71,647 research outputs found
Coupled DEM-LBM method for the free-surface simulation of heterogeneous suspensions
The complexity of the interactions between the constituent granular and
liquid phases of a suspension requires an adequate treatment of the
constituents themselves. A promising way for numerical simulations of such
systems is given by hybrid computational frameworks. This is naturally done,
when the Lagrangian description of particle dynamics of the granular phase
finds a correspondence in the fluid description. In this work we employ
extensions of the Lattice-Boltzmann Method for non-Newtonian rheology, free
surfaces, and moving boundaries. The models allows for a full coupling of the
phases, but in a simplified way. An experimental validation is given by an
example of gravity driven flow of a particle suspension
Surprising simplicity in the modeling of dynamic granular intrusion
Granular intrusions, such as dynamic impact or wheel locomotion, are complex
multiphase phenomena where the grains exhibit solid-like and fluid-like
characteristics together with an ejected gas-like phase. Despite decades of
modeling efforts, a unified description of the physics in such intrusions is as
yet unknown. Here we show that a continuum model based on the simple notions of
frictional flow and tension-free separation describes complex granular
intrusions near free surfaces. This model captures dynamics in a variety of
experiments including wheel locomotion, plate intrusions, and running legged
robots. The model reveals that three effects (a static contribution and two
dynamic ones) primarily give rise to intrusion forces in such scenarios.
Identification of these effects enables the development of a further
reduced-order technique (Dynamic Resistive Force Theory) for rapid modeling of
granular locomotion of arbitrarily shaped intruders. The continuum-motivated
strategy we propose for identifying physical mechanisms and corresponding
reduced-order relations has potential use for a variety of other materials.Comment: 41 pages including supplementary document, 10 figures, and 8 vide
Boundary conditions and Berry phase in magnetic nanostructures
The effect of micromagnetic boundary conditions on the Berry curvature and topological Hall effect in granular nanostructures is investi- gated by model calculations. Both free surfaces and grain boundaries between interacting particles or grains affect the spin structure. The Dzyaloshinskii-Moriya interactions yield corrections to the Erdmann-Weierstrass boundary conditions, but the Berry curvature remains an exclusive functional of the local spin structure, which greatly simplifies the treatment of nanostructures. An explicit example is a model nanostructure with cylindrical symmetry whose spin structure is described by Bessel function and which yields a mean-field-type Hall-effect contribution that can be related to magnetic-force-microscopy images
Localized instability of a granular layer submitted to an ascending liquid flow
International audienceUsing a very simple experimental setup, we study the response of a thin layer of immersed granular material to an ascending liquid-flow; a pressure difference Delta P is imposed between the two horizontal free surfaces of a thin layer of glass beads, such that the liquid tends to flow upwards, and the resulting flow-rate v is measured. As generally observed in fluidized beds, the layer destabilizes when the pressure force exactly compensates the weight of the grains. At the free surface, one then observes the formation of a localized fountain of granular material the characteristic size of which is found to be proportional to the grain size and, surprizingly, independent of both the flow-rate and the thickness of the granular layer. Simple theoretical arguments account for the main experimental features
Slow Kinetics of Capillary Condensation in Confined Geometry: Experiment and Theory
When two solid surfaces are brought in contact, water vapor present in the
ambient air may condense in the region of the contact to form a liquid bridge
connecting the two surfaces : this is the so-called capillary condensation.
This phenomenon has drastic consequences on the contact between solids,
modifying the macroscopic adhesion and friction properties. In this paper, we
present a survey of the work we have performed both experimentally and
theoretically to understand the microscopic foundations of the kinetics of
capillary condensation. From the theoretical point of view, we have computed
the free energy barrier associated with the condensation of the liquid from the
gas in a confined system. These calculations allow to understand the existence
of very large hysteresis, which is often associated with capillary
condensation. This results are compatible with experimental results obtained
with a surface forces apparatus in a vapor atmosphere, showing a large hysteris
of the surface energy of two parallel planes as a function of their distance.
In the second part, we present some experiments on the influence of humidity on
the avalanche angle of granular media. We show that the ageing in time of this
avalanche angle can be explained by the slow kinetics of capillary condensation
in a random confined geometry.Comment: Special Volume of Colloids and Surfaces A,Proceedings of
Nanocapillarity: Wetting of Heterogeneous Surfaces and Porous Solids,June
25-27, 2001, TRI/Princeton International Workshop, Editor: Alexander V.
Neimar
Surface flow profiles for dry and wet granular materials by Particle Tracking Velocimetry; the effect of wall roughness
Two-dimensional Particle Tracking Velocimetry (PTV) is a promising technique
to study the behaviour of granular flows. The aim is to experimentally
determine the free surface width and position of the shear band from the
velocity profile to validate simulations in a split-bottom shear cell geometry.
The position and velocities of scattered tracer particles are tracked as they
move with the bulk flow by analyzing images. We then use a new technique to
extract the continuum velocity field, applying coarse-graining with the
postprocessing toolbox MercuryCG on the discrete experimental PTV data. For
intermediate filling heights, the dependence of the shear (or angular) velocity
on the radial coordinate at the free surface is well fitted by an error
function. From the error function, we get the width and the centre position of
the shear band. We investigate the dependence of these shear band properties on
filling height and rotation frequencies of the shear cell for dry glass beads
for rough and smooth wall surfaces. For rough surfaces, the data agrees with
the existing experimental results and theoretical scaling predictions. For
smooth surfaces, particle-wall slippage is significant and the data deviates
from the predictions. We further study the effect of cohesion on the shear band
properties by using small amount of silicon oil and glycerol as interstitial
liquids with the glass beads. While silicon oil does not lead to big changes,
glycerol changes the shear band properties considerably. The shear band gets
wider and is situated further inward with increasing liquid saturation, due to
the correspondingly increasing trend of particles to stick together
Friction of a slider on a granular layer: Non-monotonic thickness dependence and effect of boundary conditions
We investigate the effective friction encountered by a mass sliding on a
granular layer as a function of bed thickness and boundary roughness
conditions. The observed friction has minima for a small number of layers
before it increases and saturates to a value which depends on the roughness of
the sliding surface. We use an index-matched interstitial liquid to probe the
internal motion of the grains with fluorescence imaging in a regime where the
liquid has no significant effect on the measured friction. The shear profiles
obtained as a function of depth show decrease in slip near the sliding surface
as the layer thickness is increased. We propose that the friction depends on
the degree of grain confinement relative to the sliding surfaces.Comment: 4 pages, 6 figure
Channel flows of granular materials and their rheological implications
While the flow of a dry granular material down an inclined channel may seem at first sight to be a relatively simple flow, the experiments which have been conducted up to now suggest sufficient complexity which may be present in all but the very simplest granular material flows; consequently it is important to our general understanding of granular material rheology that these experimental observations be fully understood. This review of the current knowledge of channel flows will focus on the basic mechanics of these flows and the contributions the observations have made to an understanding of the rheology. In order to make progress in this objective, it is necessary to avoid some of the complications which can occur in practice. Thus we shall focus only on those flows in which the interstitial fluid plays very little role in determining the rheology. In his classic paper, Bagnold (1954) was able to show that the regime in which the rheology was dominated by particle/particle or particle/wall interactions and in which the viscous stresses in the interstitial fluid played a negligible role could be defined by a single, Reynolds-number-like parameter. It transpires that the important component in this parameter is a number which we shall call the Bagnold number, Ba, defined by Ba = p₈d²δ/µF where p₈,µF are the particle density and interstitial fluid viscosity, d is the particle diameter and δ is the principal velocity gradient in the flow. In the shear flows explored by Bagnold δ is the shear rate. Bagnold (1954) found that when Ba was greater than about 450 the rheology was dominated by particle/particle and particle/wall collisions. On the other hand, for Ba < 40, the viscosity of the interstitial fluid played the dominant role. More recently Zeininger and Brennen (1985) showed that the same criteria were applicable to the extensional flows in hoppers provided the extensional velocity gradient was used for δ. This review will focus on the simpler flows at large Ba where the interstitial fluid effects are small.
Other important ancillary effects can be caused by electrical charge separation between the particles or between the particles and the boundary walls. Such effects can be essential in some flows such as those in electrostatic copying machines. Most experimenters have observed electrical effects in granular material flows, particularly when metal components of the structure are not properly grounded. The effect of such electrical forces on the rheology of the flow is a largely unexplored area of research. The lack of discussion of these effects in
this review should not be interpreted as a dismissal of their importance.
Apart from electrical and interstitial fluid effects, this review will also neglect the effects caused by non-uniformities in the size and shape of the particles. Thus, for the most part, we focus on flows of particles of spherical shape and uniform size. It is clear that while an understanding of all of these effects will be necessary in the long term, there remain some important issues which need to be resolved for even the simplest granular material flows
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