8,534 research outputs found
Nontrivial temporal scaling in a Galilean stick-slip dynamics
We examine the stick-slip fluctuating response of a rough massive
non-rotating cylinder moving on a rough inclined groove which is submitted to
weak external perturbations and which is maintained well below the angle of
repose. The experiments presented here, which are reminiscent of the Galileo's
works with rolling objects on inclines, have brought in the last years
important new insights into the friction between surfaces in relative motion
and are of relevance for earthquakes, differing from classical block-spring
models by the mechanism of energy input in the system. Robust nontrivial
temporal scaling laws appearing in the dynamics of this system are reported,
and it is shown that the time-support where dissipation occurs approaches a
statistical fractal set with a fixed value of dimension. The distribution of
periods of inactivity in the intermittent motion of the cylinder is also
studied and found to be closely related to the lacunarity of a random version
of the classic triadic Cantor set on the line.Comment: 7 pages including 6 figure
Granular Pressure and the Thickness of a Layer Jamming on a Rough Incline
Dense granular media have a compaction between the random loose and random
close packings. For these dense media the concept of a granular pressure
depending on compaction is not unanimously accepted because they are often in a
"frozen" state which prevents them to explore all their possible microstates, a
necessary condition for defining a pressure and a compressibility
unambiguously. While periodic tapping or cyclic fluidization have already being
used for that exploration, we here suggest that a succession of flowing states
with velocities slowly decreasing down to zero can also be used for that
purpose. And we propose to deduce the pressure in \emph{dense and flowing}
granular media from experiments measuring the thickness of the granular layer
that remains on a rough incline just after the flow has stopped.Comment: 10 pages, 2 figure
Inclined Surface Locomotion Strategies for Spherical Tensegrity Robots
This paper presents a new teleoperated spherical tensegrity robot capable of
performing locomotion on steep inclined surfaces. With a novel control scheme
centered around the simultaneous actuation of multiple cables, the robot
demonstrates robust climbing on inclined surfaces in hardware experiments and
speeds significantly faster than previous spherical tensegrity models. This
robot is an improvement over other iterations in the TT-series and the first
tensegrity to achieve reliable locomotion on inclined surfaces of up to
24\degree. We analyze locomotion in simulation and hardware under single and
multi-cable actuation, and introduce two novel multi-cable actuation policies,
suited for steep incline climbing and speed, respectively. We propose
compelling justifications for the increased dynamic ability of the robot and
motivate development of optimization algorithms able to take advantage of the
robot's increased control authority.Comment: 6 pages, 11 figures, IROS 201
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
Relation between dry granular flow regimes and morphology of deposits: formation of levees in pyroclastic deposits
Experiments on dry granular matter flowing down an inclined plane are
performed in order to study the dynamics of dense pyroclastic flows. The plane
is rough, and always wider than the flow, focusing this study on the case of
laterally unconfined (free boundary) flows.We found that several flow regimes
exist depending on the input fluxand on the inclination of the plane. Each flow
regime corresponds to a particular morphology of the associated deposit. In one
of these regimes, the flow reaches a steady state, and the deposit exhibits a
levee/channel morphology similar to those observed on small pyroclastic flow
deposits. The levees result from the combination between lateral static zones
on each border of the flow and the drainage of the central part of the flow
after the supply stops. Particle segregation featuresare created during the
flow, corresponding to those observed on the deposits of pyroclastic flows.
Moreover, the measurements of the deposit morphology (thickness of the channel,
height of the levees, width of the deposit) are quantitatively related to
parameters of the dynamics of the flow (flux, velocity, height of the flow),
leading to a way of studying the flow dynamics from only measurements of the
deposit. Some attempts to make extensions to natural cases are discussed
through experiments introducing the polydispersity of the particle sizes and
the particle segregation process
Sliding susceptibility of a rough cylinder on a rough inclined perturbed surface
A susceptibility function is introduced to quantify some aspects
of the intermittent stick-slip dynamics of a rough metallic cylinder of length
on a rough metallic incline submitted to small controlled perturbations and
maintained below the angle of repose. This problem is studied from the
experimental point of view and the observed power-law behavior of
is justified through the use of a general class of scaling hypotheses.Comment: 14 pages including 5 figure
Stationary shear flows of dense granular materials : a tentative continuum modelling
We propose a simple continuum model to interpret the shearing motion of
dense, dry and cohesion-less granular media. Compressibility, dilatancy and
Coulomb-like friction are the three basic ingredients. The granular stress is
split into a rate-dependent part representing the rebound-less impacts between
grains and a rate-independent part associated with long-lived contacts. Because
we consider stationary flows only, the grain compaction and the grain velocity
are the two main variables. The predicted velocity and compaction profiles are
in apparent agreement with the experimental or numerical results concerning
free-surface shear flows as well as confined shear flow
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