18,734 research outputs found
Comparative evaluation of solar, fission, fusion, and fossil energy resources, part 3
The role of nuclear fission reactors in becoming an important power source in the world is discussed. The supply of fissile nuclear fuel will be severely depleted by the year 2000. With breeder reactors the world supply of uranium could last thousands of years. However, breeder reactors have problems of a large radioactive inventory and an accident potential which could present an unacceptable hazard. Although breeder reactors afford a possible solution to the energy shortage, their ultimate role will depend on demonstrated safety and acceptable risks and environmental effects. Fusion power would also be a long range, essentially permanent, solution to the world's energy problem. Fusion appears to compare favorably with breeders in safety and environmental effects. Research comparing a controlled fusion reactor with the breeder reactor in solving our long range energy needs is discussed
Theoretical calculations of radiant heat transfer properties of particle-seeded gases
Radiant heat transfer properties of particle seeded gases, including absorption and scattering characteristics of carbon, silicon, and tungste
Unjamming a granular hopper by vibration
We present an experimental study of the outflow of a hopper continuously
vibrated by a piezoelectric device. Outpouring of grains can be achieved for
apertures much below the usual jamming limit observed for non vibrated hoppers.
Granular flow persists down to the physical limit of one grain diameter, a
limit reached for a finite vibration amplitude. For the smaller orifices, we
observe an intermittent regime characterized by alternated periods of flow and
blockage. Vibrations do not significantly modify the flow rates both in the
continuous and the intermittent regime. The analysis of the statistical
features of the flowing regime shows that the flow time significantly increases
with the vibration amplitude. However, at low vibration amplitude and small
orifice sizes, the jamming time distribution displays an anomalous statistics
Bounds on the force between black holes
We treat the problem of N interacting, axisymmetric black holes and obtain
two relations among physical parameters of the system including the force
between the black holes. The first relation involves the total mass, the
angular momenta, the distances and the forces between the black holes. The
second one relates the angular momentum and area of each black hole with the
forces acting on it.Comment: 13 pages, no figure
Force indeterminacy in the jammed state of hard disks
Granular packings of hard discs are investigated by means of contact dynamics
which is an appropriate technique to explore the allowed force-realizations in
the space of contact forces. Configurations are generated for given values of
the friction coefficient, and then an ensemble of equilibrium forces is found
for fixed contacts. We study the force fluctuations within this ensemble. In
the limit of zero friction the fluctuations vanish in accordance with the
isostaticity of the packing. The magnitude of the fluctuations has a
non-monotonous friction dependence. The increase for small friction can be
attributed to the opening of the angle of the Coulomb cone, while the decrease
as friction increases is due to the reduction of connectivity of the
contact-network, leading to local, independent clusters of indeterminacy. We
discuss the relevance of indeterminacy to packings of deformable particles and
to the mechanical response properties.Comment: 4 pages, 3 figures. Minor changes, journal reference adde
NCBO Ontology Recommender 2.0: An Enhanced Approach for Biomedical Ontology Recommendation
Biomedical researchers use ontologies to annotate their data with ontology
terms, enabling better data integration and interoperability. However, the
number, variety and complexity of current biomedical ontologies make it
cumbersome for researchers to determine which ones to reuse for their specific
needs. To overcome this problem, in 2010 the National Center for Biomedical
Ontology (NCBO) released the Ontology Recommender, which is a service that
receives a biomedical text corpus or a list of keywords and suggests ontologies
appropriate for referencing the indicated terms. We developed a new version of
the NCBO Ontology Recommender. Called Ontology Recommender 2.0, it uses a new
recommendation approach that evaluates the relevance of an ontology to
biomedical text data according to four criteria: (1) the extent to which the
ontology covers the input data; (2) the acceptance of the ontology in the
biomedical community; (3) the level of detail of the ontology classes that
cover the input data; and (4) the specialization of the ontology to the domain
of the input data. Our evaluation shows that the enhanced recommender provides
higher quality suggestions than the original approach, providing better
coverage of the input data, more detailed information about their concepts,
increased specialization for the domain of the input data, and greater
acceptance and use in the community. In addition, it provides users with more
explanatory information, along with suggestions of not only individual
ontologies but also groups of ontologies. It also can be customized to fit the
needs of different scenarios. Ontology Recommender 2.0 combines the strengths
of its predecessor with a range of adjustments and new features that improve
its reliability and usefulness. Ontology Recommender 2.0 recommends over 500
biomedical ontologies from the NCBO BioPortal platform, where it is openly
available.Comment: 29 pages, 8 figures, 11 table
Observing the evaporation transition in vibro-fluidized granular matter
By shaking a sand box the grains on the top start to jump giving the picture
of evaporating a sand bulk, and a gaseous transition starts at the surface
granular matter (GM) bed. Moreover the mixture of the grains in the whole bed
starts to move in a cooperative way which is far away from a Brownian
description. In a previous work we have shown that the key element to describe
the statistics of this behavior is the exclusion of volume principle, whereby
the system obeys a Fermi configurational approach. Even though the experiment
involves an archetypal non-equilibrium system, we succeeded in defining a
global temperature, as the quantity associated to the Lagrange parameter in a
maximum entropic statistical description. In fact in order to close our
approach we had to generalize the equipartition theorem for dissipative
systems. Therefore we postulated, found and measured a fundamental dissipative
parameter, written in terms of pumping and gravitational energies, linking the
configurational entropy to the collective response for the expansion of the
centre of mass (c.m.) of the granular bed. Here we present a kinetic approach
to describe the experimental velocity distribution function (VDF) of this
non-Maxwellian gas of macroscopic Fermi-like particles (mFp). The evaporation
transition occurs mainly by jumping balls governed by the excluded volume
principle. Surprisingly in the whole range of low temperatures that we measured
this description reveals a lattice-gas, leading to a packing factor, which is
independent of the external parameters. In addition we measure the mean free
path, as a function of the driving frequency, and corroborate our prediction
from the present kinetic theory.Comment: 6 pages, 4 figures, submitted for publication September 1st, 200
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