7,764 research outputs found
Mesh refinement in a two-dimensional large eddy simulation of a forced shear layer
A series of large eddy simulations are made of a forced shear layer and compared with experimental data. Several mesh densities were examined to separate the effect of numerical inaccuracy from modeling deficiencies. The turbulence model that was used to represent small scale, 3-D motions correctly predicted some gross features of the flow field, but appears to be structurally incorrect. The main effect of mesh refinement was to act as a filter on the scale of vortices that developed from the inflow boundary conditions
Time-accurate simulations of a shear layer forced at a single frequency
Calculations are presented for the forced shear layer studied experimentally by Oster and Wygnanski, and Weisbrot. Two different computational approaches are examined: Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). The DNS approach solves the full three dimensional Navier-Stokes equations for a temporally evolving mixing layer, while the LES approach solves the two dimensional Navier-Stokes equations with a subgrid scale turbulence model. While the comparison between these calculations and experimental data was hampered by a lack of information on the inflow boundary conditions, the calculations are shown to qualitatively agree with several aspects of the experiment. The sensitivity of these calculations to factors such as mesh refinement and Reynolds number is illustrated
Hidden Symmetries, AdS_D x S^n, and the lifting of one-time-physics to two-time-physics
The massive non-relativistic free particle in d-1 space dimensions has an
action with a surprizing non-linearly realized SO(d,2) symmetry. This is the
simplest example of a host of diverse one-time-physics systems with hidden
SO(d,2) symmetric actions. By the addition of gauge degrees of freedom, they
can all be lifted to the same SO(d,2) covariant unified theory that includes an
extra spacelike and an extra timelike dimension. The resulting action in d+2
dimensions has manifest SO(d,2) Lorentz symmetry and a gauge symmetry Sp(2,R)
and it defines two-time-physics. Conversely, the two-time action can be gauge
fixed to diverse one-time physical systems. In this paper three new gauge fixed
forms that correspond to the non-relativistic particle, the massive
relativistic particle, and the particle in AdS_(d-n) x S^n spacetime will be
discussed. The last case is discussed at the first quantized and field theory
levels as well. For the last case the popularly known symmetry is SO(d-n-1,2) x
SO(n+1), but yet we show that it is symmetric under the larger SO(d,2). In the
field theory version the action is symmetric under the full SO(d,2) provided it
is improved with a quantized mass term that arises as an anomaly from operator
ordering ambiguities. The anomalous cosmological term vanishes for AdS_2 x S^0
and AdS_n x S^n (i.e. d=2n). The strikingly larger symmetry could be
significant in the context of the proposed AdS/CFT duality.Comment: Latex, 23 pages. The term "cosmological constant" that appeared in
the original version has been changed to "mass term". My apologies for the
confusio
An overview of new supersymmetric gauge theories with 2-form gauge potentials
An overview of new 4d supersymmetric gauge theories with 2-form gauge
potentials constructed by various authors during the past five years is given.
The key role of three particular types of interaction vertices is emphasized.
These vertices are used to develop a connecting perspective on the new models
and to distinguish between them. One example is presented in detail to
illustrate characteristic features of the models. A new result on couplings of
2-form gauge potentials to Chern-Simons forms is presented.Comment: 11 pages; to appear in the proceedings of NATO ARW "Noncommutative
structures in mathematics and physics" (Kiev 09/00); table in section 3
correcte
Water adsorption on amorphous silica surfaces: A Car-Parrinello simulation study
A combination of classical molecular dynamics (MD) and ab initio
Car-Parrinello molecular dynamics (CPMD) simulations is used to investigate the
adsorption of water on a free amorphous silica surface. From the classical MD
SiO_2 configurations with a free surface are generated which are then used as
starting configurations for the CPMD.We study the reaction of a water molecule
with a two-membered ring at the temperature T=300K. We show that the result of
this reaction is the formation of two silanol groups on the surface. The
activation energy of the reaction is estimated and it is shown that the
reaction is exothermic.Comment: 12 pages, 6 figures, to be published in J. Phys.: Condens. Matte
Superconductivity and Cobalt Oxidation State in Metastable Na(x)CoO(2-delta)*yH2O (x ~ 1/3; y ~ 4x)
We report the synthesis and superconducting properties of a metastable form
of the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of using
the conventional bromine-acetonitrile mixture for sodium deintercalation, we
use an aqueous bromine solution. Using this method, we oxidize the sample to a
point that the sodium cobaltate becomes unstable, leading to formation of other
products if not controlled. This compound has the same structure as the
reported superconductor, yet it exhibits a systematic variation of the
superconducting transition temperature (Tc) as a function of time. Immediately
after synthesis, this compound is not a superconductor, even though it contains
appropriate amounts of sodium and water. The samples become superconducting
with low Tc values after ~ 90 h. Tc continually increases until it reaches a
maximum value (4.5 K) after about 260 h. Then Tc drops drastically, becoming
non-superconducting approximately 100 h later. Corresponding time-dependent
neutron powder diffraction data shows that the changes in superconductivity
exhibited by the metastable cobaltate correspond to slow formation of oxygen
vacancies in the CoO2 layers. In effect, the formation of these defects
continually reduces the cobalt oxidation state causing the sample to evolve
through its superconducting life cycle. Thus, the dome-shaped superconducting
phase diagram is mapped as a function of cobalt oxidation state using a single
sample. The width of this dome based on the formal oxidation state of cobalt is
very narrow - approximately 0.1 valence units wide. Interestingly, the maximum
Tc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, we
speculate that the maximum Tc occurs near the charge ordered insulating state
that correlates with the average cobalt oxidation state of 3.5.Comment: 22 pages, 9 figures, 1 tabl
Superform formulation for vector-tensor multiplets in conformal supergravity
The recent papers arXiv:1110.0971 and arXiv:1201.5431 have provided a
superfield description for vector-tensor multiplets and their Chern-Simons
couplings in 4D N = 2 conformal supergravity. Here we develop a superform
formulation for these theories. Furthermore an alternative means of gauging the
central charge is given, making use of a deformed vector multiplet, which may
be thought of as a variant vector-tensor multiplet. Its Chern-Simons couplings
to additional vector multiplets are also constructed. This multiplet together
with its Chern-Simons couplings are new results not considered by de Wit et al.
in hep-th/9710212.Comment: 28 pages. V2: Typos corrected and references updated; V3: References
updated and typo correcte
Microscopic calculation of the phonon dynamics of SrRuO compared with LaCuO
The phonon dynamics of the low-temperature superconductor SrRuO
is calculated quantitatively in linear response theory and compared with the
structurally isomorphic high-temperature superconductor LaCuO. Our
calculation corrects for a typical deficit of LDA-based calculations which
always predict a too large electronic -dispersion insufficient to
describe the c-axis response in the real materials. With a more realistic
computation of the electronic band structure the frequency and wavevector
dependent irreducible polarization part of the density response function is
determined and used for adiabatic and nonadiabatic phonon calculations. Our
analysis for SrRuO reveals important differences from the lattice
dynamics of - and -doped cuprates. Consistent with experimental evidence
from inelastic neutron scattering the anomalous doping related softening of the
strongly coupling high-frequency oxygen bond-stretching modes (OBSM) which is
generic for the cuprate superconductors is largely suppressed or completely
absent, respectively, depending on the actual value of the on-site Coulomb
repulsion of the Ru4d orbitals. Also the presence of a characteristic
-mode with a very steep dispersion coupling strongly with the
electrons is missing in SrRuO. Moreover, we evaluate the
possibility of a phonon-plasmon scenario for SrRuO which has been
shown recently to be realistic for LaCuO. In contrast to
LaCuO in SrRuO the very low lying plasmons are
overdamped along the c-axis.Comment: 30 pages, 16 figures, 4 tables, 33 reference
Chern-Simons Couplings and Inequivalent Vector-Tensor Multiplets
The off-shell vector-tensor multiplet is considered in an arbitrary background of N=2 vector supermultiplets. We establish the existence of two inequivalent versions, characterized by different Chern-Simons couplings. In one version the vector field of the vector-tensor multiplet is contained quadratically in the Chern-Simons term, which implies nonlinear terms in the supersymmetry transformations and equations of motion. In the second version, which requires a background of at least two abelian vector supermultiplets, the supersymmetry transformations remain at most linear in the vector-tensor components. This version is of the type known to arise from reduction of tensor supermultiplets in six dimensions. Our work applies to any number of vector-tensor multiplets
Ultrafast dynamics of a magnetic antivortex - Micromagnetic simulations
The antivortex is a fundamental magnetization structure which is the
topological counterpart of the well-known magnetic vortex. We study here the
ultrafast dynamic behavior of an isolated antivortex in a patterned Permalloy
thin-film element. Using micromagnetic simulations we predict that the
antivortex response to an ultrashort external field pulse is characterized by
the production of a new antivortex as well as of a temporary vortex, followed
by an annihilation process. These processes are complementary to the recently
reported response of a vortex and, like for the vortex, lead to the reversal of
the orientation of the antivortex core region. In addition to its fundamental
interest, this dynamic magnetization process could be used for the generation
and propagation of spin waves for novel logical circuits.Comment: 4 pages, 4 figures. To be published in Physical Review B (R
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