Universal decay of scalar turbulence

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