2,690 research outputs found
A high-Reynolds-number seal test facility: Facility description and preliminary test data
A facility has been developed for testing the leakage and rotordynamic characteristics of interstage-seal configurations for the HPFTP (High Pressure Fuel Turbopump) of the SSME (Space Shuttle Main Engine). Axial Reynolds numbers on the order of 400,000 are realized in the test facility by using a Dupont freon fluid called Halon (CBrF3). The kinematic viscosity of Halon is of the same order as the liquid hydrogen used in the HPFTP. Initial testing has focused on the current flight configurations (a three-segment, stepped unit) and a convergent-taper candidate
Sink or ecological trap for tree swallows in Central Minnesota?
Background: An ecological trap is typically defined as a low-quality habitat, incapable of sustaining a population, which is preferred over a high quality habitat. Ecological traps may lead to species extinction in populations with 1) strict habitat requirements 2) minimal information about the habitat, and 3) low population size. Tree Swallows (Tachycineta bicolor) meet the former two of these criteria.
Methods: We monitored Tree Swallow (TS) nest boxes (n=90) at two ecologically similar sites A and B with similar occupancy rates. Site B has lower reproductive fitness —suggestive of an ecological trap. Binary logistic regression was used to identify whether TS in better condition preferred site A or B and what environmental characteristics predicted nest box occupancy.
Results: Site A’s Tree Swallows were in significantly better condition and laid eggs earlier than site B’s TS. Significant predictors of nest box occupancy include distance from nearest box (p=0.000), DBH of largest stem in adjacent wooded area (p=0.026).
Conclusion: Tree Swallows seemingly recognized site A to be a superior breeding habitat over site B, suggesting that site B is a sink rather than an ecological trap. Distance from an adjacent box was an important predictor of occupancy, possibly to reduce extra-pair mating or depredation. TS favored nest boxes adjacent to an edge with smaller stems which may lessen competition by birds a nesting in naturally occurring cavities along hardwood edges. Although not a significant predictor of occupation, the average distance from the edge was greater at Site A (p = 0.012). It is possible that distance from the edge is a habitat cue utilized by Tree Swallows, independent of whether hetero- or conspecifics might be nesting in nearby natural cavities
Detection of an Extrasolar Planet Atmosphere
We report high precision spectrophotometric observations of four planetary
transits of HD 209458, in the region of the sodium resonance doublet at 589.3
nm. We find that the photometric dimming during transit in a bandpass centered
on the sodium feature is deeper by (2.32 +/- 0.57) x 10^{-4} relative to
simultaneous observations of the transit in adjacent bands. We interpret this
additional dimming as absorption from sodium in the planetary atmosphere, as
recently predicted from several theoretical modeling efforts. Our model for a
cloudless planetary atmosphere with a solar abundance of sodium in atomic form
predicts more sodium absorption than we observe. There are several
possibilities that may account for this reduced amplitude, including reaction
of atomic sodium into molecular gases and/or condensates, photoionization of
sodium by the stellar flux, a low primordial abundance of sodium, or the
presence of clouds high in the atmosphere.Comment: 26 pages, 8 figures, accepted by ApJ 2001 November 1
Preventing grain dust explosions
The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311
Field observations of shear waves in the surf zone
Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C01031, doi:10.1029/2002JC001761.Alongshore propagating meanders of the mean alongshore current in the surf zone called shear waves have periods of a few minutes and wavelengths of a few hundred meters. Here shear wave properties are estimated with arrays of current meters deployed for 4 months within 300 m of the shoreline of a sandy beach. Shear wave velocity fluctuations are approximately horizontally isotropic, with root mean square values between 10 and 40% of the mean (3-hour-averaged) alongshore current V. Cross-shore variations of the time-averaged shear wave momentum flux are consistent with shear wave energy generation close to shore where the breaking wave-driven mean alongshore current V and current shear Vx are strong and with shear wave energy dissipation and transfer back to the mean flow farther offshore where V and Vx are weak. In case studies where V is a narrow jet near the shoreline the observed strong decay of shear wave energy levels seaward of the jet, and the cross-shore and alongshore structure of shear waves within the jet, are similar to predictions based on the linearly unstable modes of the observed V. Shear wave energy levels also are high in a marginally unstable case with a strong, but weakly sheared, V.This research was supported by the Office
of Naval Research, the National Ocean Partnership Program and the
National Science Foundation
Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and Tau Bootis
Because of our relatively low spectral resolution, we compare our
observations with Gray's line bisector data by fitting observed line profiles
to an expansion in terms of orthogonal (Hermite) functions. To obtain an
accurate comparison, we model the emergent line profiles from rotating and
pulsating stars, taking the instrumental point spread function into account. We
describe this modeling process in detail.
We find no evidence for line profile or strength variations at the radial
velocity period in either 51 Peg or in Tau Boo. For 51 Peg, our upper limit for
line shape variations with 4.23-day periodicity is small enough to exclude with
10 sigma confidence the bisector curvature signal reported by Gray & Hatzes;
the bisector span and relative line depth signals reported by Gray (1997) are
also not seen, but in this case with marginal (2 sigma) confidence. We cannot,
however, exclude pulsations as the source of 51 Peg's radial velocity
variation, because our models imply that line shape variations associated with
pulsations should be much smaller than those computed by Gray & Hatzes; these
smaller signals are below the detection limits both for Gray & Hatzes' data and
for our own.
Tau Boo's large radial velocity amplitude and v*sin(i) make it easier to test
for pulsations in this star. Again we find no evidence for periodic line-shape
changes, at a level that rules out pulsations as the source of the radial
velocity variability. We conclude that the planet hypothesis remains the most
likely explanation for the existing data.Comment: 44 pages, 19 figures, plain TeX, accepted to ApJS (companion to
letter astro-ph/9712279
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