Vertical vibration is widely used to excite a granular system, inducing interesting
phenomena as pattern formation [1], segregation [2], internal collective motion
of the grains, among others [3]. The
ow of granular materials through dierent
container geometries is crucial for the improvement of several industrial processes.
Tubular containers enhance the role of the walls in the interchange of energy and
are extremely important from the industrial point of view. Many industrial processes
involve the transport of grains through pipes and are unavoidably aected
by mechanical vibrations, therefore their reliability can be greatly aected by the
knowledge of the physics of vibrationally induced granular bulk
ow [4]. An interesting
example of vibrationally induced granular transport in a pipe can be realized in
a partially lled U-tube. In this system vertical vibrations can cause the rise of one
of the columns of grains until the other one is emptied, a phenomenon often referred
to as the U-tube instability. The U-tube instability was rst mentioned by Gutman
in 1976 [5], he pointed out that the increase of the dierence in level was due to the
air pressure dierence acting across the bottom of the vibrated tube. Rajchenbach
[6], mentioned the U-tube instability a proposed a convection based mechanism to
explain it. Ohtsuki et al.