Showing papers by "Wright-Patterson Air Force Base published in 2002"

Biomimetic synthesis and patterning of silver nanoparticles

Abstract: The creation of nanoscale materials for advanced structures has led to a growing interest in the area of biomineralization. Numerous microorganisms are capable of synthesizing inorganic-based structures. For example, diatoms use amorphous silica as a structural material, bacteria synthesize magnetite (Fe3O4) particles and form silver nanoparticles, and yeast cells synthesize cadmium sulphide nanoparticles. The process of biomineralization and assembly of nanostructured inorganic components into hierarchical structures has led to the development of a variety of approaches that mimic the recognition and nucleation capabilities found in biomolecules for inorganic material synthesis. In this report, we describe the in vitro biosynthesis of silver nanoparticles using silver-binding peptides identified from a combinatorial phage display peptide library.

Efficient, tunable, and coherent 0.18 5.27-THz source based on GaSe crystal

Abstract: Continuously tunable and coherent radiation in the wide range 56.8–1618 µm (0.18–5.27 THz) has been achieved as a novel and promising terahertz source based on collinear phase-matched difference frequency generation in a GaSe crystal. This source has the advantages of high coherence, simplicity for tuning, simple alignment, and stable output. The peak output power for the terahertz radiation reaches 69.4 W at a wavelength of 196 µm (1.53 THz), which corresponds to a photon conversion efficiency of 3.3%. A simple optimization of the design can yield a compact terahertz source.

Silica-precipitating peptides isolated from a combinatorial phage display peptide library.

Abstract: Many biological organisms contain specialized structures composed of inorganic materials. Cellular processes in vivo facilitate the organized assembly of mineral building blocks into complex structures. The structural hierarchy and complexity across a range of length scales are providing new ideas and concepts for materials chemistry. Proteins that direct biomineralization can be used to control the production of nanostructured materials and facilitate the fabrication of new structures. Here, we demonstrate that some of the silica-binding peptides isolated from a combinatorial phage peptide display library can be used in precipitating silica from a solution of silicic acid. The results described in this report demonstrate that peptides displayed by phages act as templates in inorganic material synthesis and provide a means of understanding how some of the biological systems may be carrying out materials chemistry in vivo.

Flexible Ab initio boundary conditions: simulating isolated dislocations in bcc Mo and Ta

Abstract: We report the first ab initio density-functional study of the strain field and Peierls stress of isolated screw dislocations in bcc Mo and Ta. The local dislocation strain field is self-consistently coupled to the long-range elastic field using a flexible boundary condition method. This reduces the mesoscopic atomistic calculation to one involving only degrees of freedom near the dislocation core. The predicted equilibrium core for Mo is significantly different from previous atomistic results and the Peierls stress shows significant non-Schmid behavior as expected for the bcc metals.

Control of Low-Pressure Turbine Separation Using Vortex-Generator Jets

Abstract: The application of vortex-generator jets to control separation on the suction surface of a low-pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high-altitude operation of many aircraft gas-turbine engines, as well as the last stages of industrial ground-based gas turbines. Results are presented for steady blowing at jet blowing ratios from zero to four and at several chordwise positions and two freestream turbulence levels. Findings show that above a minimum blowing ratio, which is dependant on the injection location, the pressure loss in the modified blade's wake is reduced by a factor of between two and three. Boundary-layer traverses show that separation is almost completely eliminated with the application of blowing. No significant deleterious effects of vortex-generator jets are observed at higher (nonseparating) Reynolds numbers. The addition of 4% freestream turbulence to the cascade freestream lowers the separation Reynolds number of the turbine blade studied, but does not eliminate the effectiveness of the control technique. The vortex-generator jet control strategy is demonstrated to be a viable technique for low-pressure turbine separation control.

The kinetics of static globularization of Ti-6Al-4V

Abstract: The kinetics of the evolution of the lamellar-colony microstructure to an equiaxed morphology during heat treatment of a hot-worked, two-phase titanium alloy were established. For this purpose, the alpha/beta alloy Ti-6Al-4V was isothermally upset forged at 900 °C or 955 °C and subsequently annealed for times ranging from 0.5 to 100 hours. The degree of the breakup of alpha-phase lamellae into lower-aspect-ratio grains during static annealing was measured and related to the imposed strain estimated using finite-element analysis (FEA). The kinetics of the static globularization of the alpha phase were found to depend on the amount of strain and the annealing temperature but were not affected by the specific deformation temperature in the 900 °C to 955 °C range. These results demonstrated that deformation-induced dislocation substructure has a small effect on the static-globularization process. In addition, the relative globularization kinetics at 900 °C and 955 °C were rationalized in terms of classical coarsening theory.

Fretting fatigue crack initiation mechanism in Ti–6Al–4V

Abstract: r Fretting fatigue crack initiation in titanium alloy, Ti-6Al-4V, was investigated experimentally and analytically by using finite element analysis (FEA). Various types of fretting pads were used in order to determine the effects of contact geometries. Crack initiation location and crack angle orientation along the contact surface were determined by using microscopy. Finite element analysis was used in order to obtain stress state for the experimental conditions used during fretting fatigue tests. These were then used in order to investigate several critical plane based multiaxial fatigue parameters. These parameters were evaluated based on their ability to predict crack initiation location, crack orientation angle along the contact surface and the number of cycles to fretting fatigue crack initiation independent of geometry of fretting pad. These predictions were compared with their experimental counterparts in order to characterize the role of normal and shear stresses on fretting fatigue crack initiation. From these comparisons, fretting fatigue crack initiation mechanism in the tested titanium alloy appears to be governed by shear stress on the critical plane. However, normal stress on the critical plane also seems to play a role in fretting fatigue life. At present, the individual contributions/importance of shear and normal stresses in the crack initiation appears to be unclear; however, it is clear that any critical plane describing fretting fatigue crack initiation behaviour independent of geometry needs to include components of both shear and normal stresses.

Photophysical Characterization of a Series of Platinum(II)-Containing Phenyl-Ethynyl Oligomers

Abstract: A comprehensive photophysical study has been carried out on a series of platinum(II)-containing phenyl−ethynyl oligomers. The compounds are composed of a platinum center attached to two tributylpho...

X‐ray powder diffraction of polymer/layered silicate nanocomposites: Model and practice

Abstract: X-ray powder diffraction in reflection (Bragg-Brentano parafocusing geometry) is extensively used to characterize the structure of polymer/layered silicate nanocomposites (PLSNs). The large basal spacings (d 001 > 2.0 nm) necessitates the collection of data at scattering angles (2θ) of less than 10°. The calculation of an ideal scattering profile for PLSNs provides an avenue to ascertain the influence of experimental parameters and the arrangement, organization, concentration, and composition of constituents on the experimentally observed pattern. This enables better experimental technique, more complete utilization of the scattering data, insight into inconsistencies between scattering and microscopy, and minimization of incorrect interpretation or overinterpretation of data. Because of the strong θ dependence of theoretical and experimental factors at low values of 2θ, careful sample preparation and data evaluation are necessary and should be complemented by microscopic observations, especially for PLSNs with low volume fractions of organically-modified layered silicates (OLS) that are suspected of having exfoliated morphologies. X-ray powder diffraction in reflection alone is insufficient to completely characterize and ascribe PLSN morphology.

Application of Large-Eddy Simulation to Supersonic Compression Ramps

Abstract: Large-eddy simulations of supersonic compression-ramp flowfields were performed by a high-order numerical method, utilizing the Smagorinsky dynamic subgrid-scale model to account for spatially underresolved stresses. Computations were carried out at a freestream Mach number of 3.0 for ramp angles of 8, 16, 20, and 24 deg. Extensive comparisons are made between the respective solutions and available experimental data that were collected at higher Reynolds numbers. These include surface pressure, skin friction, and both mean and fluctuating velocity profiles. For the 24-deg case, a number of experimentally measured statistical quantities are compared to the simulation

Observations of Fiber Fracture and Interfacial Debonding Phenomena Using the Fragmentation Test in Single Fiber Composites

Abstract: Fiber-reinforced polymer composites are being studied for many applications because of their potential for outstanding mechanical properties. To be used in new applications, composites have to be m...

Self-consistent modeling of the flow behavior of wrought alpha/beta titanium alloys under isothermal and nonisothermal hot-working conditions

Abstract: A self-consistent model was applied to predict the plastic flow behavior during hot working of alpha/beta titanium alloys with wrought (equiaxed alpha) microstructures as a function of the flow behavior and volume fractions of the individual phases. For this purpose, constitutive relations that incorporated composition-dependent strength coefficients were determined for the alpha and beta phases. With these constitutive relations and measurements of the specific compositions and volume fractions of the two phases at hot-working temperatures, the flow stress dependence on temperature under nominally isothermal conditions and the (average) strain rates in the individual phases were predicted for Ti-6Al-4V. The effect of temperature transients during hot deformation on the flow stress under nonisothermal (conventional) forging conditions and under nominally isothermal, high strain-rate conditions was also established using the self-consistent modeling approach. In these instances, the effect of a rapid temperature drop or rise, respectively, on the retention of a metastable microstructure was quantified. The predicted flow behaviors showed good agreement with experimental measurements.

Plasma Injection for Hypersonic Blunt-Body Drag Reduction

Abstract: The summary of combined experimental and computational investigations is presented for the plasma injection from a hypersonic blunt body for drag reduction. The systematic pursuit addresses all speculated mechanisms that can generate favorable magnetoaerodynamic interactions: the counterflow jet shock aerodynamic interaction, the nonequilibrium thermodynamic and chemical phenomenon, and the electromagnetic-aerodynamic interaction. The computational study was carried out by solving both the three-dimensional and the axisymmetric, mass-averaged, Navier-Stokes equations for counterflow jet interaction. The experimental investigation of the plasma injection was conducted in a nominal Mach 6 open jet, blowdown tunnel. The weakly ionized, counterflow jet generated by a plasma torch has a vibrational temperature of 4400 K, an electron temperature around 20,000 K, and electron number density greater than 3 × 10 12 /cm 3

Nonlinear Tracking Control for Satellite Formations

Abstract: A tracking control law using a sliding mode framework is derived to control a satellite formation. Hill's relative motion equations are used to model the follower satellite's motion relative to the leader. To minimize fuel required to maintain the formation, each satellite is constrained to reside near a natural orbit. Control forces are applied only to maintain the desired relative motion by correcting for initial offsets and perturbation effects that tend to disperse the formation. The control law is modified to account for the discontinuous nature of the control forces available with the satellite propulsive thrusters. Numerical simulations using a high-fidelity, nonlinear model demonstrate the control law performance for the full nonlinear dynamics with high order perturbations.

Switchable orthorhombic F photonic crystals formed by holographic polymerization-induced phase separation of liquid crystal.

Abstract: Electrically switchable photonic crystals are simply and rapidly formed by holographic polymerization-induced phase separation of liquid crystal from a monomer-liquid crystal mixture. We report the fabrication and electro-optical properties of liquid-crystal-filled polymer photonic crystals of orthorhombic F symmetry. Inverse opal and fcc structures can also be obtained. The crystals exhibit electrically switchable Bragg diffraction at ~8-10 V/microm with crystal structure in good agreement with theoretical expectations. These photonic crystals compare favorably with liquid-crystal-imbibed colloidal crystal arrays.

Spin splitting in modulation-doped Al x Ga 1 − x N / G a N heterostructures

Abstract: We have studied the electronic properties of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructures by using Shubnikov--de Haas (SdH) measurement. Two SdH oscillations were detected on the samples of $x=0.35$ and 0.31, due to the population of the first two subbands with the energy separations of 128 and 109 meV, respectively. For the sample of $x=0.25,$ two SdH oscillations beat each other, probably due to a finite zero-field spin splitting. The spin-splitting energy is equal to 9.0 meV. The samples also showed a persistent photoconductivity effect after illuminating by blue light-emitting diode.

Hypersonic Flow over a Blunt Body with Plasma Injection

Abstract: The drag reduction of blunt body in hypersonic flow via plasma injection has been investigated by a combined experimental and computational effort. The counterflow plasma jet generated by a plasma torch has a vibrionic temperature of 4400 K, an electronic temperature around 20,000 K, and electron density greater than 3 × 1012/cm3. At a fixed injection stagnation pressure and in the absence of an applied magnetic field, the plasma injection actually increases drag above that of room-temperature air due to a decreased mass flow rate at the elevated temperature. However, at an identical mass flow rate, the plasma injection reveals a greater drag reduction than the room-temperature air counterpart through thermal energy deposition. From experimental measurements, an overwhelming major portion of the drag reduction is derived from the viscous-inviscid interaction of the counterflow jet and thermal energy deposition. The numerical results of Navier-Stokes equations with a local equilibrium plasma composition also confirm this observation.

Within-ear and across-ear interference in a cocktail-party listening task.

Abstract: Although many researchers have shown that listeners are able to selectively attend to a target speech signal when a masking talker is present in the same ear as the target speech or when a masking talker is present in a different ear than the target speech, little is known about selective auditory attention in tasks with a target talker in one ear and independent masking talkers in both ears at the same time. In this series of experiments, listeners were asked to respond to a target speech signal spoken by one of two competing talkers in their right (target) ear while ignoring a simultaneous masking sound in their left (unattended) ear. When the masking sound in the unattended ear was noise, listeners were able to segregate the competing talkers in the target ear nearly as well as they could with no sound in the unattended ear. When the masking sound in the unattended ear was speech, however, speech segregation in the target ear was substantially worse than with no sound in the unattended ear. When the masking sound in the unattended ear was time-reversed speech, speech segregation was degraded only when the target speech was presented at a lower level than the masking speech in the target ear. These results show that within-ear and across-ear speech segregation are closely related processes that cannot be performed simultaneously when the interfering sound in the unattended ear is qualitatively similar to speech.

Grain growth and texture evolution in Ti–6Al–4V during beta annealing under continuous heating conditions

Abstract: The kinetics of beta grain growth during continuous heating for two texturally-different, but microstructurally-equivalent, lots of Ti–6Al–4V material were determined using a direct-resistance-heating technique. Heating rates of 0.42, 5, 10, and 50 K s−1 were utilized. After reaching the peak temperature, a special cooling procedure was utilized to avoid complete decomposition of the beta phase during cool-down and thus enable direct determination of its texture at the end of high-temperature annealing. It was found that beta grain growth is strongly affected by texture whose evolution can give rise to a behavior which is discontinuous in nature. As a result, dramatic differences in grain-growth behavior were noted in the two lots of material and were explained in terms of variations in beta texture evolution during heat treatment.

Heat Transfer Predictions in a Laminar Hypersonic Viscous/Inviscid Interaction

Abstract: Resultsaredocumentedofablindvalidationstudytocharacterizetheaccuracyofanupwind-biasede nitevolume methodinpredictingthesurfaceloadscaused byalaminarviscous/inviscidinteraction.Theeffortsupportsarecent Research and Technology Organization code validation initiative and considers a benchmark cone guration of the Working Group 10, consisting of Mach 9.5 e owpast a 25/55 deg sharp-tipped doubleconeat a Reynolds number of 1.39435 £ 10 6 /m. Roe’ s e ux-difference splitting scheme is employed with a nominally third-order reconstruction method and harmonic limiting. An extensive iterative- and grid-convergencestudy is performed to ensure solution accuracy. Comparison with experimental data shows that overall the numerical method reproduces the features of the interaction, including location and extent of separation to a degree that may be characterized as adequate for engineering purposes. However, upstream of separation and in the narrow peak heating region, discrepancies in heat transfer rates between computation and experiment range between 10 and 15% of maximum values. An exploration of the sensitivity of the solution to small variations (up to 10%) in several e ow and numerical parameters reveals only modest ine uence on the nondimensionalized quantities.

Pharmacokinetics of toxic chemicals in breast milk: use of PBPK models to predict infant exposure.

Abstract: Factors controlling the transfer of potentially toxic chemicals in the breast milk of nursing mothers include both chemical characteristics, such as lipophilicity, and physiologic changes during lactation. Physiologically based pharmacokinetic (PBPK) models can aid in the prediction of infant exposure via breast milk. Benefits of these quantitative models include the ability to account for changing maternal physiology and transfer kinetics, as well as the chemical-specific characteristics, in order to produce more accurate estimates of neonatal risk. A recently developed PBPK model for perchlorate and iodide kinetics in the lactating and neonatal rat demonstrates the utility of PBPK modeling in predicting maternal and neonatal distribution of these two compounds. This model incorporates time-dependent changes in physiologic characteristics and includes interactions between iodide and perchlorate that alter the distribution and kinetics of iodide.

Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings. II. Experimental investigations

Abstract: We have performed a detailed study of the polarization properties and switching behavior of holographic polymer-dispersed liquid-crystal gratings. A theoretical model [ R. L. Sutherland , J. Opt. Soc. Am. B19, 2995 (2002)] is compared with a number of observed phenomena in reflection and transmission gratings made with different types of liquid crystals under a variety of experimental conditions. Anomalous polarization effects are described and interpreted. We show that a wide variation of holographic polymer-dispersed liquid-crystal grating properties can be explained in terms of the statistics of droplet orientational distributions.