8,224 research outputs found
Nonlinear transverse cascade and sustenance of MRI-turbulence in Keplerian disks with an azimuthal magnetic field
We investigate magnetohydrodynamic turbulence driven by the magnetorotational
instability (MRI) in Keplerian disks with a nonzero net azimuthal magnetic
field using shearing box simulations. As distinct from most previous studies,
we analyze turbulence dynamics in Fourier (-) space to understand its
sustenance. The linear growth of MRI with azimuthal field has a transient
character and is anisotropic in Fourier space, leading to anisotropy of
nonlinear processes in Fourier space. As a result, the main nonlinear process
appears to be a new type of angular redistribution of modes in Fourier space --
the \emph{nonlinear transverse cascade} -- rather than usual direct/inverse
cascade. We demonstrate that the turbulence is sustained by interplay of the
linear transient growth of MRI (which is the only energy supply for the
turbulence) and the transverse cascade. These two processes operate at large
length scales, comparable to box size and the corresponding small wavenumber
area, called \emph{vital area} in Fourier space is crucial for the sustenance,
while outside the vital area direct cascade dominates. The interplay of the
linear and nonlinear processes in Fourier space is generally too intertwined
for a vivid schematization. Nevertheless, we reveal the \emph{basic subcycle}
of the sustenance that clearly shows synergy of these processes in the
self-organization of the magnetized flow system. This synergy is quite robust
and persists for the considered different aspect ratios of the simulation
boxes. The spectral characteristics of the dynamical processes in these boxes
are qualitatively similar, indicating the universality of the sustenance
mechanism of the MRI-turbulence.Comment: 32 pages, 17 figures, accepted for publication in Ap
Numerical Simulations of Driven Relativistic MHD Turbulence
A wide variety of astrophysical phenomena involve the flow of turbulent
magnetized gas with relativistic velocity or energy density. Examples include
gamma-ray bursts, active galactic nuclei, pulsars, magnetars, micro-quasars,
merging neutron stars, X-ray binaries, some supernovae, and the early universe.
In order to elucidate the basic properties of the relativistic
magnetohydrodynamical (RMHD) turbulence present in these systems, we present
results from numerical simulations of fully developed driven turbulence in a
relativistically warm, weakly magnetized and mildly compressible ideal fluid.
We have evolved the RMHD equations for many dynamical times on a uniform grid
with 1024^3 zones using a high order Godunov code. We observe the growth of
magnetic energy from a seed field through saturation at about 1% of the total
fluid energy. We compute the power spectrum of velocity and density-weighted
velocity and conclude that the inertial scaling is consistent with a slope of
-5/3. We compute the longitudinal and transverse velocity structure functions
of order p up to 11, and discuss their possible deviation from the expected
scaling for non-relativistic media. We also compute the scale-dependent
distortion of coherent velocity structures with respect to the local magnetic
field, finding a weaker scale dependence than is expected for incompressible
non-relativistic flows with a strong mean field.Comment: Accepted to Ap
The relation between gas density and velocity power spectra in galaxy clusters: high-resolution hydrodynamic simulations and the role of conduction
Exploring the ICM power spectrum can help us to probe the physics of galaxy
clusters. Using high-resolution 3D plasma simulations, we study the statistics
of the velocity field and its relation with the thermodynamic perturbations.
The normalization of the ICM spectrum (density, entropy, or pressure) is
linearly tied to the level of large-scale motions, which excite both gravity
and sound waves due to stratification. For low 3D Mach number M~0.25, gravity
waves mainly drive entropy perturbations, traced by preferentially tangential
turbulence. For M>0.5, sound waves start to significantly contribute, passing
the leading role to compressive pressure fluctuations, associated with
isotropic (or slightly radial) turbulence. Density and temperature fluctuations
are then characterized by the dominant process: isobaric (low M), adiabatic
(high M), or isothermal (strong conduction). Most clusters reside in the
intermediate regime, showing a mixture of gravity and sound waves, hence
drifting towards isotropic velocities. Remarkably, regardless of the regime,
the variance of density perturbations is comparable to the 1D Mach number. This
linear relation allows to easily convert between gas motions and ICM
perturbations, which can be exploited by Chandra, XMM data and by the
forthcoming Astro-H. At intermediate and small scales (10-100 kpc), the
turbulent velocities develop a Kolmogorov cascade. The thermodynamic
perturbations act as effective tracers of the velocity field, broadly
consistent with the Kolmogorov-Obukhov-Corrsin advection theory. Thermal
conduction acts to damp the gas fluctuations, washing out the filamentary
structures and steepening the spectrum, while leaving unaltered the velocity
cascade. The ratio of the velocity and density spectrum thus inverts the
downtrend shown by the non-diffusive models, allowing to probe the presence of
significant conductivity in the ICM.Comment: Accepted by A&A; 15 pages, 10 figures; added insights and references
- thank you for the positive feedbac
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
Galaxy formation hydrodynamics: From cosmic flows to star-forming clouds
Major progress has been made over the last few years in understanding
hydrodynamical processes on cosmological scales, in particular how galaxies get
their baryons. There is increasing recognition that a large part of the baryons
accrete smoothly onto galaxies, and that internal evolution processes play a
major role in shaping galaxies - mergers are not necessarily the dominant
process. However, predictions from the various assembly mechanisms are still in
large disagreement with the observed properties of galaxies in the nearby
Universe. Small-scale processes have a major impact on the global evolution of
galaxies over a Hubble time and the usual sub-grid models account for them in a
far too uncertain way. Understanding when, where and at which rate galaxies
formed their stars becomes crucial to understand the formation of galaxy
populations. I discuss recent improvements and current limitations in
"resolved" modelling of star formation, aiming at explicitely capturing
star-forming instabilities, in cosmological and galaxy-sized simulations. Such
models need to develop three-dimensional turbulence in the ISM, which requires
parsec-scale resolution at redshift zero.Comment: To appear in the proceedings for IAU Symposium 270: Computational
Star Formation (eds. Alves, Elmegreen, Girart, Trimble
Quantum turbulence at finite temperature: the two-fluids cascade
To model isotropic homogeneous quantum turbulence in superfluid helium, we
have performed Direct Numerical Simulations (DNS) of two fluids (the normal
fluid and the superfluid) coupled by mutual friction. We have found evidence of
strong locking of superfluid and normal fluid along the turbulent cascade, from
the large scale structures where only one fluid is forced down to the vorticity
structures at small scales. We have determined the residual slip velocity
between the two fluids, and, for each fluid, the relative balance of inertial,
viscous and friction forces along the scales. Our calculations show that the
classical relation between energy injection and dissipation scale is not valid
in quantum turbulence, but we have been able to derive a temperature--dependent
superfluid analogous relation. Finally, we discuss our DNS results in terms of
the current understanding of quantum turbulence, including the value of the
effective kinematic viscosity
Large Eddy Simulations in Astrophysics
In this review, the methodology of large eddy simulations (LES) is introduced
and applications in astrophysics are discussed. As theoretical framework, the
scale decomposition of the dynamical equations for neutral fluids by means of
spatial filtering is explained. For cosmological applications, the filtered
equations in comoving coordinates are also presented. To obtain a closed set of
equations that can be evolved in LES, several subgrid scale models for the
interactions between numerically resolved and unresolved scales are discussed,
in particular the subgrid scale turbulence energy equation model. It is then
shown how model coefficients can be calculated, either by dynamical procedures
or, a priori, from high-resolution data. For astrophysical applications,
adaptive mesh refinement is often indispensable. It is shown that the subgrid
scale turbulence energy model allows for a particularly elegant and physically
well motivated way of preserving momentum and energy conservation in AMR
simulations. Moreover, the notion of shear-improved models for inhomogeneous
and non-stationary turbulence is introduced. Finally, applications of LES to
turbulent combustion in thermonuclear supernovae, star formation and feedback
in galaxies, and cosmological structure formation are reviewed.Comment: 64 pages, 23 figures, submitted to Living Reviews in Computational
Astrophysic
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