1 research outputs found
Active and passive fields face to face
The statistical properties of active and passive scalar fields transported by
the same turbulent flow are investigated. Four examples of active scalar have
been considered: temperature in thermal convection, magnetic potential in
two-dimensional magnetohydrodynamics, vorticity in two-dimensional Ekman
turbulence and potential temperature in surface flows. In the cases of
temperature and vorticity, it is found that the active scalar behavior is akin
to that of its co-evolving passive counterpart. The two other cases indicate
that this similarity is in fact not generic and differences between passive and
active fields can be striking: in two-dimensional magnetohydrodynamics the
magnetic potential performs an inverse cascade while the passive scalar
cascades toward the small-scales; in surface flows, albeit both perform a
direct cascade, the potential temperature and the passive scalar have different
scaling laws already at the level of low-order statistical objects. These
dramatic differences are rooted in the correlations between the active scalar
input and the particle trajectories. The role of such correlations in the issue
of universality in active scalar transport and the behavior of dissipative
anomalies is addressed.Comment: 36 pages, 20 eps figures, for the published version see
http://www.iop.org/EJ/abstract/1367-2630/6/1/07