ABSTRACT
Dynamical instabilities in fluid mechanics can give rise to a variety of complex behaviors, ranging from pattern formation, to spontaneous chaotic mixing and turbulence (Lamb 1932; Chandrasekhar 1961). These are common phenomena, as for instance, the Faraday instability in a two fluid system under vertical oscillations, or the Kelvin-Helmholtz like instability arising under time-periodic shear in a two fluid system, or in a liquid-sand system where it leads to the formation of ripples as those observed on the shoreline (Scherer et al. 1999; Wunenburger et al. 2000). Fluid dynamical instabilities and pattern formation has just begun to be discovered in granular media (Goldfarb et al. 2002; Conway et al. 2004; Jaeger et al. 1996; Shinbrot et al. 1999; Shinbrot & Muzio 2000). Up to now the vast majority of fluid dynamical instabilities which have been reproduced in a granular medium involves a one component system; this is the case, for instance, of the Faraday instability (Moon et al. 2002), of the Couette-Taylor instability (Conway et al. 2004), and of the Rayleigh-Bernard (Sunthar & Kumaran 2001) instability. Recently the two-components Kelvin-Helmholtz instability has been reproduced in a granular medium (Goldfarb et al. 2002): the initially flat interface between two parallel streams of particles forced to flow with different velocities evolves, leading the formation of curly-like structures.
