1,628 research outputs found
Large-scale lattice Boltzmann simulations of complex fluids: advances through the advent of computational grids
During the last two years the RealityGrid project has allowed us to be one of
the few scientific groups involved in the development of computational grids.
Since smoothly working production grids are not yet available, we have been
able to substantially influence the direction of software development and grid
deployment within the project. In this paper we review our results from large
scale three-dimensional lattice Boltzmann simulations performed over the last
two years. We describe how the proactive use of computational steering and
advanced job migration and visualization techniques enabled us to do our
scientific work more efficiently. The projects reported on in this paper are
studies of complex fluid flows under shear or in porous media, as well as
large-scale parameter searches, and studies of the self-organisation of liquid
cubic mesophases.
Movies are available at
http://www.ica1.uni-stuttgart.de/~jens/pub/05/05-PhilTransReview.htmlComment: 18 pages, 9 figures, 4 movies available, accepted for publication in
Phil. Trans. R. Soc. London Series
Structural Transitions in Colloidal Suspensions
In suspensions of colloidal particles different types of interactions are in a subtle interplay. In this report we are interested in sub-micro meter sized Al2O3 particles which are suspended in water. Their interactions can be adjusted by tuning the pH-value and the salt concentration. In this manner different microscopic structures can be obtained. Industrial processes for the production of ceramics can be optimized by taking advantage of specific changes of the microscopic structure. To investigate the influences of the pH-value and the salt concentration on the microscopic structure and the properties of the suspension, we have developed a coupled Stochastic Rotation Dynamics (SRD) and Molecular Dynamics (MD) simulation code. The code has been parallelized using MPI. We utilize the pair correlation function and the structure factor to analyze the structure of the suspension. The results are summarized in a stability diagram. For selected conditions we study the process of cluster formation in large scale simulations of dilute suspensions
Using computational steering to explore the parameter space of stability in a suspension
We simulate a suspension of model colloidal particles interacting via DLVO (Derjaguin, Landau, Vervey, Overbeek) potentials. The interaction potentials can be related to experimental conditions, defined by the pH-value, the salt concentration and the volume fraction of solid particles suspended in the acqueous solvent. Depending on these parameters, the system shows different structural properties, including cluster formation, a glass-like repulsive structure, or a stable suspension. To explore the parameter space many simulations are required. In order to reduce the computational effort and data storage requirements, we developed a steering approach to control a running simulation and to detect interesting transitions from one region in the configuration space to another. In this article we describe the implementation of the steering in the simulation program and illustrate its applicability by several example cases
Order-disorder transition in nanoscopic semiconductor quantum rings
Using the path integral Monte Carlo technique we show that semiconductor
quantum rings with up to six electrons exhibit a temperature, ring diameter,
and particle number dependent transition between spin ordered and disordered
Wigner crystals. Due to the small number of particles the transition extends
over a broad temperature range and is clearly identifiable from the electron
pair correlation functions.Comment: 4 pages, 5 figures, For recent information on physics of small
systems see http://www.smallsystems.d
Blood crystal: emergent order of red blood cells under wall-confined shear flow
Driven or active suspensions can display fascinating collective behavior,
where coherent motions or structures arise on a scale much larger than that of
the constituent particles. Here, we report experiments and numerical
simulations revealing that red blood cells (RBCs) assemble into regular
patterns in a confined shear flow. The order is of pure hydrodynamic and
inertialess origin, and emerges from a subtle interplay between (i)
hydrodynamic repulsion by the bounding walls which drives deformable cells
towards the channel mid-plane and (ii) intercellular hydrodynamic interactions
which can be attractive or repulsive depending on cell-cell separation. Various
crystal-like structures arise depending on RBC concentration and confinement.
Hardened RBCs in experiments and rigid particles in simulations remain
disordered under the same conditions where deformable RBCs form regular
patterns, highlighting the intimate link between particle deformability and the
emergence of order. The difference in structuring ability of healthy
(deformable) and diseased (stiff) RBCs creates a flow signature potentially
exploitable for diagnosis of blood pathologies
Penning traps with unitary architecture for storage of highly charged ions
Penning traps are made extremely compact by embedding rare-earth permanent
magnets in the electrode structure. Axially-oriented NdFeB magnets are used in
unitary architectures that couple the electric and magnetic components into an
integrated structure. We have constructed a two- magnet Penning trap with
radial access to enable the use of laser or atomic beams, as well as the
collection of light. An experimental apparatus equipped with ion optics is
installed at the NIST electron beam ion trap (EBIT) facility, constrained to
fit within 1 meter at the end of a horizontal beamline for transporting highly
charged ions. Highly charged ions of neon and argon, extracted with initial
energies up to 4000 eV per unit charge, are captured and stored to study the
confinement properties of a one-magnet trap and a two-magnet trap. Design
considerations and some test results are discussed
Simulations of slip flow on nanobubble-laden surfaces
On microstructured hydrophobic surfaces, geometrical patterns may lead to the
appearance of a superhydrophobic state, where gas bubbles at the surface can
have a strong impact on the fluid flow along such surfaces. In particular, they
can strongly influence a detected slip at the surface. We present two-phase
lattice Boltzmann simulations of a flow over structured surfaces with attached
gas bubbles and demonstrate how the detected slip depends on the pattern
geometry, the bulk pressure, or the shear rate. Since a large slip leads to
reduced friction, our results allow to assist in the optimization of
microchannel flows for large throughput.Comment: 22 pages, 12 figure
Implementation of on-site velocity boundary conditions for D3Q19 lattice Boltzmann
On-site boundary conditions are often desired for lattice Boltzmann
simulations of fluid flow in complex geometries such as porous media or
microfluidic devices. The possibility to specify the exact position of the
boundary, independent of other simulation parameters, simplifies the analysis
of the system. For practical applications it should allow to freely specify the
direction of the flux, and it should be straight forward to implement in three
dimensions. Furthermore, especially for parallelized solvers it is of great
advantage if the boundary condition can be applied locally, involving only
information available on the current lattice site. We meet this need by
describing in detail how to transfer the approach suggested by Zou and He to a
D3Q19 lattice. The boundary condition acts locally, is independent of the
details of the relaxation process during collision and contains no artificial
slip. In particular, the case of an on-site no-slip boundary condition is
naturally included. We test the boundary condition in several setups and
confirm that it is capable to accurately model the velocity field up to second
order and does not contain any numerical slip.Comment: 13 pages, 4 figures, revised versio
Shear Viscosity of claylike colloids in computer simulations and experiments
Dense suspensions of small strongly interacting particles are complex systems that are rarely understood on the microscopic level. We investigate properties of dense suspensions and sediments of small spherical Al2O3 particles in a shear cell by means of a combined molecular-dynamics and stochastic rotation dynamics simulation. We study structuring effects and the dependence of the suspension’s viscosity on the shear rate and shear thinning for systems of varying salt concentration and pH value. To show the agreement of our results with experimental data, the relation between the bulk pH value and surface charge of spherical colloidal particles is modeled by Debye-Hückel theory in conjunction with a 2 pK charge regulation model. © 2006 The American Physical Societ
Lattice Boltzmann simulations of apparent slip in hydrophobic microchannels
Various experiments have found a boundary slip in hydrophobic microchannel
flows, but a consistent understanding of the results is still lacking. While
Molecular Dynamics (MD) simulations cannot reach the low shear rates and large
system sizes of the experiments, it is often impossible to resolve the needed
details with macroscopic approaches. We model the interaction between
hydrophobic channel walls and a fluid by means of a multi-phase lattice
Boltzmann model. Our mesoscopic approach overcomes the limitations of MD
simulations and can reach the small flow velocities of known experiments. We
reproduce results from experiments at small Knudsen numbers and other
simulations, namely an increase of slip with increasing liquid-solid
interactions, the slip being independent of the flow velocity, and a decreasing
slip with increasing bulk pressure. Within our model we develop a semi-analytic
approximation of the dependence of the slip on the pressure.Comment: 7 pages, 4 figure
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