Environmental Engineering Reference
In-Depth Information
Fig. 2
Spouting can jets:
a
liquid and
b
non cohesive dry sand
to the liquid jets, another hint is that the wall thickness,
w
, affects a lot the intensity
and shape of sand jets. In both cases the competition among the friction force and
the gravity force will affect the pressure and the jet properties. This work is devoted
to clarify the origin of these facts.
In order to reach our goal, the plan of the paper is as follows: in Sect.
2
we give
a historical outline of the dry friction phenomenon, in a general context, and how
it has been included in the theoretical treatments of confined granular material. In
Sect.
3
, we give a short review of the experimental studies of the gravity flows and
mass flow rates when the orifices are made in lateral walls. We also describe some
experiments where the influence of
D
,
w
and
d
, the grain diameter, are so important
that they control the flow. In Sect.
4
, we study tilted bins and we propose, on the
basis of our experimental results, a correlation that embraces both changes in
D
and
w
and the tilt angle of the bins,
. In Sect.
5
we discuss some new results based on
a modification to the Janssen law for the pressure in a bin and finally, in Sect.
6
,we
give the main conclusions of this work.
ʲ
2 Dry Friction: A Historical Review
The late Egyptians were already occupied with the problem of friction. From histor-
ical traditions we know already that they wetted the sand on which they transported
their stones. Today, we know that this reduces the friction coefficient.
The macroscopic friction laws, that we know, have been discovered during the last
centuries, mainly by Leonardo da Vinci (1452-1519), Guillaume Amontons (1663-
1705), Leonhard Euler (1707-1783) and Charles Augustin Coulomb (1736-1806).
Leonardo da Vinci (Fig.
3
) postulated in the Codex Atlanticus (da Vinci
1480
)
that friction is proportional to load and independent of the area of the item being
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