1,151 research outputs found
Homoclinic Signatures of Dynamical Localization
It is demonstrated that the oscillations in the width of the momentum
distribution of atoms moving in a phase-modulated standing light field, as a
function of the modulation amplitude, are correlated with the variation of the
chaotic layer width in energy of an underlying effective pendulum. The maximum
effect of dynamical localization and the nearly perfect delocalization are
associated with the maxima and minima, respectively, of the chaotic layer
width. It is also demonstrated that kinetic energy is conserved as an almost
adiabatic invariant at the minima of the chaotic layer width, and that the
system is accurately described by delta-kicked rotors at the zeros of the
Bessel functions J_0 and J_1. Numerical calculations of kinetic energy and
Lyapunov exponents confirm all the theoretical predictions.Comment: 7 pages, 4 figures, enlarged versio
Low impact velocity wastage in FBCs : experimental results and comparison between abrasion and erosion theories
The use of technologies related to combustion of coal in fluidized bed combustors (FBCs) present attractive advantages over conventional pulverized coal units. Some of the outstanding characteristics are: excellent heat transfer, low emission of contaminants, good combustion efficiencies and good fuel flexibility. However, FBC units can suffer materials deterioration due to particle interaction of solid particles with the heat transfer tubes immersed on the bed (Hou, 2004, Oka, 2004, Rademarkers et al., 1990). Among other issues, some of the most important factors believed to cause wear problems are: the motion of slowly but relatively coarse particles, particles loaded onto the surface by other particles, erosion by relatively fast-moving particles associated with bubbles, and abrasion by blocks of particles thrown into the surface by bubble collapse. Thus, erosion or abrasion processes can occur by a variety of causes. For the case of particle movement against in-bed surfaces, it has been suggested that there is no difference in the ability to cause degradation between solid particle erosion and low stress three body abrasion, and distinctions between the two forms of wear should not to be made (Levy, 1987)
X-ray flares on the UV Ceti-type star CC Eridani: a "peculiar" time-evolution of spectral parameters
Context: Weak flares are supposed to be an important heating agent of the
outer layers of stellar atmospheres. However, due to instrumental limitations,
only large X-ray flares have been studied in detail until now.
Aims: We used an XMM-Newton observation of the very active BY-Dra type binary
star CC Eri in order to investigate the properties of two flares that are
weaker than those typically studied in the literature.
Methods: We performed time-resolved spectroscopy of the data taken with the
EPIC-PN CCD camera. A multi-temperature model was used to fit the spectra. We
inferred the size of the flaring loops using the density-temperature diagram.
The loop scaling laws were applied for deriving physical parameters of the
flaring plasma. We also estimated the number of loops involved in the observed
flares.
Results: A large X-ray variability was found. Spectral analysis showed that
all the regions in the light curve, including the flare segments, are
well-described by a 3-T model with variable emission measures but,
surprisingly, with constant temperatures (values of 3, 10 and 22 MK). The
analysed flares lasted ~ 3.4 and 7.1 ks, with flux increases of factors
1.5-1.9. They occurred in arcades made of a few tens of similar coronal loops.
The size of the flaring loops is much smaller than the distance between the
stellar surfaces in the binary system, and even smaller than the radius of each
of the stars. The obtained results are consistent with the following ideas: (i)
the whole X-ray light curve of CC Eri could be the result of a superposition of
multiple low-energy flares, and (ii) stellar flares can be scaled-up versions
of solar flares.Comment: 14 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
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