37,903 research outputs found
Design of a video teleconference facility for a synchronous satellite communications link
The system requirements, design tradeoffs, and final design of a video teleconference facility are discussed, including proper lighting, graphics transmission, and picture aesthetics. Methods currently accepted in the television broadcast industry are used in the design. The unique problems associated with using an audio channel with a synchronous satellite communications link are discussed, and a final audio system design is presented
Transient radiation and conduction in a slotted slab and a hollow cylinder
Transient radiation and conduction in slotted slab and hollow cylinde
Direct N-body Simulations of Rubble Pile Collisions
There is increasing evidence that many km-sized bodies in the Solar System
are piles of rubble bound together by gravity. We present results from a
project to map the parameter space of collisions between km-sized spherical
rubble piles. The results will assist in parameterization of collision outcomes
for Solar System formation models and give insight into fragmentation scaling
laws. We use a direct numerical method to evolve the positions and velocities
of the rubble pile particles under the constraints of gravity and physical
collisions. We test the dependence of the collision outcomes on impact
parameter and speed, impactor spin, mass ratio, and coefficient of restitution.
Speeds are kept low (< 10 m/s, appropriate for dynamically cool systems such as
the primordial disk during early planet formation) so that the maximum strain
on the component material does not exceed the crushing strength. We compare our
results with analytic estimates and hydrocode simulations. Off-axis collisions
can result in fast-spinning elongated remnants or contact binaries while fast
collisions result in smaller fragments overall. Clumping of debris escaping
from the remnant can occur, leading to the formation of smaller rubble piles.
In the cases we tested, less than 2% of the system mass ends up orbiting the
remnant. Initial spin can reduce or enhance collision outcomes, depending on
the relative orientation of the spin and orbital angular momenta. We derive a
relationship between impact speed and angle for critical dispersal of mass in
the system. We find that our rubble piles are relatively easy to disperse, even
at low impact speed, suggesting that greater dissipation is required if rubble
piles are the true progenitors of protoplanets.Comment: 30 pages including 4 tables, 8 figures. Revised version to be
published in Icarus
Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations
Plasma velocity and magnetic field measurements from the Voyager 2 mission
are used to study solar wind turbulence in the slow solar wind at two different
heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to
provide information on the radial evolution of this turbulence. The magnetic
helicity and the cross-helicity, which express the correlation between the
plasma velocity and the magnetic field, are used to characterize the
turbulence. Wave number spectra are computed by means of the Taylor hypothesis
applied to time resolved single point Voyager 2 measurements. The overall
picture we get is complex and difficult to interpret. A substantial decrease of
the cross-helicity at smaller scales (over 1-3 hours of observation) with
increasing heliocentric distance is observed. At 5 AU the only peak in the
probability density of the normalized residual energy is negative, near -0.5.
At 29 AU the probability density becomes doubly peaked, with a negative peak at
-0.5 and a smaller peak at a positive values of about 0.7. A decrease of the
cross-helicity for increasing heliocentric distance is observed, together with
a reduction of the unbalance toward the magnetic energy of the energy of the
fluctuations. For the smaller scales, we found that at 29 AU the normalized
polarization is small and positive on average (about 0.1), it is instead zero
at 5 AU. For the larger scales, the polarization is low and positive at 5 AU
(average around 0.1) while it is negative (around - 0.15) at 29 AU.Comment: 14 pages 5 figures. Accepted for publication on European Journal of
Mechanics B/Fluids (5/8/2015
Turbulence in the solar wind: spectra from Voyager 2 data at 5 AU
Fluctuations in the flow velocity and magnetic fields are ubiquitous in the
Solar System. These fluctuations are turbulent, in the sense that they are
disordered and span a broad range of scales in both space and time. The study
of solar wind turbulence is motivated by a number of factors all keys to the
understanding of the Solar Wind origin and thermodynamics. The solar wind
spectral properties are far from uniformity and evolve with the increasing
distance from the sun. Most of the available spectra of solar wind turbulence
were computed at 1 astronomical unit, while accurate spectra on wide frequency
ranges at larger distances are still few. In this paper we consider solar wind
spectra derived from the data recorded by the Voyager 2 mission during 1979 at
about 5 AU from the sun. Voyager 2 data are an incomplete time series with a
voids/signal ratio that typically increases as the spacecraft moves away from
the sun (45% missing data in 1979), making the analysis challenging. In order
to estimate the uncertainty of the spectral slopes, different methods are
tested on synthetic turbulence signals with the same gap distribution as V2
data. Spectra of all variables show a power law scaling with exponents between
-2.1 and -1.1, depending on frequency subranges. Probability density functions
(PDFs) and correlations indicate that the flow has a significant intermittency.Comment: 14 pages, 7 figures. Discussion improved since the previous versio
Novel Fiber Design for Wideband Conversion and Amplification in Multimode Fibers
We propose an operating principle to achieve broadband and highly tunable
mode conversion and amplification exploiting inter-modal four wave mixing in a
multimode fiber. A bandwidth of 30 nanometers is demonstrated by properly
designing a simple step-index silica fiber.Comment: Ecoc conference 201
Crystallization of a Mos1 transposase-inverted-repeat DNA complex: biochemical and preliminary crystallographic analyses
A complex formed between Mos1 transposase and its inverted-repeat DNA has been crystallized. The crystals diffract to 3.25 Å resolution and exhibit monoclinic (P2(1)) symmetry, with unit-cell parameters a = 120.8, b = 85.1, c = 131.6 Å, β = 99.3°. The X-ray diffraction data display noncrystallographic twofold symmetry and characteristic dsDNA diffraction at ∼3.3 Å. Biochemical analyses confirmed the presence of DNA and full-length protein in the crystals. The relationship between the axis of noncrystallographic symmetry, the unit-cell axes and the DNA diffraction pattern are discussed. The data are consistent with the previously proposed model of the paired-ends complex containing a dimer of the transposase
Biofilm-forming capability of highly virulent, multidrug-resistant Candida auris
The emerging multidrug-resistant yeast pathogen Candida auris has attracted considerable attention as a source of healthcare–associated infections. We report that this highly virulent yeast has the capacity to form antifungal resistant biofilms sensitive to the disinfectant chlorhexidine in vitro
Improvements in estimating proportions of objects from multispectral data
Methods for estimating proportions of objects and materials imaged within the instantaneous field of view of a multispectral sensor were developed further. Improvements in the basic proportion estimation algorithm were devised as well as improved alien object detection procedures. Also, a simplified signature set analysis scheme was introduced for determining the adequacy of signature set geometry for satisfactory proportion estimation. Averaging procedures used in conjunction with the mixtures algorithm were examined theoretically and applied to artificially generated multispectral data. A computationally simpler estimator was considered and found unsatisfactory. Experiments conducted to find a suitable procedure for setting the alien object threshold yielded little definitive result. Mixtures procedures were used on a limited amount of ERTS data to estimate wheat proportion in selected areas. Results were unsatisfactory, partly because of the ill-conditioned nature of the pure signature set
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