2,370 research outputs found
Stress response and structural transitions in sheared gyroidal and lamellar amphiphilic mesophases: lattice-Boltzmann simulations
We report on the stress response of gyroidal and lamellar amphiphilic
mesophases to steady shear simulated using a bottom-up lattice-Boltzmann model
for amphiphilic fluids and sliding periodic (Lees-Edwards) boundary conditions.
We study the gyroid per se (above the sponge-gyroid transition, of high
crystallinity) and the molten gyroid (within such a transition, of
shorter-range order). We find that both mesophases exhibit shear-thinning, more
pronounced and at lower strain rates for the molten gyroid. At late times after
the onset of shear, the skeleton of the crystalline gyroid becomes a structure
of interconnected irregular tubes and toroidal rings, mostly oriented along the
velocity ramp imposed by the shear, in contradistinction with free-energy
Langevin-diffusion studies which yield a much simpler structure of disentangled
tubes. We also compare the shear stress and deformation of lamellar mesophases
with and without amphiphile when subjected to the same shear flow applied
normal to the lamellae. We find that the presence of amphiphile allows (a) the
shear stress at late times to be higher than in the case without amphiphile,
and (b) the formation of rich patterns on the sheared interface, characterised
by alternating regions of high and low curvature.Comment: 15 pages, 10 figures, Physical Review E, in pres
Large-scale grid-enabled lattice-Boltzmann simulations of complex fluid flow in porous media and under shear
Well designed lattice-Boltzmann codes exploit the essentially embarrassingly
parallel features of the algorithm and so can be run with considerable
efficiency on modern supercomputers. Such scalable codes permit us to simulate
the behaviour of increasingly large quantities of complex condensed matter
systems. In the present paper, we present some preliminary results on the large
scale three-dimensional lattice-Boltzmann simulation of binary immiscible fluid
flows through a porous medium derived from digitised x-ray microtomographic
data of Bentheimer sandstone, and from the study of the same fluids under
shear. Simulations on such scales can benefit considerably from the use of
computational steering and we describe our implementation of steering within
the lattice-Boltzmann code, called LB3D, making use of the RealityGrid steering
library. Our large scale simulations benefit from the new concept of capability
computing, designed to prioritise the execution of big jobs on major
supercomputing resources. The advent of persistent computational grids promises
to provide an optimal environment in which to deploy these mesoscale simulation
methods, which can exploit the distributed nature of compute, visualisation and
storage resources to reach scientific results rapidly; we discuss our work on
the grid-enablement of lattice-Boltzmann methods in this context.Comment: 17 pages, 6 figures, accepted for publication in
Phil.Trans.R.Soc.Lond.
Ludwig: A parallel Lattice-Boltzmann code for complex fluids
This paper describes `Ludwig', a versatile code for the simulation of
Lattice-Boltzmann (LB) models in 3-D on cubic lattices. In fact `Ludwig' is not
a single code, but a set of codes that share certain common routines, such as
I/O and communications. If `Ludwig' is used as intended, a variety of complex
fluid models with different equilibrium free energies are simple to code, so
that the user may concentrate on the physics of the problem, rather than on
parallel computing issues. Thus far, `Ludwig''s main application has been to
symmetric binary fluid mixtures. We first explain the philosophy and structure
of `Ludwig' which is argued to be a very effective way of developing large
codes for academic consortia. Next we elaborate on some parallel implementation
issues such as parallel I/O, and the use of MPI to achieve full portability and
good efficiency on both MPP and SMP systems. Finally, we describe how to
implement generic solid boundaries, and look in detail at the particular case
of a symmetric binary fluid mixture near a solid wall. We present a novel
scheme for the thermodynamically consistent simulation of wetting phenomena, in
the presence of static and moving solid boundaries, and check its performance.Comment: Submitted to Computer Physics Communication
Arrest stress of uniformly sheared wet granular matter
We conduct extensive independent numerical experiments considering
frictionless disks without internal degrees of freedom (rotation etc.) in two
dimensions. We report here that for a large range of the packing fractions
below random-close packing, all components of the stress tensor of wet granular
materials remain finite in the limit of zero shear rate. This is direct
evidence for a fluid-to-solid arrest transition. The offset value of the shear
stress characterizes plastic deformation of the arrested state {which
corresponds to {\em dynamic yield stress} of the system}. {Based on an
analytical line of argument, we propose that the mean number of capillary
bridges per particle, , follows a non-trivial dependence on the packing
fraction, , and the capillary energy, \vareps. Most noticeably, we show
that is a generic and universal quantity which does not depend on the
driving protocol.} Using this universal quantity, we calculate the arrest
stress, , analytically based on a balance of the energy injection
rate due to the external force driving the flow and the dissipation rate
accounting for the rupture of capillary bridges. The resulting prediction of
is a non-linear function of the packing fraction , and the
capillary energy \vareps. This formula provides an excellent, parameter-free
prediction of the numerical data. Corrections to the theory for small and large
packing fractions are connected to the emergence of shear bands and of
contributions to the stress from repulsive particle interactions, respectively.Comment: 7 pages, g figure
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