96 research outputs found
Evidence of reverse and intermediate size segregation in dry granular flows down a rough incline
In a dry granular flow, size segregation behave differently for a mixture
containing a few large beads with a size ratio (S) above 5 (Thomas, Phys.Rev.E
62,96(2000)). For moderate large S, large beads migrate to an intermediate
depth in the bed: this is called intermediate segregation. For the largest S,
large beads migrate to the bottom: this is called reverse segregation (in
contrast with surface segregation). As the reversal and intermediate depth
values depend on the bead fraction, this numerical study mainly uses a single
large tracer. Small fractions are also computed showing the link between a
tracer behavior and segregation process. For half-filled rotating drum and for
rough incline, two and three (3D) dimensional cases are studied. In the
tumbler, trajectories of a large tracer show that it reaches a constant depth
during the flow. For large S, this depth is intermediate with a progressive
sinking when S increases. Largest S correspond to tracers at the bottom of the
flow. All 3D simulation are in quantitative agreement with the experiments. In
the flow down an incline, a large tracer reaches an equilibrium depth during
flow. For large S, its depth is intermediate, inside the bed. For the largest
S, its depth is reverse, near the bottom. Results are slightly different for
thin or thick flow. For 3D thick flows, the reversal between surface and bottom
positions occurs within a short range of S: no tracer stabilizes near
mid-height and two reachable intermediate depth layers exist, below the surface
and above the bottom. For 3D thin flows, all intermediate depths are reachable,
depending on S. The numerical study of larger tracer fractions (5-10%) shows
the 3 segregation patterns (surface, intermediate, reverse) corresponding to
the 3 types of equilibrium depth. The reversal is smoother than for a single
tracer. It happens around S=4.5, in agreement with experiments.Comment: 18 pages, 27 figure
Influence of Rough and Smooth Walls on Macroscale Flows in Tumblers
Walls in discrete element method simulations of granular flows are sometimes
modeled as a closely packed monolayer of fixed particles, resulting in a rough
wall rather than a geometrically smooth wall. An implicit assumption is that
the resulting rough wall differs from a smooth wall only locally at the
particle scale. Here we test this assumption by considering the impact of the
wall roughness at the periphery of the flowing layer on the flow of
monodisperse particles in a rotating spherical tumbler. We find that varying
the wall roughness significantly alters average particle trajectories even far
from the wall. Rough walls induce greater poleward axial drift of particles
near the flowing layer surface, but decrease the curvature of the trajectories.
Increasing the volume fill level in the tumbler has little effect on the axial
drift for rough walls, but increases the drift while reducing curvature of the
particle trajectories for smooth walls. The mechanism for these effects is
related to the degree of local slip at the bounding wall, which alters the
flowing layer thickness near the walls, affecting the particle trajectories
even far from the walls near the equator of the tumbler. Thus, the proper
choice of wall conditions is important in the accurate simulation of granular
flows, even far from the bounding wall.Comment: 32 pages, 19 figures, regular article, accepted for publication in
Physical Review E 200
Slow axial drift in three-dimensional granular tumbler flow
Models of monodisperse particle flow in partially filled three-dimensional
tumblers often assume that flow along the axis of rotation is negligible. We
test this assumption, for spherical and double cone tumblers, using experiments
and discrete element method simulations. Cross sections through the particle
bed of a spherical tumbler show that, after a few rotations, a colored band of
particles initially perpendicular to the axis of rotation deforms: particles
near the surface drift toward the pole, while particles deeper in the flowing
layer drift toward the equator. Tracking of mm-sized surface particles in
tumblers with diameters of 8-14 cm shows particle axial displacements of one to
two particle diameters, corresponding to axial drift that is 1-3% of the
tumbler diameter, per pass through the flowing layer. The surface axial drift
in both double cone and spherical tumblers is zero at the equator, increases
moving away from the equator, and then decreases near the poles. Comparing
results for the two tumbler geometries shows that wall slope causes axial
drift, while drift speed increases with equatorial diameter. The dependence of
axial drift on axial position for each tumbler geometry is similar when both
are normalized by their respective maximum values
Design of Sliding Mode Techniques for a CMG-based Testbed Attitude Control System
Precise pointing accuracy and rapid maneuvering are two key features for attitude control missions of small spacecraft. Control moment gyroscopes (CMGs) are applied as ideal actuator for large torque output capability but are usually limited due to the problem of inherent mechanical singularity. This paper proposes a robust attitude control methodology, based on Sliding Mode Control (SMC) techniques, in presence of CMG practical restrictions and disturbances. Two second-order SMC techniques are designed, to guarantee accuracy and limited convergence time. Moreover, attitude control torques are generated by means of four single gimbal CMGs in pyramidal configuration, considering the design of an experimental testbed. The effectiveness of the proposed methodologies are shown in simulations, for different mission scenarios, including singularity points
Why antiplectic metachronal cilia waves are optimal to transport bronchial mucus
International audienceThe coordinated beating of epithelial cilia in human lungs is a fascinating problem from the hydrodynamics perspective. The phase lag between neighboring cilia is able to generate collective cilia motions, known as metachronal waves. Different kinds of waves can occur, antiplectic or symplectic, depending on the direction of the wave with respect to the flow direction. It is shown here, using a coupled lattice Boltzmann-immersed boundary solver, that the key mechanism responsible for their transport efficiency is a blowing-suction effect that displaces the interface between the periciliary liquid and the mucus phase. The contribution of this mechanism on the average flow generated by the cilia is compared to the contribution of the lubrication effect. The results reveal that the interface displacement is the main mechanism responsible for the better efficiency of antiplectic metachronal waves over symplectic ones to transport bronchial mucus. The conclusions drawn here can be extended to any two-layer fluid configuration having different viscosities, and put into motion by cilia-shaped or comb-plate structures, having a back-and-forth motion with phase lags
Granular Flows in a Rotating Drum: the Scaling Law between Velocity and Thickness of the Flow
The flow of dry granular material in a half-filled rotating drum is studied.
The thickness of the flowing zone is measured for several rotation speeds, drum
sizes and beads sizes (size ratio between drum and beads ranging from 47 to
7400). Varying the rotation speed, a scaling law linking mean velocity vs
thickness of the flow, , is deduced for each couple (beads, drum).
The obtained exponent is not always equal to 1, value previously reported
in a drum, but varies with the geometry of the system. For small size ratios,
exponents higher than 1 are obtained due to a saturation of the flowing zone
thickness. The exponent of the power law decreases with the size ratio, leading
to exponents lower than 1 for high size ratios. These exponents imply that the
velocity gradient of a dry granular flow in a rotating drum is not constant.
More fundamentally, these results show that the flow of a granular material in
a rotating drum is very sensible to the geometry, and that the deduction of the
``rheology'' of a granular medium flowing in such a geometry is not obvious
Écoulement granulaire et ségrégation en tambour tournant lisse ou rugueux
La ségrégation axiale en tambour tournant sphérique est étudiée à travers l'influence du taux de remplissage et de la rugosité des parois. L'étude des écoulements monodisperses a mis en évidence la courbure des trajectoires des particules et l'existence de cellules de convection. Nous montrons comment ces deux caractéristiques, couplées à la ségrégation amène à une organisation axiale en trois bandes petites/grandes/petites ou grandes/petites/grandes selon le phénomène dominant
Profils de vitesse des écoulements granulaires
Nous présentons une étude expérimentale et
numérique de l'écoulement sur plan incliné de milieux granulaires monodisperse et
bidisperse ségrégé. Après présentation des profils de vitesse théoriques attendus, et
ceux observés expérimentalement et numériquement, que l'augmentation de la vitesse
d'écoulement par la présence de grandes particules à la surface a lieu sur les
écoulements de faibles épaisseurs. Dès que l'écoulement est plus épais, on retrouve
numériquement la superposition des deux profils monodisperses, contrairement au cas
expérimental
Calibration of soil moisture sensors for a long-term field experiment
In the framework of the ICOS RI network, a field site in Lonzée, Belgium, is equipped to provide long-term data
on greenhouse gas emissions from an agricultural field and the associated environmental variables. Soil moisture
is one of the state variables which are monitored with high temporal resolution and with several repetitions in the
field to take into account soil heterogeneity. In order to facilitate field installation in combination with agricultural
practices, Sentek Enviroscan sensors, a collection of FDR sensors at different depths on a stick, were chosen
to measure soil moisture. In this contribution, we will discuss the results of a detailed calibration experiment
we performed for this sensor type and compare it to the results we got from a different FDR sensor: the ML3
Thetaprobe. We calibrated the probes for the different soil horizons at 3 different locations in the field using big
reconstructed soil columns which were brought to defined soil moisture levels in the lab. The results showed that
the universal calibration relationship of the sensors gave quite similar results as the soil-specific calibration up till
a moisture content of 40%. We also observed that the higher the soil moisture content becomes, the more difficult
it is to obtain a homogeneous distribution of the water in the calibration column which might have an impact on
the sensor readings
- …