The present paper deals with another example of granular inherent
behavior, which has been referred to as the U-tube instability
(Rajchenbach, 1991; Darias et al., 2011). It occurs when a U shaped
tube, partially filled with grains, is vibrated vertically, and the granular
material tends to migrate to one side of the tube (see Fig. 1).
The height differenceh between the level of grains in each branch
grows exponentially with time for low frequencies of oscillation
(∼10 Hz) (Darias et al., 2011). In the present work, we will address
the effect of wall friction on the exponential growth rate of h.