31,898 research outputs found
Analysis of opposed jet hydrogen-air counter flow diffusion flame
A computational simulation of the opposed-jet diffusion flame is performed to study its structure and extinction limits. The present analysis concentrates on the nitrogen-diluted hydrogen-air diffusion flame, which provides the basic information for many vehicle designs such as the aerospace plane for which hydrogen is a candidate as the fuel. The computer program uses the time-marching technique to solve the energy and species equations coupled with the momentum equation solved by the collocation method. The procedure is implemented in two stages. In the first stage, a one-step forward overal chemical reaction is chosen with the gas phase chemical reaction rate determined by comparison with experimental data. In the second stage, a complete chemical reaction mechanism is introduced with detailed thermodynamic and transport property calculations. Comparison between experimental extinction data and theoretical predictions is discussed. The effects of thermal diffusion as well as Lewis number and Prandtl number variations on the diffusion flame are also presented
Semimetalic graphene in a modulated electric potential
The -electronic structure of graphene in the presence of a modulated
electric potential is investigated by the tight-binding model. The low-energy
electronic properties are strongly affected by the period and field strength.
Such a field could modify the energy dispersions, destroy state degeneracy, and
induce band-edge states. It should be noted that a modulated electric potential
could make semiconducting graphene semimetallic, and that the onset period of
such a transition relies on the field strength. There exist infinite
Fermi-momentum states in sharply contrast with two crossing points (Dirac
points) for graphene without external fields. The finite density of states
(DOS) at the Fermi level means that there are free carriers, and, at the same
time, the low DOS spectrum exhibits many prominent peaks, mainly owing to the
band-edge states.Comment: 12pages, 5 figure
Aspects of Floquet Bands and Topological Phase Transitions in a Continuously Driven Superlattice
Recently the creation of novel topological states of matter by a periodic
driving field has attracted great attention. To motivate further experimental
and theoretical studies, we investigate interesting aspects of Floquet bands
and topological phase transitions in a continuously driven Harper model. In
such a continuously driven system with an odd number of Floquet bands, the
bands are found to have nonzero Chern numbers in general and topological phase
transitions take place as we tune various system parameters, such as the
amplitude or the period of the driving field. The nontrivial Floquet band
topology results in a quantized transport of Wannier states in the lattice
space. For certain parameter choices, very flat yet topologically nontrivial
Floquet bands may also emerge, a feature that is potentially useful for the
simulation of physics of strongly correlated systems. Some cases with an even
number of Floquet bands may also have intriguing Dirac cones in the spectrum.
Under open boundary conditions, anomalous counter-propagating chiral edge modes
and degenerate zero modes are also found as the system parameters are tuned.
These results should be of experimental interest because a continuously driven
system is easier to realize than a periodically kicked system.Comment: 29 pages, 9 figures. Comments are welcom
The inferior caval vein draining into the left atrial cavity : a rare case
The inferior vena cava (IVC) draining into the left atrium (LA) is exceedingly rare in the setting of the usual atrial arrangement (situs solitus). This article describes a patient with this unique anomaly, and its repair.peer-reviewe
Determination of a set of constitutive equations for an al-li alloy at SPF conditions
© 2015 The Authors.Uniaxial tensile tests of aluminium-lithium alloy AA1420wereconducted at superplastic forming conditions. The mechanical properties of this Al-Li alloy were then modelled by a set of physicallybased constitutive equations. The constitutive equations describe the isotropic work hardening,recovery and damage by dislocation density changes and grain size evolution. Based on a recent upgraded optimisation technique, the material constants for these constitutive equations were determined
Outflow-Dominated Emission from the Quiescent Massive Black Holes in NGC 4621 and NGC 4697
The nearby elliptical galaxies NGC 4621 and NGC 4697 each host a supermassive
black hole with a mass more than 1e8 Solar masses. Analysis of archival Chandra
data and new NRAO Very Large Array data shows that each galaxy contains a
low-luminosity active galactic nucleus (LLAGN), identified as a faint, hard
X-ray source that is astrometrically coincident with a faint 8.5-GHz source.
The latter has a diameter less than 0.3 arcsec (26 pc for NGC 4621, 17 pc for
NGC 4697). The black holes energizing these LLAGNs have Eddington ratios L(2-10
keV) / L(Edd) ~ 1e-9, placing them in the so-called quiescent regime. The
emission from these quiescent black holes is radio-loud, with log Rx = log
nuLnu(8.5 GHz) / L(2-10 keV) ~ -2, suggesting the presence of a radio outflow.
Also, application of the radio-X-ray-mass relation from Yuan & Cui for
quiescent black holes predicts the observed radio luminosities nuLnu(8.5 GHz)
to within a factor of a few. Significantly, that relation invokes X-ray
emission from the outflow rather than from an accretion flow. The faint, but
detectable, emission from these two massive black holes is therefore consistent
with being outflow-dominated. Observational tests of this finding are
suggested.Comment: 11 pages; 4 figures: emulateapj; to appear in Ap
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