Two-particle dispersion by correlated random velocity fields

We consider the two-particle dispersion in a velocity field, where the relative two-point velocity scales according to v(2)(r)xr(alpha) and the corresponding correlation time scales as tau(r)alpha r(beta). We show that for alpha/2+P <1 the diffusion approximation holds, and the increase in the interparticle distances is governed by the distance-dependent diffusion coefficient K(r)xr(alpha+beta). The possible regimes outside of the validity of diffusion approximation are discussed. The Kolmogorov scaling in turbulent how alpha=beta=2/3 corresponds to a borderline situation. The experimental data for this case suggest that the separation regime is probably ballistic. [S1063-651X(99)05011-4].