5,581 research outputs found
Selection of dune shapes and velocities. Part 2: A two-dimensional modelling
We present in this paper a simplification of the dune model proposed by
Sauermann et al. which keeps the basic mechanisms but allows analytical and
parametric studies. Two kinds of purely propagative two dimensional solutions
are exhibited: dunes and domes, which, by contrast to the former, do not show
avalanche slip face. Their shape and velocity can be predicted as a function of
their size. We recover in particular that dune profiles are not scale invariant
(small dunes are flatter than the large ones), and that the inverse of the
velocity grows almost linearly with the dune size. We furthermore get the
existence of a critical mass below which no stable dune exists. However, the
linear stability analysis of a flat sand sheet shows that it is unstable at
large wavelengths and suggests a mechanism of dune initiation.Comment: submitted to Eur. Phys. J. B, 13 pages, 17 figure
Comment on "Minimal size of a barchan dune"
It is now an accepted fact that the size at which dunes form from a flat sand
bed as well as their `minimal size' scales on the flux saturation length. This
length is by definition the relaxation length of the slowest mode toward
equilibrium transport. The model presented by Parteli, Duran and Herrmann
[Phys. Rev. E 75, 011301 (2007)] predicts that the saturation length decreases
to zero as the inverse of the wind shear stress far from the threshold. We
first show that their model is not self-consistent: even under large wind, the
relaxation rate is limited by grain inertia and thus can not decrease to zero.
A key argument presented by these authors comes from the discussion of the
typical dune wavelength on Mars (650 m) on the basis of which they refute the
scaling of the dune size with the drag length evidenced by Claudin and
Andreotti [Earth Pla. Sci. Lett. 252, 30 (2006)]. They instead propose that
Martian dunes, composed of large grains (500 micrometers), were formed in the
past under very strong winds. We show that this saltating grain size, estimated
from thermal diffusion measurements, is not reliable. Moreover, the microscopic
photographs taken by the rovers on Martian aeolian bedforms show a grain size
of 87 plus or minus 25 micrometers together with hematite spherules at
millimetre scale. As those so-called ``blueberries'' can not be entrained by
reasonable winds, we conclude that the saltating grains on Mars are the small
ones, which gives a second strong argument against the model of Parteli et al.Comment: A six page comment on ``Minimal size of a barchan dune'' by Parteli,
Duran and Herrmann [Phys. Rev. E 75, 011301 (2007) arXiv:0705.1778
Dynamic and instability of submarine avalanches
We perform a laboratory-scale experiment of submarine avalanches on a rough
inclined plane. A sediment layer is prepared and thereafter tilted up to an
angle lower than the spontaneous avalanche angle. The sediment is scrapped
until an avalanche is triggered. Based on the stability diagram of the sediment
layer, we investigate different structures for the avalanche front dynamics.
First we see a straight front descending the slope, and then a transverse
instability occurs. Eventually, a fingering instability shows up similar to
rivulets appearing for a viscous fluid flowing down an incline. The mechanisms
leading to this new instability and the wavelength selection are discussed.Comment: 4 pages, 6 figures, to appear in the proceedings of Powders and
Grains 200
Evidence of Raleigh-Hertz surface waves and shear stiffness anomaly in granular media
Due to the non-linearity of Hertzian contacts, the speed of sound in granular
matter increases with pressure. Under gravity, the non-linear elastic
description predicts that acoustic propagation is only possible through surface
modes, called Rayleigh-Hertz modes and guided by the index gradient. Here we
directly evidence these modes in a controlled laboratory experiment and use
them to probe the elastic properties of a granular packing under vanishing
confining pressure. The shape and the dispersion relation of both transverse
and sagittal modes are compared to the prediction of non-linear elasticity that
includes finite size effects. This allows to test the existence of a shear
stiffness anomaly close to the jamming transition.Comment: 4 pages 4 figure
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