1,913 research outputs found
Some Finite Size Effects in Simulations of Glass Dynamics
We present the results of a molecular dynamics computer simulation in which
we investigate the dynamics of silica. By considering different system sizes,
we show that in simulations of the dynamics of this strong glass former
surprisingly large finite size effects are present. In particular we
demonstrate that the relaxation times of the incoherent intermediate scattering
function and the time dependence of the mean squared displacement are affected
by such finite size effects. By compressing the system to high densities, we
transform it to a fragile glass former and find that for that system these
types of finite size effects are much weaker.Comment: 12 pages of RevTex, 4 postscript figures available from W. Ko
Slow Dynamics in Ion-Conducting Sodium Silicate Melts: Simulation and Mode-Coupling Theory
A combination of molecular-dynamics (MD) computer simulation and
mode-coupling theory (MCT) is used to elucidate the structure-dynamics relation
in sodium-silicate melts (NSx) of varying sodium concentration. Using only the
partial static structure factors from the MD as an input, MCT reproduces the
large separation in relaxation time scales of the sodium and the silicon/oxygen
components. This confirms the idea of sodium diffusion channels which are
reflected by a prepeak in the static structure factors around 0.95 A^-1, and
shows that it is possible to explain the fast sodium-ion dynamics peculiar to
these mixtures using a microscopic theory.Comment: 4 pages, 4 figure
Interface free energies in p-spin glass models
The replica method has been used to calculate the interface free energy
associated with the change from periodic to anti-periodic boundary conditions
in finite-dimensional p-spin glass models in the phase which at mean-field
level has one-step replica symmetry breaking (1RSB). In any finite dimension
the interface free energy is exponentially small for a large system. This
result implies that in finite dimensions, the 1RSB state does not exist, as it
is destroyed by thermal excitation of arbitrarily large droplets. The
implications of this for the theory of structural glasses are discussed.Comment: 4 page
Molecular Dynamics Computer Simulation of the Dynamics of Supercooled Silica
We present the results of a large scale computer simulation of supercooled
silica. We find that at high temperatures the diffusion constants show a
non-Arrhenius temperature dependence whereas at low temperature this dependence
is also compatible with an Arrhenius law. We demonstrate that at low
temperatures the intermediate scattering function shows a two-step relaxation
behavior and that it obeys the time temperature superposition principle. We
also discuss the wave-vector dependence of the nonergodicity parameter and the
time and temperature dependence of the non-Gaussian parameter.Comment: 5 pages, Latex, 6 postscript figure
Stock Returns Versus Bond Returns: Actual Historical Data 1926-2008
Robert J. Angell, D.B.A., is professor of finance, Department of Economics & Finance, North Carolina Agricultural and Technology State University, Greensboro, NC 27411.
Charles W. Cole, Ph.D., is assistant professor of finance, Department of Economics & Finance, North Carolina Agricultural and Technology State University, Greensboro, N
Effect of dynamic stall on the aerodynamics of vertical-axis wind turbines
Accurate simulations of the aerodynamic performance of vertical-axis wind turbines pose a significant challenge for computational fluid dynamics methods. The aerodynamic interaction between the blades of the rotor and the wake that is produced by the blades requires a high-fidelity representation of the convection of vorticity within the wake. In addition, the cyclic motion of the blades induces large variations in the angle of attack on the blades that can manifest as dynamic stall. The present paper describes the application of a numerical model that is based on the vorticity transport formulation of the NavierâStokes equations, to the prediction of the aerodynamics of a verticalaxis wind turbine that consists of three curved rotor blades that are twisted helically around the rotational axis of the rotor. The predicted variation of the power coefficient with tip speed ratio compares very favorably with experimental measurements. It is demonstrated that helical blade twist reduces the oscillation of the power coefficient that is an inherent feature of turbines with non-twisted blade configurations
Inhomogeneous elastic response of silica glass
Using large scale molecular dynamics simulations we investigate the
properties of the {\em non-affine} displacement field induced by macroscopic
uniaxial deformation of amorphous silica,a strong glass according to Angell's
classification. We demonstrate the existence of a length scale
characterizing the correlations of this field (corresponding to a volume of
about 1000 atoms), and compare its structure to the one observed in a standard
fragile model glass. The "Boson-peak'' anomaly of the density of states can be
traced back in both cases to elastic inhomogeneities on wavelengths smaller
than , where classical continuum elasticity becomes simply unapplicable
Thermodynamic behaviour and structural properties of an aqueous sodium chloride solution upon supercooling
We present the results of a molecular dynamics simulation study of
thermodynamic and structural properties upon supercooling of a low
concentration sodium chloride solution in TIP4P water and the comparison with
the corresponding bulk quantities. We study the isotherms and the isochores for
both the aqueous solution and bulk water. The comparison of the phase diagrams
shows that thermodynamic properties of the solution are not merely shifted with
respect to the bulk. Moreover, from the analysis of the thermodynamic curves,
both the spinodal line and the temperatures of maximum density curve can be
calculated. The spinodal line appears not to be influenced by the presence of
ions at the chosen concentration, while the temperatures of maximum density
curve displays both a mild shift in temperature and a shape modification with
respect to bulk. Signatures of the presence of a liquid-liquid critical point
are found in the aqueous solution. By analysing the water-ion radial
distribution functions of the aqueous solution we observe that upon changing
density, structural modifications appear close to the spinodal. For low
temperatures additional modifications appear also for densities close to that
corresponding to a low density configurational energy minimum.Comment: 10 pages, 13 figures, 2 tables. To be published in J. Chem. Phy
Trends in total column ozone measurements
It is important to ensure the best available data are used in any determination of possible trends in total ozone in order to have the most accurate estimates of any trends and the associated uncertainties. Accordingly, the existing total ozone records were examined in considerable detail. Once the best data set has been produced, the statistical analysis must examine the data for any effects that might indicate changes in the behavior of global total ozone. The changes at any individual measuring station could be local in nature, and herein, particular attention was paid to the seasonal and latitudinal variations of total ozone, because two dimensional photochemical models indicate that any changes in total ozone would be most pronounced at high latitudes during the winter months. The conclusions derived from this detailed examination of available total ozone can be split into two categories, one concerning the quality and the other the statistical analysis of the total ozone record
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