88 research outputs found
Control of Local Mass Transfer in the Separated and Reattaching Flow by a Periodic Forcing
A numerical study based on the Large Eddy Simulation (LES) methodology was made of mass transfer in locally
forced turbulent separated and reattaching flow over a backward facing step. The local forcing was given to the flow
by a sinusoidally blowing /suction of the fluid into a separated shear layer. The Reynolds number was fixed at 33000
and Schmidt number at 1. The forcing frequency was varied in the range 0 St 2, where St is the Strouhal number
of forcing. The obtained results revealed the existence of an optimum forcing frequency value, St = 0.25, in terms of
the reduced reattachment length. At this frequency the mass transfer is significantly enhanced in the recirculation
zone. The influence of the frequency and the amplitude of forcing, in the maximum mass transfer positions and the
maximum Sherwood number, are analyzed
Simulation of the Periodically Perturbed Separated and Reattaching Flow over a Backward-Facing Step
In this work, the Large Eddy Simulation (LES) methodology is used to study the effects of a periodic perturbation introduced into a separated shear layer behind a backward-facing step. This study carried out by acting on the two parameters characteristics of the perturbation: frequency and amplitude. The obtained results reveal the existence of an optimum perturbation frequency value, Stp = 0.25, in terms of the reduced reattachment length. At this perturbation frequency value, we observed an increase in the vortical shedding frequency in the reattachment zone with a significant change of the structure of the flow. The value of the optimum frequency appears to be independent of the perturbation amplitude. At this frequency the maximum decrease of reattachment length is 50% and the maximum increase of vortical shedding frequency is 43 % compared to the unperturbed case
Resonant Andreev reflections in superconductor-carbon-nanotube devices
Resonant Andreev reflection through superconductor-carbon-nanotube devices
was investigated theoretically with a focus on the superconducting proximity
effect. Consistent with a recent experiment, we find that for high transparency
devices on-resonance, the Andreev current is characterized by a large value and
a resistance dip; low-transparency off-resonance devices give the opposite
result. We also give evidence that the observed low-temperature transport
anomaly may be a natural result of Andreev reflection process
Carbon nanotube-based quantum pump in the presence of superconducting lead
Parametric electron pump through superconductor-carbon-nanotube based
molecular devices was investigated. It is found that a dc current, which is
assisted by resonant Andreev reflection, can be pumped out from such molecular
device by a cyclic variation of two gate voltages near the nanotube. The pumped
current can be either positive or negative under different system parameters.
Due to the Andreev reflection, the pumped current has the double peak structure
around the resonant point. The ratio of pumped current of N-SWNT-S system to
that of N-SWNT-N system (I^{NS}/I^N) is found to approach four in the weak
pumping regime near the resonance when there is exactly one resonant level at
Fermi energy inside the energy gap. Numerical results confirm that in the weak
pumping regime the pumped current is proportional to the square of the pumping
amplitude V_p, but in the strong pumping regime the pumped current has the
linear relation with V_p. Our numerical results also predict that pumped
current can be obtained more easily by using zigzag tube than by using armchair
tube
From favorable atomic configurations to supershell structures: a new interpretation of conductance histograms
Title: From favorable atomic configurations to supershell structures: a new
interpretation of conductance histograms Authors: A. Hasmy (IVIC), E. Medina
(IVIC), P.A. Serena (CSIC,IVIC) Comments: 7 pages, 3 figures,
cond-mat.anwar.10825 Subj-class: Soft Condensed MatterComment: 7 pages, 3 figuresSubject: fput HMS.tex HMS-FIG1.ps HMS-FIG2.ps
HMS-FIG3.p
Tight-binding parameters for charge transfer along DNA
We systematically examine all the tight-binding parameters pertinent to
charge transfer along DNA. The molecular structure of the four DNA bases
(adenine, thymine, cytosine, and guanine) is investigated by using the linear
combination of atomic orbitals method with a recently introduced
parametrization. The HOMO and LUMO wavefunctions and energies of DNA bases are
discussed and then used for calculating the corresponding wavefunctions of the
two B-DNA base-pairs (adenine-thymine and guanine-cytosine). The obtained HOMO
and LUMO energies of the bases are in good agreement with available
experimental values. Our results are then used for estimating the complete set
of charge transfer parameters between neighboring bases and also between
successive base-pairs, considering all possible combinations between them, for
both electrons and holes. The calculated microscopic quantities can be used in
mesoscopic theoretical models of electron or hole transfer along the DNA double
helix, as they provide the necessary parameters for a tight-binding
phenomenological description based on the molecular overlap. We find that
usually the hopping parameters for holes are higher in magnitude compared to
the ones for electrons, which probably indicates that hole transport along DNA
is more favorable than electron transport. Our findings are also compared with
existing calculations from first principles.Comment: 15 pages, 3 figures, 7 table
Effectiveness of esterified whey proteins fractions against Egyptian Lethal Avian Influenza A (H5N1)
<p>Abstract</p> <p>Background</p> <p>Avian influenza A (H5N1) virus is one of the most important public health concerns worldwide. The antiviral activity of native and esterified whey proteins fractions (α- lactalbumin, β- lactoglobulin, and lactoferrin) was evaluated against A/chicken/Egypt/086Q-NLQP/2008 HPAI (H5N1) strain of clade 2.2.1 (for multiplicity of infection (1 MOI) after 72 h of incubation at 37°C in the presence of 5% CO<sub>2</sub>) using MDCK cell lines.</p> <p>Result</p> <p>Both the native and esterified lactoferrin seem to be the most active antiviral protein among the tested samples, followed by β- lactoglobulin. α-Lactalbumin had less antiviral activity even after esterification.</p> <p>Conclusion</p> <p>Esterification of whey proteins fractions especially lactoferrin and β-lactoglobulin enhanced their antiviral activity against H5N1 in a concentration dependent manner.</p
Global economic burden of unmet surgical need for appendicitis
Background: There is a substantial gap in provision of adequate surgical care in many low-and middle-income countries. This study aimed to identify the economic burden of unmet surgical need for the common condition of appendicitis. Methods: Data on the incidence of appendicitis from 170 countries and two different approaches were used to estimate numbers of patients who do not receive surgery: as a fixed proportion of the total unmet surgical need per country (approach 1); and based on country income status (approach 2). Indirect costs with current levels of access and local quality, and those if quality were at the standards of high-income countries, were estimated. A human capital approach was applied, focusing on the economic burden resulting from premature death and absenteeism. Results: Excess mortality was 4185 per 100 000 cases of appendicitis using approach 1 and 3448 per 100 000 using approach 2. The economic burden of continuing current levels of access and local quality was US 73 141 million using approach 2. The economic burden of not providing surgical care to the standards of high-income countries was 75 666 million using approach 2. The largest share of these costs resulted from premature death (97.7 per cent) and lack of access (97.0 per cent) in contrast to lack of quality. Conclusion: For a comparatively non-complex emergency condition such as appendicitis, increasing access to care should be prioritized. Although improving quality of care should not be neglected, increasing provision of care at current standards could reduce societal costs substantially
Metallic, magnetic and molecular nanocontacts
Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained
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