45 research outputs found
Comment on ``Force Balance at the Transition from Selective Withdrawal to Viscous Entrainment
Comment on paper by Blanchette and Zhang, Phys. Rev. Lett. 102, 144501
(2009)
Electrically induced tunable cohesion in granular systems
Experimental observations of confined granular materials in the presence of
an electric field that induces cohesive forces are reported. The angle of
repose is found to increase with the cohesive force. A theoretical model for
the stability of a granular heap, including both the effect of the sidewalls
and cohesion is proposed. A good agreement between this model and the
experimental results is found. The steady-state flow angle is practically
unaffected by the electric field except for high field strengths and low flow
rates.Comment: accepted for publication in "Journal of Statistical Mechanics: Theory
and Experiment
The angle of repose of spherical grains in granular Hele-Shaw cells: A molecular dynamics study
We report the results of three dimensional molecular dynamic simulations on
the angle of repose of a sandpile formed by pouring mono-sized cohesionless
spherical grains into a granular Hele-Shaw cell. In particular, we are
interested to investigate the effects of those variables which may impact
significantly on pattern formation of granular mixtures in Hele-Shaw cells. The
results indicate that the frictional forces influence remarkably the formation
of pile on the grain level. Furthermore, We see that increasing grain insertion
rate decreases slightly the angle of repose. We also find that in accordance
with experimental results, the cell thickness is another significant factor and
the angle of repose decays exponentially by increasing the cell thickness. It
is shown that this effect can be interpreted as a cross-over from two to three
dimensions. In fact, using grains with different sizes shows that the behaviour
of the angle of repose when both size and cell thickness are varied is
controlled by a scaled function of the ratio of these two variables.Comment: 8 pages, 12 figures, equation(7) correcte
Wall effects on granular heap stability
We investigate the effects of lateral walls on the angle of movement and on
the angle of repose of a granular pile. Our experimental results for beads
immersed in water are similar to previous results obtained in air and to recent
numerical simulations. All of these results, showing an increase of pile angles
with a decreasing gap width, are explained by a model based on the redirection
of stresses through the granular media. Two regimes are observed depending on
the bead diameter. For large beads, the range of wall effects corresponds to a
constant number of beads whereas it corresponds to a constant characteristic
length for small beads as they aggregate via van der Waals forces
Methane storms as a driver of Titan's dune orientation
Titan's equatorial regions are covered by eastward propagating linear dunes.
This direction is opposite to mean surface winds simulated by Global Climate
Models (GCMs), which are oriented westward at these latitudes, similar to trade
winds on Earth. Different hypotheses have been proposed to address this
apparent contradiction, involving Saturn's gravitational tides, large scale
topography or wind statistics, but none of them can explain a global eastward
dune propagation in the equatorial band. Here we analyse the impact of
equinoctial tropical methane storms developing in the superrotating atmosphere
(i.e. the eastward winds at high altitude) on Titan's dune orientation. Using
mesoscale simulations of convective methane clouds with a GCM wind profile
featuring superrotation, we show that Titan's storms should produce fast
eastward gust fronts above the surface. Such gusts dominate the aeolian
transport, allowing dunes to extend eastward. This analysis therefore suggests
a coupling between superrotation, tropical methane storms and dune formation on
Titan. Furthermore, together with GCM predictions and analogies to some
terrestrial dune fields, this work provides a general framework explaining
several major features of Titan's dunes: linear shape, eastward propagation and
poleward divergence, and implies an equatorial origin of Titan's dune sand.Comment: Published online on Nature Geoscience on 13 April 201
Singular regional brightening events on Titan as seen by Cassini/VIMS
Titan, the largest satellite of Saturn, is the only satellite in the solar system with a dense atmosphere. The close and continuous observations of Titan by the Cassini spacecraft, in orbit around Saturn since July 2004, bring us evidences that Titan tropo-sphere and low stratosphere experience an exotic, but complete meteorological cycle similar to the Earth hy-drological cycle, with hydrocarbons evaporation, con-densation in clouds, and rainfall. Cassini monitoring campaigns also demonstrate that Titan’s cloud cover-age and climate vary with latitude. Titan’s tropics, with globally weak meteorological activity and widespread dune fields, seem to be slightly more arid than the poles, where extensive and numerous liquid reservoirs and sustained cloud activity were discovered.
Only a few tropospheric clouds have been observed at Titan’s tropics during the southern summer [2-4]. As equinox was approaching (in August 2009), they oc-curred more frequently and appeared to grow in strength and size [5-7]
Hele-Shaw beach creation by breaking waves: a mathematics-inspired experiment
Fundamentals of nonlinear wave-particle interactions are studied experimentally in a Hele-Shaw configuration with wave breaking and a dynamic bed. To design this configuration, we determine, mathematically, the gap width which allows inertial flows to survive the viscous damping due to the side walls. Damped wave sloshing experiments compared with simulations confirm that width-averaged potential-flow models with linear momentum damping are adequately capturing the large scale nonlinear wave motion. Subsequently, we show that the four types of wave breaking observed at real-world beaches also emerge on Hele-Shaw laboratory beaches, albeit in idealized forms. Finally, an experimental parameter study is undertaken to quantify the formation of quasi-steady beach morphologies due to nonlinear, breaking waves: berm or dune, beach and bar formation are all classified. Our research reveals that the Hele-Shaw beach configuration allows a wealth of experimental and modelling extensions, including benchmarking of forecast models used in the coastal engineering practice, especially for shingle beaches