156 research outputs found
A preliminary study of the detection of geomorphological features over northeast Africa by satellite radiation measurements in the visible and infrared
Detecting geomorphological features over Northeast Africa by Nimbus 2 visible and infrared radiation measurement
Inverse cascades in turbulence and the case of rotating flows
We first summarize briefly several properties concerning the dynamics of
two-dimensional (2D) turbulence, with an emphasis on the inverse cascade of
energy to the largest accessible scale of the system. In order to study a
similar phenomenon in three-dimensional (3D) turbulence undergoing strong
solid-body rotation, we test a previously developed Large Eddy Simulation (LES)
model against a high-resolution direct numerical simulation of rotating
turbulence on a grid of points. We then describe new numerical results
on the inverse energy cascade in rotating flows using this LES model and
contrast the case of 2D versus 3D forcing, as well as non-helical forcing
(i.e., with weak overall alignment between velocity and vorticity) versus the
fully helical Beltrami case, both for deterministic and random forcing. The
different scaling of the inverse energy cascade can be attributed to the
dimensionality of the forcing, with, in general, either a or a
energy spectrum of slow modes at large scales, perpendicular
referring to the direction of rotation. We finally invoke the role of shear in
the case of a strongly anisotropic deterministic forcing, using the so-called
ABC flow.Comment: 10 pages, 3 figure
Conformal invariance in three-dimensional rotating turbulence
We examine three--dimensional turbulent flows in the presence of solid-body
rotation and helical forcing in the framework of stochastic Schramm-L\"owner
evolution curves (SLE). The data stems from a run on a grid of points,
with Reynolds and Rossby numbers of respectively 5100 and 0.06. We average the
parallel component of the vorticity in the direction parallel to that of
rotation, and examine the resulting field for
scaling properties of its zero-value contours. We find for the first time for
three-dimensional fluid turbulence evidence of nodal curves being conformal
invariant, belonging to a SLE class with associated Brownian diffusivity
. SLE behavior is related to the self-similarity of the
direct cascade of energy to small scales in this flow, and to the partial
bi-dimensionalization of the flow because of rotation. We recover the value of
with a heuristic argument and show that this value is consistent with
several non-trivial SLE predictions.Comment: 4 pages, 3 figures, submitted to PR
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
Numerical study of dynamo action at low magnetic Prandtl numbers
We present a three--pronged numerical approach to the dynamo problem at low
magnetic Prandtl numbers . The difficulty of resolving a large range of
scales is circumvented by combining Direct Numerical Simulations, a
Lagrangian-averaged model, and Large-Eddy Simulations (LES). The flow is
generated by the Taylor-Green forcing; it combines a well defined structure at
large scales and turbulent fluctuations at small scales. Our main findings are:
(i) dynamos are observed from down to ; (ii) the critical
magnetic Reynolds number increases sharply with as turbulence sets
in and then saturates; (iii) in the linear growth phase, the most unstable
magnetic modes move to small scales as is decreased and a Kazantsev
spectrum develops; then the dynamo grows at large scales and modifies
the turbulent velocity fluctuations.Comment: 4 pages, 4 figure
Depletion of Nonlinearity in Magnetohydrodynamic Turbulence: Insights from Analysis and Simulations
We build on recent developments in the study of fluid turbulence [Gibbon
\textit{et al.} Nonlinearity 27, 2605 (2014)] to define suitably scaled,
order- moments, , of , where
and are, respectively, the vorticity and current density in
three-dimensional magnetohydrodynamics (MHD). We show by mathematical analysis,
for unit magnetic Prandtl number , how these moments can be used to
identify three possible regimes for solutions of the MHD equations; these
regimes are specified by inequalities for and . We then
compare our mathematical results with those from our direct numerical
simulations (DNSs) and thus demonstrate that 3D MHD turbulence is like its
fluid-turbulence counterpart insofar as all solutions, which we have
investigated, remain in \textit{only one of these regimes}; this regime has
depleted nonlinearity. We examine the implications of our results for the
exponents that characterize the power-law dependences of the energy
spectra on the wave number , in the inertial range of
scales. We also comment on (a) the generalization of our results to the case
and (b) the relation between and the order- moments
of gradients of hydrodynamic fields, which are used in characterizing
intermittency in turbulent flows.Comment: 14 pages, 3 figure
Depletion of nonlinearity in magnetohydrodynamic turbulence: insights from analysis and simulations
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