51,511 research outputs found
A design assessment of multiwall, metallic stand-off, and RSI reusable thermal protection systems including space shuttle application
The design and assessment of reusable surface insulation (RSI), metallic stand off and multiwall thermal protection systems (TPS) is discussed. Multiwall TPS is described in some detail, and analyses useful for design of multiwall are included. Results indicate that multiwall has the potential to satisfy the TPS design goals better than the other systems. The total mass of the stand-off TPS and of the metallic systems require less primary structure mass than the RSI system, since the nonbuckling skin criteria required for RSI may be removed. Continued development of multiwall TPS is required to verify its potential and to provide the necessary data base for design
Computer program to determine pressure distributions and forces on blunt bodies of revolution
Program was written to include integration of surface pressure in order to obtain axial-force, normal-force, and pitching-moment coefficients. Program was written in CDC FORTRAN for the CDC-6600 computer system
Chiral-logarithmic Corrections to the S and T Parameters in Higgsless Models
Recently, Higgsless models have proven to be viable alternatives to the
Standard Model (SM) and supersymmetric models in describing the breaking of the
electroweak symmetry. Whether extra-dimensional in nature or their
deconstructed counterparts, the physical spectrum of these models typically
consists of ``towers'' of massive vector gauge bosons which carry the same
quantum numbers as the SM W and Z. In this paper, we calculate the one-loop,
chiral-logarithmic corrections to the S and T parameters from the lightest
(i.e. SM) and the next-to-lightest gauge bosons using a novel application of
the Pinch Technique. We perform our calculation using generic Feynman rules
with generic couplings such that our results can be applied to various models.
To demonstrate how to use our results, we calculate the leading
chiral-logarithmic corrections to the S and T parameters in the deconstructed
three site Higgsless model. As we point out, however, our results are not
exclusive to Higgsless models and may, in fact, be used to calculate the
one-loop corrections from additional gauge bosons in models with fundamental
(or composite) Higgs bosons.Comment: 45 pages, 15 figures, added references, analysis of three site model
expanded to include delocalized fermion
Ethane steam reforming over a platinum/alumina catalyst: effect of sulphur poisoning
In this study we have examined the adsorption of hydrogen sulfide and methanethiol over platinum catalysts and examined the effect of these poisons on the steam reforming of ethane. Adsorption of hydrogen sulfide was measured at 293 and 873 K. At 873 K the adsorbed state of hydrogen sulfide in the presence of hydrogen was SH rather than S, even though the Pt:S ratio was unity. The effect of 11.2 ppm hydrogen sulfide or methanethiol on the steam reforming of ethane was studied at 873 K and 20 barg. Both poisons deactivated the catalyst over a number of hours, but methanethiol was found to be more deleterious, reducing the conversion by almost an order of magnitude, possibly due to the co-deposition of sulfur and carbon. Changes in the selectivity revealed that the effect of sulfur was not uniform on the reactions occurring, with the production of methane reduced proportionally more than the other products, due to the surface sensitivity of the hydrogenolysis and methanation reactions. The water-gas shift reaction was affected to a lesser extent. No regeneration was observed when hydrogen sulfide was removed from the feedstream in agreement with adsorption studies. A slight regeneration was observed when methanethiol was removed from the feed, but this was believed to be due to the removal of carbon rather than sulfur. The overall effect of sulfur poisoning was to reduce activity and enhance hydrogen selectivity
An Oculist Looks at Endocrine Exophthalmos
Based on a talk delivered before the Royal Medical Society on Friday. 12th December 1958.When it was suggested to me that I should talk around some subject that held interest beyond the strictly ophthalmological one I saw a chance to present the ophthalmologist as someoneother than a man confined within the bony boundaries of his orbit, often fighting stern actions to defeat the invasive tactics of his neighbours, the neuro surgeons and the nose and throat doctors. Here was an opportunity to demonstrate the many connections of our subject and the many other branches of medical practice with which we are at times associated, ranging from neurology and rheumatology, through paediatrics, midwifery, and dermatology, to metabolic disease and endocrinology--to mention but a few of the pies in which anophthalmological finger may be found
Power system applications of fiber optic sensors
This document is a progress report of work done in 1985 on the Communications and Control for Electric Power Systems Project at the Jet Propulsion Laboratory. These topics are covered: Electric Field Measurement, Fiber Optic Temperature Sensing, and Optical Power transfer. Work was done on the measurement of ac and dc electric fields. A prototype sensor for measuring alternating fields was made using a very simple electroscope approach. An electronic field mill sensor for dc fields was made using a fiber optic readout, so that the entire probe could be operated isolated from ground. There are several instances in which more precise knowledge of the temperature of electrical power apparatus would be useful. This report describes a number of methods whereby the distributed temperature profile can be obtained using a fiber optic sensor. The ability to energize electronics by means of an optical fiber has the advantage that electrical isolation is maintained at low cost. In order to accomplish this, it is necessary to convert the light energy into electrical form by means of photovoltaic cells. JPL has developed an array of PV cells in gallium arsenide specifically for this purpose. This work is described
Interband, intraband and excited-state direct photon absorption of silicon and germanium nanocrystals embedded in a wide band-gap lattice
Embedded Si and Ge nanocrystals (NCs) in wide band-gap matrices are studied
theoretically using an atomistic pseudopotential approach. From small clusters
to large NCs containing on the order of several thousand atoms are considered.
Effective band-gap values as a function of NC diameter reproduce very well the
available experimental and theoretical data. It is observed that the highest
occupied molecular orbital for both Si and Ge NCs and the lowest unoccupied
molecular orbital for Si NCs display oscillations with respect to size among
the different irreducible representations of the point group to which
these spherical NCs belong. Based on this electronic structure, first the
interband absorption is thoroughly studied which shows the importance of
surface polarization effects that significantly reduce the absorption when
included. This reduction is found to increase with decreasing NC size or with
increasing permittivity mismatch between the NC core and the host matrix.
Reasonable agreement is observed with the experimental absorption spectra where
available. The deformation of spherical NCs into prolate or oblate ellipsoids
are seen to introduce no pronounced effects for the absorption spectra. Next,
intraconduction and intravalence band absorption coefficients are obtained in
the wavelength range from far-infrared to visible region. These results can be
valuable for the infrared photodetection prospects of these NC arrays. Finally,
excited-state absorption at three different optical pump wavelengths, 532 nm,
355 nm and 266 nm are studied for 3- and 4 nm-diameter NCs. This reveals strong
absorption windows in the case of holes and a broad spectrum in the case of
electrons which can especially be relevant for the discussions on achieving
gain in these structures.Comment: Published version, 13 pages, 15 figures, local field effects include
Massive spheroids can form in single minor mergers
Accepted for publication in MNRAS, 12 pages, 6 figuresUnderstanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of ‘ex situ’ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (M ∗ > 10 11M) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex situ mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11–0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong in situ star formation. While this channel produces a minority (∼5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers – or any significant merging at all – contrary to what is classically believed.Peer reviewedFinal Accepted Versio
On the origin dependence of multipole moments in electromagnetism
The standard description of material media in electromagnetism is based on
multipoles. It is well known that these moments depend on the point of
reference chosen, except for the lowest order. It is shown that this "origin
dependence" is not unphysical as has been claimed in the literature but forms
only part of the effect of moving the point of reference. When also the
complementary part is taken into account then different points of reference
lead to different but equivalent descriptions of the same physical reality.
This is shown at the microscopic as well as at the macroscopic level. A similar
interpretation is valid regarding the "origin dependence" of the reflection
coefficients for reflection on a semi infinite medium. We show that the
"transformation theory" which has been proposed to remedy this situation (and
which is thus not needed) is unphysical since the transformation considered
does not leave the boundary conditions invariant.Comment: 14 pages, 0 figure
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