786 research outputs found
Quasi-static magnetohydrodynamic turbulence at high Reynolds number
We analyse the anisotropy of homogeneous turbulence in an electrically
conducting fluid submitted to a uniform magnetic field, for low magnetic
Reynolds number, in the quasi- static approximation. We interpret disagreeing
previous predictions between linearized theory and simulations: in the linear
limit, the kinetic energy of transverse velocity components, normal to the
magnetic field, decays faster than the kinetic energy of the axial component,
along the magnetic field (Moffatt (1967)); whereas many numerical studies
predict a final state characterised by dominant energy of transverse velocity
components. We investigate the corresponding nonlinear phenomenon using Direct
Numerical Simulations of freely-decaying turbulence, and a two-point
statistical spectral closure based on the Eddy Damped Quasi-Normal Markovian
model. The transition from the three-dimensional turbulent flow to a
"two-and-a-half-dimensional" flow (Montgomery & Turner (1982)) is a result of
the combined effects of short-time linear Joule dissipation and longer time
nonlinear creation of polarisation anisotropy. It is this combination of linear
and nonlinear effects which explains the disagreement between predictions from
linearized theory and results from numerical simulations. The transition is
characterized by the elongation of turbulent structures along the applied
magnetic field, and by the strong anisotropy of directional two-point
correlation spectra, in agreement with experimental evidence. Inertial
equatorial transfers in both DNS and the model are presented to describe in
detail the most important equilibrium dynamics. Spectral scalings are
maintained in high Reynolds number turbulence attainable only with the EDQNM
model, which also provides simplified modelling of the asymptotic state of
quasi-static MHD turbulence.Comment: Journal of Fluid Mechanics, 201
The decay of Batchelor and Saffman rotating turbulence
The decay rate of isotropic and homogeneous turbulence is known to be
affected by the large-scale spectrum of the initial perturbations, associated
with at least two cannonical self-preserving solutions of the von
K\'arm\'an-Howarth equation: the so-called Batchelor and Saffman spectra. The
effect of long-range correlations in the decay of anisotropic flows is less
clear, and recently it has been proposed that the decay rate of rotating
turbulence may be independent of the large-scale spectrum of the initial
perturbations. We analyze numerical simulations of freely decaying rotating
turbulence with initial energy spectra (Batchelor turbulence) and
(Saffman turbulence) and show that, while a self-similar decay
cannot be identified for the total energy, the decay is indeed affected by
long-range correlations. The decay of two-dimensional and three-dimensional
modes follows distinct power laws in each case, which are consistent with
predictions derived from the anisotropic von K\'arm\'an-Howarth equation, and
with conservation of anisotropic integral quantities by the flow evolution
Chaotic motion of charged particles in toroidal magnetic configurations
We study the motion of a charged particle in a tokamak magnetic field and
discuss its chaotic nature. Contrary to most of recent studies, we do not make
any assumption on any constant of the motion and solve numerically the
cyclotron gyration using Hamiltonian formalism. We take advantage of a
symplectic integrator allowing us to make long-time simulations. First
considering an idealized magnetic configuration, we add a non generic
perturbation corresponding to a magnetic ripple, breaking one of the invariant
of the motion. Chaotic motion is then observed and opens questions about the
link between chaos of magnetic field lines and chaos of particle trajectories.
Second, we return to a axi-symmetric configuration and tune the safety factor
(magnetic configuration) in order to recover chaotic motion. In this last
setting with two constants of the motion, the presence of chaos implies that no
third global constant exists, we highlight this fact by looking at variations
of the first order of the magnetic moment in this chaotic setting. We are
facing a mixed phase space with both regular and chaotic regions and point out
the difficulties in performing a global reduction such as gyrokinetics
Scaling and energy transfer in rotating turbulence
The inertial-range properties of quasi-stationary hydrodynamic turbulence
under solid-body rotation are studied via high-resolution direct numerical
simulations. For strong rotation the nonlinear energy cascade exhibits
depletion and a pronounced anisotropy with the energy flux proceeding mainly
perpendicularly to the rotation axis. This corresponds to a transition towards
a quasi-two-dimensional flow similar to a linear Taylor-Proudman state. In
contrast to the energy spectrum along the rotation axis which does not scale
self-similarly, the perpendicular spectrum displays an inertial range with
-behavior. A new phenomenology gives a rationale for the
observations. The scaling exponents of structure functions up to
order measured perpendicular to the rotation axis indicate reduced
intermittency with increasing rotation rate. The proposed phenomenology is
consistent with the inferred asymptotic non-intermittent behavior
.Comment: to be published in Europhysics Letters (www.epletters.net), minor
changes to match version in prin
The CERN Cryogenic Test Facility for the Atlas Barrel Toroid Magnets
The superconducting magnet system of the ATLAS detector will consist of a central solenoid, two end-cap toroidal magnets (ECT) and the barrel toroid magnet (BT) made of eight coils symmetrically placed around the central axis of the detector. The magnets will be tested individually in a 5000 m2 experimental area prior to their final installation at an underground cavern of the LHC Collider. For the BT magnets, a dedicated cryogenic test facility has been designed which is currently under the construction and commissioning phase. A liquid nitrogen pre-cooling unit and a 1200 [email protected] refrigerator will allow flexible operating conditions via a rather complex distribution and transfer line system. Flow of two-phase helium for cooling the coils is provided by centrifugal pumps immersed in a saturated liquid helium bath. The integration of the pumps in an existing cryostat required the adoption of novel mechanical solutions. Tests conducted permitted the validation of the technical design of the cryostat and its instrumentation. The characteristics of one pump were measured and pressure rise of 300 mbar at nominal flow of 80 g/s confirmed the specifications
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