6,904 research outputs found
DynamO: A free O(N) general event-driven molecular-dynamics simulator
Molecular-dynamics algorithms for systems of particles interacting through
discrete or "hard" potentials are fundamentally different to the methods for
continuous or "soft" potential systems. Although many software packages have
been developed for continuous potential systems, software for discrete
potential systems based on event-driven algorithms are relatively scarce and
specialized. We present DynamO, a general event-driven simulation package which
displays the optimal O(N) asymptotic scaling of the computational cost with the
number of particles N, rather than the O(N log(N)) scaling found in most
standard algorithms. DynamO provides reference implementations of the best
available event-driven algorithms. These techniques allow the rapid simulation
of both complex and large (>10^6 particles) systems for long times. The
performance of the program is benchmarked for elastic hard sphere systems,
homogeneous cooling and sheared inelastic hard spheres, and equilibrium
Lennard-Jones fluids. This software and its documentation are distributed under
the GNU General Public license and can be freely downloaded from
http://marcusbannerman.co.uk/dynamo
Simulations of dense granular flow: Dynamic Arches and Spin Organization
We present a numerical model for a two dimensional (2D) granular assembly,
falling in a rectangular container when the bottom is removed. We observe the
occurrence of cracks splitting the initial pile into pieces, like in
experiments. We study in detail various mechanisms connected to the
`discontinuous decompaction' of this granular material. In particular, we focus
on the history of one single long range crack, from its origin at one side
wall, until it breaks the assembly into two pieces. This event is correlated to
an increase in the number of collisions, i.e. strong pressure, and to a
momentum wave originated by one particle. Eventually, strong friction reduces
the falling velocity such that the crack may open below the slow, high pressure
`dynamic arch'. Furthermore, we report the presence of large, organized
structures of the particles' angular velocities in the dense parts of the
granulate when the number of collisions is large.Comment: Submitted to J. Phys.
Ligand-Receptor Interactions
The formation and dissociation of specific noncovalent interactions between a
variety of macromolecules play a crucial role in the function of biological
systems. During the last few years, three main lines of research led to a
dramatic improvement of our understanding of these important phenomena. First,
combination of genetic engineering and X ray cristallography made available a
simultaneous knowledg of the precise structure and affinity of series or
related ligand-receptor systems differing by a few well-defined atoms. Second,
improvement of computer power and simulation techniques allowed extended
exploration of the interaction of realistic macromolecules. Third, simultaneous
development of a variety of techniques based on atomic force microscopy,
hydrodynamic flow, biomembrane probes, optical tweezers, magnetic fields or
flexible transducers yielded direct experimental information of the behavior of
single ligand receptor bonds. At the same time, investigation of well defined
cellular models raised the interest of biologists to the kinetic and mechanical
properties of cell membrane receptors. The aim of this review is to give a
description of these advances that benefitted from a largely multidisciplinar
approach
Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions
We use interface-resolved simulations to study finite-size effects in
turbulent channel flow of neutrally-buoyant spheres. Two cases with particle
sizes differing by a factor of 2, at the same solid volume fraction of 20% and
bulk Reynolds number are considered. These are complemented with two reference
single-phase flows: the unladen case, and the flow of a Newtonian fluid with
the effective suspension viscosity of the same mixture in the laminar regime.
As recently highlighted in Costa et al. (PRL 117, 134501), a particle-wall
layer is responsible for deviations of the statistics from what is observed in
the continuum limit where the suspension is modeled as a Newtonian fluid with
an effective viscosity. Here we investigate the fluid and particle dynamics in
this layer and in the bulk. In the particle-wall layer, the near wall
inhomogeneity has an influence on the suspension micro-structure over a
distance proportional to the particle size. In this layer, particles have a
significant (apparent) slip velocity that is reflected in the distribution of
wall shear stresses. This is characterized by extreme events (both much higher
and much lower than the mean). Based on these observations we provide a scaling
for the particle-to-fluid apparent slip velocity as a function of the flow
parameters. We also extend the flow scaling laws in to second-order Eulerian
statistics in the homogeneous suspension region away from the wall. Finite-size
effects in the bulk of the channel become important for larger particles, while
negligible for lower-order statistics and smaller particles. Finally, we study
the particle dynamics along the wall-normal direction. Our results suggest that
1-point dispersion is dominated by particle-turbulence (and not
particle-particle) interactions, while differences in 2-point dispersion and
collisional dynamics are consistent with a picture of shear-driven
interactions
Density profiles of a colloidal liquid at a wall under shear flow
Using a dynamical density functional theory we analyze the density profile of
a colloidal liquid near a wall under shear flow. Due to the symmetries of the
system considered, the naive application of dynamical density functional theory
does not lead to a shear induced modification of the equilibrium density
profile, which would be expected on physical grounds. By introducing a
physically motivated dynamic mean field correction we incorporate the missing
shear induced interparticle forces into the theory. We find that the shear flow
tends to enhance the oscillations in the density profile of hard-spheres at a
hard-wall and, at sufficiently high shear rates, induces a nonequilibrium
transition to a steady state characterized by planes of particles parallel to
the wall. Under gravity, we find that the center-of-mass of the density
distribution increases with shear rate, i.e., shear increases the potential
energy of the particles
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