3,813 research outputs found
Flowfield measurements in a model scramjet combustion using laser-induced iodine fluorescence
Preliminary designs were completed for an iodine mixing chamber and the optical setup to be used with a modified wind tunnel in obtaining accurate, spatially resolved measurements of variables in the flowfield of a model nonreacting scramjet combustor. Schematics of the iodine-seeded wind tunnel and a sketch of the charcoal filter for removing the iodine are included along with a cutaway section of the laboratory
Dental Pathology in Selected Carnivores from Arkansas
The occurrence and kinds of dental pathology in wild carnivore populations in Arkansas were investigated through examination of 1295 skulls of bobcat (Felis rufus), river otter (Lutra canadensis), gray fox (Urocyon cinereoargenteus), coyote (Canis latrans), and raccoon (Procyon lotor). Pulpitis or abscesses in broken or otherwise exposed teeth were noted in all species, but bobcats had the fewest exposures while otter and gray foxhad the most. Osteomyelitis occurred in all species except the bobcat. Caries were noted in the coyote, raccoon, and gray fox. Otter and coyote had alveolar thinning, and coyote had enamel dysplasia and maxillary sinusitis
In-flight acoustic measurements on a light twin-engined turboprop airplane
Four series of flight tests were conducted to measure sound pressure levels inside and outside the cabin of a twin-engined turboprop airplane. Particular emphasis was placed on harmonics of the propeller blade passage frequency. The cabin was unfurnished for the first three flights, when the main objective was to investigate the repeatability of the data. For the fourth flight, the cabin was treated with fiberglass batts. Typically, the exterior sound pressure levels were found to vary 3 to 5 dB for a given harmonic, but variations as high as 8 dB were observed. The variability of harmonic levels within the cabin was slightly higher but depended on control of the relative phase between the propellers; when phase was not controlled the average variability was about 10 dB. Noise reductions provided by the fuselage structure were in the range of 20 to 40 dB, when an exterior microphone in the plane of rotation of the propeller was used as reference
Investigation of laser-induced iodine fluorescence for the measurement of density in compressible flows
Laser induced fluorescence is an attractive nonintrusive approach for measuring molecular number density in compressible flows although this technique does not produce a signal that is directly related to the number density. Saturation and frequency detuned excitation are explored as means for minimizing the quenching effect using iodine as the molecular system because of its convenient absorption spectrum. Saturation experiments indicate that with available continuous wave laser sources of Gaussian transverse intensity distribution only partial saturation could be achieved in iodine at the pressures of interest in gas dynamics. Using a fluorescence lineshape theory, it is shown that for sufficiently large detuning of a narrow bandwidth laser from a molecular transition, the quenching can be cancelled by collisional broadening over a large range of pressures and temperatures. Experimental data obtained in a Mach 4.3 underexpanded jet of nitrogen seeded with iodine for various single mode argon laser detunings from a strong iodine transition at 5145 A are discussed
Theory of imaging a photonic crystal with transmission near-field optical microscopy
While near-field scanning optical microscopy (NSOM) can provide optical
images with resolution much better than the diffraction limit, analysis and
interpretation of these images is often difficult. We present a theory of
imaging with transmission NSOM that includes the effects of tip field,
tip/sample coupling, light propagation through the sample and light collection.
We apply this theory to analyze experimental NSOM images of a nanochannel glass
(NCG) array obtained in transmission mode. The NCG is a triangular array of
dielectric rods in a dielectric glass matrix with a two-dimensional photonic
band structure. We determine the modes for the NCG photonic crystal and
simulate the observed data. The calculations show large contrast at low
numerical aperture (NA) of the collection optics and detailed structure at high
NA consistent with the observed images. We present calculations as a function
of NA to identify how the NCG photonic modes contribute to and determine the
spatial structure in these images. Calculations are presented as a function of
tip/sample position, sample index contrast and geometry, and aperture size to
identify the factors that determine image formation with transmission NSOM in
this experiment.Comment: 28 pages of ReVTex, 14 ps figures, submitted to Phys. Rev.
Re-entrant ferroelectricity in liquid crystals
The ferroelectric (Sm C) -- antiferroelectric (Sm C) -- reentrant
ferroelectric (re Sm C) phase temperature sequence was observed for system
with competing synclinic - anticlinic interactions. The basic properties of
this system are as follows (1) the Sm C phase is metastable in temperature
range of the Sm C stability (2) the double inversions of the helix
handedness at Sm C -- Sm C and Sm C% -- re-Sm C phase
transitions were found (3) the threshold electric field that is necessary to
induce synclinic ordering in the Sm C phase decreases near both Sm
C -- Sm C and Sm C -- re-Sm C phase boundaries, and it has
maximum in the middle of the Sm C stability region. All these properties
are properly described by simple Landau model that accounts for nearest
neighboring layer steric interactions and quadrupolar ordering only.Comment: 10 pages, 5 figures, submitted to PR
Velocity Visualization in Gaseous Flows
Techniques yielding simultaneous, multiple point measurements of velocity in reacting or nonreacting flow fields have the potential to significantly impact basic and applied studies of fluid mechanics. Several candidate schemes which could provide such measurement capability were investigated. The concepts utilize laser sources which lead to scattered light which can be monitored by a film based camera or a multielement solid state camera. Velocity measurements in supersonic flows using a novel Doppler modulated fluorescence concept are presented
Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors
The effect of disorder on transport and magnetization in ferromagnetic III-V
semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that
Coulomb-induced correlations of the defect positions are crucial for the
transport and magnetic properties of these highly compensated materials. We
employ Monte Carlo simulations to obtain the correlated defect distributions.
Exact diagonalization gives reasonable results for the spectrum of valence-band
holes and the metal-insulator transition only for correlated disorder. Finally,
we show that the mean-field magnetization also depends crucially on defect
correlations.Comment: 4 pages RevTeX4, 5 figures include
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