91 research outputs found
Micro-macro transition and simplified contact models for wet granular materials
Wet granular materials in a quasi-static steady state shear flow have been
studied with discrete particle simulations. Macroscopic quantities, consistent
with the conservation laws of continuum theory, are obtained by time averaging
and spatial coarse-graining. Initial studies involve understanding the effect
of liquid content and liquid properties like the surface tension on the
macroscopic quantities. Two parameters of the liquid bridge contact model have
been studied as the constitutive parameters that define the structure of this
model (i) the rupture distance of the liquid bridge model, which is
proportional to the liquid content, and (ii) the maximum adhesive force, as
controlled by the surface tension of the liquid. Subsequently a correlation is
developed between these micro parameters and the steady state cohesion in the
limit of zero confining pressure. Furthermore, as second result, the
macroscopic torque measured at the walls, which is an experimentally accessible
parameter, is predicted from our simulation results as a dependence on the
micro-parameters. Finally, the steady state cohesion of a realistic non-linear
liquid bridge contact model scales well with the steady state cohesion for a
simpler linearized irreversible contact model with the same maximum adhesive
force and equal energy dissipated per contact
Friction dependence of shallow granular flows from discrete particle simulations
A shallow-layer model for granular flows is completed with a closure relation for the macroscopic bed friction or basal roughness obtained from micro-scale discrete particle simulations of steady flows. We systematically vary the bed friction by changing the contact friction coefficient between basal and flowing particles, while the base remains geometrically rough. By simulating steady uniform flow over a wide parameter range, we obtain a friction law that is a function of both flow and bed variables. Surprisingly, we find that the macroscopic bed friction is only weakly dependent on the contact friction of bed particles and predominantly determined by the properties of the flowing particles
Closure Relations for Shallow Granular Flows from Particle Simulations
The Discrete Particle Method (DPM) is used to model granular flows down an
inclined chute. We observe three major regimes: static piles, steady uniform
flows and accelerating flows. For flows over a smooth base, other
(quasi-steady) regimes are observed where the flow is either highly energetic
and strongly layered in depth for small inclinations, or non-uniform and
oscillating for larger inclinations.
For steady uniform flows, depth profiles of density, velocity and stress have
been obtained using an improved coarse-graining method, which allows accurate
statistics even at the base of the flow. A shallow-layer model for granular
flows is completed with macro-scale closure relations obtained from micro-scale
DPM simulations of steady flows. We thus obtain relations for the effective
basal friction, shape factor, mean density, and the normal stress anisotropy as
functions of layer thickness, flow velocity and basal roughness. For
collisional flows, the functional dependencies are well determined and have
been obtained.Comment: Will be presented at PARTICLES 2011 - CIMN
Segregation of large particles in dense granular flows: A granular Saffman effect?
We report on the scaling between the lift force and the velocity lag
experienced by a single particle of different size in a monodisperse dense
granular chute flow. The similarity of this scaling to the Saffman lift force
in (micro) fluids, suggests an inertial origin for the lift force responsible
for segregation of (isolated, large) intruders in dense granular flows. We also
observe an anisotropic pressure/stress field surrounding the particle, which
potentially lies at the origin of the velocity lag. These findings are relevant
for modelling and theoretical predictions of particle-size segregation. At the
same time, the suggested interplay between polydispersity and inertial effects
in dense granular flows with stress- and strain-gradients, implies striking new
parallels between fluids, suspensions and granular flows with wide application
perspectives
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
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