Nonasymptotic properties of transport and mixing

We study relative dispersion of passive scalar in nonideal cases, i.e., in situations in which asymptotic techniques cannot be applied; typically when the characteristic length scale of the Eulerian velocity field is not much smaller than the domain size. Of course, in such a situation usual asympto tic quantities (the diffusion coefficients) do not give any relevant inform ation about the transport mechanisms. On the other hand, we shall show that the Finite Size Lyapunov Exponent, originally introduced for the predictab ility problem, appears to be rather powerful in approaching the nonasymptot ic transport properties. This technique is applied in a series of numerical experiments in simple flows with chaotic behaviors, in experimental data a nalysis of drifter and to study relative dispersion in fully developed turb ulence. (C) 2000 American Institute of Physics. [S1054-1500(00)01001-6].