Hydrodynamic Uctuations and Averaging Problems in Dense Granular Ows

We analize the properties of dense granular systems by assuming a hydrodynamical description , based on conservation laws for the particle number density and linear momentum. We combine analytical methods and experimental and numerical results obtained by ensemble-averaging of data on creep during compaction and molecular dynamics simulations of convective ow. 1 INTRODUCTION In spite of the early and in many aspects successful hydrodynamical approach applied by Haa (P. K. Haa 1983 1986) to granular systems, there still remain many unsolved questions, since the behavior of a granular mass is fundamentally diier-ent from that of typical uids. For instance, in addition to the ordinary source of hydrodynamic uctuations {which in this case can't be removed by taking the thermodynamic limit, the fact that solid grains in a dense arrangement can't be regarded as points in any length scale leads to a second source of uctuations; it operates at distances much larger than the typical diameter of the particles and is related to the appearance of extensive arrangements inside the system. Given a mean density not far away from the close-packing limit, diierent actual realizations in the conngura-tion space of the particles may lead to departing dynamic properties and, ultimately, in generating completely diierent time-sequences. This contribution is what we call non-local noise. Both, local and non-local noise, are always present but their relative importance strongly depends on the forcing applied to the system, measured by the param