2,113 research outputs found
Long surface wave instability in dense granular flows
In this paper we present an experimental study of the long surface wave
instability that can develop when a granular material flows down a rough
inclined plane. The threshold and the dispersion relation of the instability
are precisely measured by imposing a controlled perturbation at the entrance of
the flow and measuring its evolution along the slope. The results are compared
with the prediction of a linear stability analysis conducted in the framework
of the depth-averaged or Saint-Venant equations. We show that when the friction
law proposed in Pouliquen (1999a) is introduced in the Saint-Venant equations,
the theory is able to predict quantitatively the stability threshold and the
phase velocity of the waves but fails in predicting the observed cutoff
frequency. The instability is shown to be of the same nature as the long wave
instability observed in classical fluids but with characteristics that can
dramatically differ due to the specificity of the granular rheology.Comment: 29 pages, 20 figures, to be published in Journal of Fluid Mechanic
Friction law for dense granular flows: application to the motion of a mass down a rough inclined plane
The problem of the spreading of a granular mass released at the top of a
rough inclined plane was investigated. We experimentally measure the evolution
of the avalanche from the initiation up to the deposit using a Moir\'e image
processing technique. The results are quantitatively compared with the
prediction of an hydrodynamic model based on depth averaged equations. In the
model, the interaction between the flowing layer and the rough bottom is
described by a non trivial friction force whose expression is derived from
measurements on steady uniform flows. We show that the spreading of the mass is
quantitatively predicted by the model when the mass is released on a plane free
of particles. When an avalanche is triggered on an initially static layer, the
model fails in quantitatively predicting the propagation but qualitatively
captures the evolution.Comment: 19 pages, 10 figures, to be published in J. Fluid Mec
Longitudinal Vortices in Granular Flows
We present a new instability observed in rapid granular flows down rough
inclined planes. For high inclinations and flow rates, the free surface of the
flow experiences a regular deformation in the transverse direction.
Measurements of the surface velocities imply that this instability is
associated with the formation of longitudinal vortices in the granular flow.
From the experimental observations, we propose a mechanism for the
longitudinal vortex formation based on the concept of granular temperature.Comment: 4 pages, 4 figure
Velocity correlations in dense granular flows
Velocity fluctuations of grains flowing down a rough inclined plane are
experimentally studied. The grains at the free surface exhibit fluctuating
motions, which are correlated over few grains diameters. The characteristic
correlation length is shown to depend on the inclination of the plane and not
on the thickness of the flowing layer. This result strongly supports the idea
that dense granular flows are controlled by a characteristic length larger than
the particle diameter
Crucial role of side walls for granular surface flows: consequences for the rheology
In this paper we study the steady uniform flows that develop when granular
material is released from a hopper on top of a static pile in a channel. We
more specifically focus on the role of side walls by carrying out experiments
in setup of different widths, from narrow channels 20 particle diameters wide
to channels 600 particle diameters wide. Results show that steady flows on pile
are entirely controlled by side wall effects. A theoretical model, taking into
account the wall friction and based on a simple local constitutive law recently
proposed for other granular flow configurations (GDR MiDi 2004), gives
predictions in quantitative agreement with the measurements. This result gives
new insights in our understanding of free surface granular flows and strongly
supports the relevance of the constitutive law proposed.Comment: a forgotten square root in Appendix B (Eq B4), and corrected
coefficients in Appendix C; 25 pages, 17 figures, published in J. Fluid Mec
Compaction of a granular material under cyclic shear
In this paper we present experimental results concerning the compaction of a
granular assembly of spheres under periodic shear deformation. The dynamic of
the system is slow and continuous when the amplitude of the shear is constant,
but exhibits rapid evolution of the volume fraction when a sudden change in
shear amplitude is imposed. This rapid response is shown to be to be
uncorrelated with the slow compaction process.Comment: 7 pages, 9 figures, accepted for publication in European Physical
Journal
The influence of surface roughness on the rheology of immersed and dry frictional spheres
Pressure-imposed rheometry is used to examine the influence of surface
roughness on the rheology of immersed and dry frictional spheres in the dense
regime. The quasi-static value of the effective friction coefficient is not
significantly affected by particle roughness while the critical volume fraction
at jamming decreases with increasing roughness. These values are found to be
similar in immersed and dry conditions. Rescaling the volume fraction by the
maximum volume fraction leads to collapses of rheological data on master
curves. The asymptotic behaviors are examined close to the jamming transition
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