A simple lattice gas model with random fields and gravity is introduced to
describe a system of grains moving in a disordered environment. Off equilibrium
relaxations of bulk density and its two time correlation functions are
numerically found to show logarithmic time dependences and "aging" effects.
Similitudes with dry granular media are stressed. The connections with off
equilibrium dynamics in others kinds of "frustrated" lattice models in presence
of a directional driving force (gravity) are discussed to single out the
appearance of universal features in the relaxation process.Comment: 15 pages, latex, 7 figures include
