14,905 research outputs found
The slimming effect of advection on black-hole accretion flows
At super-Eddington rates accretion flows onto black holes have been described
as slim (aspect ratio ) or thick (H/R >1) discs, also known as
tori or (Polish) doughnuts. The relation between the two descriptions has never
been established, but it was commonly believed that at sufficiently high
accretion rates slim discs inflate, becoming thick. We wish to establish under
what conditions slim accretion flows become thick. We use analytical equations,
numerical 1+1 schemes, and numerical radiative MHD codes to describe and
compare various accretion flow models at very high accretion rates.We find that
the dominant effect of advection at high accretion rates precludes slim discs
becoming thick. At super-Eddington rates accretion flows around black holes can
always be considered slim rather than thick.Comment: 8 pages, 5 figures. Astronomy & Astrophysics, in pres
Optimization conditions of UV-C radiation combined with ultrasound-assisted extraction of cherry tomato (Lycopersicon esculentum) lycopene extract
The aim of this work was to study the effect of UV-C radiation on ultrasound assisted extraction
(UAE) of cherry tomato bioactive compounds. Cherry tomatoes were exposed to two UV-C radiation
doses (0.5 and 1.0 J cm−2
) and stored at 20 ± 0.5 oC for 7 days. Next, they were lyophilized, and
the bioactive compounds were extracted by UAE at 20 KHz. To evaluate the effectiveness of the
extraction process of the bioactive compounds, a CCRD (central composite rotational design) was
used together with RSM (response surface methodology), for extraction times from 4 to 12 minutes
and concentrations (g of lyophilized product / L of ethanol) of 1:10, 1:20 and 1:30. The extracts
obtained from the irradiated tomatoes presented 5.8 times more lycopene content than the controls
and higher antioxidant activity was obtained for 4 and 8 min, in the concentrations 1:10 and 1:20 (m
v−1). Through numerical model optimization, optimal extraction conditions were obtained. The results
demonstrated that by previously irradiating tomatoes with UV-C light, the UAE yielded considerably
higher amounts of lycopene and other bioactives.CNPq (National Council of Technological and Scientific
Development, Brazil), Erasmus Mundus action 2; Fellow
Mundus Project; Department of Chemical Engineering and Food Engineering
(UFSC - Brazil) and the Department of Food Engineering (UAlg - Portugal) .info:eu-repo/semantics/publishedVersio
One-loop conformal anomaly in an implicit momentum space regularization framework
In this paper we consider matter fields in a gravitational background in
order to compute the breaking of the conformal current at one-loop order.
Standard perturbative calculations of conformal symmetry breaking expressed by
the non-zero trace of the energy-momentum tensor have shown that some violating
terms are regularization dependent, which may suggest the existence of spurious
breaking terms in the anomaly. Therefore, we perform the calculation in a
momentum space regularization framework in which regularization dependent terms
are judiciously parametrized. We compare our results with those obtained in the
literature and conclude that there is an unavoidable arbitrariness in the
anomalous term .Comment: in European Physical Journal C, 201
Ion motion in the wake driven by long particle bunches in plasmas
We explore the role of the background plasma ion motion in self-modulated
plasma wakefield accelerators. We employ J. Dawson's plasma sheet model to
derive expressions for the transverse plasma electric field and ponderomotive
force in the narrow bunch limit. We use these results to determine the on-set
of the ion dynamics, and demonstrate that the ion motion could occur in
self-modulated plasma wakefield accelerators. Simulations show the motion of
the plasma ions can lead to the early suppression of the self-modulation
instability and of the accelerating fields. The background plasma ion motion
can nevertheless be fully mitigated by using plasmas with heavier plasmas.Comment: 23 pages, 6 figure
Fractal analysis of weld defect patterns obtained by radiographic tests
This paper presents a fractal analysis of radiographic patterns obtained from
specimens with three types of inserted welding defects: lack of fusion, lack of
penetration, and porosity. The study focused on patterns of carbon steel beads
from radiographs of the International Institute of Welding (IIW). The
radiographs were scanned using a greyscale with 256 levels, and the fractal
features of the surfaces constructed from the radiographic images were
characterized by means of Hurst, detrended-fluctuation, and minimal-cover
analyses. A Karhunen-Loeve transformation was then used to classify the curves
obtained from the fractal analyses of the various images, and a study of the
classification errors was performed. The obtained results indicate that fractal
analyses can be an effective additional tool for pattern recognition of weld
defects in radiographic tests.Comment: 7 pages, 2 figures. To appear AIP Conference Proceedings - QNDE 200
Phase diagram of a model for a binary mixture of nematic molecules on a Bethe lattice
We investigate the phase diagram of a discrete version of the Maier-Saupe
model with the inclusion of additional degrees of freedom to mimic a
distribution of rodlike and disklike molecules. Solutions of this problem on a
Bethe lattice come from the analysis of the fixed points of a set of nonlinear
recursion relations. Besides the fixed points associated with isotropic and
uniaxial nematic structures, there is also a fixed point associated with a
biaxial nematic structure. Due to the existence of large overlaps of the
stability regions, we resorted to a scheme to calculate the free energy of
these structures deep in the interior of a large Cayley tree. Both
thermodynamic and dynamic-stability analyses rule out the presence of a biaxial
phase, in qualitative agreement with previous mean-field results
All-optical trapping and acceleration of heavy particles
A scheme for fast, compact, and controllable acceleration of heavy particles
in vacuum is proposed, in which two counterpropagating lasers with variable
frequencies drive a beat-wave structure with variable phase velocity, thus
allowing for trapping and acceleration of heavy particles, such as ions or
muons. Fine control over the energy distribution and the total charge of the
beam is obtained via tuning of the frequency variation. The acceleration scheme
is described with a one-dimensional theory, providing the general conditions
for trapping and scaling laws for the relevant features of the particle beam.
Two-dimensional, electromagnetic particle-in-cell simulations confirm the
validity and the robustness of the physical mechanism.Comment: 10 pages, 3 figures, to appear in New Journal of Physic
The ion motion in self-modulated plasma wakefield accelerators
The effects of plasma ion motion in self-modulated plasma based accelerators
is examined. An analytical model describing ion motion in the narrow beam limit
is developed, and confirmed through multi-dimensional particle-in-cell
simulations. It is shown that the ion motion can lead to the early saturation
of the self-modulation instability, and to the suppression of the accelerating
gradients. This can reduce the total energy that can be transformed into
kinetic energy of accelerated particles. For the parameters of future
proton-driven plasma accelerator experiments, the ion dynamics can have a
strong impact. Possible methods to mitigate the effects of the ion motion in
future experiments are demonstrated.Comment: 11 pages, 3 figures, accepted for publication in Phys. Rev. Let
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