40,511 research outputs found
Space shuttle main engine: Interactive design challenges
The operating requirements established by NASA for the SSME were considerably more demanding than those for earlier rocket engines used in the military launch vehicles or Apollo program. The SSME, in order to achieve the high performance, low weight, long life, reusable objectives, embodied technical demands far in excess of its predecessor rocket engines. The requirements dictated the use of high combustion pressure and the staged combustion cycle which maximizes performance through total use of all propellants in the main combustion process. This approach presented a myriad of technical challenges for maximization of performance within attainable state of the art capabilities for operating pressures, operating temperatures and rotating machinery efficiencies. Controlling uniformity of the high pressure turbomachinery turbine temperature environment was a key challenge for thrust level and life capability demanding innovative engineering. New approaches in the design of the components were necessary to accommodate the multiple use, minimum maintenance objectives. Included were the use of line replaceable units to facilitate field maintenance automatic checkout and internal inspection capabilities
Inverting a Supernova: Neutrino Mixing, Temperatures and Binding Energy
We show that the temperatures of the emergent non-electron neutrinos and the
binding energy released by a galactic Type II supernova are determinable,
assuming the Large Mixing Angle (LMA) solution is correct, from observations at
the Sudbury Neutrino Observatory (SNO) and at Super-Kamiokande (SK). If the
neutrino mass hierarchy is inverted, either a lower or upper bound can be
placed on the neutrino mixing angle , and the hierarchy can be
deduced for adiabatic transitions. For the normal hierarchy, neither can
be constrained nor can the hierarchy be determined. Our
conclusions are qualitatively unchanged for the proposed Hyper-Kamiokande
detector.Comment: Following astro-ph/0208035, we adopt electron and non-electron
neutrino spectra with very small differences. Conclusions change
Phonon entropy of alloying and ordering of Cu-Au
Inelastic neutron scattering spectra were measured with a time-of-flight spectrometer on six disordered Cu-Au alloys at 300 K. The neutron-weighted phonon density of states was obtained from a conventional analysis of these spectra. Several methods were developed to account for this neutron weighting and obtain the phonon entropy of the disordered alloys. The phonon entropies of formation of disordered fcc Cu-Au alloys obtained in this way were generally mutually consistent, and were also consistent with predictions from a cluster approximation obtained from ab-initio calculations by Ozolin[underaccent cedilla [below] s-breve, Wolverton, and Zunger. We estimate a phonon entropy of disordering of 0.15±0.05kB/atom in Cu3Au at 300 K. A resonance mode associated with the motions of the heavy Au atoms in the Cu-rich alloys was observed at 9 meV. An analysis of the resonance mode provided a check on the partial phonon entropy of Au atoms
Optical and near-infrared spectrophotometric properties of Long Period Variables and other luminous red stars
Based on a new and large sample of optical and near-infrared spectra obtained
at the Mount Stromlo and Siding Spring Observatories (Lancon & Wood 1998;
Lancon & Wood, in preparation), spectrophotometric properties of cool oxygen-
and carbon-rich Long Period Variables and supergiants are presented.
Temperatures of oxygen-rich stars are assigned by comparison with synthetic
spectra computed from up-to-date oxygen-rich model atmosphere grids. The
existence of reliable optical and near-infrared temperature indicators is
investigated. A narrow relation between the bolometric correction BC(I) and the
broad-band colour I-J is obtained for oxygen-rich cool stars. The ability of
specific near-infrared indices to separate luminosity classes, atmospheric
chemistry or variability subtypes is discussed. Some comments are also given on
extinction effects, water band strengths in Long Period Variables and the
evaluation of 12CO/13CO ratio in red giants.Comment: 14 pages, 21 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/~ralvarez
A Coronal Hole's Effects on CME Shock Morphology in the Inner Heliosphere
We use STEREO imagery to study the morphology of a shock driven by a fast
coronal mass ejection (CME) launched from the Sun on 2011 March 7. The source
region of the CME is located just to the east of a coronal hole. The CME ejecta
is deflected away from the hole, in contrast with the shock, which readily
expands into the fast outflow from the coronal hole. The result is a CME with
ejecta not well centered within the shock surrounding it. The shock shape
inferred from the imaging is compared with in situ data at 1 AU, where the
shock is observed near Earth by the Wind spacecraft, and at STEREO-A. Shock
normals computed from the in situ data are consistent with the shock morphology
inferred from imaging.Comment: to appear in The Astrophysical Journa
A Long-Term Hydrologically-Based Data Set of Land Surface Fluxes and States for the Conterminous United States
A frequently encountered difficulty in assessing model-predicted land–atmosphere exchanges of moisture and energy is the absence of comprehensive observations to which model predictions can be compared at the spatial and temporal resolutions at which the models operate. Various methods have been used to evaluate the land surface schemes in coupled models, including comparisons of model-predicted evapotranspiration with values derived from atmospheric water balances, comparison of model-predicted energy and radiative fluxes with tower measurements during periods of intensive observations, comparison of model-predicted runoff with observed streamflow, and comparison of model predictions of soil moisture with spatial averages of point observations. While these approaches have provided useful model diagnostic information, the observation-based products used in the comparisons typically are inconsistent with the model variables with which they are compared—for example, observations are for points or areas much smaller than the model spatial resolution, comparisons are restricted to temporal averages, or the spatial scale is large compared to that resolved by the model. Furthermore, none of the datasets available at present allow an evaluation of the interaction of the water balance components over large regions for long periods. In this study, a model-derived dataset of land surface states and fluxes is presented for the conterminous United States and portions of Canada and Mexico. The dataset spans the period 1950–2000, and is at a 3-h time step with a spatial resolution of ⅛ degree. The data are distinct from reanalysis products in that precipitation is a gridded product derived directly from observations, and both the land surface water and energy budgets balance at every time step. The surface forcings include precipitation and air temperature (both gridded from observations), and derived downward solar and longwave radiation, vapor pressure deficit, and wind. Simulated runoff is shown to match observations quite well over large river basins. On this basis, and given the physically based model parameterizations, it is argued that other terms in the surface water balance (e.g., soil moisture and evapotranspiration) are well represented, at least for the purposes of diagnostic studies such as those in which atmospheric model reanalysis products have been widely used. These characteristics make this dataset useful for a variety of studies, especially where ground observations are lacking
Hegemonic conceptualizations of empowerment in entrepreneurship and their suitability for collective contexts
The relationship between empowerment and entrepreneurship in collective societies is, in our view, insufficiently examined. Accepted definitions of empowerment and the assumptions underlying programs and research designs based on them result in outcomes that self-fulfil and, as a result, disappoint. Several issues are prevalent: the empowerment potential of programs is overestimated and the dominant view of what constitutes an ‘empowered self’ does not go deep enough to explore, and reframe, the self and its relationship to agency—two issues at the core of empowerment definitions and formulations. In this conceptual article, we examine the entrepreneurship and empowerment literature to suggest ways forward for the future health and relevance of the subject area. We highlight a serious methodological and perceptual issue within the literature, which offers many opportunities for theory development in the field
Element-centric clustering comparison unifies overlaps and hierarchy
Clustering is one of the most universal approaches for understanding complex
data. A pivotal aspect of clustering analysis is quantitatively comparing
clusterings; clustering comparison is the basis for many tasks such as
clustering evaluation, consensus clustering, and tracking the temporal
evolution of clusters. In particular, the extrinsic evaluation of clustering
methods requires comparing the uncovered clusterings to planted clusterings or
known metadata. Yet, as we demonstrate, existing clustering comparison measures
have critical biases which undermine their usefulness, and no measure
accommodates both overlapping and hierarchical clusterings. Here we unify the
comparison of disjoint, overlapping, and hierarchically structured clusterings
by proposing a new element-centric framework: elements are compared based on
the relationships induced by the cluster structure, as opposed to the
traditional cluster-centric philosophy. We demonstrate that, in contrast to
standard clustering similarity measures, our framework does not suffer from
critical biases and naturally provides unique insights into how the clusterings
differ. We illustrate the strengths of our framework by revealing new insights
into the organization of clusters in two applications: the improved
classification of schizophrenia based on the overlapping and hierarchical
community structure of fMRI brain networks, and the disentanglement of various
social homophily factors in Facebook social networks. The universality of
clustering suggests far-reaching impact of our framework throughout all areas
of science
Comprehensive Observations of a Solar Minimum CME with STEREO
We perform the first kinematic analysis of a CME observed by both imaging and
in situ instruments on board STEREO, namely the SECCHI, PLASTIC, and IMPACT
experiments. Launched on 2008 February 4, the CME is tracked continuously from
initiation to 1 AU using the SECCHI imagers on both STEREO spacecraft, and is
then detected by the PLASTIC and IMPACT particle and field detectors on board
STEREO-B. The CME is also detected in situ by ACE and SOHO/CELIAS at Earth's L1
Lagrangian point. The CME hits STEREO-B, ACE, and SOHO on 2008 February 7, but
misses STEREO-A entirely. This event provides a good example of just how
different the same event can look when viewed from different perspectives. We
also demonstrate many ways in which the comprehensive and continuous coverage
of this CME by STEREO improves confidence in our assessment of its kinematic
behavior, with potential ramifications for space weather forecasting. The
observations provide several lines of evidence in favor of the observable part
of the CME being narrow in angular extent, a determination crucial for deciding
how best to convert observed CME elongation angles from Sun-center to actual
Sun-center distances.Comment: 27 pages, 10 figures, AASTEX v5.2, accepted by Ap
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