1,397 research outputs found
Moment method boundary conditions for multiphase Lattice Boltzmann simulations with partially-wetted walls
We propose a lattice Boltzmann approach for simulating contact angle phenomena in multiphase fluid systems. Boundary conditions for partially-wetted walls are introduced using the moment method. The algorithm with our boundary conditions allows for a maximum density ratio of 200000 for neutral wetting. The achievable density ratio decreases as the contact angle departs from 90°, but remains of the order O(102) for all but extreme contact angles. In all simulations an excellent agreement between the simulated and nominal contact angles is observe
Representations of integers by certain positive definite binary quadratic forms
We prove part of a conjecture of Borwein and Choi concerning an estimate on
the square of the number of solutions to n=x^2+Ny^2 for a squarefree integer N.Comment: 8 pages, submitte
What fraction of stars formed in infrared galaxies at high redshift?
Star formation happens in two types of environment: ultraviolet-bright
starbursts (like 30 Doradus and HII galaxies at low redshift and Lyman-break
galaxies at high redshift) and infrared-bright dust-enshrouded regions (which
may be moderately star-forming like Orion in the Galaxy or extreme like the
core of Arp 220). In this work I will estimate how many of the stars in the
local Universe formed in each type of environment, using observations of
star-forming galaxies at all redshifts at different wavelengths and of the
evolution of the field galaxy population.Comment: 7 pages, 0 figs, to appear in proceedings of "Starbursts - From 30
Doradus to Lyman break galaxies", edited by Richard de Grijs and Rosa M.
Gonzalez Delgado, published by Kluwe
XUV digital in-line holography using high-order harmonics
A step towards a successful implementation of timeresolved digital in-line
holography with extreme ultraviolet radiation is presented. Ultrashort XUV
pulses are produced as high-order harmonics of a femtosecond laser and a
Schwarzschild objective is used to focus harmonic radiation at 38 nm and to
produce a strongly divergent reference beam for holographic recording.
Experimental holograms of thin wires are recorded and the objects
reconstructed. Descriptions of the simulation and reconstruction theory and
algorithms are also given. Spatial resolution of few hundreds of nm is
potentially achievable, and micrometer resolution range is demonstrated.Comment: 8 pages, 8 figure
Magnetic spin excitations in Mn doped GaAs : A model study
We provide a quantitative theoretical model study of the dynamical magnetic
properties of optimally annealed GaMnAs. This model has already
been shown to reproduce accurately the Curie temperatures for
GaMnAs. Here we show that the calculated spin stiffness are in
excellent agreement with those which were obtained from ab-initio based
studies. In addition, an overall good agreement is also found with available
experimental data. We have also evaluated the magnon density of states and the
typical density of states from which the "mobility edge", separating the
extended from localized magnon states, was determined. The power of the model
lies in its ability to be generalized for a broad class of diluted magnetic
semiconductor materials, thus it bridges the gap between first principle
calculations and model based studies.Comment: 5 pages, 5 figures, Text and some figures revised to match the
accepted versio
Automated Coronal Hole Detection using Local Intensity Thresholding Techniques
We identify coronal holes using a histogram-based intensity thresholding
technique and compare their properties to fast solar wind streams at three
different points in the heliosphere. The thresholding technique was tested on
EUV and X-ray images obtained using instruments onboard STEREO, SOHO and
Hinode. The full-disk images were transformed into Lambert equal-area
projection maps and partitioned into a series of overlapping sub-images from
which local histograms were extracted. The histograms were used to determine
the threshold for the low intensity regions, which were then classified as
coronal holes or filaments using magnetograms from the SOHO/MDI. For all three
instruments, the local thresholding algorithm was found to successfully
determine coronal hole boundaries in a consistent manner. Coronal hole
properties extracted using the segmentation algorithm were then compared with
in situ measurements of the solar wind at 1 AU from ACE and STEREO. Our results
indicate that flux tubes rooted in coronal holes expand super-radially within 1
AU and that larger (smaller) coronal holes result in longer (shorter) duration
high-speed solar wind streams
Svestka's Research: Then and Now
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of the
areas that particularly interested him and assess them in a "then and now"
style. His insights in each case were quite sound, although of course in the
modern era we have learned things that he could not readily have envisioned.
His own views about his research life have been published recently in this
journal, to which he contributed so much, and his memoir contains much
additional scientific and personal information (Svestka, 2010).Comment: Invited review for "Solar and Stellar Flares," a conference in honour
of Prof. Zden\v{e}k \v{S}vestka, Prague, June 23-27, 2014. This is a
contribution to a Topical Issue in Solar Physics, based on the presentations
at this meeting (Editors Lyndsay Fletcher and Petr Heinzel
A-dependence of nuclear transparency in quasielastic A(e,e'p) at high Q^2
The A-dependence of the quasielastic A(e,e'p) reaction has been studied at
SLAC with H-2, C, Fe, and Au nuclei at momentum transfers Q^2 = 1, 3, 5, and
6.8 (GeV/c)^2. We extract the nuclear transparency T(A,Q^2), a measure of the
average probability that the struck proton escapes from the nucleus A without
interaction. Several calculations predict a significant increase in T with
momentum transfer, a phenomenon known as Color Transparency. No significant
rise within errors is seen for any of the nuclei studied.Comment: 5 pages incl. 2 figures, Caltech preprint OAP-73
Deconfining Phase Transition as a Matrix Model of Renormalized Polyakov Loops
We discuss how to extract renormalized from bare Polyakov loops in SU(N)
lattice gauge theories at nonzero temperature in four spacetime dimensions.
Single loops in an irreducible representation are multiplicatively renormalized
without mixing, through a renormalization constant which depends upon both
representation and temperature. The values of renormalized loops in the four
lowest representations of SU(3) were measured numerically on small, coarse
lattices. We find that in magnitude, condensates for the sextet and octet loops
are approximately the square of the triplet loop. This agrees with a large
expansion, where factorization implies that the expectation values of loops in
adjoint and higher representations are just powers of fundamental and
anti-fundamental loops. For three colors, numerically the corrections to the
large relations are greatest for the sextet loop, ; these
represent corrections of for N=3. The values of the renormalized
triplet loop can be described by an SU(3) matrix model, with an effective
action dominated by the triplet loop. In several ways, the deconfining phase
transition for N=3 appears to be like that in the matrix model of
Gross and Witten.Comment: 24 pages, 7 figures; v2, 27 pages, 12 figures, extended discussion
for clarity, results unchange
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