289 research outputs found
Design improvement of a pump wear ring labyrinth seal
The investigation was successful in obtaining two improved designs for the impeller wear ring seal of the liquid hydrogen turbopump of interest. A finite difference computer code was extensively used in a parametric computational study in determining a cavity configuration with high flow resistance due to turbulence dissipation. These two designs, along with that currently used, were fabricated and tested. The improved designs were denoted Type O and Type S. The measurements showed that Type O and Type S given 67 and 30 percent reduction in leakage over the current design, respectively. It was found that the number of cavities, the step height and the presence of a small stator groove are quite important design features. Also, the tooth thickness is of some significance. Finally, the tooth height and an additional large cavity cut out from the stator (upstream of the step) are of negligible importance
COMPUTED EFFECT OF RUB-GROOVE SIZE ON STEPPED LABYRINTH SEAL PERFORMANCE
ABSTRACT A numerical study was undertaken to explore the effects of the size of wear-in mb grooves that are typically cut into the abradable land of stepped labyrinth seals. The elliptic form of the 2-D axisymmetric Navier-Stokes equations for compressible turbulent flow were solved. The relationships among the friction coefficient, the leakage Reynolds number, the groove depth and width and the pre-rub radial clearance were examined. It was found that the standard k-s turbulence model and wall functions are effective for computing the friction coefficient and leakage for labyrinth seals with honeycomb land surfaces, both with and without the presence of mb grooves. The so-called rub grooves are the result of labyrinth teeth cutting wear grooves into the abradable surface of the land (stationary housing of the seal). Furthermore, it was found that the case of a small pre-mb tooth radial clearance, a wide rub groove and an intermediate step height is the most sensitive to the presence of a rub groove, with a leakage increase over the no-groove case of about 100 percent and 194 percent for the shallow and deep grooves, respectively. It was also found, for example, that the leakage varied with pre-rub clearance and groove width, in order from lowest to highest leakage, as: (a) small clearance and narrow groove, (b) small clearance and wide groove, (c) large clearance and narrow groove and (d) large clearance and wide groove
A Test of the Standard Hypothesis for the Origin of the HI Holes in Holmberg II
The nearby irregular galaxy Holmberg II has been extensively mapped in HI
using the Very Large Array (VLA), revealing intricate structure in its
interstellar gas component (Puche et al. 1992). An analysis of these structures
shows the neutral gas to contain a number of expanding HI holes. The formation
of the HI holes has been attributed to multiple supernova events occurring
within wind-blown shells around young, massive star clusters, with as many as
10-200 supernovae required to produce many of the holes. From the sizes and
expansion velocities of the holes, Puche et al. assigned ages of ~10^7 to 10^8
years. If the supernova scenario for the formation of the HI holes is correct,
it implies the existence of star clusters with a substantial population of
late-B, A and F main sequence stars at the centers of the holes. Many of these
clusters should be detectable in deep ground-based CCD images of the galaxy. In
order to test the supernova hypothesis for the formation of the HI holes, we
have obtained and analyzed deep broad-band BVR and narrow-band H-alpha images
of Ho II. We compare the optical and HI data and search for evidence of the
expected star clusters in and around the HI holes. We also use the HI data to
constrain models of the expected remnant stellar population. We show that in
several of the holes the observed upper limits for the remnant cluster
brightness are strongly inconsistent with the SNe hypothesis described in Puche
et al. Moreover, many of the HI holes are located in regions of very low
optical surface brightness which show no indication of recent star formation.
Here we present our findings and explore possible alternative explanations for
the existence of the HI holes in Ho II, including the suggestion that some of
the holes were produced by Gamma-ray burst events.Comment: 30 pages, including 6 tables and 3 images. To appear in Astron.
Journal (June 1999
Orion Crew Module Aerodynamic Testing
The Apollo-derived Orion Crew Exploration Vehicle (CEV), part of NASA s now-cancelled Constellation Program, has become the reference design for the new Multi-Purpose Crew Vehicle (MPCV). The MPCV will serve as the exploration vehicle for all near-term human space missions. A strategic wind-tunnel test program has been executed at numerous facilities throughout the country to support several phases of aerodynamic database development for the Orion spacecraft. This paper presents a summary of the experimental static aerodynamic data collected to-date for the Orion Crew Module (CM) capsule. The test program described herein involved personnel and resources from NASA Langley Research Center, NASA Ames Research Center, NASA Johnson Space Flight Center, Arnold Engineering and Development Center, Lockheed Martin Space Sciences, and Orbital Sciences. Data has been compiled from eight different wind tunnel tests in the CEV Aerosciences Program. Comparisons are made as appropriate to highlight effects of angle of attack, Mach number, Reynolds number, and model support system effects
Exploring the Structures and Substructures of the Andromeda Satellite Dwarf Galaxies Cassiopeia III, Perseus I, and Lacerta I
We present results from wide-field imaging of the resolved stellar
populations of the dwarf spheroidal galaxies Cassiopeia III (And XXXII) and
Perseus I (And XXXIII), two satellites in the outer stellar halo of the
Andromeda galaxy (M31). Our WIYN pODI photometry traces the red giant star
population in each galaxy to ~2.5-3 half-light radii from the galaxy center. We
use the Tip of the Red Giant Branch (TRGB) method to derive distances of
(m-M)_0 = 24.62+/-0.12 mag (839 (+48,-450) kpc, or 156 (+16,-13) kpc from M31)
for Cas III and 24.47+/-0.13 mag (738 (+48,-45) kpc, or 351 (+17,-16) kpc from
M31) for Per I. These values are consistent within the errors with TRGB
distances derived from a deeper Hubble Space Telescope study of the galaxies'
inner regions. For each galaxy, we derive structural parameters, total
magnitude, and central surface brightness. We also place upper limits on the
ratio of neutral hydrogen gas mass to optical luminosity, confirming the
gas-poor nature of both galaxies. We combine our data set with corresponding
data for the M31 satellite galaxy Lacerta I (And XXXI) from earlier work, and
search for substructure within the RGB star populations of Cas III, Per I, and
Lac I. We find an overdense region on the west side of Lac I at a significance
level of 2.5-3-sigma and a low-significance filament extending in the direction
of M31. In Cas III, we identify two modestly significant overdensities near the
center of the galaxy and another at two half-light radii. Per I shows no
evidence for substructure in its RGB star population, which may reflect this
galaxy's isolated nature.Comment: 26 pages, 15 figures, 5 tables. Accepted for publication in The
Astronomical Journa
Dry Creek Revisited: New Excavations, Radiocarbon Dates, and Site Formation Inform on the Peopling of Eastern Beringia
The multicomponent Dry Creek site, located in the Nenana Valley, central Alaska, is arguably one of the most important archaeological sites in Beringia. Original work in the 1970s identified two separate cultural layers, called Components 1 and 2, thought to date to the terminal Pleistocene and suggesting that the site was visited by Upper Paleolithic huntergatherers between about 13,000 and 12,000 calendar years before present (cal B.P.). The oldest of these became the typeassemblage for the Nenana complex. Recently, some have questioned the geoarchaeological integrity of the site's early deposits, suggesting that the separated cultural layers resulted from natural postdepositional disturbances. In 2011, we revisited Dry Creek to independently assess the site's age and formation. Here we present our findings and reaffirm original interpretations of clear separation of two terminal Pleistocene cultural occupations. For the first time, we report direct radiocarbon dates on cultural features associated with both occupation zones, one dating to 13,485-13,305 and the other to 11,060-10,590 cal B.P.</jats:p
GRIPS - Gamma-Ray Imaging, Polarimetry and Spectroscopy
We propose to perform a continuously scanning all-sky survey from 200 keV to
80 MeV achieving a sensitivity which is better by a factor of 40 or more
compared to the previous missions in this energy range. The Gamma-Ray Imaging,
Polarimetry and Spectroscopy (GRIPS) mission addresses fundamental questions in
ESA's Cosmic Vision plan. Among the major themes of the strategic plan, GRIPS
has its focus on the evolving, violent Universe, exploring a unique energy
window. We propose to investigate -ray bursts and blazars, the
mechanisms behind supernova explosions, nucleosynthesis and spallation, the
enigmatic origin of positrons in our Galaxy, and the nature of radiation
processes and particle acceleration in extreme cosmic sources including pulsars
and magnetars. The natural energy scale for these non-thermal processes is of
the order of MeV. Although they can be partially and indirectly studied using
other methods, only the proposed GRIPS measurements will provide direct access
to their primary photons. GRIPS will be a driver for the study of transient
sources in the era of neutrino and gravitational wave observatories such as
IceCUBE and LISA, establishing a new type of diagnostics in relativistic and
nuclear astrophysics. This will support extrapolations to investigate star
formation, galaxy evolution, and black hole formation at high redshifts.Comment: to appear in Exp. Astron., special vol. on M3-Call of ESA's Cosmic
Vision 2010; 25 p., 25 figs; see also www.grips-mission.e
Expression of Distal-less, dachshund, and optomotor blind in Neanthes arenaceodentata (Annelida, Nereididae) does not support homology of appendage-forming mechanisms across the Bilateria
The similarity in the genetic regulation of
arthropod and vertebrate appendage formation has been
interpreted as the product of a plesiomorphic gene
network that was primitively involved in bilaterian
appendage development and co-opted to build appendages
(in modern phyla) that are not historically related
as structures. Data from lophotrochozoans are needed to
clarify the pervasiveness of plesiomorphic appendage forming
mechanisms. We assayed the expression of three
arthropod and vertebrate limb gene orthologs, Distal-less
(Dll), dachshund (dac), and optomotor blind (omb), in
direct-developing juveniles of the polychaete Neanthes
arenaceodentata. Parapodial Dll expression marks premorphogenetic
notopodia and neuropodia, becoming restricted
to the bases of notopodial cirri and to ventral
portions of neuropodia. In outgrowing cephalic appendages,
Dll activity is primarily restricted to proximal
domains. Dll expression is also prominent in the brain. dac
expression occurs in the brain, nerve cord ganglia, a pair
of pharyngeal ganglia, presumed interneurons linking a
pair of segmental nerves, and in newly differentiating
mesoderm. Domains of omb expression include the brain,
nerve cord ganglia, one pair of anterior cirri, presumed
precursors of dorsal musculature, and the same pharyngeal
ganglia and presumed interneurons that express dac.
Contrary to their roles in outgrowing arthropod and
vertebrate appendages, Dll, dac, and omb lack comparable
expression in Neanthes appendages, implying independent
evolution of annelid appendage development. We infer
that parapodia and arthropodia are not structurally or
mechanistically homologous (but their primordia might
be), that Dll’s ancestral bilaterian function was in sensory
and central nervous system differentiation, and that
locomotory appendages possibly evolved from sensory
outgrowths
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