  The asymptotic decay of passive scalar fields is solved analytically for the
Kraichnan model, where the velocity has a short correlation time. At long
times, two universality classes are found, both characterized by a distribution
of the scalar -- generally non-Gaussian -- with global self-similar evolution
in time. Analogous behavior is found numerically with a more realistic flow
resulting from an inverse energy cascade.

Chaotic Dynamics;Statistical Mechanics