The dependence of with the amplitude of vibration (keeping
the frequency fixed at 10 Hz) is shown in Fig. 4(right). We
also plot the results as a function of the maximum dimensionless
acceleration = a(2f)2/g (with a the amplitude of oscillation,
f the frequency and g the acceleration of gravity), because it is a
commonly used parameter in vibrated granular materials research.
Segmented lines in Fig. 4(right) are the numerical solutions of a
model proposed by Sánchez et al. (2009), depicted in the next section.
The range of displayed is due to the fact that lower values
of did not show the instability and higher values could not be
reached with our current experimental setup. Let us discuss each
of the four experimental data sets plotted in Fig. 4(right). The red
curve (open circles) corresponds to the tube with smooth walls.