1,036 research outputs found
Network algebra for synchronous dataflow
We develop an algebraic theory of synchronous dataflow networks. First, a
basic algebraic theory of networks, called BNA (Basic Network Algebra), is
introduced. This theory captures the basic algebraic properties of networks.
For synchronous dataflow networks, it is subsequently extended with additional
constants for the branching connections that occur between the cells of
synchronous dataflow networks and axioms for these additional constants. We
also give two models of the resulting theory, the one based on stream
transformers and the other based on processes as considered in process algebra.Comment: 24 page
Directional solidification of flake and nodular cast iron during KC-135 low-g maneuvers
Alloys solidified in a low-gravity environment can, due to the elimination of sedimentation and convection, form unique and often desirable microstructures. One method of studying the effects of low-gravity (low-g) on alloy solidification was the use of the NASA KC-135 aircraft flying repetitive low-g maneuvers. Each maneuver gives from 20 to 30 seconds of low-g which is between about 0.1 and 0.001 gravity. A directional solidification furnace was used to study the behavior of off eutectic composition case irons in a low-g environment. The solidification interface of hypereutectic flake and spheroidal graphite case irons was slowly advanced through a rod sample, 5 mm in diameter. Controlled solidification was continued through a number of aircraft parabolas. The known solidification rate of the sample was then correlated with accelerometer data to determine the gravity level during solidification for any location of the sample. The thermal gradient and solidification rate were controlled independently. Samples run on the KC-135 aircraft exhibited bands of coarser graphite or of larger nodules usually corresponding to the regions solidified under low-g. Samples containing high phosphorous (used in order to determine the eutectic cell) exhibited larger eutectic cells in the low-g zone, followed by a band of coarser graphite
Preliminary science report on the directional solidification of hypereutectic cast iron during KC-135 low-G maneuvers
An ADSS-P directional solidification furnace was reconfigured for operation on the KC-135 low-g aircraft. The system offers many advantages over quench ingot methods for study of the effects of sedimentation and convection on alloy formation. The directional sodification furnace system was first flown during the September 1982 series of flights. The microstructure of the hypereutectic cast iron sample solidified on one of these flights suggests a low-g effect on graphite morphology. Further experiments are needed to ascertain that this effect is due to low-gravity and to deduce which of the possible mechanisms is responsible for it
A 10B-based neutron detector with stacked Multiwire Proportional Counters and macrostructured cathodes
We present the results of the measurements of the detection efficiency for a
4.7 \r{A} neutron beam incident upon a detector incorporating a stack of up to
five MultiWire Proportional Counters (MWPC) with Boron-coated cathodes. The
cathodes were made of Aluminum and had a surface exhibiting millimeter-deep
V-shaped grooves of 45{\deg}, upon which the thin Boron film was deposited by
DC magnetron sputtering. The incident neutrons interacting with the converter
layer deposited on the sidewalls of the grooves have a higher capture
probability, owing to the larger effective absorption film thickness. This
leads to a higher overall detection efficiency for the grooved cathode when
compared to a cathode with a flat surface. Both the experimental results and
the predictions of the GEANT4 model suggests that a 5-counter detector stack
with coated grooved cathodes has the same efficiency as a 7-counter stack with
flat cathodes. The reduction in the number of counters in the stack without
altering the detection efficiency will prove highly beneficial for large-area
position-sensitive detectors for neutron scattering applications, for which the
cost-effective manufacturing of the detector and associated readout electronics
is an important objective. The proposed detector concept could be a
technological option for one of the new chopper spectrometers and other
instruments planned to be built at the future European Spallation Source in
Sweden. These results with macrostructured cathodes generally apply not just to
MWPCs but to other gaseous detectors as well.Comment: 14 pages, 9 figure
Fractional calculus analysis of the cosmic microwave background
Cosmic microwave background (CMB) radiation is the imprint from an early stage of the Universe and investigation of its properties is crucial for understanding the fundamental laws governing the structure and evolution of the Universe.
Measurements of the CMB anisotropies are decisive to cosmology, since any cosmological model must explain it. The brightness, strongest at the microwave frequencies, is almost uniform in all directions, but tiny variations reveal a spatial pattern of small anisotropies. Active research is being developed seeking better interpretations of the phenomenon. This paper analyses the recent data in the perspective of fractional calculus.
By taking advantage of the inherent memory of fractional operators some hidden properties are captured and described
On assessment of processing variables on vertical centrifugal casting technique
The aim of the present study is to investigate the influence of the vertical centrifugal casting
technique over mechanical and metallurgical properties of a hypereutectic Al–18Si alloy. Due to
the inherent vibration of the centrifugal casting technique, and in order to study and understand
the individual effects of the equipment vibration and the centrifugal force itself (pressure or fluid
dynamics), as well as the combined effect of both, three different tests were performed: gravity
casting, gravity casting with vibration and centrifugal casting. It was concluded that the
metallurgical and mechanical properties of castings obtained by the centrifugal casting process
depend on the combined effect of the centrifugal pressure and/or fluid dynamics and on the
inherent vibration of the technique itself. Correlations between the different casting techniques
and obtained mechanical and metallurgical properties are presentedFCT - Fundação para a Ciência e Tecnologi
Very fast optical flaring from a possible new Galactic magnetar
Highly luminous rapid flares are characteristic of processes around compact
objects like white dwarfs, neutron stars or black holes. In the high energy
regime of X- and gamma-rays, outbursts with variability time-scales of seconds
and faster are routinely observed, e.g. in gamma-ray bursts or Soft Gamma
Repeaters. In the optical, flaring activity on such time-scales has never been
observed outside the prompt phase of GRBs. This is mostly due to the fact that
outbursts with strong, fast flaring usually are discovered in the high-energy
regime. Most optical follow-up observations of such transients employ
instruments with integration times exceeding tens of seconds, which are
therefore unable to resolve fast variability. Here we show the observation of
extremely bright and rapid optical flaring in the galactic transient SWIFT
J195509.6+261406. Flaring of this kind has never previously been reported. Our
optical light-curves are phenomenologically similar to high energy light-curves
of Soft Gamma Repeaters and Anomalous X-ray Pulsars, which are thought to be
neutron stars with extremely high magnetic fields (magnetars). This suggests
similar emission processes may be at work, but in contrast to the other known
magnetars with strong emission in the optical.Comment: 8 pages, 3 figures. A substantially revised version of this
manuscript was published in Nature. Due to license issues, the accepted
manuscript will only be put on astro-ph as v2 6 months after this versio
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