70 research outputs found
A model for ripple instabilities in granular media
We extend the model of surface granular flow proposed in \cite{bcre} to
account for the effect of an external `wind', which acts as to dislodge
particles from the static bed, such that a stationary state of flowing grains
is reached. We discuss in detail how this mechanism can be described in a
phenomenological way, and show that a flat bed is linearly unstable against
ripple formation in a certain region of parameter space. We focus in particular
on the (realistic) case where the migration velocity of the instability is much
smaller than the grains' velocity. In this limit, the full dispersion relation
can be established. We find that the critical wave vector is of the order of
the saltation length. We provide an intuitive interpretation of the
instability.Comment: 11 pages, latex, 2 encapsulated postscript figure
Selection of dune shapes and velocities. Part 1: Dynamics of sand, wind and barchans
Almost fifty years of investigations of barchan dunes morphology and dynamics
is reviewed, with emphasis on the physical understanding of these objects. The
characteristics measured on the field (shape, size, velocity) and the physical
problems they rise are presented. Then, we review the dynamical mechanisms
explaining the formation and the propagation of dunes. In particular a complete
and original approach of the sand transport over a flat sand bed is proposed
and discussed. We conclude on open problems by outlining future research
directions.Comment: submitted to Eur. Phys. J. B, 20 pages, 20 figure
Direct numerical simulations of aeolian sand ripples
Aeolian sand beds exhibit regular patterns of ripples resulting from the
interaction between topography and sediment transport. Their characteristics
have been so far related to reptation transport caused by the impacts on the
ground of grains entrained by the wind into saltation. By means of direct
numerical simulations of grains interacting with a wind flow, we show that the
instability turns out to be driven by resonant grain trajectories, whose length
is close to a ripple wavelength and whose splash leads to a mass displacement
towards the ripple crests. The pattern selection results from a compromise
between this destabilizing mechanism and a diffusive downslope transport which
stabilizes small wavelengths. The initial wavelength is set by the ratio of the
sediment flux and the erosion/deposition rate, a ratio which increases linearly
with the wind velocity. We show that this scaling law, in agreement with
experiments, originates from an interfacial layer separating the saltation zone
from the static sand bed, where momentum transfers are dominated by mid-air
collisions. Finally, we provide quantitative support for the use the
propagation of these ripples as a proxy for remote measurements of sediment
transport.Comment: 21 pages, 12 figure
Active dry granular flows: rheology and rigidity transitions
The constitutive relations of a dense granular flow composed of
self-propelling frictional hard particles are investigated by means of DEM
numerical simulations. We show that the rheology, which relates the dynamical
friction and the volume fraction to the inertial number ,
depends on a dimensionless number , which compares the active
force to the confining pressure. Two liquid/solid transitions -- in the Maxwell
rigidity sense -- are observed. As soon as the activity is turned on, the
packing becomes an `active solid' with a mean number of particle contacts
larger than the isostatic value. The quasi-static values of and
decrease with . At a finite value of the activity ,
corresponding to the isostatic condition, a second `active rigidity transition'
is observed beyond which the quasi-static values of the friction vanishes and
the rheology becomes Newtonian. For , we provide
evidence for a highly intermittent dynamics of this 'active fluid'.Comment: 7 pages, 7 figures, final version, accepted for publication in
Europhys. Let
Transition from viscous to inertial regime in dense suspensions
Non-Brownian suspensions present a transition from Newtonian behavior in the
zero-shear limit to a shear thickening behaviour at a large shear rate, none of
which is clearly understood so far. Here, we carry out numerical simulations of
such an athermal dense suspension under shear, at an imposed confining
pressure. This set-up is conceptually identical to the recent experiments of
Boyer and co-workers [Phys. Rev. Lett. 107,188301 (2011)]. Varying the
interstitial fluid viscosities, we recover the Newtonian and Bagnoldian regimes
and show that they correspond to a dissipation dominated by viscous and contact
forces respectively. We show that the two rheological regimes can be unified as
a function of a single dimensionless number, by adding the contributions to the
dissipation at a given volume fraction.Comment: 4 pages, 3 figure
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