correlations; to be able to determine them directly from the statistical properties of the velocity field would be therefore of some interest.

It must be said that most of the prescriptions entering a Lagrangian dispersion model could be obtained, in practice, by dimensional reasoning or by experiments. In some cases, like in the presence of particles endowed with inertia, this turns out, however, to be a difficult task [@csanady63; @sawford91a]. It is very difficult, for instance, to make assumptions on the preference of solid particles to lie in certain regions of the flow instead of others [@wang92; @elgobashi92; @wang93]. Solid particle transport by a turbulent flow is an example of a situation in which careful treatment of the time dependent statistics of the velocity field is essential. It is precisely the interplay between the response time of the solid particle to the fluid, i.e. the Stokes time $\tau_S$, and the characteristic times of the turbulent flow [@olla01], which determines the dynamics, and this is clearly lost when all the turbulent times are sent to zero.

Recently, there has been strong theoretical interest on the problem of turbulence induced concentration fluctuations in a heavy particle suspension. In [@elperin00], the role of 