31,767 research outputs found
Radioelectric Field Features of Extensive Air Showers Observed with CODALEMA
Based on a new approach to the detection of radio transients associated with
extensive air showers induced by ultra high energy cosmic rays, the
experimental apparatus CODALEMA is in operation, measuring about 1 event per
day corresponding to an energy threshold ~ 5. 10^16 eV. Its performance makes
possible for the first time the study of radio-signal features on an
event-by-event basis. The sampling of the magnitude of the electric field along
a 600 meters axis is analyzed. It shows that the electric field lateral spread
is around 250 m (FWHM). The possibility to determine with radio both arrival
directions and shower core positions is discussed.Comment: Accepted for publication in Astroparticle Physic
Echoes of multiple outbursts of Sagittarius A* revealed by Chandra
The relatively rapid spatial and temporal variability of the X-ray radiation
from some molecular clouds near the Galactic center shows that this emission
component is due to the reflection of X-rays generated by a source that was
luminous in the past, most likely the central supermassive black hole,
Sagittarius A*. Studying the evolution of the molecular cloud reflection
features is therefore a key element to reconstruct Sgr A*'s past activity. The
aim of the present work is to study this emission on small angular scales in
order to characterize the source outburst on short time scales. We use Chandra
high-resolution data collected from 1999 to 2011 to study the most rapid
variations detected so far, those of clouds between 5' and 20' from Sgr A*
towards positive longitudes. Our systematic spectral-imaging analysis of the
reflection emission, notably of the Fe Kalpha line at 6.4 keV and its
associated 4-8 keV continuum, allows us to characterize the variations down to
15" angular scale and 1-year time scale. We reveal for the first time abrupt
variations of few years only and in particular a short peaked emission, with a
factor of 10 increase followed by a comparable decrease, that propagates along
the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts
with the slower 10-year linear variations we reveal in all the other molecular
structures of the region. Based on column density constraints, we argue that
these two different behaviors are unlikely to be due to the same illuminating
event. The variations are likely due to a highly variable active phase of Sgr
A* sometime within the past few hundred years, characterized by at least two
luminous outbursts of a few-year time scale and during which the Sgr A*
luminosity went up to at least 10^39 erg/s.Comment: 17 pages, 16 figures, Accepted for publication in Astronomy &
Astrophysic
Probabilistic Flow Regime Map Modeling of Two-Phase Flow
The purpose of this investigation is to develop models for two-phase heat transfer, void fraction, and
pressure drop, three key design parameters, in single, smooth, horizontal tubes using a common probabilistic two-phase
flow regime basis. Probabilistic two-phase flow maps are experimentally developed for R134a at 25 ??C, 35
??C, and 50 ??C, R410A at 25 ??C, mass fluxes from 100 to 600 kg/m2-s, qualities from 0 to 1 in 8.00 mm, 5.43 mm,
3.90 mm, and 1.74 mm I.D. horizontal, smooth, adiabatic tubes in order to extend probabilistic two-phase flow map
modeling to single tubes. An automated flow visualization technique, utilizing image recognition software and a
new optical method, is developed to classify the flow regimes present in approximately one million captured images.
The probabilistic two-phase flow maps developed are represented as continuous functions and generalized based on
physical parameters. Condensation heat transfer, void fraction, and pressure drop models are developed for single
tubes utilizing the generalized flow regime map developed. The condensation heat transfer model is compared to
experimentally obtained condensation data of R134a at 25 ??C in 8.915 mm diameter smooth copper tube with mass
fluxes ranging from 100 to 300 kg/m2-s and a full quality range. The condensation heat transfer, void fraction, and
pressure drop models developed are also compared to data found in the literature for a wide range of tube sizes,
refrigerants, and flow conditions.Air Conditioning and Refrigeration Project 18
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