3,049 research outputs found
Two-stage fan. 2: Data and performance with redesigned second stage rotor uniform and distorted inlet flows
A two-stage fan with a first rotor tip speed of 1450 ft/sec (441.96 m/sec) and no inlet guide vanes was tested with uniform and distorted inlet flows, with a redesigned second rotor having a part span shroud to prevent flutter, with variable-stagger stators set in nominal positions, and without rotor casing treatment. The fan achieved a pressure ratio 2.8 at a corrected flow of 185.4 lbm/sec (84.0 kg/sec), an adiabatic efficiency of 85.0 percent, and a stall margin of 12 percent. The redesigned second rotor did not flutter. Tip radial distortion reduced the stall margin at intermediate speed, but had little effect on stall margin at high or low speeds. Hub radial distortion reduced the stall margin at design speed but increased stall margin at low speed. Circumferential distortion reduced stall pressure ratio and flow to give approximately the same stall lines with uniform inlet flow. Distortions were attenuated by the fan. For Vol. 1, see N74-11421
Electron-Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons
With the high-resolution spectroscopy now available in the optical and
satellite UV, it is possible to determine the neutral/ionized column density
ratios for several different elements in a single cloud. Assuming ionization
equilibrium for each element, one can make several independent determinations
of the electron density. For the clouds for which such an analysis has been
carried out, these different estimates disagree by large factors, suggesting
that some process (or processes) besides photoionization and radiative
recombination might play an important role in the ionization balance. One
candidate process is collisions of ions with dust grains.
Making use of recent work quantifying the abundances of polycyclic aromatic
hydrocarbon molecules and other grains in the interstellar medium, as well as
recent models for grain charging, we estimate the grain-assisted ion
recombination rates for several astrophysically important elements. We find
that these rates are comparable to the rates for radiative recombination for
conditions typical of the cold neutral medium. Including grain-assisted ion
recombination in the ionization equilibrium analysis leads to increased
consistency in the various electron density estimates for the gas along the
line of sight to 23 Orionis. However, not all of the discrepancies can be
eliminated in this way; we speculate on some other processes that might play a
role. We also note that grain-assisted recombination of H+ and He+ leads to
significantly lower electron fractions than usually assumed for the cold
neutral medium.Comment: LaTeX(12 pages, 8 figures, uses emulateapj5.sty, apjfonts.sty);
submitted to ApJ; corrected typo
Genetic diversity of \u3ci\u3eDanthonia spicata\u3c/i\u3e (L.) Beauv. based on genomic simple sequence repeat markers
Danthonia spicata (L.) Beauv., commonly known as poverty oatgrass, is a perennial bunch-type grass native to North America. D. spicata is often found in low input turfgrass areas on the East Coast of the United States and has potential for development as a new native low input turfgrass species. Roche 454 sequenced randomly sheared genomic DNA reads of D. spicata were mined for SSR markers using the MIcroSAtellite identification tool. A total of 66,553 singlet sequences (approximately 37.5 Mbp) were examined, and 3454 SSR markers were identified. Trinucleotide motifs with greater than six repeats and possessing unique PCR priming sites within the genome, as determined by Primer-BLAST, were evaluated visually for heterozygosity and mutation consistent with stepwise evolution using CLC Genomics software. Sixty-three candidate markers were selected for testing from the trinucleotide SSR marker sites meeting these in silico criteria. Ten primer pairs that amplified polymorphic loci in preliminary experiments were used to screen 91 individual plants composed of at least 3â5 plants from each of 23 different locations. The primer pairs amplified 54 alleles ranging in size from 71 to 246 bp. Minimum and maximum numbers of alleles per locus were two and 12, respectively, with an average of 5.4. A dendrogram generated by unweighted pair group method with arithmetic mean cluster analysis using the Jaccardâs similarity coefficient was in agreement with the grouping obtained by Structure v2.3. The analyses were dominated by clonal groupings and lack evidence for gene flow with some alleles present in a single plant from a single location. Fourteen multilocus genotype groups were observed providing strong evidence for asexual reproduction in the studied D. spicata populations
The Structure and Star-Formation History of NGC 5461
We compute photoionization models for the giant extragalactic H II region NGC
5461, and compare their predictions to several observational constraints. Since
we aim at reproducing not only the global properties of the region, but its
local structure also, the models are constrained to reproduce the observed
density profile, and our analysis takes into consideration the bias introduced
by the shapes and sizes of the slits used by different observers. We find that
an asymmetric nebula with a gaussian density distribution, powered by a young
burst of 3.1 Myr, satisfactorily reproduces most of the constraints, and that
the star-formation efficiency inferred from the model agrees with current
estimates. Our results strongly depend on the assumed density law, since
constant density models overestimate the hardness of the ionizing field,
affecting the deduced properties of the central stellar cluster. We illustrate
the features of our best model, and discuss the possible sources of errors and
uncertainties affecting the outcome of this type of studies.Comment: 33 pages (LaTeX), 3 .eps figures. to be published in ApJ, May 200
Oxygen Gas Phase Abundance Revisited
We present new measurements of the interstellar gas-phase oxygen abundance
along the sight lines towards 19 early-type galactic stars at an average
distance of 2.6 kpc. We derive O {\small I} column densities from {\it
HST}/STIS observations of the weak 1355 \AA intersystem transition. We derive
total hydrogen column densities [N(H {\small I})+2N(H)] using {\it
HST}/STIS observations of \lya and {\it FUSE} observations of molecular
hydrogen. The molecular hydrogen content of these sight lines ranges from
f(H) = 2N(H)/[N(H {\small I})+2N(H)] = 0.03 to 0.47. The average
of 6.3 cm mag with a standard
deviation of 15% is consistent with previous surveys. The mean oxygen abundance
along these sight lines, which probe a wide range of galactic environments in
the distant ISM, is 10 \oh = (1 in the mean). %(1 ). We see no evidence for decreasing
gas-phase oxygen abundance with increasing molecular hydrogen fraction and the
relative constancy of \oh suggests that the component of dust containing the
oxygen is not readily destroyed. We estimate that, if 60% of the dust grains
are resilient against destruction by shocks, the distant interstellar total
oxygen abundance can be reconciliated with the solar value derived from the
most recent measurements %by Holweger and by Allende Prieto, Lambert & Asplund:
of 10 \oh = 517 58 (1 ). We note that the smaller
oxygen abundances derived for the interstellar gas within 500 pc %by Meyer,
Cardelli & Jura or from nearby B star surveys are consistent with a local
elemental deficit.Comment: 9 figures, 37 page
Near-Infrared Classification Spectroscopy: H-band Spectra of Fundamental MK Standards
We present a catalogue of H-band spectra for 85 stars of approximately solar
abundance observed at a resolving power of 3000 with the KPNO Mayall 4m FTS.
The atlas covers spectral types O7-M5 and luminosity classes I-V as defined on
the MK system. We identify both atomic and molecular indices and line-ratios
which are temperature and luminosity sensitive allowing spectral classification
to be carried out in the H-band. The line ratios permit spectral classification
in the presence of continuum excess emission, which is commonly found in
pre-main sequence and evolved stars. We demonstrate that with spectra of R =
1000 obtained at SNR > 50 it is possible to derive spectral types within +- 2
subclasses for late-type stars. These data are available electronically through
the Astronomical Data Center in addition to being served on the World-Wide-Web.Comment: To appear in the November 20, 1998 issue of ApJ (Volume 508, #1
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Poverty, Disease, and the Ecology of Complex Systems
Understanding why some human populations remain persistently poor remains a significant challenge for both the social and natural sciences. The extremely poor are generally reliant on their immediate natural resource base for subsistence and suffer high rates of mortality due to parasitic and infectious diseases. Economists have developed a range of models to explain persistent poverty, often characterized as poverty traps, but these rarely account for complex biophysical processes. In this Essay, we argue that by coupling insights from ecology and economics, we can begin to model and understand the complex dynamics that underlie the generation and maintenance of poverty traps, which can then be used to inform analyses and possible intervention policies. To illustrate the utility of this approach, we present a simple coupled model of infectious diseases and economic growth, where poverty traps emerge from nonlinear relationships determined by the number of pathogens in the system. These nonlinearities are comparable to those often incorporated into poverty trap models in the economics literature, but, importantly, here the mechanism is anchored in core ecological principles. Coupled models of this sort could be usefully developed in many economically important biophysical systemsâsuch as agriculture, fisheries, nutrition, and land use changeâto serve as foundations for deeper explorations of how fundamental ecological processes influence structural poverty and economic development
Precious, Tiny and Shiny: Close range photogrammetry for artists and others
Close Range Photogrammetry is increasingly being used to create digital assets across a range of disciplines, and can provide 3D âscansâ and surface âtexturesâ to very high resolution. The techniques of capture and processing usually present challenges for objects that are very small, monochromatic, reflective or relatively lacking in features.
This ongoing visual arts project seeks to capture, re-examine and ultimately transform specific military and religious insignia as part of an investigation into their interrelated iconographies and histories. The materiality (highly reflective) and scale (very detailed embroidery) of these objects has required different approaches to achieving the accurate 3D models required.
This paper will present the apparatus, methods and devices that have been developed to help solve these problems. Together they offer affordable, reliable, and scalable solutions to the traditional challenges of â3D scanningâ for particular types of objects, and robust methods applicable to a wide range of situations. They also help reduce the investment (of capital cost, time and labour) required to achieve accurate digital models with high resolution textures, opening up creative possibilities further down the virtual or material âpipelineâ
Assessing direct contributions of morphological awareness and prosodic sensitivity to childrenâs word reading and reading comprehension
We examined the independent contributions of prosodic sensitivity and morphological awareness to word reading, text reading accuracy, and reading comprehension. We did so in a longitudinal study of English-speaking children (N = 70). At 5 to 7 years of age, children completed the metalinguistic measures along with control measures of phonological awareness and vocabulary. Children completed the reading measures two years later. Morphological awareness, but not prosodic sensitivity made a significant independent contribution to word reading, text reading accuracy and reading comprehension. The effects of morphological awareness on reading comprehension remained after controls for word reading. These results suggest that morphological awareness needs to be considered seriously in models of reading development and that prosodic sensitivity might have primarily indirect relations to reading outcomes.
Keywords: Morphological Awareness; Prosody; Word Reading; Reading Comprehension
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