37,404 research outputs found
Engineering analysis and test results of the pre-stage planetary gear trains for wrist rotation and pitch assembly and azimuth and elevation assembly of the extendable stiff arm manipulator kit assembly
In order to improve the performance capability of the Extendable Stiff Arm Manipulator (ESAM) it was necessary to increase the overall gear ratio by a factor of approximately four. This is accomplished with minimum effect to existing hardware by the interposition of a planetary gear transmission between the respective drive motors and the harmonic drive transmissions. The engineering analysis in support of this design approach and the subsequent no-load test results are reported
Uniform Silicon Isotope Ratios Across the Milky Way Galaxy
We report the relative abundances of the three stable isotopes of silicon,
Si, Si and Si, across the Galaxy using the transition of silicon monoxide. The chosen sources represent a range in
Galactocentric radii () from 0 to 9.8 kpc. The high spectral
resolution and sensitivity afforded by the GBT permit isotope ratios to be
corrected for optical depths. The optical-depth-corrected data indicate that
the secondary-to-primary silicon isotope ratios
and vary much less than predicted on the basis of
other stable isotope ratio gradients across the Galaxy. Indeed, there is no
detectable variation in Si isotope ratios with . This lack of an
isotope ratio gradient stands in stark contrast to the monotonically decreasing
trend with exhibited by published secondary-to-primary oxygen
isotope ratios. These results, when considered in the context of the
expectations for chemical evolution, suggest that the reported oxygen isotope
ratio trends, and perhaps that for carbon as well, require further
investigation. The methods developed in this study for SiO isotopologue ratio
measurements are equally applicable to Galactic oxygen, carbon and nitrogen
isotope ratio measurements, and should prove useful for future observations of
these isotope systems.Comment: 18 pages, 12 figures, 2 tables. Published in The Astrophysical
Journal, Volume 839, Issue
Scramjet sidewall burning: Preliminary shock tunnel results
Experiments performed with a two dimensional model scramjet with particular emphasis on the effect of fuel injection from a wall are reported. Air low with a nominal Mach number of 3.5 and varied enthalpies was produced. It was found that neither hydrogen injection angle nor combustor divergence angle had any appreciable effect on thrust values while increased combustor length appeared to increase thrust levels. Specific impulse was observed to peak when hydrogen was injected at an equivalence ratio of about 2. Lowering the Mach number of the injected hydrogen at low equivalence ratios, less than 4, appeared to benefit specific impulse while hydrogen Mach number had little effect at higher equivalence ratios. When a 1:1 mixture by volume of nitrogen and oxygen is used instead of air as a test gas, it is found that hydrogen combustion is enhanced but only at high enthalpies
A new mid-infrared map of the BN/KL region using the Keck telescope
We present a new mid-infrared (12.5micron) map of the BN/KL high-mass
star-forming complex in Orion using the LWS instrument at Keck I. Despite poor
weather we achieved nearly diffraction-limited images (FWHM = 0.38'') over a
roughly 25'' X 25'' region centered on IRc2 down to a flux limit of ~250 mJy.
Many of the known infrared (IR) sources in the region break up into smaller
sub-components. We have also detected 6 new mid-IR sources. Nearly all of the
sources are resolved in our mosaic. The near-IR source ''n'' is slightly
elongated in the mid-IR along a NW--SE axis and perfectly bisects the
double-peaked radio source ''L''. Source n has been identified as a candidate
for powering the large IR luminosity of the BN/KL region (L = 10^5 L_sun). We
postulate that the 12 micron emission arises in a circumstellar disk
surrounding source n. The morphology of the mid-IR emission and the Orion ''hot
core'' (as seen in NH_3 emission), along with the location of water and OH
masers, is very suggestive of a bipolar cavity centered on source n and aligned
with the rotation axis of the hypothetical circumstellar disk. IRc2, once
thought to be the dominant energy source for the BN/KL region, clearly breaks
into 4 sub-sources in our mosaic, as seen previously at 3.8 -- 5.0 micron. The
anti-correlation of mid-IR emission and NH_3 emission from the nearby hot core
indicates that the IRc2 sources are roughly coincident (or behind) the dense
hot core. The nature of IRc2 is not clear: neither self-luminous sources
(embedded protostars) nor external heating by source I can be definitively
ruled out. We also report the discovery of a new arc-like feature SW of the BN
object, and some curious morphology surrounding near-IR source ''t".Comment: To appear in The Astronomical Journal, July 2004 (16 pages, 7
figures
Further shock tunnel studies of scramjet phenomena
Scramjet phenomena were studied using the shock tunnel T3 at the Australian National University. Simple two dimensional models were used with a combination of wall and central injectors. Silane as an additive to hydrogen fuel was studied over a range of temperatures and pressures to evaluate its effect as an ignition aid. The film cooling effect of surface injected hydrogen was measured over a wide range of equivalence. Heat transfer measurements without injection were repeated to confirm previous indications of heating rates lower than simple flat plate predictions for laminar boundary layers in equilibrium flow. The previous results were reproduced and the discrepancies are discussed in terms of the model geometry and departures of the flow from equilibrium. In the thrust producing mode, attempts were made to increase specific impulse with wall injection. Some preliminary tests were also performed on shock induced ignition, to investigate the possibility in flight of injecting fuel upstream of the combustion chamber, where it could mix but not burn
Flexible conformable hydrophobized surfaces for turbulent flow drag reduction
In recent years extensive work has been focused onto using superhydrophobic surfaces for drag reduction applications. Superhydrophobic surfaces retain a gas layer, called a plastron, when submerged underwater in the Cassie-Baxter state with water in contact with the tops of surface roughness features. In this state the plastron allows slip to occur across the surface which results in a drag reduction. In this work we report flexible and relatively large area superhydrophobic surfaces produced using two different methods: Large roughness features were created by electrodeposition on copper meshes; Small roughness features were created by embedding carbon nanoparticles (soot) into Polydimethylsiloxane (PDMS). Both samples were made into cylinders with a diameter under 12 mm. To characterize the samples, scanning electron microscope (SEM) images and confocal microscope images were taken. The confocal microscope images were taken with each sample submerged in water to show the extent of the plastron. The hydrophobized electrodeposited copper mesh cylinders showed drag reductions of up to 32% when comparing the superhydrophobic state with a wetted out state. The soot covered cylinders achieved a 30% drag reduction when comparing the superhydrophobic state to a plain cylinder. These results were obtained for turbulent flows with Reynolds numbers 10,000 to 32,500
Psychological Issues in Online Adaptive Task Allocation
Adaptive aiding is an idea that offers potential for improvement over many current approaches to aiding in human-computer systems. The expected return of tailoring the system to fit the user could be in the form of improved system performance and/or increased user satisfaction. Issues such as the manner in which information is shared between human and computer, the appropriate division of labor between them, and the level of autonomy of the aid are explored. A simulated visual search task was developed. Subjects are required to identify targets in a moving display while performing a compensatory sub-critical tracking task. By manipulating characteristics of the situation such as imposed task-related workload and effort required to communicate with the computer, it is possible to create conditions in which interaction with the computer would be more or less desirable. The results of preliminary research using this experimental scenario are presented, and future directions for this research effort are discussed
Growth to early adulthood following extremely preterm birth: the EPICure study.
OBJECTIVE: To investigate growth trajectories from age 2.5 to 19 years in individuals born before 26 weeks of gestation (extremely preterm; EP) compared with term-born controls. METHODS: Multilevel modelling of growth data from the EPICure study, a prospective 1995 birth cohort of 315 EP participants born in the UK and Ireland and 160 term-born controls recruited at school age. Height, weight, head circumference and body mass index (BMI) z-scores were derived from UK standards at ages 2.5, 6, 11 and 19 years. RESULTS: 129 (42%) EP children were assessed at 19 years. EP individuals were on average 4.0 cm shorter and 6.8 kg lighter with a 1.5 cm smaller head circumference relative to controls at 19 years. Relative to controls, EP participants grew faster in weight by 0.06 SD per year (95% CI 0.05 to 0.07), in head circumference by 0.04 SD (95% CI 0.03 to 0.05), but with no catch-up in height. For the EP group, because of weight catch-up between 6 and 19 years, BMI was significantly elevated at 19 years to +0.32 SD; 23.4% had BMI >25 kg/m2 and 6.3% >30 kg/m2 but these proportions were similar to those in control subjects. EP and control participants showed similar pubertal development in early adolescence, which was not associated with height at 19 years in either study group. Growth through childhood was related to birth characteristics and to neonatal feeding practices. CONCLUSIONS: EP participants remained shorter and lighter and had smaller head circumferences than reference data or controls in adulthood but had elevated BMI
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