In those patients with radiographic response to treatment, weight gain was accomplished with oral feedings in four of five patients, whereas those with stable disease required nasogastric, nasojejunal, or gastrostomy tube supplementation to maintain weight. Disease progression occurred at a median of 24 months after initiation of chemotherapy, and two patients remained free of progressive disease at last follow-up. Five patients were alive a median of 59 months from diagnosis. The need for radiation or other therapies was delayed in six of seven children. Therapy was tolerated without significant toxicities. CONCLUSION The authors conclude that treatment of DS with a carboplatin and vincristine regimen results in demonstrable weight gain, may result in tumor shrinkage, and in some cases, significantly delays the need for alternative therapies. Cancer 1998;83:166-172. © 1998 American Cancer Society.

Interaction of a vortex with a flat flame formed between opposing jets of hydrogen and air

Abstract: Studies on individual vortex-flame interactions constitute important elements for the understanding of the turbulent-flame structure. Vortices having sufficiently high normal velocity can pass through the flame by extinguishing it locally. In several circumstances they deform the flame surface significantly before attaining extinction conditions. The development of curvature on the flame surface, especially in hydrogen flames, could lead to different quenching patterns. An experimental/numerical investigation is performed to explore possible quenching patterns in opposing-jet diffusion flames. A diluted hydrogen-nitrogen mixture is used as the fuel. Vortices are driven toward the flame surface with different velocities from the air side. The changes in the structure of the flame during its interaction with the incoming vortex are recorded by measuring instantaneous OH-concentration field using the laser-induced fluorescence (LIF) technique. A time-dependent CFDC code that incorporates 13 species and 74 reactions is used for the simulation of these vortex-flame interactions. Both the experiments and calculations have identified two types of quenching patterns: namely, point and annular. It is found that when an air-side vortex is forced toward the flame at a relatively high speed, then the flame at the stagnation line quenches, resulting in a well-known point-quenching pattern. On the other hand, when the vortex is forced at a moderate speed, the flame surface deforms significantly, and quenching develops in an annular ring away from the stagnation line, resulting in an unusual annular-quenching pattern. Detailed analyses performed just before the development of annular quenching and 1 ms later suggest that this unusual annular quenching did not result from the strain rate. Based on the understanding gained from previous investigations on curvature effects in coaxial hydrogen jet flames and the findings made in the present study, it is argued that such quenching develops as a result of the combined effect of preferential diffusion and flame curvature.

Automated modular fixture planning: Geometric analysis

Abstract: Attendant Processes such as fixture and die design are often a necessary but time-consuming and expensive component of a production cycle. Coupling such attendant processes to product design via feature-based CAD will lead to more responsive and affordable product design and redesign. In the context of on-going research in automating fixture configuration design, this paper presents a fundamental study of automated fixture planning with a focus on geometric analysis. The initial conditions for modular fixture assembly are established together with geometric relationships between fixture components and the workpiece to be analyzed. Of particular focus is the design of alternative locating points and components, together with examples of 3-D fixture designs.

Empirical Models Based on Free-Modulus Magnitude Estimation of Perceived Presence in Virtual Environments:

Abstract: A series of 3 studies was conducted to test free-modulus magnitude estimation as a measure of perceived presence in virtual environments (VEs) and to model the first- and second-order effects of 11 VE system parameters on perceived presence across 5 subtasks. Sequential experimentation techniques were used to build 4 empirical models using polynomial regression. An integrated empirical model of data combined across 2 experiments demonstrated that all significant factors had a positive effect on perceived presence. Three of these parameters--field of view, sound, and head tracking--had almost 3 times as much influence on presence than the other 4 significant parameters, which were visual display resolution, texture mapping, stereopsis, and scene update rate. Sequential experimentation was an efficient tool for building empirical models of perceived presence, but the subjective nature of this phenomenon and individual differences made data bridging across sequential studies problematic. It was concluded that magnitude estimation is a useful measure of perceived presence, and the resulting polynomial regression models can be used to facilitate VE system design decisions. This research has broad application in the selection and design of VE system components and overall design of VE systems.