A granular instability driven by gravity is studied experimentally and
numerically. The instability arises as grains fall in a closed Hele-Shaw cell
where a layer of dense granular material is positioned above a layer of air.
The initially flat front defined by the grains subsequently develops into a
pattern of falling granular fingers separated by rising bubbles of air. A
transient coarsening of the front is observed right from the start by a finger
merging process. The coarsening is later stabilized by new fingers growing from
the center of the rising bubbles. The structures are quantified by means of
Fourier analysis and quantitative agreement between experiment and computation
is shown. This analysis also reveals scale invariance of the flow structures
under overall change of spatial scale.Comment: 4 pages, 11 figure
