25,736 research outputs found
Coverage and Connectivity in Three-Dimensional Networks
Most wireless terrestrial networks are designed based on the assumption that
the nodes are deployed on a two-dimensional (2D) plane. However, this 2D
assumption is not valid in underwater, atmospheric, or space communications. In
fact, recent interest in underwater acoustic ad hoc and sensor networks hints
at the need to understand how to design networks in 3D. Unfortunately, the
design of 3D networks is surprisingly more difficult than the design of 2D
networks. For example, proofs of Kelvin's conjecture and Kepler's conjecture
required centuries of research to achieve breakthroughs, whereas their 2D
counterparts are trivial to solve. In this paper, we consider the coverage and
connectivity issues of 3D networks, where the goal is to find a node placement
strategy with 100% sensing coverage of a 3D space, while minimizing the number
of nodes required for surveillance. Our results indicate that the use of the
Voronoi tessellation of 3D space to create truncated octahedral cells results
in the best strategy. In this truncated octahedron placement strategy, the
transmission range must be at least 1.7889 times the sensing range in order to
maintain connectivity among nodes. If the transmission range is between 1.4142
and 1.7889 times the sensing range, then a hexagonal prism placement strategy
or a rhombic dodecahedron placement strategy should be used. Although the
required number of nodes in the hexagonal prism and the rhombic dodecahedron
placement strategies is the same, this number is 43.25% higher than the number
of nodes required by the truncated octahedron placement strategy. We verify by
simulation that our placement strategies indeed guarantee ubiquitous coverage.
We believe that our approach and our results presented in this paper could be
used for extending the processes of 2D network design to 3D networks.Comment: To appear in ACM Mobicom 200
In situ performance measurements of the mitre photovoltaic array
A data acquisition system was developed to provide more accurate and consistent measurement of the degradation of solar arrays. A technique was developed for in-situ measurement of photovoltaic panels of sufficient quality to permit evaluation of electrical performance over extended periods of several years
Broad-line region structure and kinematics in the radio galaxy 3C 120
Broad emission lines originate in the surroundings of supermassive black
holes in the centers of active galactic nuclei (AGN). One method to investigate
the extent, structure, and kinematics of the BLR is to study the continuum and
line profile variability in AGN. We selected the radio-loud Seyfert 1 galaxy 3C
120 as a target for this study. We took spectra with a high signal-to-noise
ratio of 3C 120 with the 9.2m Hobby-Eberly Telescope between Sept. 2008 and
March 2009. In parallel, we photometrically monitored the continuum flux at the
Wise observatory. We analyzed the continuum and line profile variations in
detail (1D and 2D reverberation mapping) and modeled the geometry of the
line-emitting regions based on the line profiles. We show that the BLR in 3C
120 is stratified with respect to the distance of the line-emitting regions
from the center with respect to the line widths (FWHM) of the rms profiles and
with respect to the variability amplitude of the emission lines. The emission
line wings of H{\alpha} and H{\beta} respond much faster than their central
region. This is explained by accretion disk models. In addition, these lines
show a stronger response in the red wings. However, the velocity-delay maps of
the helium lines show a stronger response in the blue wing. Furthermore, the
HeII{\lambda}4686 line responds faster in the blue wing in contradiction to
observations made one and a half years later when the galaxy was in a lower
state. The faster response in the blue wing is an indication for central
outflow motions when this galaxy was in a bright state during our observations.
The vertical BLR structure in 3C 120 coincides with that of other AGN. We
confirm the general trend: the emission lines of narrow line AGN originate at
larger distances from the midplane than AGN with broader emission lines.Comment: 18 pages, 25 figures, Astronomy & Astrophysics in pres
Electron transport in Coulomb- and tunnel-coupled one-dimensional systems
We develop a linear theory of electron transport for a system of two
identical quantum wires in a wide range of the wire length L, unifying both the
ballistic and diffusive transport regimes. The microscopic model, involving the
interaction of electrons with each other and with bulk acoustical phonons
allows a reduction of the quantum kinetic equation to a set of coupled
equations for the local chemical potentials for forward- and backward-moving
electrons in the wires. As an application of the general solution of these
equations, we consider different kinds of electrical contacts to the
double-wire system and calculate the direct resistance, the transresistance, in
the presence of tunneling and Coulomb drag, and the tunneling resistance. If L
is smaller than the backscattering length l_P, both the tunneling and the drag
lead to a negative transresistance, while in the diffusive regime (L >>l_P) the
tunneling opposes the drag and leads to a positive transresistance. If L is
smaller than the phase-breaking length, the tunneling leads to interference
oscillations of the resistances that are damped exponentially with L.Comment: Text 14 pages in Latex/Revtex format, 4 Postscript figure
Elastic -transfer in the elastic scattering of OC
The elastic scattering OC angular distributions at O
bombarding energies of 100.0, 115.9 and 124.0 MeV and their optical model
description including the -particle exchange contribution calculated in
the Coupled Reaction Channel approach are presented. The angular distributions
show not only the usual diffraction pattern but also, at larger angles,
intermediate structure of refractive origin on which finer oscillations are
superimposed. The large angle features can be consistently described including
explicitly the elastic -transfer process and using a refractive optical
potential with a deep real part and a weakly absorptive imaginary part.Comment: 3 pages, 2 figures, accepted in Eur.Phys.J A (Short note
The disordered-free-moment phase: a low-field disordered state in spin-gap antiferromagnets with site dilution
Site dilution of spin-gapped antiferromagnets leads to localized free
moments, which can order antiferromagnetically in two and higher dimensions.
Here we show how a weak magnetic field drives this order-by-disorder state into
a novel disordered-free-moment phase, characterized by the formation of local
singlets between neighboring moments and by localized moments aligned
antiparallel to the field. This disordered phase is characterized by the
absence of a gap, as it is the case in a Bose glass. The associated
field-driven quantum phase transition is consistent with the universality of a
superfluid-to-Bose-glass transition. The robustness of the
disordered-free-moment phase and its prominent features, in particular a series
of pseudo-plateaus in the magnetization curve, makes it accessible and relevant
to experiments.Comment: 4 pages, 4 figure
Cold-air performance of a tip turbine designed to drive a lift fan. 3: Effect of simulated fan leakage on turbine performance
Performance data were obtained experimentally for a 0.4 linear scale version of the LF460 lift fan turbine for a range of scroll inlet total to diffuser exit static pressure ratios at design equivalent speed with simulated fan leakage air. Tests were conducted for full and partial admission operation with three separate combinations of rotor inlet and rotor exit leakage air. Data were compared to the results obtained from previous investigations in which no leakage air was present. Results are presented in terms of mass flow, torque, and efficiency
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