Testing a simple model for the instability
In Fig. 4(right), the numerical solution of a model proposed by
Sánchez et al. (2009) is compared with our experimental results.
The model is based on the idea that if the granular medium were
continuously fluidized (as when shaken at sufficiently large frequencies),
we would expect a typical fluid behavior: a tendency
of h to decrease. However, at low frequencies the granular
medium does not remain continuously fluidized. It changes from
an expanded to a compacted state as it slightly detaches from