Il comportamento di un sistema granulare e' un problema non banale.
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TI: Surface flows of granular mixtures - III. Canonical model
AU: Boutreux-T; Makse-HA; de-Gennes-PG
SO: EUROPEAN-PHYSICAL-JOURNAL-B. MAY 1999; 9 (1) : 105-115.
PY: 1999
IS: 1434-6028
AB: We present the generalization of the minimal model fur surface flows
of granular mixtures proposed by Boutreux and de Gennes [J. Phys. I
France 6, 1295 (1996)]. The minimal model was valid for grains differing
only in their surface properties. The present model also takes into
account differences in thr: sire of the grains. We apply the model to
study segregation in two-dimensional silos of mixtures of grains
differing in size and/or surface properties. When the difference in size
is small, the model predicts that a continuous segregation appears in
the static phase during the filling of a silo. When the difference in
size is wide, we take into account the segregation of the grains in the
rolling phase, and the model predicts complete segregation and
stratification in agreement with experimental observations.
TI: Granular matter: a tentative view
AU: de-Gennes-PG
SO: REVIEWS-OF-MODERN-PHYSICS. MAR 1999; 71 Special Iss. SI :
S374-S382.
PY: 1999
IS: 0034-6861
AB: Granular matter describes large collections of small grains, under
conditions in which the Brownian motion of the grains is negligible
(sizes d>1 micrometer). The grains can exhibit solidlike behavior and
fluidlike behavior, but the description of these states is still
controversial. The present discussion is restricted to static problems,
for which the main approach is to describe properly the initial state of
each volume element, when it was deposited from a fluid
how.[S0034-6861(99)02202-3].
PT: Journal
TI: Thermal expansion effects in a silo
AU: de-Gennes-PG
COMPTES-RENDUS-DE-L-ACADEMIE-DES-SCIENCES-SERIE-II-FASCICULE-B-MECANIQUE
-PHYSIQUE-ASTRONOMIE. FEB-MAR 1999; 327 (2-3) : 267-274.
PY: 1999
IS: 1287-4620
AB: We consider a vertical container filled with granular material and
exposed to temperature cycles which induce changes in the pressure
distribution. Our starting point is an elastic model for the grains
leading to classical Janssen behaviour in isothermal conditions. We
assume complete mobilisation of the wall frictions. The dynamics
superposes static friction plus a viscous friction. After a T jump
favouring an expansion, we predict that the resulting overpressures are
relaxed only in a region (of size comparable to the Janssen screening
length lambda) near the top. When T returns to the initial value, we
expect decohesion in a certain layer near the top (for rapid cooling).
For slow cooling, we expect a smooth return to the original Janssen
state. We also discuss a different, but related, effect: instead of
changing the temperature, we switch off the gravity effects (on a small
column) by suddenly rotating the column from vertical to horizontal.
Here, a rarefaction wave should invade the whole column.
TI: Reflections on the mechanics of granular matter
AU: de-Gennes-PG
SO: PHYSICA-A. DEC 15 1998; 261 (3-4) : 267-293.
PY: 1998
IS: 0378-4371
AB: During recent pears, a rather basic conflict has emerged between
departments of mechanics physics concerning the description of granular
media. Experts from mechanics measure stress: strain relations, using
the so-called triaxial tests, and then use these data to predict the
behavior of a sample under given boundary conditions. Some physicists
have a different view: they have claimed that it is not possible to
define a proper displacement field in a heap of sand, and that the
notion of strains is thus ambiguous. In the present text, we conclude
that the crucial features are the following: (a) a heap is usually
formed from a flowing phase of sand, (b) there is (empirically) a sharp
intel face between the flowing phase and the frozen heap below, (c) we
may define for each grain a displacement, which is measured from the
moment when it froze. (This displacement is due, for instance, to a
compaction of the heap under its own weight.) We also present some
aspects of the dynamics, including dune motions and surface flows of
grains. Bouchaud, Cates and coworkers have constructed a very compact
description for thin flows. We discuss the practical consequences of
this picture, emphasizing some possible extensions for thicker flows.
This suggests a number of possible experiments on avalanches.
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Received on Thu Nov 09 2000 - 00:00:00 CET