4,350 research outputs found
Thermophysical properties of warm dense hydrogen
We study the thermophysical properties of warm dense hydrogen using quantum
molecular dynamics simulations. New results are presented for the pair
distribution functions, the equation of state, the Hugoniot curve, and the
reflectivity. We compare with available experimental data and predictions of
the chemical picture. Especially, we discuss the nonmetal-to-metal transition
which occurs at about 40 GPa in the dense fluid
Buildup of Magnetic Shear and Free Energy During Flux Emergence and Cancellation
We examine a simulation of flux emergence and cancellation, which shows a
complex sequence of processes that accumulate free magnetic energy in the solar
corona essential for the eruptive events such as coronal mass ejections (CMEs),
filament eruptions and flares. The flow velocity at the surface and in the
corona shows a consistent shearing pattern along the polarity inversion line
(PIL), which together with the rotation of the magnetic polarities, builds up
the magnetic shear. Tether-cutting reconnection above the PIL then produces
longer sheared magnetic field lines that extend higher into the corona, where a
sigmoidal structure forms. Most significantly, reconnection and upward
energy-flux transfer are found to occur even as magnetic flux is submerging and
appears to cancel at the photosphere. A comparison of the simulated coronal
field with the corresponding coronal potential field graphically shows the
development of nonpotential fields during the emergence of the magnetic flux
and formation of sunspots
An exact Riemann solver based solution for regular shock refraction
We study the classical problem of planar shock refraction at an oblique
density discontinuity, separating two gases at rest. When the shock impinges on
the density discontinuity, it refracts and in the hydrodynamical case 3 signals
arise. Regular refraction means that these signals meet at a single point,
called the triple point.
After reflection from the top wall, the contact discontinuity becomes
unstable due to local Kelvin-Helmholtz instability, causing the contact surface
to roll up and develop the Richtmyer-Meshkov instability. We present an exact
Riemann solver based solution strategy to describe the initial self similar
refraction phase, by which we can quantify the vorticity deposited on the
contact interface. We investigate the effect of a perpendicular magnetic field
and quantify how addition of a perpendicular magnetic field increases the
deposition of vorticity on the contact interface slightly under constant Atwood
number. We predict wave pattern transitions, in agreement with experiments, von
Neumann shock refraction theory, and numerical simulations performed with the
grid-adaptive code AMRVAC. These simulations also describe the later phase of
the Richtmyer-Meshkov instability.Comment: 21 pages, 17 figures in 41 ps-files, accepted by J. Fluid Mec
Making Anti-de Sitter Black Holes
It is known from the work of Banados et al. that a space-time with event
horizons (much like the Schwarzschild black hole) can be obtained from 2+1
dimensional anti-de Sitter space through a suitable identification of points.
We point out that this can be done in 3+1 dimensions as well. In this way we
obtain black holes with event horizons that are tori or Riemann surfaces of
genus higher than one. They can have either one or two asymptotic regions.
Locally, the space-time is isometric to anti-de Sitter space.Comment: LaTeX, 10 pages, 6 postscript figures, uses epsf.te
Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium
We have performed quantum molecular dynamics simulations for dense helium to
study the nonmetal-to-metal transition at high pressures. We present new
results for the equation of state and the Hugoniot curve in the warm dense
matter region. The optical conductivity is calculated via the Kubo-Greenwood
formula from which the dc conductivity is derived. The nonmetal-to-metal
transition is identified at about 1 g/ccm. We compare with experimental results
as well as with other theoretical approaches, especially with predictions of
chemical models.Comment: 4 pages, 5 figure
Prediction of a double-antireflection coating made solely with SiN x in a single, directional deposition step
Silicon solar cell modules, where the EVA layer is replaced by an air gap, are able to produce the same electric power as standard modules with EVA only if their anti-reflective properties are enhanced. We propose a method to do this by exploiting the fact that, on Si surfaces textured with random pyramids, light incident from near normal angle always hits at least two pyramidal faces before being reflected back toward the sun. If these two faces are covered with an anti-reflective coating (ARC) made of one and the same material but with two different thicknesses, the coating acts as a double ARC. Such a coating can be produced by depositing the SiNx layer from an oblique angle, optimally from 14.7°. Our detailed raytracing analysis predicts that J sc can then be improved by 0.2 mA/cm2 for normal incident sunlight and AM1.5g standard illumination, and is improved for all angles within a cone with an apex angle of approximately 64°. Furthermore, the coating can be optimized for modules in vertical mounting, where a Jsc gain of 0.1 mA/cm2 is predicted for an angle of incidence of 40°
De Sitter Space and Spatial Topology
Morrow-Jones and Witt have shown that generic spatial topologies admit
initial data that evolve to locally de Sitter spacetimes under Einstein's
equations. We simplify their arguments, make them a little more general, and
solve for the global time evolution of the wormhole initial data considered by
them. Finally we give explicit examples of locally de Sitter domains of
development whose universal covers cannot be embedded in de Sitter space.Comment: 21 pages, 7 figure
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