7,807 research outputs found
Marangoni driven turbulence in high energy surface melting processes
Experimental observations of high-energy surface melting processes, such as
laser welding, have revealed unsteady, often violent, motion of the free
surface of the melt pool. Surprisingly, no similar observations have been
reported in numerical simulation studies of such flows. Moreover, the published
simulation results fail to predict the post-solidification pool shape without
adapting non-physical values for input parameters, suggesting the neglect of
significant physics in the models employed. The experimentally observed violent
flow surface instabilities, scaling analyses for the occurrence of turbulence
in Marangoni driven flows, and the fact that in simulations transport
coefficients generally have to be increased by an order of magnitude to match
experimentally observed pool shapes, suggest the common assumption of laminar
flow in the pool may not hold, and that the flow is actually turbulent. Here,
we use direct numerical simulations (DNS) to investigate the role of turbulence
in laser melting of a steel alloy with surface active elements. Our results
reveal the presence of two competing vortices driven by thermocapillary forces
towards a local surface tension maximum. The jet away from this location at the
free surface, separating the two vortices, is found to be unstable and highly
oscillatory, indeed leading to turbulence-like flow in the pool. The resulting
additional heat transport, however, is insufficient to account for the observed
differences in pool shapes between experiment and simulations
Topological strings on noncommutative manifolds
We identify a deformation of the N=2 supersymmetric sigma model on a
Calabi-Yau manifold X which has the same effect on B-branes as a noncommutative
deformation of X. We show that for hyperkahler X such deformations allow one to
interpolate continuously between the A-model and the B-model. For generic
values of the noncommutativity and the B-field, properties of the topologically
twisted sigma-models can be described in terms of generalized complex
structures introduced by N. Hitchin. For example, we show that the path
integral for the deformed sigma-model is localized on generalized holomorphic
maps, whereas for the A-model and the B-model it is localized on holomorphic
and constant maps, respectively. The geometry of topological D-branes is also
best described using generalized complex structures. We also derive a
constraint on the Chern character of topological D-branes, which includes
A-branes and B-branes as special cases.Comment: 36 pages, AMS latex. v2: a reference to a related work has been
added. v3: An error in the discussion of the Fourier-Mukai transform for
twisted coherent sheaves has been fixed, resulting in several changes in
Section 2. The rest of the paper is unaffected. v4: an incorrect statement
concerning Lie algebroid cohomology has been fixe
The structure of one-relator relative presentations and their centres
Suppose that G is a nontrivial torsion-free group and w is a word in the
alphabet G\cup\{x_1^{\pm1},...,x_n^{\pm1}\} such that the word w' obtained from
w by erasing all letters belonging to G is not a proper power in the free group
F(x_1,...,x_n). We show how to reduce the study of the relative presentation
\^G= to the case n=1. It turns out that an
"n-variable" group \^G can be constructed from similar "one-variable" groups
using an explicit construction similar to wreath product. As an illustration,
we prove that, for n>1, the centre of \^G is always trivial. For n=1, the
centre of \^G is also almost always trivial; there are several exceptions, and
all of them are known.Comment: 15 pages. A Russian version of this paper is at
http://mech.math.msu.su/department/algebra/staff/klyachko/papers.htm . V4:
the intoduction is rewritten; Section 1 is extended; a short introduction to
Secton 5 is added; some misprints are corrected and some cosmetic
improvements are mad
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Asymmetrically severe internal auditory canal hypoplasia: A case report.
We present a case of an otherwise healthy 20-month-old with congenital sensorineural hearing loss. CT and MR imaging demonstrated bilateral asymmetrically severe hypoplasia of the internal auditory canals and vestibulocochlear nerves. Additional developmental inner ear anomalies were present in this patient, including unilateral semicircular canal hypoplasia and suspected bilateral cochlear hypoplasia. The patient retained normal facial nerve function bilaterally. We highlight the current research and understanding of congenital IAC abnormalities
The Ages and Abundances of the M87 Globular Clusters
A subset of 150 globular clusters in M87 has been selected on the basis of
S/N ratio for abundance and age determinations from the sample of Paper I.
Indices measuring the strength of the strongest spectral features were
determined for the M87 GCs and from new data for twelve galactic GCs. Combining
the new and existing data for the galactic GCs and comparing the colors
and the line indices gives qualitative indications for the ages and abundances
of the GCs. Quantitative results are obtained by applying the Worthey (1994)
models for the integrated light of stellar systems of a single age, calibrated
by observations of galactic GCs, to deduce abundances and ages for the objects
in our sample.
We find that the M87 GCs span a wide range in metallicity, from very metal
poor to somewhat above solar metallicity. The mean [Fe/H] of -0.95 dex is
higher than that of the galactic GC system, and there is a metal rich tail that
reaches to higher [Fe/H] than one finds among the galactic GCs. The mean
metallicity of the M87 GC system is about a factor of four lower than that of
the M87 stellar halo at a fixed projected radius . The metallicity inferred
from the X-ray studies is similar to that of the M87 stellar halo, not to that
of GCs. We infer the relative abundances of Na, Mg, and Fe in the M87 GCs from
the strength of their spectral features. The behavior of these elements between
the metal rich and metal poor M87 GCs is similar to that shown by the galactic
GCs and by halo stars in the Galaxy. The pattern of chemical evolution in these
disparate old stellar systems is indistinguishable. We obtain a median age for
the M87 GC system of 13 Gyr, similar to that of the galactic GCs, with a small
dispersion about this value.Comment: 56 pages with included postscript figures; added derived M87 GC
metallicities to Table 2, a statistical analysis of possible bimodality, an
appendix on the metallicity calibration of U-R and the Washington system, and
other smaller changes. Accepted for publication in ApJ. (See paper for
complete version of the Abstract.
Competing orders II: the doped quantum dimer model
We study the phases of doped spin S=1/2 quantum antiferromagnets on the
square lattice, as they evolve from paramagnetic Mott insulators with valence
bond solid (VBS) order at zero doping, to superconductors at moderate doping.
The interplay between density wave/VBS order and superconductivity is
efficiently described by the quantum dimer model, which acts as an effective
theory for the total spin S=0 sector. We extend the dimer model to include
fermionic S=1/2 excitations, and show that its mean-field, static gauge field
saddle points have projective symmetries (PSGs) similar to those of `slave'
particle U(1) and SU(2) gauge theories. We account for the non-perturbative
effects of gauge fluctuations by a duality mapping of the S=0 dimer model. The
dual theory of vortices has a PSG identical to that found in a previous paper
(L. Balents et al., cond-mat/0408329) by a duality analysis of bosons on the
square lattice. The previous theory therefore also describes fluctuations
across superconducting, supersolid and Mott insulating phases of the present
electronic model. Finally, with the aim of describing neutron scattering
experiments, we present a phenomenological model for collective S=1 excitations
and their coupling to superflow and density wave fluctuations.Comment: 22 pages, 10 figures; part I is cond-mat/0408329; (v2) changed title
and added clarification
Consistent Batalin--Fradkin quantization of Infinitely Reducible First Class Constraints
We reconsider the problem of BRST quantization of a mechanics with infinitely
reducible first class constraints. Following an earlier recipe [Phys. Lett. B
381, 105, (1996)], the original phase space is extended by purely auxiliary
variables, the constraint set in the enlarged space being first stage of
reducibility. The BRST charge involving only a finite number of ghost variables
is explicitly constructed.Comment: 5 pages, LaTex. Minor corrections including the title. The version to
appear in Phys. Rev.
Chronic Sleep Disturbance Impairs Glucose Homeostasis in Rats
Epidemiological studies have shown an association between short or disrupted sleep and an increased risk for metabolic disorders. To assess a possible causal relationship, we examined the effects of experimental sleep disturbance on glucose regulation in Wistar rats under controlled laboratory conditions. Three groups of animals were used: a sleep restriction group (RS), a group subjected to moderate sleep disturbance without restriction of sleep time (DS), and a home cage control group. To establish changes in glucose regulation, animals were subjected to intravenous glucose tolerance tests (IVGTTs) before and after 1 or 8 days of sleep restriction or disturbance. Data show that both RS and DS reduce body weight without affecting food intake and also lead to hyperglycemia and decreased insulin levels during an IVGTT. Acute sleep disturbance also caused hyperglycemia during an IVGTT, yet, without affecting the insulin response. In conclusion, both moderate and severe disturbances of sleep markedly affect glucose homeostasis and body weight control
Experimental realization of a quantum game on a one-way quantum computer
We report the first demonstration of a quantum game on an all-optical one-way
quantum computer. Following a recent theoretical proposal we implement a
quantum version of Prisoner's Dilemma, where the quantum circuit is realized by
a 4-qubit box-cluster configuration and the player's local strategies by
measurements performed on the physical qubits of the cluster. This
demonstration underlines the strength and versatility of the one-way model and
we expect that this will trigger further interest in designing quantum
protocols and algorithms to be tested in state-of-the-art cluster resources.Comment: 13 pages, 4 figure
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