Scalar fields transported by turbulent flows are commonly encountered in many natural phenomena and engineering problems as, e.g, convection (temperature). When the transported scalar influences the velocity field we speak about active scalars, and of passive scalars if the feedback of the scalar field is negligible. This lecture focuses on the statistical properties of active and passive scalar fields transported by the same turbulent flow. By examining a few cases, we shall see that one can have situations in which the two scalars share the same statistics and others in which they are blatantly different. These differences are traced back to the correlations between the active scalar input and the particle trajectories. These observations clearly raise the problem of the universality in active scalar transport.

The presented material will be based on the review:

"Active and passive fields face to face"

A. Celani, M. Cencini, A. Mazzino and M. Vergassola, New J. Phys. 6, 72 (2004).

Freely available on http://www.iop.org/EJ/abstract/1367-2630/6/1/072