47 research outputs found
Structure, Stresses and Local Dynamics in Glasses
The interrelations between short range structural and elastic aspects in
glasses and glass forming liquids pose important and yet unresolved questions.
In this paper these relations are analyzed for mono-atomic glasses and stressed
liquids with a short range repulsive-attractive pair potentials. Strong
variations of the local pressure are found even in a zero temperature glass,
whereas the largest values of pressure are the same in both glasses and
liquids. The coordination number z(J) and the effective first peak radius
depend on the local pressures J's. A linear relation was found between
components of site stress tensor and the local elastic constants. A linear
relation was also found between the trace of the squares of the local
frequencies and the local pressures. Those relations hold for glasses at zero
temperature and for liquids. We explain this by a relation between the
structure and the potential terms. A structural similarity between liquids and
solids is manifested by similar dependencies of the coordination number on the
pressures.Comment: 7 pages, 11 figure
Systematic coarse-graining of the dynamics of entangled polymer melts: the road from chemistry to rheology
For optimal processing and design of entangled polymeric materials it is
important to establish a rigorous link between the detailed molecular
composition of the polymer and the viscoelastic properties of the macroscopic
melt. We review current and past computer simulation techniques and critically
assess their ability to provide such a link between chemistry and rheology. We
distinguish between two classes of coarse-graining levels, which we term
coarse-grained molecular dynamics (CGMD) and coarse-grained stochastic dynamics
(CGSD). In CGMD the coarse-grained beads are still relatively hard, thus
automatically preventing bond crossing. This also implies an upper limit on the
number of atoms that can be lumped together and therefore on the longest chain
lengths that can be studied. To reach a higher degree of coarse-graining, in
CGSD many more atoms are lumped together, leading to relatively soft beads. In
that case friction and stochastic forces dominate the interactions, and actions
must be undertaken to prevent bond crossing. We also review alternative methods
that make use of the tube model of polymer dynamics, by obtaining the
entanglement characteristics through a primitive path analysis and by
simulation of a primitive chain network. We finally review super-coarse-grained
methods in which an entire polymer is represented by a single particle, and
comment on ways to include memory effects and transient forces.Comment: Topical review, 31 pages, 10 figure
Supercooled Water and the Kinetic Glass Transition II: Collective Dynamics
In this article we study in detail the Q-vector dependence of the collective
dynamics in simulated deeply supercooled SPC/E water. The evolution of the
system has been followed for 250 ns at low T, allowing a clear identification
of a two step relaxation process. We present evidence in favor of the use of
the mode coupling theory for supercooled liquid as framework for the
description of the slow alpha-relaxation dynamics in SPC/E water,
notwithstanding the fact that the cage formation in this system is controlled
by the formation of an open network of hydrogen bonds as opposed to packing
constraints, as in the case of simple liquids.Comment: rev-tex + 9 figure
A semi-schematic model for the center of mass dynamics in supercooled molecular liquids
We introduce a semi-schematic mode-coupling model to describe the slow
dynamics in molecular liquids, retaining explicitly only the description of the
center of mass degrees of freedom. Angular degrees of freedom are condensed in
a q-vector independent coupling parameter. We compare the time and q-dependence
of the density fluctuation correlators with numerical data from a 250 ns long
molecular dynamics simulation. Notwithstanding the choice of a network-forming
liquid as a model for comparing theory and simulation, the model describes the
main static and dynamic features of the relaxation in a broad q-vector range.Comment: Revtex, 2 figure
Effect of turbulence on the drag and lift of a particle
A direct numerical simulation ~DNS! is used to study the effect of a freestream isotropic turbulent
flow on the drag and lift forces on a spherical particle. The particle diameter is about 1.5???10 times
the Kolmogorov scale, the particle Reynolds number is about 60???600, and the freestream turbulence
intensity is about 10%???25%. The isotropic turbulent field considered here is stationary, i.e., frozen
in time. It is shown that the freestream turbulence does not have a substantial and systematic effect
on the time-averaged mean drag. The standard drag correlation based on the instantaneous or mean
relative velocity results in a reasonably accurate prediction of the mean drag obtained from the
DNS. However, the accuracy of prediction of the instantaneous drag decreases with increasing
particle size. For the smaller particles, the low frequency oscillations in the DNS drag are well
captured by the standard drag, but for the larger particles significant differences exist even for the
low frequency components. Inclusion of the added-mass and history forces, computed based on the
fluid velocity at the center of the particle, does not improve the prediction. Different estimates of the
fluid velocity seen by the particle are examined. It is shown that the mean drag is insensitive to the
fluid velocity measured at the particle center, or obtained by averaging over a fluid volume of the
order of the particle size. The fluctuations diminish as the size of the averaging volume increases.
The effect of increasing freestream turbulence intensity for the same particle size is studied.
Fluctuations in the drag and lift forces are shown to scale with the mean drag and freestream
intensity. The standard drag without the added-mass and history forces provides the best
approximation to the DNS result.published or submitted for publicationis peer reviewe
Wall effect for high Reynolds number motion of spheres in shear thinning fluids
This article does not have an abstract
Effect of turbulence on the drag and lift of a particle
A direct numerical simulation (DNS) is used to study the effect of a freestream isotropic turbulent flow on the drag and lift forces on a spherical particle. The particle diameter is about 1.5 to 10 times the Kolmogorov scale, the particle Reynolds number is about 60 to 600, and the freestream turbulence intensity is about 10 to 25%. It is shown that the freestream turbulence does not have a substantial and systematic effect on the time-averaged mean drag. The standard drag correlation based on the instantaneous or mean slip velocity results in a reasonably accurate prediction of the mean drag obtained from the DNS. However, the accuracy of prediction of the instantaneous drag decreases with increasing particle size. For the smaller particles, the low frequency oscillations in the DNS drag are well captured by the standard drag, but for the larger particles significant differences exist even for the low frequency components. Inclusion of the added-mass and history forces, computed based on the fluid velocity at the center of the particle, does not improve the prediction. Different estimates of the fluid velocity seen by the particle are examined. It is shown that the mean drag is insensitive to the fluid velocity measured at the particle center, or obtained by averaging over a fluid volume of the order of the particle size. The fluctuations diminish as the size of the averaging volume increases. The effect of increasing freestream turbulence intensity for the same particle size is studied. Fluctuations in the drag and lift forces are shown to scale with the mean drag and freestream intensity. The standard drag without the added-mass and history forces provides the best approximation to the DNS result.published or submitted for publicationis peer reviewe
Estimation of zero-shear viscosity of polymer solutions from falling sphere data
This article does not have an abstract
The influence of fluid elasticity on wall effects for creeping sphere motion in cylindrical tubes
Experimental results are presented of wall effect for the slow motion of spheres in elastic, constant-viscosity liquids. The results are correlated in terms of diameter ratio for d/D < 0.3, and Weissenberg number We < 5. Weissenberg number is defined as We = 2θVm/d, with θ the Maxwellian relaxation time (θ = N1/2τγ). The wall effect is found to be adequately described by Newtonian expressions for small Weissenberg number, We < 0.01. For larger values of the Weissenberg number, We > 0.2, virtually no wall effect is discernible; the small effect observed is correlated by the wall factor expression The wall effect observed is ascribed to the influence of fluid elasticity alone, since all the fluids used were elastic to a greater or lesser extent, but showed no shear thinning. f=1-0.17d/D