12,888 research outputs found
From invasion percolation to flow in rock fracture networks
The main purpose of this work is to simulate two-phase flow in the form of
immiscible displacement through anisotropic, three-dimensional (3D) discrete
fracture networks (DFN). The considered DFNs are artificially generated, based
on a general distribution function or are conditioned on measured data from
deep geological investigations. We introduce several modifications to the
invasion percolation (MIP) to incorporate fracture inclinations, intersection
lines, as well as the hydraulic path length inside the fractures. Additionally
a trapping algorithm is implemented that forbids any advance of the invading
fluid into a region, where the defending fluid is completely encircled by the
invader and has no escape route. We study invasion, saturation, and flow
through artificial fracture networks, with varying anisotropy and size and
finally compare our findings to well studied, conditioned fracture networks.Comment: 18 pages, 10 figure
Phase boundary anisotropy and its effects on the maze-to-lamellar transition in a directionally solidified Al-Al2Cu eutectic
Solid-solid phase boundary anisotropy is a key factor controlling the
selection and evolution of non-faceted eutectic patterns during directional
solidification. This is most remarkably observed during the so-called
maze-to-lamellar transition. By using serial sectioning, we followed the
spatio-temporal evolution of a maze pattern over long times in a large Al-Al2Cu
eutectic grain with known crystal orientation of the Al and Al2Cu phases, hence
known crystal orientation relationship (OR). The corresponding phase boundary
energy anisotropy (-plot) was also known, as being previously estimated
from molecular-dynamics computations. The experimental observations reveal the
time-scale of the maze-to-lamellar transition and shed light on the processes
involved in the gradual alignment of the phase boundaries to one distinct
energy minimum which nearly corresponds to one distinct plane from the family
. This particular plane is selected
due to a crystallographic bias induced by a small disorientation of the
crystals relative to the perfect OR. The symmetry of the OR is thus slightly
broken, which promotes lamellar alignment. Finally, the maze-to-lamellar
transition leaves behind a network of fault lines inherited from the phase
boundary alignment process. In the maze pattern, the fault lines align along
the corners of the Wulff shape, thus allowing us to propose a link between the
pattern defects and missing orientations in the Wulff shapeComment: 26 pages, 6 figure
Superconductivity of SrTiO_{3-\delta}
Superconducting SrTiO_{3-\delta} was obtained by annealing single crystalline
SrTiO_3 samples in ultra high vacuum. An analysis of the V(I) characteristics
revealed very small critical currents I_c which can be traced back to a
unavoidable doping inhomogeneity. R(T) curves were measured for a range of
magnetic fields B at I<<I_c, thereby probing only the sample regions with the
highest doping level. The resulting curves B_{c2}(T) show upward curvature,
both at small and strong doping. These results are discussed in the context of
bipolaronic and conventional superconductivity with Fermi surface anisotropy.
We conclude that the special superconducting properties of SrTiO_{3-\delta} can
be related to its Fermi surface and compare this finding with properties of the
recently discovered superconductor MgB_2.Comment: EPJ style, 6 pages, 8 figures; minor changes, Fig. 5 replaced; use
PDF version for printout
Field structure and electron life times in the MEFISTO Electron Cyclotron Resonance Ion Source
The complex magnetic field of the permanent-magnet electron cyclotron
resonance (ECR) ion source MEFISTO located at the University of Bern have been
numerically simulated. For the first time the magnetized volume qualified for
electron cyclotron resonance at 2.45 GHz and 87.5 mT has been analyzed in
highly detailed 3D simulations with unprecedented resolution. New results were
obtained from the numerical simulation of 25211 electron trajectories. The
evident characteristic ion sputtering trident of hexapole confined ECR sources
has been identified with the field and electron trajectory distribution.
Furthermore, unexpected long electron trajectory lifetimes were found.Comment: 11 pages, 18 figure
3D Radio and X-Ray Modeling and Data Analysis Software: Revealing Flare Complexity
We have undertaken a major enhancement of our IDL-based simulation tools
developed earlier for modeling microwave and X-ray emission. The object-based
architecture provides an interactive graphical user interface that allows the
user to import photospheric magnetic field maps and perform magnetic field
extrapolations to almost instantly generate 3D magnetic field models, to
investigate the magnetic topology of these models by interactively creating
magnetic field lines and associated magnetic flux tubes, to populate the flux
tubes with user-defined nonuniform thermal plasma and anisotropic, nonuniform,
nonthermal electron distributions; to investigate the spatial and spectral
properties of radio and X-ray emission calculated from the model, and to
compare the model-derived images and spectra with observational data. The
application integrates shared-object libraries containing fast gyrosynchrotron
emission codes developed in FORTRAN and C++, soft and hard X-ray codes
developed in IDL, a FORTRAN-based potential-field extrapolation routine and an
IDL-based linear force free field extrapolation routine. The interactive
interface allows users to add any user-defined radiation code that adheres to
our interface standards, as well as user-defined magnetic field extrapolation
routines. Here we use this tool to analyze a simple single-loop flare and use
the model to constrain the 3D structure of the magnetic flaring loop and 3D
spatial distribution of the fast electrons inside this loop. We iteratively
compute multi-frequency microwave and multi-energy X-ray images from realistic
magnetic fluxtubes obtained from an extrapolation of a magnetogram taken prior
to the flare, and compare them with imaging data obtained by SDO, NoRH, and
RHESSI instruments. We use this event to illustrate use of the tool for general
interpretation of solar flares to address disparate problems in solar physics.Comment: 12 pages, 11 figures, ApJ accepte
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