6,654 research outputs found
NASA metric system study
Statistical analysis of metric and English unit usage and trends in technical documents generated by NASA Center
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Strategic, tactical decisions and information in Rapid Manufacturing supply chain
The efficiency and agility of its supply chain is vital to the commercial success of any product. Sharing strategic and tactical information effectively within the supply chain is often a key factor in achieving this goal. This paper proposes a framework to identify strategic, tactical decisions and information. The framework is used to conduct a sector based analysis of the Rapid Manufacturing (RM) industry. The decisions and information identified include amongst others various supply chain strategies and technical information
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Model granularity and related concepts
Models are integral to engineering design and basis for many decisions. Therefore, it is necessary to comprehend how a model’s properties might influence its behaviour. Model granularity is an important property but has so far only received limited attention. The terminology used to describe granularity and related phenomena varies and pertinent concepts are distributed across communities. This article positions granularity in the theoretical background of models, collects formal definitions for relevant terms from a range of communities and discusses the implications for engineering design
Properties of gas clumps and gas clumping factor in the intra cluster medium
The spatial distribution of gas matter inside galaxy clusters is not
completely smooth, but may host gas clumps associated with substructures. These
overdense gas substructures are generally a source of unresolved bias of X-ray
observations towards high density gas, but their bright luminosity peaks may be
resolved sources within the ICM, that deep X-ray exposures may be (already)
capable to detect. In this paper we aim at investigating both features, using a
set of high-resolution cosmological simulations with ENZO. First, we monitor
how the bias by unresolved gas clumping may yield incorrect estimates of global
cluster parameters and affects the measurements of baryon fractions by X-ray
observations. We find that based on X-ray observations of narrow radial strips,
it is difficult to recover the real baryon fraction to better than 10 - 20
percent uncertainty. Second, we investigated the possibility of observing
bright X-ray clumps in the nearby Universe (z<=0.3). We produced simple mock
X-ray observations for several instruments (XMM, Suzaku and ROSAT) and
extracted the statistics of potentially detectable bright clumps. Some of the
brightest clumps predicted by simulations may already have been already
detected in X- ray images with a large field of view. However, their small
projected size makes it difficult to prove their existence based on X-ray
morphology only. Preheating, AGN feedback and cosmic rays are found to have
little impact on the statistical properties of gas clumps.Comment: 17 pages, 11 figures. MNRAS accepte
Gas clumping in galaxy clusters
The reconstruction of galaxy cluster's gas density profiles is usually
performed by assuming spherical symmetry and averaging the observed X-ray
emission in circular annuli. In the case of a very inhomogeneous and asymmetric
gas distribution, this method has been shown to return biased results in
numerical simulations because of the dependence of the X-ray emissivity.
We propose a method to recover the true density profiles in the presence of
inhomogeneities, based on the derivation of the azimuthal median of the surface
brightness in concentric annuli. We demonstrate the performance of this method
with numerical simulations, and apply it to a sample of 31 galaxy clusters in
the redshift range 0.04-0.2 observed with ROSAT/PSPC. The clumping factors
recovered by comparing the mean and the median are mild and show a slight trend
of increasing bias with radius. For , we measure a clumping factor
, which indicates that the thermodynamic properties and
hydrostatic masses measured in this radial range are only mildly affected by
this effect. Comparing our results with three sets of hydrodynamical numerical
simulations, we found that non-radiative simulations significantly overestimate
the level of inhomogeneities in the ICM, while the runs including cooling, star
formation, and AGN feedback reproduce the observed trends closely. Our results
indicate that most of the accretion of X-ray emitting gas is taking place in
the diffuse, large-scale accretion patterns rather than in compact structures.Comment: 12 pages, 11 figures, accepted for publication in MNRAS.
Largely-improved version compared to v1, method and comparison with
simulations update
Microwave Driven Magnetic Plasma Accelerator Studies (CYCLOPS)
A microwave-driven cyclotron resonance plasma acceleration device was investigated using argon, krypton, xenon, and mercury as propellants. Limited ranges of propellant flow rate, input power, and magnetic field strength were used. Over-all efficiencies (including the 65% efficiency of the input polarizer) less than 10% were obtained for specific impulse values between 500 and 1500 sec. Power transfer efficiencies, however, approached 100% of the input power available in the right-hand component of the incident circularly polarized radiation. Beam diagnostics using Langmuir probes, cold gas mapping, r-f mapping and ion energy analyses were performed in conjunction with an engine operating in a pulsed mode. Measurements of transverse electron energies at the position of cyclotron resonant absorption yielded energy values more than an order of magnitude lower than anticipated. The measured electron energies were, however, consistent with the low values of average ion energy measured by retarding potential techniques. The low values of average ion energy were also consistent with the measured thrust values. It is hypothesized that ionization and radiation limit the electron kinetic energy to low-values thus limiting the energy which is finally transferred to the ion. Thermalization by electron-electron collision was also identified as an additional loss mechanism. The use of light alkali metals, which have relatively few low lying energy levels to excite, with the input power to mass ratio selected so as to limit the electron energies to less than the second ionization potential, is suggested. It is concluded, however, that the over-all efficiency for such propellants would be less than 40 per cent
Towards Understanding Spontaneous Speech: Word Accuracy vs. Concept Accuracy
In this paper we describe an approach to automatic evaluation of both the
speech recognition and understanding capabilities of a spoken dialogue system
for train time table information. We use word accuracy for recognition and
concept accuracy for understanding performance judgement. Both measures are
calculated by comparing these modules' output with a correct reference answer.
We report evaluation results for a spontaneous speech corpus with about 10000
utterances. We observed a nearly linear relationship between word accuracy and
concept accuracy.Comment: 4 pages PS, Latex2e source importing 2 eps figures, uses icslp.cls,
caption.sty, psfig.sty; to appear in the Proceedings of the Fourth
International Conference on Spoken Language Processing (ICSLP 96
Towards computational insights into the large-scale structure of spin foams
Understanding the large-scale physics is crucial for the spin foam approach to quantum gravity. We tackle this challenge from a statistical physics perspective using simplified, yet feature-rich models. In particular, this allows us to explicitly answer whether broken symmetries will be restored by renormalization: We observe a weak phase transition in both Migdal-Kadanoff and tensor network renormalization. In this work we give a concise presentation of the concepts, results and promises of this new direction of research
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