1,211 research outputs found
Eliminating spurious velocities in the free energy lattice Boltzmann method
Spurious velocities are unphysical currents that appear close to curved
interfaces in diffuse interface methods. We analyse the causes of these
spurious velocities in the free energy lattice Boltzmann algorithm. By making a
suitable choice of the equilibrium distribution, and by finding the best way to
numerically calculate derivatives, we show that these velocities may be
decreased by an order of magnitude compared to previous models. Furthermore, we
propose a momentum conserving forcing method that reduces spurious velocities
by another factor of ~5. In three dimensions we find that 19 velocity vectors
is the minimum number necessary.Comment: 8 pages, 3 figure
Radio and X-ray observations of an exceptional radio flare in the extreme z=4.72 blazar GB B1428+4217
We report on the extreme behaviour of the high redshift blazar GB B1428+4217
at z=4.72. A continued programme of radio measurements has revealed an
exceptional flare in the lightcurve, with the 15.2 GHz flux density rising by a
factor ~3 from ~140 mJy to ~430 mJy in a rest-frame timescale of only ~4 months
-- much larger than any previous flares observed in this source. In addition to
new measurements of the 1.4-43 GHz radio spectrum we also present the analysis
and results of a target-of-opportunity X-ray observation using XMM-Newton, made
close to the peak in radio flux. Although the X-ray data do not show a flare in
the high energy lightcurve, we are able to confirm the X-ray spectral
variability hinted at in previous observations. GB B1428+4217 is one of several
high-redshift radio-loud quasars that display a low energy break in the X-ray
spectrum, probably due to the presence of excess absorption in the source.
X-ray spectral analysis of the latest XMM-Newton data is shown to be consistent
with the warm absorption scenario which we have hypothesized previously. Warm
absorption is also consistent with the observed X-ray spectral variability of
the source, in which the spectral changes can be successfully accounted-for
with a fixed column density of material in which the ionization state is
correlated with hardness of the underlying power-law emission.Comment: 8 pages, 5 figures, MNRAS accepte
Interface Width and Bulk Stability: requirements for the simulation of Deeply Quenched Liquid-Gas Systems
Simulations of liquid-gas systems with extended interfaces are observed to
fail to give accurate results for two reasons: the interface can get ``stuck''
on the lattice or a density overshoot develops around the interface. In the
first case the bulk densities can take a range of values, dependent on the
initial conditions. In the second case inaccurate bulk densities are found. In
this communication we derive the minimum interface width required for the
accurate simulation of liquid gas systems with a diffuse interface. We
demonstrate this criterion for lattice Boltzmann simulations of a van der Waals
gas. When combining this criterion with predictions for the bulk stability we
can predict the parameter range that leads to stable and accurate simulation
results. This allows us to identify parameter ranges leading to high density
ratios of over 1000. This is despite the fact that lattice Boltzmann
simulations of liquid-gas systems were believed to be restricted to modest
density ratios of less than 20.Comment: 5 pages, 3 figure
Transport coefficients of a mesoscopic fluid dynamics model
We investigate the properties of stochastic rotation dynamics
(Malevanets-Kapral method), a mesoscopic model used for simulating fluctuating
hydrodynamics. Analytical results are given for the transport coefficients. We
discuss the most efficient way of measuring the transport properties and obtain
excellent agreement between the theoretical and numerical calculations.Comment: 12 pages, 9 figures, submitted to J. Chem. Phy
Modeling microscopic swimmers at low Reynolds number
We employ three numerical methods to explore the motion of low Reynolds
number swimmers, modeling the hydrodynamic interactions by means of the Oseen
tensor approximation, lattice Boltzmann simulations and multiparticle collision
dynamics. By applying the methods to a three bead linear swimmer, for which
exact results are known, we are able to compare and assess the effectiveness of
the different approaches. We then propose a new class of low Reynolds number
swimmers, generalized three bead swimmers that can change both the length of
their arms and the angle between them. Hence we suggest a design for a
microstructure capable of moving in three dimensions. We discuss multiple bead,
linear microstructures and show that they are highly efficient swimmers. We
then turn to consider the swimming motion of elastic filaments. Using
multiparticle collision dynamics we show that a driven filament behaves in a
qualitatively similar way to the micron-scale swimming device recently
demonstrated by Dreyfus et al.Comment: 12 pages, 10 figure
HST/NICMOS Observations of Fast Infrared Flickering in the Microquasar GRS 1915+105
We report infrared observations of the microquasar GRS 1915+105 using the
NICMOS instrument of the Hubble Space Telescope during 9 visits in April-June
2003. During epochs of high X-ray/radio activity near the beginning and end of
this period, we find that the \um infrared flux is generally low ( mJy) and relatively steady. However, during the X-ray/radio ``plateau''
state between these epochs, we find that the infrared flux is significantly
higher ( mJy), and strongly variable. In particular, we find events
with amplitudes % occurring on timescales of s
(e-folding timescales of s). These flickering timescales are several
times faster than any previously-observed infrared variability in GRS 1915+105
and the IR variations exceed corresponding X-ray variations at the same () timescale. These results suggest an entirely new type of infrared
variability from this object. Based on the properties of this flickering, we
conclude that it arises in the plateau-state jet outflow itself, at a distance
AU from the accretion disk. We discuss the implications of this work and
the potential of further flickering observations for understanding jet
formation around black holes.Comment: 19 pages, incl. 4 figures; accepted for publication in Ap
Spectral state dependence of the 0.4-2 MeV polarized emission in Cygnus X-1 seen with INTEGRAL/IBIS, and links with the AMI radio data
Polarization of the >~400 keV hard tail of the microquasar Cygnus X-1 has
been independently reported by INTEGRAL/IBIS, and INTEGRAL/SPI and interpreted
as emission from a compact jet. These conclusions were, however, based on the
accumulation of all INTEGRAL data regardless of the spectral state. We utilize
additional INTEGRAL exposure accumulated until December 2012, and include the
AMI/Ryle (15 GHz) radio data in our study. We separate the observations into
hard, soft, and intermediate/transitional states and detect radio emission from
a compact jet in hard and intermediate states, but not in the soft. The 10-400
keV INTEGRAL (JEM-X and IBIS) state resolved spectra are well modeled with
thermal Comptonization and reflection components. We detect a hard tail in the
0.4-2 MeV range for the hard state only. We extract the state dependent
polarigrams of Cyg X-1, which all are compatible to no or undetectable level of
polarization except in 400-2000 keV range in the hard state where the
polarization fraction is 7532 % and the polarization angle 40.0 +-14 deg.
An upper limit on the 0.4-2 MeV soft state polarization fraction is 70%. Due to
the short exposure, we obtain no meaningful constraint for the intermediate
state. The likely detection of a >400 keV polarized tail in the hard state,
together with the simultaneous presence of a radio jet, reinforce the notion of
a compact jet origin of the 400 keV emission.Comment: 13 pages, 5 figures, accepted for publication in Ap
- …