1,930 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
Two-Gaussian excitations model for the glass transition
We develop a modified "two-state" model with Gaussian widths for the site
energies of both ground and excited states, consistent with expectations for a
disordered system. The thermodynamic properties of the system are analyzed in
configuration space and found to bridge the gap between simple two state models
("logarithmic" model in configuration space) and the random energy model
("Gaussian" model in configuration space). The Kauzmann singularity given by
the random energy model remains for very fragile liquids but is suppressed or
eliminated for stronger liquids. The sharp form of constant volume heat
capacity found by recent simulations for binary mixed Lennard Jones and soft
sphere systems is reproduced by the model, as is the excess entropy and heat
capacity of a variety of laboratory systems, strong and fragile. The ideal
glass in all cases has a narrow Gaussian, almost invariant among molecular and
atomic glassformers, while the excited state Gaussian depends on the system and
its width plays a role in the thermodynamic fragility. The model predicts the
existence of first-order phase transition for fragile liquids.Comment: 12 pages, 12 figure
A lattice mesoscopic model of dynamically heterogeneous fluids
We introduce a mesoscopic three-dimensional Lattice Boltzmann Model which
attempts to mimick the physical features associated with cage effects in
dynamically heterogeneous fluids. To this purpose, we extend the standard
Lattice Boltzmann dynamics with self-consistent constraints based on the
non-local density of the surrounding fluid. The resulting dynamics exhibits
typical features of dynamic heterogeneous fluids, such as non-Gaussian density
distributions and long-time relaxation. Due to its intrinsically parallel
dynamics, and absence of statistical noise, the method is expected to compute
significantly faster than molecular dynamics, Monte Carlo and lattice glass
models.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
A microscopic description of the aging dynamics: fluctuation-dissipation relations, effective temperature and heterogeneities
We consider the dynamics of a diluted mean-field spin glass model in the
aging regime. The model presents a particularly rich heterogeneous behavior. In
order to catch this behavior, we perform a **spin-by-spin analysis** for a
**given disorder realization**. The results compare well with the outcome of a
static calculation which uses the ``survey propagation'' algorithm of Mezard,
Parisi, and Zecchina [Sciencexpress 10.1126/science.1073287 (2002)]. We thus
confirm the connection between statics and dynamics at the level of single
degrees of freedom. Moreover, working with single-site quantities, we can
introduce a new response-vs-correlation plot, which clearly shows how
heterogeneous degrees of freedom undergo coherent structural rearrangements.
Finally we discuss the general scenario which emerges from our work and
(possibly) applies to more realistic glassy models. Interestingly enough, some
features of this scenario can be understood recurring to thermometric
considerations.Comment: 4 pages, 5 figures (7 eps files
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 Navier–Stokes 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
The Ising M-p-spin mean-field model for the structural glass: continuous vs. discontinuous transition
The critical behavior of a family of fully connected mean-field models with
quenched disorder, the Ising spin glass, is analyzed, displaying a
crossover between a continuous and a random first order phase transition as a
control parameter is tuned. Due to its microscopic properties the model is
straightforwardly extendable to finite dimensions in any geometry.Comment: 10 pages, 1 figure, 1 tabl
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
Molecular structural order and anomalies in liquid silica
The present investigation examines the relationship between structural order,
diffusivity anomalies, and density anomalies in liquid silica by means of
molecular dynamics simulations. We use previously defined orientational and
translational order parameters to quantify local structural order in atomic
configurations. Extensive simulations are performed at different state points
to measure structural order, diffusivity, and thermodynamic properties. It is
found that silica shares many trends recently reported for water [J. R.
Errington and P. G. Debenedetti, Nature 409, 318 (2001)]. At intermediate
densities, the distribution of local orientational order is bimodal. At fixed
temperature, order parameter extrema occur upon compression: a maximum in
orientational order followed by a minimum in translational order. Unlike water,
however, silica's translational order parameter minimum is broad, and there is
no range of thermodynamic conditions where both parameters are strictly
coupled. Furthermore, the temperature-density regime where both structural
order parameters decrease upon isothermal compression (the structurally
anomalous regime) does not encompass the region of diffusivity anomalies, as
was the case for water.Comment: 30 pages, 8 figure
Liquid Limits: The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids
The liquid-gas spinodal and the glass transition define ultimate boundaries
beyond which substances cannot exist as (stable or metastable) liquids. The
relation between these limits is analyzed {\it via} computer simulations of a
model liquid. The results obtained indicate that the liquid - gas spinodal and
the glass transition lines intersect at a finite temperature, implying a glass
- gas mechanical instability locus at low temperatures. The glass transition
lines obtained by thermodynamic and dynamic criteria agree very well with each
other.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
Ehrenfest relations at the glass transition: solution to an old paradox
In order to find out whether there exists a thermodynamic description of the
glass phase, the Ehrenfest relations along the glass transition line are
reconsidered. It is explained that the one involving the compressibility is
always satisfied, and that the one involving the specific heat is principally
incorrect.
Thermodynamical relations are presented for non-ergodic systems with a
one-level tree in phase space. They are derived for a spin glass model, checked
for other models, and expected to apply, e.g., to glass forming liquids. The
second Ehrenfest relation gets a contribution from the configurational entropy.Comment: 4 pages revtex, to appear in Phys. Rev. Let
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