470 research outputs found
On the effect of rotation on magnetohydrodynamic turbulence at high magnetic Reynolds number
This article is focused on the dynamics of a rotating electrically conducting
fluid in a turbulent state. As inside the Earth's core or in various industrial
processes, a flow is altered by the presence of both background rotation and a
large scale magnetic field. In this context, we present a set of 3D direct
numerical simulations of incompressible decaying turbulence. We focus on
parameters similar to the ones encountered in geophysical and astrophysical
flows, so that the Rossby number is small, the interaction parameter is large,
but the Elsasser number, defining the ratio between Coriolis and Lorentz
forces, is about unity. These simulations allow to quantify the effect of
rotation and thus inertial waves on the growth of magnetic fluctuations due to
Alfv\'en waves. Rotation prevents the occurrence of equipartition between
kinetic and magnetic energies, with a reduction of magnetic energy at
decreasing Elsasser number {\Lambda}. It also causes a decrease of energy
transfer mediated by cubic correlations. In terms of flow structure, a decrease
of {\Lambda} corresponds to an increase in the misalignment of velocity and
magnetic field.Comment: 18 pages, 12 figure
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
On the two-dimensionalization of quasistatic magnetohydrodynamic turbulence
We analyze the anisotropy of turbulence in an electrically conducting fluid
in the presence of a uniform magnetic field, for low magnetic Reynolds number,
using the quasi-static approximation. In the linear limit, the kinetic energy
of velocity components normal to the magnetic field decays faster than the
kinetic energy of component along the magnetic field [Moffatt, JFM 28, 1967].
However, numerous numerical studies predict a different behaviour, wherein the
final state is characterized by dominant horizontal energy. We investigate the
corresponding nonlinear phenomenon using Direct Numerical Simulations. The
initial temporal evolution of the decaying flow indicates that the turbulence
is very similar to the so-called "two-and-a-half-dimensional" flow [Montgomery
& Turner, Phys. Fluids 25(2), 1982] and we offer an explanation for the
dominance of horizontal kinetic energy.Comment: 17 pages, 8 figure
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
Signatures of two-dimensionalisation of 3D turbulence in presence of rotation
A reason has been given for the inverse energy cascade in the
two-dimensionalised rapidly rotating 3D incompressible turbulence. For such
system, literature shows a possibility of the exponent of wavenumber in the
energy spectrum's relation to lie between -2 and -3. We argue the existence of
a more strict range of -2 to -7/3 for the exponent in the case of rapidly
rotating turbulence which is in accordance with the recent experiments. Also, a
rigorous derivation for the two point third order structure function has been
provided helping one to argue that even with slow rotation one gets, though
dominated, a spectrum with the exponent -2.87, thereby hinting at the
initiation of the two-dimensionalisation effect with rotation.Comment: An extended and typos-corrected version of the earlier submissio
On two-dimensionalization of three-dimensional turbulence in shell models
Applying a modified version of the Gledzer-Ohkitani-Yamada (GOY) shell model,
the signatures of so-called two-dimensionalization effect of three-dimensional
incompressible, homogeneous, isotropic fully developed unforced turbulence have
been studied and reproduced. Within the framework of shell models we have
obtained the following results: (i) progressive steepening of the energy
spectrum with increased strength of the rotation, and, (ii) depletion in the
energy flux of the forward forward cascade, sometimes leading to an inverse
cascade. The presence of extended self-similarity and self-similar PDFs for
longitudinal velocity differences are also presented for the rotating 3D
turbulence case
DesafĂos teĂłricos y prácticos del universalismo proporcional: una revisiĂłn
Objective. In 2010, the principle of proportionate universalism (PU) has been proposed as a solution to reduce health inequalities. It had a great resonance but does not seem to have been widely applied and no guidelines exist on how to implement it. The two specific objectives of this scoping review were: (1) to describe the theoretical context in which PU was established, (2) to describe how researchers apply PU and related methodological issues. Methods. We searched for all articles published until 6th of February 2020, mentioning “Proportionate Universalism” or its synonyms “Targeted universalism” OR “Progressive Universalism” as a topic in all Web of Science databases. Results. This review of 55 articles allowed us a global vision around the question of PU regarding its theoretical foundations and practical implementation. PU principle is rooted in the social theories of universalism and targeting. It proposes to link these two aspects in order to achieve an effective reduction of health inequalities. Regarding practical implementation, PU interventions were rare and led to different interpretations. There are still many methodological and ethical challenges regarding conception and evaluation of PU interventions, including how to apply proportionality, and identification of needs. Conclusion. This review mapped available scientific literature on PU and its related concepts. PU principle originates from social theories. As highlighted by authors who implemented PU interventions, application raises many challenges from design to evaluation. Analysis of PU applications provided in this review answered to some of them but remaining methodological challenges could be addressed in further research
Theoretical and practical challenges of proportionate universalism: A review
Objective. In 2010, the principle of proportionate universalism (PU) has been proposed as a solution to reduce health inequalities. It had a great resonance but does not seem to have been widely applied and no guidelines exist on how to implement it. The two specific objectives of this scoping review were: (1) to describe the theoretical context in which PU was established, (2) to describe how researchers apply PU and related methodological issues. Methods. We searched for all articles published until 6th of February 2020, mentioning “Proportionate Universalism” or its synonyms “Targeted universalism” OR “Progressive Universalism” as a topic in all Web of Science databases. Results. This review of 55 articles allowed us a global vision around the question of PU regarding its theoretical foundations and practical implementation. PU principle is rooted in the social theories of universalism and targeting. It proposes to link these two aspects in order to achieve an effective reduction of health inequalities. Regarding practical implementation, PU interventions were rare and led to different interpretations. There are still many methodological and ethical challenges regarding conception and evaluation of PU interventions, including how to apply proportionality, and identification of needs. Conclusion. This review mapped available scientific literature on PU and its related concepts. PU principle originates from social theories. As highlighted by authors who implemented PU interventions, application raises many challenges from design to evaluation. Analysis of PU applications provided in this review answered to some of them but remaining methodological challenges could be addressed in further research
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