285 research outputs found
WhiskyMHD: a new numerical code for general relativistic magnetohydrodynamics
The accurate modelling of astrophysical scenarios involving compact objects
and magnetic fields, such as the collapse of rotating magnetized stars to black
holes or the phenomenology of gamma-ray bursts, requires the solution of the
Einstein equations together with those of general-relativistic
magnetohydrodynamics. We present a new numerical code developed to solve the
full set of general-relativistic magnetohydrodynamics equations in a dynamical
and arbitrary spacetime with high-resolution shock-capturing techniques on
domains with adaptive mesh refinements. After a discussion of the equations
solved and of the techniques employed, we present a series of testbeds carried
out to validate the code and assess its accuracy. Such tests range from the
solution of relativistic Riemann problems in flat spacetime, over to the
stationary accretion onto a Schwarzschild black hole and up to the evolution of
oscillating magnetized stars in equilibrium and constructed as consistent
solutions of the coupled Einstein-Maxwell equations.Comment: minor changes to match the published versio
Magnetized Accretion Inside the Marginally Stable Orbit around a Black Hole
Qualitative arguments are presented to demonstrate that the energy density of
magnetic fields in matter accreting onto a black hole inside the marginally
stable orbit is automatically comparable to the rest-mass energy density of the
accretion flow. Several consequences follow: magnetic effects must be
dynamically significant, but cannot be so strong as to dominate; outward energy
transport in Alfven waves may alter the effective efficiency of energy
liberation; and vertical magnetic stresses in this region may contribute to
"coronal" activity.Comment: to appear in Ap. J. Letter
Numerical 3+1 general relativistic magnetohydrodynamics: a local characteristic approach
We present a general procedure to solve numerically the general relativistic
magnetohydrodynamics (GRMHD) equations within the framework of the 3+1
formalism. The work reported here extends our previous investigation in general
relativistic hydrodynamics (Banyuls et al. 1997) where magnetic fields were not
considered. The GRMHD equations are written in conservative form to exploit
their hyperbolic character in the solution procedure. All theoretical
ingredients necessary to build up high-resolution shock-capturing schemes based
on the solution of local Riemann problems (i.e. Godunov-type schemes) are
described. In particular, we use a renormalized set of regular eigenvectors of
the flux Jacobians of the relativistic magnetohydrodynamics equations. In
addition, the paper describes a procedure based on the equivalence principle of
general relativity that allows the use of Riemann solvers designed for special
relativistic magnetohydrodynamics in GRMHD. Our formulation and numerical
methodology are assessed by performing various test simulations recently
considered by different authors. These include magnetized shock tubes,
spherical accretion onto a Schwarzschild black hole, equatorial accretion onto
a Kerr black hole, and magnetized thick accretion disks around a black hole
prone to the magnetorotational instability.Comment: 18 pages, 8 figures, submitted to Ap
Magnetohydrodynamics in full general relativity: Formulation and tests
A new implementation for magnetohydrodynamics (MHD) simulations in full
general relativity (involving dynamical spacetimes) is presented. In our
implementation, Einstein's evolution equations are evolved by a BSSN formalism,
MHD equations by a high-resolution central scheme, and induction equation by a
constraint transport method. We perform numerical simulations for standard test
problems in relativistic MHD, including special relativistic magnetized shocks,
general relativistic magnetized Bondi flow in stationary spacetime, and a
longterm evolution for self-gravitating system composed of a neutron star and a
magnetized disk in full general relativity. In the final test, we illustrate
that our implementation can follow winding-up of the magnetic field lines of
magnetized and differentially rotating accretion disks around a compact object
until saturation, after which magnetically driven wind and angular momentum
transport inside the disk turn on.Comment: 28 pages, to be published in Phys. Rev.
U redox state and speciation of U in contact with magnetite nanoparticles : High resolution XANES, EXAFS, XPS and TEM study
Long-term storage of high-level radioactive waste is associated with potential radioecological hazards.
One chemical element of high interest is uranium (U), which can mainly exists as a mobile U(VI)
(oxidizing conditions) and sparingly soluble U(IV) (reducing conditions) species. It is expected that
the main inorganic reducing agent for U(VI) in the environment are ferrous species in magnetite,
formed on the steel canisters surface as an intermediate iron (Fe) corrosion product [1]. Results
obtained from laboratory experiments for the interaction of U(VI) with magnetite nanoparticles point
to partial reduction of U(VI) [2] or the formation of ~3 nm uranium dioxide (UO₂) particles on the
surface layer [3]. The evidence for U(VI) reduction to intermediate U(V) state was found with no
direct evidence of U(IV), which is in contradiction with thermodynamic calculations [4]. Continuous
interaction and related phase dissolution/recrystallization processes can also lead to U redox changes
and structural U incorporation into Fe oxides, resulting in U immobilization [5]. U redox state and
speciation analyses are still very challenging due to simultaneous formation of several different
species in such mineral systems. New advanced spectroscopic methods for characterization of such
systems will provide more precise results from speciation studies. The main goal of our investigation
is to assess the U M4 edge high energy resolution X-ray absorption near edge structure (HR-XANES)
spectroscopy technique for detection of U(V) possibly co-existing with U(IV) and U(VI) under
reducing conditions on/in Fe containing minerals. The U M4 edge HR-XANES has an advantage
compared to the conventional U L3 edge XANES, as the measured spectra are less dominated by corehole lifetime broadening effects and therefore have narrower spectral features [6-8]. This technique
facilitates the detection of minor contribution of one oxidation state in mixtures.
We have investigated the U redox states and speciation in a set of samples where U coprecipitated
with magnetite nanoparticles (~ 20 nm) with U concentrations varying in the 1000-10000 ppm range
(1000, 3000, 6000 and 10000 ppm). In addition to U M4 edge HR-XANES, U L3 edge extended X-ray
absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) and transmission electron
microscopy (TEM) techniques have been applied. The studied system models the interaction of U(VI)
with magnetite in aqueous solution, important for the understanding of the retarding effect of Fe
corrosion products on U in the context of deep geological spent nuclear fuel disposal. These
spectroscopic results can be compared with thermodynamic calculations and geochemical models
describing this interaction.
After 10 days U interaction with magnetite U M4 edge HR-XANES results indicate the formation of
U(IV), U(V) and U(VI) mixtures in varying ratios, depending on the initial U loading. Going from
10000 to 3000 ppm, the U(VI) content decreases continuously and is no longer found in the 1000 ppm
sample. At the same time the U(IV) and U(V) fractions increase. U(V) is stabilized as the main U
redox state in the 1000 ppm sample along with a smaller U(IV) contribution. After 20 days of contact
time XPS data show the predominance of U(IV) and U(V) species in the 6000 ppm sample. However,
mostly U(V) and some U(IV) is found for the 1000 ppm sample. For all samples aged for 240 days U
L3 XANES and EXAFS strongly suggest the formation of a UO₂ phase, UO₂ is the dominating species
in the 10000 ppm sample with U-O bond distance 2.33. Å as determined by EXAFS. UO₂ crystalline clusters with about 5 nm size formed on the surface of the magnetite nanoparticles are also found by TEM in the 10000 and 3000 ppm samples. The major and minor contributions of U(V) and U(IV), respectively, for the 1000 ppm sample after 240 days confirm the assumption that the U redox kinetics has completed within less than 10 days at this U concentration. EXAFS analyses reveal U(V)-Fe interaction in the second U coordination sphere, which substantially increases from the 10000 to 1000 ppm sample and is the dominating species in the 1000 ppm sample
Efficient error correction and haplotypes reconstruction for deep sequencing of hepatitis c amplicons
Секция 1. Защита информации и компьютерный анализ данныхWe present two new highly efficient pyrosequencing error correction algorithms:
(i) k-mer – based error correction (KEC); and (ii) empirical frequency threshold
(ET). Both were compared to the recently published clustering algorithm
SHORAH to evaluate the relative performance using 24 experimental datasets obtained
by 454-sequencing of amplicons with known sequences. We found that all
three algorithms showed similar performance in terms of finding true haplotypes, but
KEC and ET methods significantly outperformed SHORAH both in terms of their
ability to remove false haplotypes and to estimate the frequency of true ones
Three-Dimensional Hydrodynamic Simulations of Accretion Tori in Kerr Spacetimes
This paper presents results of three-dimensional simulations of global
hydrodynamic instabilities in black hole tori, extending earlier work by Hawley
to Kerr spacetimes. This study probes a three-dimensional parameter space of
torus angular momentum, torus size, and black hole angular momentum. We have
observed the growth of the Papaloizou-Pringle instability for a range of torus
configurations and the resultant formation of m=1 planets. We have also
observed the quenching of this instability in the presence of early accretion
flows; however, in one simulation both early accretion and planet formation
occurred. Though most of the conclusions reached in Hawley's earlier work on
Schwarzschild black holes carry over to Kerr spacetime, the presence of frame
dragging in the Kerr geometry adds an element of complexity to the simulations;
we have seen especially clear examples of this phenomenon in the accretion
flows that arise from retrograde tori.Comment: Accepted for publication in ApJ. 23 pages, 11 figures, animations
available at http://www.astro.virginia.edu/~jd5v/h91redux/h91redux.htm
Amorphous NiCu Thin Films Sputtered on TiO2 Nanotube Arrays: A Noble-Metal Free Photocatalyst for Hydrogen Evolution
In this work, NiCu co-catalysts on TiO2 are studied for photocatalytic hydrogen evolution. NiCu co-catalyst films are deposited at room temperature by argon plasma sputtering on high aspect-ratio anodic TiO2 nanotubes. To tune the Ni : Cu atomic ratio, alloys of various compositions were used as sputtering targets. Such co-catalyst films are found to be amorphous with small nanocrystalline domains. A series of parameters is investigated, i. e., i) Ni : Cu relative ratio in the sputtered films, ii) NiCu film thickness, and iii) thickness of the TiO2 nanotube layers. The highest photocatalytic activity is obtained with 8 μm long TiO2 nanotubes, sputter-coated with a 10 nm-thick NiCu films with a 1 : 1 Ni : Cu atomic ratio. This photocatalyst reaches a stable hydrogen evolution rate of 186 μL h−1 cm−2, 4.6 and 3 times higher than that of Ni- and Cu-TiO2, respectively, demonstrating a synergistic co-catalytic effect of Ni and Cu in the alloy co-catalyst film
Structural tale of two novel (Cr, Mn)C carbides in steel
Chromium (Cr), manganese (Mn) and carbon (C) are well known alloying elements used in technologically important alloy steels and advanced high strength steels. It is known that binary CrCx and MnCx carbides can be formed in steels, but in this study we reveal for the first time that Cr and Mn were found combined in novel ternary cementite type (Cr, Mn)C carbides. Electron diffraction experiments showed that Cr, Mn and C formed two distinct carbide phases possessing orthorhombic and monoclinic crystal structures. Density functional theory calculations were performed on these phases and excellent agreement was found between calculations and experiments on the lattice parameters and relative atomic positions. The calculations showed that the combination of Mn and Cr resulted in a very high thermodynamic stability of the (Cr, Mn)C carbides, and that local structural relaxations are associated with carbon additions. Possible implications of these ternary carbides for novel applications in steel design and manufacturing are discussed
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