156,830 research outputs found
Effect of a Flared Renal Stent on the Performance of Fenestrated Stent-Grafts at Rest and Exercise Conditions
Purpose: To quantify the hemodynamic impact of a flared renal stent on the performance of fenestrated stent-grafts (FSGs) by analyzing flow patterns and wall shear stress–derived parameters in flared and nonflared FSGs in different physiologic scenarios. Methods: Hypothetical models of FSGs were created with and without flaring of the proximal portion of the renal stent. Flared FSGs with different dilation angles and protrusion lengths were examined, as well as a nonplanar flared FSG to account for lumbar curvature. Laminar and pulsatile blood flow was simulated by numerically solving Navier-Stokes equations. A physiologically realistic flow rate waveform was prescribed at the inlet, while downstream vasculature was modeled using a lumped parameter 3-element windkessel model. No slip boundary conditions were imposed at the FSG walls, which were assumed to be rigid. While resting simulations were performed on all the FSGs, exercise simulations were also performed on a flared FSG to quantify the effect of flaring in different physiologic scenarios. Results: For cycle-averaged inflow of 2.94 L/min (rest) and 4.63 L/min (exercise), 27% of blood flow was channeled into each renal branch at rest and 21% under exercise for all the flared FSGs examined. Although the renal flow waveform was not affected by flaring, flow within the flared FSGs was disturbed. This flow disturbance led to high endothelial cell activation potential (ECAP) values at the renal ostia for all the flared geometries. Reducing the dilation angle or protrusion length and exercise lowered the ECAP values for flared FSGs. Conclusion: Flaring of renal stents has a negligible effect on the time dependence of renal flow rate waveforms and can maintain sufficient renal perfusion at rest and exercise. Local flow patterns are, however, strongly dependent on renal flaring, which creates a local flow disturbance and may increase the thrombogenicity at the renal ostia. Smaller dilation angles, shorter protrusion lengths, and moderate lower limb exercise are likely to reduce the risk of thrombosis in flared geometries
Inertial effects on two-particle relative dispersion in turbulent flows
We report experimental results on the relative motion of pairs of solid
spheric particles with initial separations in the inertial range of fully
developed turbulence in water. The particle densities were in the range of , \textit{i.e.}, from neutrally
buoyant to highly inertial; and their sizes were of the Kolmogorov scale. For
all particles, we observed a Batchelor like regime, in which particles
separated ballistically. Similar to the Batchelor regime for tracers, this
regime was observed in the early stages of the relative separation for times with determined by the turbulence energy dissipation
rate and the initial separation between particle pairs. In this time interval
heavier particles separated faster than fluid tracers. The second order
Eulerian velocity structure functions was found to increase with density. In
other words, both observations show that the relative velocity between inertial
particles was larger than that between tracers. Based on the widely used,
simplified equation of motion for inertial point-particles, we derived a model
that shows an increase in relative velocity between inertial particles. In its
scale dependence, however, it disagrees quantitatively with the experimental
results. This we attribute to the preferential sampling of the flow field by
inertial particles, which is not captured by the model.Comment: 6 pages, 5 figures, 2 tables, epl2.cls, submitted to EP
Control of fast electron propagation in foam target by high-Z doping
The influence of high-Z dopant (Bromine) in low-Z foam (polystyrene) target
on laser-driven fast electron propagation is studied by the 3D hybrid
particle-in-cell (PIC)/fluid code HEETS.It is found that the fast electrons are
better confined in doped targets due to the increasing resistivity of the
target, which induces a stronger resistive magnetic field which acts to
collimate the fast electron propagation.The energy deposition of fast electrons
into the background target is increased slightly in the doped target, which is
beneficial for applications requiring long distance propagation of fast
electrons, such as fast ignition
Impact of Resonant Magnetic Perturbations on Zonal Modes, Drift-Wave Turbulence and the L-H Transition Threshold
We study the effects of Resonant Magnetic Perturbations (RMPs) on turbulence,
flows and confinement in the framework of resistive drift-wave turbulence. This
work was motivated, in parts, by experiments reported at the IAEA 2010
conference [Y. Xu {\it et al}, Nucl. Fusion \textbf{51}, 062030] which showed a
decrease of long-range correlations during the application of RMPs. We derive
and apply a zero-dimensional predator-prey model coupling the Drift-Wave Zonal
Mode system [M. Leconte and P.H. Diamond, Phys. Plasmas \textbf{19}, 055903] to
the evolution of mean quantities. This model has both density gradient drive
and RMP amplitude as control parameters and predicts a novel type of transport
bifurcation in the presence of RMPs. This model allows a description of the
full L-H transition evolution with RMPs, including the mean sheared flow
evolution. The key results are: i) The L-I and I-H power thresholds \emph{both}
increase with RMP amplitude |\bx|, the relative increase of the L-I threshold
scales as \Delta P_{\rm LI} \propto |\bx|^2 \nu_*^{-2} \gyro^{-2}, where
is edge collisionality and \gyro is the sound gyroradius. ii) RMPs
are predicted to \emph{decrease} the hysteresis between the forward and
back-transition. iii) Taking into account the mean density evolution, the
density profile - sustained by the particle source - has an increased turbulent
diffusion compared with the reference case without RMPs which provides one
possible explanation for the \emph{density pump-out} effect.Comment: 30 pages, IAEA-based articl
Modulo Three Problem With A Cellular Automaton Solution
An important global property of a bit string is the number of ones in it. It
has been found that the parity (odd or even) of this number can be found by a
sequence of deterministic, translational invariant cellular automata with
parallel update in succession for a total of O(N^2) time. In this paper, we
discover a way to check if this number is divisible by three using the same
kind of cellular automata in O(N^3) time. We also speculate that the method
described here could be generalized to check if it is divisible by four and
other positive integers.Comment: 10 pages in revtex 4.0, using amsfont
The optical/UV excess of isolated neutron stars in the RCS model
The X-ray dim isolated neutron stars (XDINSs) are peculiar pulsar-like
objects, characterized by their very well Planck-like spectrum. In studying
their spectral energy distributions, the optical/UV excess is a long standing
problem. Recently, Kaplan et al. (2011) have measured the optical/UV excess for
all seven sources, which is understandable in the resonant cyclotron scattering
(RCS) model previously addressed. The RCS model calculations show that the RCS
process can account for the observed optical/UV excess for most sources . The
flat spectrum of RX J2143.0+0654 may due to contribution from bremsstrahlung
emission of the electron system in addition to the RCS process.Comment: 6 pages, 2 figures, 1 table, accepted for publication in Research in
Astronomy and Astrophysic
AXPs and SGRs in the outer gap model: confronting Fermi observations
Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are
magnetar candidates, i.e., neutron stars powered by strong magnetic field. If
they are indeed magnetars, they will emit high-energy gamma-rays which are
detectable by Fermi-LAT according to the outer gap model. However, no
significant detection is reported in recent Fermi-LAT observations of all known
AXPs and SGRs. Considering the discrepancy between theory and observations, we
calculate the theoretical spectra for all AXPs and SGRs with sufficient
observational parameters. Our results show that most AXPs and SGRs are
high-energy gamma-ray emitters if they are really magnetars. The four AXPs 1E
1547.0-5408, XTE J1810-197, 1E 1048.1-5937, and 4U 0142+61 should have been
detected by Fermi-LAT. Then there is conflict between out gap model in the case
of magnetars and Fermi observations. Possible explanations in the magnetar
model are discussed. On the other hand, if AXPs and SGRs are fallback disk
systems, i.e., accretion-powered for the persistent emissions, most of them are
not high-energy gamma-ray emitters. Future deep Fermi-LAT observations of AXPs
and SGRs will help us make clear whether they are magnetars or fallback disk
systems.Comment: 15 pages, 3 figures, 1 table, accepted for publication in The
Astrophysical Journa
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