217 research outputs found
Tridimensional to bidimensional transition in magnetohydrodynamic turbulence with a guide field and kinetic helicity injection
We study the transition in dimensionality of a three-dimensional
magnetohydrodynamic flow forced only mechanically, when the strength of a
magnetic guiding field is gradually increased. We use numerical simulations to
consider cases in which the mechanical forcing injects (or not) helicity in the
flow. As the guiding field is increased, the strength of the magnetic field
fluctuations decrease as a power law of the guiding field intensity. We show
that for strong enough guiding fields, the helical magnetohydrodynamic flow can
become almost two-dimensional. In this case, the mechanical energy can undergo
a process compatible with an inverse cascade, being transferred preferentially
towards scales larger than the forcing scale. The presence of helicity changes
the spectral scaling of the small magnetic field fluctuations, and affects the
statistics of the velocity field and of the velocity gradients. Moreover, at
small scales the dynamics of the flow becomes dominated by a direct cascade of
helicity, which can be used to derive scaling laws for the velocity field.Comment: 11 pages, 11 figure
Generation of turbulence through frontogenesis in sheared stratified flows
The large-scale structures in the ocean and the atmosphere are in geostrophic
balance, and a conduit must be found to channel the energy to the small scales
where it can be dissipated. In turbulence this takes the form of an energy
cascade, whereas one possible mechanism in a balanced flow at large scales is
through the formation of fronts, a common occurrence in geophysical dynamics.
We show in this paper that an iconic configuration in laboratory and numerical
experiments for the study of turbulence, that of the Taylor-Green or von
K\'arm\'an swirling flow, can be suitably adapted to the case of fluids with
large aspect ratios, leading to the creation of an imposed large-scale vertical
shear. To this effect we use direct numerical simulations of the Boussinesq
equations without net rotation and with no small-scale modeling, and with this
idealized Taylor-Green set-up. Various grid spacings are used, up to
spatial points. The grids are always isotropic, with box
aspect ratios of either or . We find that when shear and
stratification are comparable, the imposed shear layer resulting from the
forcing leads to the formation of multiple fronts and filaments which
destabilize and further evolve into a turbulent flow in the bulk, with a
sizable amount of dissipation and mixing, and with a cycle of front creation,
instability, and development of turbulence. The results depend on the vertical
length scales for shear and for stratification, with stronger large-scale
gradients being generated when the two length scales are comparable.Comment: 19 pages, 15 figures, several simulations added in this new versio
Turbulent magnetic dynamo excitation at low magnetic Prandtl number
Planetary and stellar dynamos likely result from turbulent motions in
magnetofluids with kinematic viscosities that are small compared to their
magnetic diffusivities. Laboratory experiments are in progress to produce
similar dynamos in liquid metals. This work reviews recent computations of
thresholds in critical magnetic Reynolds number above which dynamo
amplification can be expected for mechanically-forced turbulence (helical and
non-helical, short wavelength and long wavelength) as a function of the
magnetic Prandtl number . New results for helical forcing are discussed,
for which a dynamo is obtained at . The fact that the
kinetic turbulent spectrum is much broader in wavenumber space than the
magnetic spectrum leads to numerical difficulties which are bridged by a
combination of overlapping direct numerical simulations and subgrid models of
magnetohydrodynamic turbulence. Typically, the critical magnetic Reynolds
number increases steeply as the magnetic Prandtl number decreases, and then
reaches an asymptotic plateau at values of at most a few hundred. In the
turbulent regime and for magnetic Reynolds numbers large enough, both small and
large scale magnetic fields are excited. The interactions between different
scales in the flow are also discussed.Comment: 8 pages, 8 figures, to appear in Physics of Plasma
Intermittency in Hall-magnetohydrodynamics with a strong guide field
We present a detailed study of intermittency in the velocity and magnetic
field fluctuations of compressible Hall-magnetohydrodynamic turbulence with an
external guide field. To solve the equations numerically, a reduced model valid
when a strong guide field is present is used. Different values for the ion skin
depth are considered in the simulations. The resulting data is analyzed
computing field increments in several directions perpendicular to the guide
field, and building structure functions and probability density functions. In
the magnetohydrodynamic limit we recover the usual results with the magnetic
field being more intermittent than the velocity field. In the presence of the
Hall effect, field fluctuations at scales smaller than the ion skin depth show
a substantial decrease in the level of intermittency, with close to monofractal
scaling.Comment: 10 pages, 8 figure
Helical rotating turbulence. Part II. Intermittency, scale invariance and structures
We study the intermittency properties of the energy and helicity cascades in
two 1536^3 direct numerical simulations of helical rotating turbulence.
Symmetric and anti-symmetric velocity increments are examined, as well as
probability density functions of the velocity field and of the helicity
density. It is found that the direct cascade of energy to small scales is scale
invariant and non-intermittent, whereas the direct cascade of helicity is
highly intermittent. Furthermore, the study of structure functions of different
orders allows us to identify a recovery of isotropy of strong events at very
small scales in the flow. Finally, we observe the juxtaposition in space of
strong laminar and persistent helical columns next to time-varying vortex
tangles, the former being associated with the self-similarity of energy and the
latter with the intermittency of helicity.Comment: 11 pages, 10 figure
On the inverse cascade of magnetic helicity
We study the inverse cascade of magnetic helicity in conducting fluids by
investigating the detailed transfer of helicity between different spherical
shells in Fourier space in direct numerical simulations of three-dimensional
magnetohydrodynamics (MHD). Two different numerical simulations are used, one
where the system is forced with an electromotive force in the induction
equation, and one in which the system is forced mechanically with an ABC flow
and the magnetic field is solely sustained by a dynamo action. The magnetic
helicity cascade at the initial stages of both simulations is observed to be
inverse and local (in scale space) in the large scales, and direct and local in
the small scales. When saturation is approached most of the helicity is
concentrated in the large scales and the cascade is non-local. Helicity is
transfered directly from the forced scales to the largest scales. At the same
time, a smaller in amplitude direct cascade is observed from the largest scale
to small scales.Comment: Submitted to PR
Shell to shell energy transfer in MHD, Part I: steady state turbulence
We investigate the transfer of energy from large scales to small scales in
fully developed forced three-dimensional MHD-turbulence by analyzing the
results of direct numerical simulations in the absence of an externally imposed
uniform magnetic field. Our results show that the transfer of kinetic energy
from the large scales to kinetic energy at smaller scales, and the transfer of
magnetic energy from the large scales to magnetic energy at smaller scales, are
local, as is also found in the case of neutral fluids, and in a way that is
compatible with Kolmogorov (1941) theory of turbulence. However, the transfer
of energy from the velocity field to the magnetic field is a highly non-local
process in Fourier space. Energy from the velocity field at large scales can be
transfered directly into small scale magnetic fields without the participation
of intermediate scales. Some implications of our results to MHD turbulence
modeling are also discussed.Comment: Submitted to PR
Helicity cascades in rotating turbulence
The effect of helicity (velocity-vorticity correlations) is studied in direct
numerical simulations of rotating turbulence down to Rossby numbers of 0.02.
The results suggest that the presence of net helicity plays an important role
in the dynamics of the flow. In particular, at small Rossby number, the energy
cascades to large scales, as expected, but helicity then can dominate the
cascade to small scales. A phenomenological interpretation in terms of a direct
cascade of helicity slowed down by wave-eddy interactions leads to the
prediction of new inertial indices for the small-scale energy and helicity
spectra.Comment: 7 pages, 8 figure
Multifunctional peri-urban agriculture: Some ecosystem services of a sustainable olive grove
This study reports the influence of a sustainable management model which entails the recycling of urban wastewater and distribution by drip irrigation, recycling of polygenic carbon sources internal to the olive orchard (cover crops, pruning material) on yield, soil water holding capacity, soil biodiversity. Sustainable management practices were applied for a 15-year period in a 2-ha olive orchard located in an hilly peri-urban zone of southern Italy, where olive tree represents the dominant crop and has a key role inside the traditional landscape. A comparison between sustainable and conventional management (soil tillage, burning of the pruning residues, mineral fertilization, empirical irrigation) was carried out. This study suggests some guidelines of a sustainable management of peri-urban olive groves, with benefits to the whole agro-ecosystem stability and to the near town, recognizing the multifunctional role of agriculture that enhances the creation of synergies between urban and rural areas
Inertial Range Scaling, Karman-Howarth Theorem and Intermittency for Forced and Decaying Lagrangian Averaged MHD in 2D
We present an extension of the Karman-Howarth theorem to the Lagrangian
averaged magnetohydrodynamic (LAMHD-alpha) equations. The scaling laws
resulting as a corollary of this theorem are studied in numerical simulations,
as well as the scaling of the longitudinal structure function exponents
indicative of intermittency. Numerical simulations for a magnetic Prandtl
number equal to unity are presented both for freely decaying and for forced two
dimensional MHD turbulence, solving directly the MHD equations, and employing
the LAMHD-alpha equations at 1/2 and 1/4 resolution. Linear scaling of the
third-order structure function with length is observed. The LAMHD-alpha
equations also capture the anomalous scaling of the longitudinal structure
function exponents up to order 8.Comment: 34 pages, 7 figures author institution addresses added magnetic
Prandtl number stated clearl
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