718 research outputs found
Energy spectra stemming from interactions of Alfven waves and turbulent eddies
We present a numerical analysis of an incompressible decaying
magnetohydrodynamic turbulence run on a grid of 1536^3 points. The Taylor
Reynolds number at the maximum of dissipation is ~1100, and the initial
condition is a superposition of large scale ABC flows and random noise at small
scales, with no uniform magnetic field. The initial kinetic and magnetic
energies are equal, with negligible correlation. The resulting energy spectrum
is a combination of two components, each moderately resolved. Isotropy obtains
in the large scales, with a spectral law compatible with the
Iroshnikov-Kraichnan theory stemming from the weakening of nonlinear
interactions due to Alfven waves; scaling of structure functions confirms the
non-Kolmogorovian nature of the flow in this range. At small scales, weak
turbulence emerges with a k_{\perp}^{-2} spectrum, the perpendicular direction
referring to the local quasi-uniform magnetic field.Comment: 4 pages, 4 figure
The Anisotropy of MHD Alfv\'{e}nic Turbulence
We perform direct 3-dimensional numerical simulations for magnetohydrodynamic
(MHD) turbulence in a periodic box of size threaded by strong uniform
magnetic fields. We use a pseudo-spectral code with hyperviscosity and
hyperdiffusivity to solve the incompressible MHD equations. We analyze the
structure of the eddies as a function of scale. A straightforward calculation
of anisotropy in wavevector space shows that the anisotropy is scale-{\it
independent}. We discuss why this is {\it not} the true scaling law and how the
curvature of large-scale magnetic fields affects the power spectrum and leads
to the wrong conclusion. When we correct for this effect, we find that the
anisotropy of eddies depends on their size: smaller eddies are more elongated
than larger ones along {\it local} magnetic field lines. The results are
consistent with the scaling law proposed by Goldreich and Sridhar (1995, 1997). Here
(and ) are wavenumbers measured relative to
the local magnetic field direction. However, we see some systematic deviations
which may be a sign of limitations to the model, or our inability to fully
resolve the inertial range of turbulence in our simulations.Comment: 13 pages (11 NEW figures), ApJ, in press (Aug 10, 2000?
MHD Turbulence Revisited
Kraichnan (1965) proposed that MHD turbulence occurs as a result of
collisions between oppositely directed Alfv\'en wave packets. Recent work has
generated some controversy over the nature of non linear couplings between
colliding Alfv\'en waves. We find that the resolution to much of the confusion
lies in the existence of a new type of turbulence, intermediate turbulence, in
which the cascade of energy in the inertial range exhibits properties
intermediate between those of weak and strong turbulent cascades. Some
properties of intermediate MHD turbulence are: (i) in common with weak
turbulent cascades, wave packets belonging to the inertial range are long
lived; (ii) however, components of the strain tensor are so large that, similar
to the situation in strong turbulence, perturbation theory is not applicable;
(iii) the breakdown of perturbation theory results from the divergence of
neighboring field lines due to wave packets whose perturbations in velocity and
magnetic fields are localized, but whose perturbations in displacement are not;
(iv) 3--wave interactions dominate individual collisions between wave packets,
but interactions of all orders make comparable contributions to the
intermediate turbulent energy cascade; (v) successive collisions are correlated
since wave packets are distorted as they follow diverging field lines; (vi) in
common with the weak MHD cascade, there is no parallel cascade of energy, and
the cascade to small perpendicular scales strengthens as it reaches higher wave
numbers; (vii) For an appropriate weak excitation, there is a natural
progression from a weak, through an intermediate, to a strong cascade.Comment: 25 pages, to appear in The Astrophysical Journa
Strong Imbalanced Turbulence
We consider stationary, forced, imbalanced, or cross-helical MHD Alfvenic
turbulence where the waves traveling in one direction have higher amplitudes
than the opposite waves. This paper is dedicated to so-called strong
turbulence, which cannot be treated perturbatively. Our main result is that the
anisotropy of the weak waves is stronger than the anisotropy of a strong waves.
We propose that critical balance, which was originally conceived as a causality
argument, has to be amended by what we call a propagation argument. This
revised formulation of critical balance is able to handle the imbalanced case
and reduces to old formulation in the balanced case. We also provide
phenomenological model of energy cascading and discuss possibility of
self-similar solutions in a realistic setup of driven turbulence.Comment: this is shorter, 5 page version of what is to appear in ApJ 682, Aug.
1, 200
Loading rates in California inferred from aftershocks
International audienceWe estimate the loading rate in southern California and the change in stress induced by a transient slip event across the San Andreas fault (SAF) system in central California, using a model of static fatigue. We analyze temporal properties of aftershocks in order to determine the time delay before the onset of the power law aftershock decay rate. In creep-slip and stick-slip zones, we show that the rate of change of this delay is related to seismic and aseismic deformation across the SAF system. Furthermore, we show that this rate of change is proportional to the deficit of slip rate along the SAF. This new relationship between geodetic and seismological data is in good agreement with predictions from a Limited Power Law model in which the evolution of the duration of a linear aftershock decay rate over short time results from variations in the load of the brittle upper crust
Role of cross helicity in magnetohydrodynamic turbulence
Strong incompressible three-dimensional magnetohydrodynamic turbulence is
investigated by means of high resolution direct numerical simulations. The
simulations show that the configuration space is characterized by regions of
positive and negative cross-helicity, corresponding to highly aligned or
anti-aligned velocity and magnetic field fluctuations, even when the average
cross-helicity is zero. To elucidate the role of cross-helicity, the spectra
and structure of turbulence are obtained in imbalanced regions where
cross-helicity is non-zero. When averaged over regions of positive and negative
cross-helicity, the result is consistent with the simulations of balanced
turbulence. An analytical explanation for the obtained results is proposed.Comment: 4 pages, 4 figure
Radio-wave propagation through a medium containing electron-density fluctuations described by an anisotropic Goldreich-Sridhar spectrum
We study the propagation of radio waves through a medium possessing density
fluctuations that are elongated along the ambient magnetic field and described
by an anisotropic Goldreich-Sridhar power spectrum. We derive general formulas
for the wave phase structure function, visibility, angular broadening,
diffraction-pattern length scales, and scintillation time scale for arbitrary
distributions of turbulence along the line of sight, and specialize these
formulas to idealized cases.Comment: 25 pages, 3 figures, submitted to Ap
Predicting Failure using Conditioning on Damage History: Demonstration on Percolation and Hierarchical Fiber Bundles
We formulate the problem of probabilistic predictions of global failure in
the simplest possible model based on site percolation and on one of the
simplest model of time-dependent rupture, a hierarchical fiber bundle model. We
show that conditioning the predictions on the knowledge of the current degree
of damage (occupancy density or number and size of cracks) and on some
information on the largest cluster improves significantly the prediction
accuracy, in particular by allowing to identify those realizations which have
anomalously low or large clusters (cracks). We quantify the prediction gains
using two measures, the relative specific information gain (which is the
variation of entropy obtained by adding new information) and the
root-mean-square of the prediction errors over a large ensemble of
realizations. The bulk of our simulations have been obtained with the
two-dimensional site percolation model on a lattice of size and hold true for other lattice sizes. For the hierarchical fiber
bundle model, conditioning the measures of damage on the information of the
location and size of the largest crack extends significantly the critical
region and the prediction skills. These examples illustrate how on-going damage
can be used as a revelation of both the realization-dependent pre-existing
heterogeneity and the damage scenario undertaken by each specific sample.Comment: 7 pages + 11 figure
Diffusion and dispersion of passive tracers: Navier-Stokes versus MHD turbulence
A comparison of turbulent diffusion and pair-dispersion in homogeneous,
macroscopically isotropic Navier-Stokes (NS) and nonhelical magnetohydrodynamic
(MHD) turbulence based on high-resolution direct numerical simulations is
presented. Significant differences between MHD and NS systems are observed in
the pair-dispersion properties, in particular a strong reduction of the
separation velocity in MHD turbulence as compared to the NS case. It is shown
that in MHD turbulence the average pair-dispersion is slowed down for
, being
the Kolmogorov time, due to the alignment of the relative Lagrangian tracer
velocity with the local magnetic field. Significant differences in turbulent
single-particle diffusion in NS and MHD turbulence are not detected. The fluid
particle trajectories in the vicinity of the smallest dissipative structures
are found to be characterisically different although these comparably rare
events have a negligible influence on the statistics investigated in this work.Comment: Europhysics Letters, in prin
Imbalanced Strong MHD Turbulence
We present a phenomenological model of imbalanced MHD turbulence in an
incompressible magnetofluid. The steady-state cascades, of waves traveling in
opposite directions along the mean magnetic field, carry unequal energy fluxes
to small length scales, where they decay due to viscous and resistive
dissipation. The inertial-range scalings are well-understood when both cascades
are weak. We study the case when both cascades are, in a sense, strong. The
inertial-range of this imbalanced cascade has the following properties: (i) the
ratio of the r.m.s. Elsasser amplitudes is independent of scale, and is equal
to the ratio of the corresponding energy fluxes; (ii) in common with the
balanced strong cascade, the energy spectra of both Elsasser waves are of the
anisotropic Kolmogorov form, with their parallel correlation lengths equal to
each other on all scales, and proportional to the two-thirds power of the
transverse correlation length; (iii) the equality of cascade time and
waveperiod (critical balance) that characterizes the strong balanced cascade
does not apply to the Elsasser field with the larger amplitude. Instead, the
more general criterion that always applies to both Elsasser fields is that the
cascade time is equal to the correlation time of the straining imposed by
oppositely-directed waves. Our results are particularly relevant for turbulence
in the solar wind. Spacecraft measurements have established that, in the
inertial range of solar wind turbulence, waves travelling away from the sun
have higher amplitudes than those travelling towards it. Result (i) allows us
to infer the turbulent flux ratios from the amplitude ratios, thus providing
insight into the origin of the turbulence
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