73 research outputs found
Kelvin-Helmholtz versus Hall Magneto-shear instability in astrophysical flows
We study the stability of shear flows in a fully ionized plasma.
Kelvin-Helmholtz is a well known, macroscopic and ideal shear-driven
instability. In sufficiently low density plasmas, also the microscopic Hall
magneto-shear instability can take place. We performed three-dimensional
simulations of the Hall-MHD equations where these two instabilities are
present, and carried out a comparative study. We find that when the shear flow
is so intense that its vorticity surpasses the ion-cyclotron frequency of the
plasma, the Hall magneto-shear instability is not only non-negligible, but it
actually displays growth rates larger than those of the Kelvin-Helmholtz
instability
Single-particle dispersion in stably stratified turbulence
We present models for single-particle dispersion in vertical and horizontal
directions of stably stratified flows. The model in the vertical direction is
based on the observed Lagrangian spectrum of the vertical velocity, while the
model in the horizontal direction is a combination of a continuous-time
eddy-constrained random walk process with a contribution to transport from
horizontal winds. Transport at times larger than the Lagrangian turnover time
is not universal and dependent on these winds. The models yield results in good
agreement with direct numerical simulations of stratified turbulence, for which
single-particle dispersion differs from the well studied case of homogeneous
and isotropic turbulence
Inverse cascades and resonant triads in rotating and stratified turbulence
Kraichnan’s seminal ideas on inverse cascades yielded new tools to study common phenomena in geophysical turbulent flows. In the atmosphere and the oceans, rotation and stratification result in a flow that can be approximated as two-dimensional at very large scales but which requires considering three-dimensional effects to fully describe turbulent transport processes and non-linear phenomena. Motions can thus be classified into two classes: fast modes consisting of inertia-gravity waves and slow quasi-geostrophic modes for which the Coriolis force and horizontal pressure gradients are close to balance. In this paper, we review previous results on the strength of the inverse cascade in rotating and stratified flows and then present new results on the effect of varying the strength of rotation and stratification (measured by the inverse Prandtl ratio N/f, of the Coriolis frequency to the Brunt-Väisäla frequency) on the amplitude of the waves and on the flow quasi-geostrophic behavior. We show that the inverse cascade is more efficient in the range of N/f for which resonant triads do not exist, /2≤N/f≤21/2≤N/f≤2. We then use the spatio-temporal spectrum to show that in this range slow modes dominate the dynamics, while the strength of the waves (and their relevance in the flow dynamics) is weaker.Fil: Oks, D.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Mininni, Pablo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Marino, R.. Universite Lyon 2; FranciaFil: Pouquet, A.. State University of Colorado Boulder; Estados Unido
Simulations of the Kelvin-Helmholtz instability driven by coronal mass ejections in the turbulent corona
Recent high resolution AIA/SDO images show evidence of the development of the
Kelvin-Helmholtz instability, as coronal mass ejections (CMEs) expand in the
ambient corona. A large-scale magnetic field mostly tangential to the interface
is inferred, both on the CME and on the background sides. However, the magnetic
field component along the shear flow is not strong enough to quench the
instability. There is also observational evidence that the ambient corona is in
a turbulent regime, and therefore the criteria for the development of the
instability are a-priori expected to differ from the laminar case. To study the
evolution of the Kelvin-Helmholtz instability with a turbulent background, we
perform three-dimensional simulations of the incompressible magnetohydrodynamic
equations. The instability is driven by a velocity profile tangential to the
CME-corona interface, which we simulate through a hyperbolic tangent profile.
The turbulent background is generated by the application of a stationary
stirring force. We compute the instability growth-rate for different values of
the turbulence intensity, and find that the role of turbulence is to attenuate
the growth. The fact that the Kelvin-Helmholtz instability is observed, sets an
upper limit to the correlation length of the coronal background turbulence
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
On the emergence of helicity in rotating stratified turbulence
We perform numerical simulations of decaying rotating stratified turbulence
and show, in the Boussinesq framework, that helicity (velocity-vorticity
correlation), as observed in super-cell storms and hurricanes, is spontaneously
created due to an interplay between buoyancy and rotation common to large-scale
atmospheric and oceanic flows. Helicity emerges from the joint action of eddies
and of inertia-gravity waves (with inertia and gravity with respective
associated frequencies and ), and it occurs when the waves are
sufficiently strong. For the amount of helicity produced is correctly
predicted by a quasi-linear balance equation. Outside this regime, and up to
the highest Reynolds number obtained in this study, namely ,
helicity production is found to be persistent for as large as , and for and respectively as large as and
.Comment: 10 pages, 5 figure
Hall-MHD small-scale dynamos
Much of the progress in our understanding of dynamo mechanisms has been made
within the theoretical framework of magnetohydrodynamics (MHD). However, for
sufficiently diffuse media, the Hall effect eventually becomes non-negligible.
We present results from three dimensional simulations of the Hall-MHD equations
subjected to random non-helical forcing. We study the role of the Hall effect
in the dynamo efficiency for different values of the Hall parameter, using a
pseudospectral code to achieve exponentially fast convergence. We also study
energy transfer rates among spatial scales to determine the relative importance
of the various nonlinear effects in the dynamo process and in the energy
cascade. The Hall effect produces a reduction of the direct energy cascade at
scales larger than the Hall scale, and therefore leads to smaller energy
dissipation rates. Finally, we present results stemming from simulations at
large magnetic Prandtl numbers, which is the relevant regime in hot and diffuse
media such a the interstellar medium.Comment: 11 pages and 11 figure
Energy transfer in Hall-MHD turbulence: cascades, backscatter, and dynamo action
Scale interactions in Hall MHD are studied using both the mean field theory
derivation of transport coefficients, and direct numerical simulations in three
space dimensions. In the magnetically dominated regime, the eddy resistivity is
found to be negative definite, leading to large scale instabilities. A direct
cascade of the total energy is observed, although as the amplitude of the Hall
effect is increased, backscatter of magnetic energy to large scales is found, a
feature not present in MHD flows. The coupling between the magnetic and
velocity fields is different than in the MHD case, and backscatter of energy
from small scale magnetic fields to large scale flows is also observed. For the
magnetic helicity, a strong quenching of its transfer is found. We also discuss
non-helical magnetically forced Hall-MHD simulations where growth of a large
scale magnetic field is observed.Comment: 25 pages, 16 figure
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