11,839 research outputs found
Recertification of the air and methane storage vessels at the Langley 8-foot high-temperature structures tunnel
This center operates a number of sophisticated wind tunnels in order to fulfill the needs of its researchers. Compressed air, which is kept in steel storage vessels, is used to power many of these tunnels. Some of these vessels have been in use for many years, and Langley is currently recertifying these vessels to insure their continued structural integrity. One of the first facilities to be recertified under this program was the Langley 8-foot high-temperature structures tunnel. This recertification involved (1) modification, hydrotesting, and inspection of the vessels; (2) repair of all relevant defects; (3) comparison of the original design of the vessel with the current design criteria of Section 8, Division 2, of the 1974 ASME Boiler and Pressure Vessel Code; (4) fracture-mechanics, thermal, and wind-induced vibration analyses of the vessels; and (5) development of operating envelopes and a future inspection plan for the vessels. Following these modifications, analyses, and tests, the vessels were recertified for operation at full design pressure (41.4 MPa (6000 psi)) within the operating envelope developed
Constraints on Association of Single-pulse Gamma-ray Bursts and Supernovae
We explore the hypothesis, similar to one recently suggested by Bloom and
colleagues, that some nearby supernovae are associated with smooth,
single-pulse gamma-ray bursts, possibly having no emission above ~ 300 keV. We
examine BATSE bursts with durations longer than 2 s, fitting those which can be
visually characterized as single-pulse events with a lognormal pulse model. The
fraction of events that can be reliably ascertained to be temporally and
spectrally similar to the exemplar, GRB 980425 - possibly associated with SN
1998bw - is 4/1573 or 0.25%. This fraction could be as high as 8/1573 (0.5%) if
the dimmest bursts are included. Approximately 2% of bursts are morphologically
similar to GRB 980425 but have emission above ~ 300 keV. A search of supernova
catalogs containing 630 detections during BATSE's lifetime reveals only one
burst (GRB 980425) within a 3-month time window and within the total 3-sigma
BATSE error radius that could be associated with a type Ib/c supernova. There
is no tendency for any subset of single-pulse GRBs to fall near the
Supergalactic Plane, whereas SNe of type Ib/c do show this tendency. Economy of
hypotheses leads us to conclude that nearby supernovae generally are not
related to smooth, single-pulse gamma-ray bursts.Comment: 25 pages, 5 figure
Cloning and characterization of a conjugated bile acid hydrolase gene from Clostridium perfringens.
The gene encoding a conjugated bile acid hydrolase (CBAH) from Clostridium perfringens 13 has been cloned and expressed in Escherichia coli, and its nucleotide sequence has been determined. Nucleotide and predicted amino acid sequence analyses indicated that the gene product is related to two previously characterized amidases, a CBAH from Lactobacillus plantarum (40% identity) and a penicillin V amidase from Bacillus sphaericus (34% identity). The product is apparently unrelated to a CBAH from C. perfringens for which N-terminal sequence information was determined. The gene product was purified from recombinant E. coli and used to raise antibody in rabbits. The presence of the protein in C. perfringens was then confirmed by immunoblot analysis. The protein was shown to have a native molecular weight of 147,000 and a subunit molecular weight of 36,100, indicating its probable existence as a tetramer. Disruption of the chromosomal C. perfringens CBAH gene with a chloramphenicol resistance cartridge resulted in a mutant strain which retained partial CBAH activity. Polyacrylamide gel electrophoresis followed by enzymatic activity staining and immunoblotting indicated that the mutant strain no longer expressed the cloned CBAH (CBAH-1) but did express at least one additional CBAH (CBAH-2). CBAH-2 was immunologically distinct from CBAH-1, and its mobility on native polyacrylamide gels was different from that of CBAH-1. Furthermore, comparisons of pH optima and substrate specificities of CBAH activities from recombinant E. coli and wild-type and mutant C. perfringens provided further evidence for the presence of multiple CBAH activities in C. perfringens. Originally published Applied and Environmental Microbiology, Vol. 61, No. 7, July 199
Nodal Quasiparticle Lifetimes in Cuprate Superconductors
A new generation of angular-resolved photoemission spectroscopy (ARPES)
measurements on the cuprate superconductors offer the promise of enhanced
momentum and energy resolution. In particular, the energy and temperature
dependence of the on-shell nodal (k_x=k_y) quasiparticle scattering rate can be
studied. In the superconducting state, low temperature transport measurements
suggest that one can describe nodal quasiparticles within the framework of a
BCS d-wave model by including forward elastic scattering and spin-fluctuation
inelastic scattering. Here, using this model, we calculate the temperature and
frequency dependence of the on-shell nodal quasiparticle scattering rate in the
superconducting state which determines the momentum width of the ARPES momentum
distribution curves. For a zero-energy quasiparticle at the nodal momentum k_N,
both the elastic and inelastic scattering rate show a sudden decrease as the
temperature drops below Tc, reflecting the onset of the gap amplitude. At low
temperatures the scattering rate decreases as T^3 and approaches a zero
temperature value determined by the elastic impurity scattering. For T>T_c, we
find a quasilinear dependence on T. At low reduced temperatures, the elastic
scattering rate for the nodal quasiparticles exhibits a quasilinear increase at
low energy which arises from elastic scattering processes. The inelastic
spin-fluctuation scattering leads to a low energy omega^3 dependence which, for
omega>~Delta_0, crosses over to a quasilinear behavior.Comment: 8 pages, 7 figures, minor revision
MODISTools - downloading and processing MODIS remotely sensed data in R
Remotely sensed data – available at medium to high resolution across global spatial and temporal scales – are a valuable resource for ecologists. In particular, products from NASA's MODerate-resolution Imaging Spectroradiometer (MODIS), providing twice-daily global coverage, have been widely used for ecological applications. We present MODISTools, an R package designed to improve the accessing, downloading, and processing of remotely sensed MODIS data. MODISTools automates the process of data downloading and processing from any number of locations, time periods, and MODIS products. This automation reduces the risk of human error, and the researcher effort required compared to manual per-location downloads. The package will be particularly useful for ecological studies that include multiple sites, such as meta-analyses, observation networks, and globally distributed experiments. We give examples of the simple, reproducible workflow that MODISTools provides and of the checks that are carried out in the process. The end product is in a format that is amenable to statistical modeling. We analyzed the relationship between species richness across multiple higher taxa observed at 526 sites in temperate forests and vegetation indices, measures of aboveground net primary productivity. We downloaded MODIS derived vegetation index time series for each location where the species richness had been sampled, and summarized the data into three measures: maximum time-series value, temporal mean, and temporal variability. On average, species richness covaried positively with our vegetation index measures. Different higher taxa show different positive relationships with vegetation indices. Models had high R2 values, suggesting higher taxon identity and a gradient of vegetation index together explain most of the variation in species richness in our data. MODISTools can be used on Windows, Mac, and Linux platforms, and is available from CRAN and GitHub (https://github.com/seantuck12/MODISTools)
Ab Initio Calculation of Impurity Effects in Copper Oxide Materials
We describe a method for calculating, within density functional theory, the
electronic structure associated with typical defects which substitute for Cu in
the CuO2 planes of high-Tc superconducting materials. The focus is primarily on
Bi2Sr2CaCu2O8, the material on which most STM measurements of impurity
resonances in the superconducting state have been performed. The magnitudes of
the effective potentials found for Zn, Ni and vacancies on the in-plane Cu
sites in this host material are remarkably consistent with phenomenological
fits of potential scattering models to STM resonance energies. The effective
potential ranges are quite short, of order 1 A with weak long range tails, in
contrast to some current models of extended potentials which attempt to fit STM
data. For the case of Zn and Cu vacancies, the effective potentials are
strongly repulsive, and states on the impurity site near the Fermi level are
simply removed. The local density of states (LDOS) just above the impurity is
nevertheless found to be a maximum in the case of Zn and a local minimum in
case of the vacancy, in agreement with experiment. The Zn and Cu vacancy
patterns are explained as due to the long-range tails of the effective impurity
potential at the sample surface. The case of Ni is richer due to the Ni atom's
strong hybridization with states near the Fermi level; in particular, the short
range part of the potential is attractive, and the LDOS is found to vary
rapidly with distance from the surface and from the impurity site. We propose
that the current controversy surrounding the observed STM patterns can be
resolved by properly accounting for the effective impurity potentials and
wave-functions near the cuprate surface. Other aspects of the impurity states
for all three species are discussed.Comment: 37 pp. pdf including figures, submitted to Phys. Rev.
Galaxy Distances in the Nearby Universe: Corrections For Peculiar Motions
By correcting the redshift--dependent distances for peculiar motions through
a number of peculiar velocity field models, we recover the true distances of a
wide, all-sky sample of nearby galaxies (~ 6400 galaxies with velocities
cz<5500 km/s), which is complete up to the blue magnitude B=14 mag. Relying on
catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups
and the remaining objects as field galaxies.
We model the peculiar velocity field using: i) a cluster dipole
reconstruction scheme; ii) a multi--attractor model fitted to the Mark II and
Mark III catalogs of galaxy peculiar velocities. According to Mark III data the
Great Attractor has a smaller influence on local dynamics than previously
believed, whereas the Perseus-Pisces and Shapley superclusters acquire a
specific dynamical role. Remarkably, the Shapley structure, which is found to
account for nearly half the peculiar motion of the Local Group, is placed by
Mark III data closer to the zone of avoidance with respect to its optical
position.
Our multi--attractor model based on Mark III data favors a cosmological
density parameter Omega ~ 0.5 (irrespective of a biasing factor of order
unity). Differences among distance estimates are less pronounced in the ~ 2000
- 4000 km/s distance range than at larger or smaller distances. In the last
regions these differences have a serious impact on the 3D maps of the galaxy
distribution and on the local galaxy density --- on small scales.Comment: 24 pages including (9 eps figures and 7 tables). Figures 1,2,3,4 are
available only upon request. Accepted by Ap
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