Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Helicity dynamics, inverse and bi-directional cascades in fluid and MHD
  turbulence: A brief review   We briefly review helicity dynamics, inverse and bi-directional cascades in
fluid and magnetohydrodynamic (MHD) turbulence, with an emphasis on the latter.
The energy of a turbulent system, an invariant in the non-dissipative case, is
transferred to small scales through nonlinear mode coupling. Fifty years ago,
it was realized that, for a two-dimensional fluid, energy cascades instead to
larger scales, and so does magnetic helicity in three-dimensional MHD. However,
evidence obtained recently indicates that in fact, for a range of governing
parameters, there are systems for which their ideal invariants can be
transferred, with constant fluxes, to both the large scales and the small
scales, as for MHD or rotating stratified flows, in the latter case including
with quasi-geostrophic forcing. Such bi-directional, split, cascades directly
affect the rate at which mixing and dissipation occur in these flows in which
nonlinear eddies interact with fast waves with anisotropic dispersion laws, due
for example to imposed rotation, stratification or uniform magnetic fields. The
directions of cascades can be obtained in some cases through the use of
phenomenological arguments, one of which we derive here following classical
lines in the case of the inverse magnetic helicity cascade in electron MHD.
With more highly-resolved data sets stemming from large laboratory experiments,
high-performance computing and in-situ satellite observations, machine-learning
tools are bringing novel perspectives to turbulence research, e.g. in helping
devise new explicit sub-grid scale parameterizations, which may lead to
enhanced physical insight, including in the future in the case of these new
bi-directional cascades.

8
The categories of this paper are: hep-ex
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
26
The categories of this paper are: bayes-an
6
The categories of this paper are: astro-ph
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
15
The categories of this paper are: nucl-ex