31,476 research outputs found
Development of advanced fuel cell system (phase 4)
For abstract, see N76-23686
The bimodal spiral galaxy surface brightness distribution
We have assessed the significance of Tully and Verheijen's (1997) bimodal
Ursa Major Cluster spiral galaxy near-infrared surface brightness distribution,
focussing on whether this bimodality is simply an artifact of small number
statistics. A Kolmogorov-Smirnov style of significance test shows that the
total distribution is fairly represented by a single-peaked distribution, but
that their isolated galaxy subsample (with no significant neighbours within a
projected distance of around 80 kpc) is bimodal at the 96 per cent level. We
have also investigated the assumptions underlying the isolated galaxy surface
brightness distribution, finding that the (often large) inclination corrections
used in the construction of this distribution reduce the significance of the
bimodality. We conclude that the Ursa Major Cluster dataset is insufficient to
establish the presence of a bimodal near-infrared surface brightness
distribution: an independent sample of around 100 isolated, low inclination
galaxies is required to establish bimodality at the 99 per cent level.Comment: 5 pages LaTeX; 2 embedded figures; re-submitted to MNRAS after
replying to referee's comment
Amplitude control of quantum interference
Usually, the oscillations of interference effects are controlled by relative
phases. We show that varying the amplitudes of quantum waves, for instance by
changing the reflectivity of beam splitters, can also lead to quantum
oscillations and even to Bell violations of local realism. We first study
theoretically a generalization of the Hong-Ou-Mandel experiment to arbitrary
source numbers and beam splitter transmittivity. We then consider a Bell type
experiment with two independent sources, and find strong violations of local
realism for arbitrarily large source number ; for small , one operator
measures essentially the relative phase of the sources and the other their
intensities. Since, experimentally, one can measure the parity of the number of
atoms in an optical lattice more easily than the number itself, we assume that
the detectors measure parity.Comment: 4 pages; 4 figure
Development of advanced fuel cell system, phase 3
A multiple task research and development program was performed to improve the weight, life, and performance characteristics of hydrogen-oxygen alkaline fuel cells for advanced power systems. Gradual wetting of the anode structure and subsequent long-term performance loss was determined to be caused by deposition of a silicon-containing material on the anode. This deposit was attributed to degradation of the asbestos matrix, and attention was therefore placed on development of a substitute matrix of potassium titanate. An 80 percent gold 20 percent platinum catalyst cathode was developed which has the same performance and stability as the standard 90 percent gold - 10 percent platinum cathode but at half the loading. A hybrid polysulfone/epoxy-glass fiber frame was developed which combines the resistance to the cell environment of pure polysulfone with the fabricating ease of epoxy-glass fiber laminate. These cell components were evaluated in various configurations of full-size cells. The ways in which the baseline engineering model system would be modified to accommodate the requirements of the space tug application are identified
Development of advanced fuel cell system, phase 2
A multiple task research and development program was performed to improve the weight, life, and performance characteristics of hydrogen-oxygen alkaline fuel cells for advanced power systems. Development and characterization of a very stable gold alloy catalyst was continued from Phase I of the program. A polymer material for fabrication of cell structural components was identified and its long term compatibility with the fuel cell environment was demonstrated in cell tests. Full scale partial cell stacks, with advanced design closed cycle evaporative coolers, were tested. The characteristics demonstrated in these tests verified the feasibility of developing the engineering model system concept into an advanced lightweight long life powerplant
Lifetime Constraints for Late Dark Matter Decay
We consider a class of late-decaying dark-matter models, in which a dark
matter particle decays to a heavy stable daughter of approximately the same
mass, together with one or more relativistic particles which carry away only a
small fraction of the parent rest mass. Such decays can affect galactic halo
structure and evolution, and have been invoked as a remedy to some of the small
scale structure-formation problems of cold dark matter. There are existing
stringent limits on the dark matter lifetime if the decays produce photons. By
considering examples in which the relativistic decay products instead consist
of neutrinos or electron-position pairs, we derive stringent limits on these
scenarios for a wide range of dark matter masses. We thus eliminate a sizable
portion of the parameter space for these late decay models if the dominant
decay channel involves Standard Model final states.Comment: 13 pages, 7 figures. Replaced to match published version. Discussion
expanded. References added. Accepted by Phys. Rev
Observer with a constant proper acceleration
Relying on the equivalence principle, a first approach of the general theory
of relativity is presented using the spacetime metric of an observer with a
constant proper acceleration. Within this non inertial frame, the equation of
motion of a freely moving object is studied and the equation of motion of a
second accelerated observer with the same proper acceleration is examined. A
comparison of the metric of the accelerated observer with the metric due to a
gravitational field is also performed.Comment: 5 figure
Bayesian inferencing for wind resource characterisation
The growing role of wind power in power systems has motivated R&D on methodologies to characterise the wind resource at sites for which no wind speed data is available. Applications such as feasibility assessment of prospective installations and system integration analysis of future scenarios, amongst others, can greatly benefit from such methodologies. This paper focuses on the inference of wind speeds for such potential sites using a Bayesian approach to characterise the spatial distribution of the resource. To test the approach, one year of wind speed data from four weather stations was modelled and used to derive inferences for a fifth site. The methodology used is described together with the model employed and simulation results are presented and compared to the data available for the fifth site. The results obtained indicate that Bayesian inference can be a useful tool in spatial characterisation of wind
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