10 research outputs found
Scale interactions in magnetohydrodynamic turbulence
This article reviews recent studies of scale interactions in
magnetohydrodynamic turbulence. The present day increase of computing power,
which allows for the exploration of different configurations of turbulence in
conducting flows, and the development of shell-to-shell transfer functions, has
led to detailed studies of interactions between the velocity and the magnetic
field and between scales. In particular, processes such as induction and dynamo
action, the damping of velocity fluctuations by the Lorentz force, or the
development of anisotropies, can be characterized at different scales. In this
context we consider three different configurations often studied in the
literature: mechanically forced turbulence, freely decaying turbulence, and
turbulence in the presence of a uniform magnetic field. Each configuration is
of interest for different geophysical and astrophysical applications. Local and
non-local transfers are discussed for each case. While the transfer between
scales of solely kinetic or solely magnetic energy is local, transfers between
kinetic and magnetic fields are observed to be local or non-local depending on
the configuration. Scale interactions in the cascade of magnetic helicity are
also reviewed. Based on the results, the validity of several usual assumptions
in hydrodynamic turbulence, such as isotropy of the small scales or
universality, is discussed.Comment: Posted with permission from the Annual Review of Fluid Mechanics,
Volume 43 (c) 2011 by Annual Reviews, http://www.annualreviews.or
On the magnetic fields generated by experimental dynamos
We review the results obtained by three successful fluid dynamo experiments
and discuss what has been learnt from them about the effect of turbulence on
the dynamo threshold and saturation. We then discuss several questions that are
still open and propose experiments that could be performed to answer some of
them.Comment: 40 pages, 13 figure