Onset and evolution of a wavy instability in shaken sand

A novel small wavelength instability occurs in experiments for vertically shaken (z = acos(omega t)) sand, for small particle diameters d and when gas effects are important. At onset, the energy (a omega)(2)/gd is proportional to a/d, as previously found for traveling waves. The instability begins as a surface perturbation with wavelength lambda(0) similar to 6 mm; subsequently lambda grows exponentially in time. (C) 1997 Published by Elsevier Science B.V.