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Search Results: 1 - 10 of 448394 matches for " J. Léorat "
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Nonlinear dynamo action in a precessing cylindrical container
C. Nore,J. Léorat,J. -L. Guermond,F. Luddens
Physics , 2011, DOI: 10.1103/PhysRevE.84.016317
Abstract: It is numerically demonstrated by means of a magnetohydrodynamics (MHD) code that precession can trigger the dynamo effect in a cylindrical container. This result adds credit to the hypothesis that precession can be strong enough to be one of the sources of the dynamo action in some astrophysical bodies.
Remarks on the stability of the Navier-Stokes equations supplemented with stress-free boundary conditions
J. -L. Guermond,J. Léorat,F. Luddens,C. Nore
Physics , 2012, DOI: 10.1016/j.euromechflu.2012.11.003
Abstract: The purpose of this note is to analyze the long term stability of the Navier-Stokes equations supplemented with the Coriolis force and the stress-free boundary condition. It is shown that, if the flow domain is axisymmetric, spurious stability behaviors can occur depending whether the Coriolis force is active or not.
Evidence of a cyclonic regime in a precessing cylindrical container
W. Mouhali,T. Lehner,J. Léorat,R. Vitry
Physics , 2014,
Abstract: We report experimental observations obtained by particule image velocimetry (PIV) of the behavior of a flow driven by rotation and precession of a cylindrical container. Various hydrodynamical regimes are identified according to the value of the control parameter which is the ratio of the precession frequency to the rotation frequency. In particular when this parameter is increased from small values, we have observed an induced differential rotation followed by the apparition of permanent cyclonic vortices.
Impact of impellers on the axisymmetric magnetic mode in the VKS2 dynamo experiment
R. Laguerre,C. Nore,A. Ribeiro,J. Léorat,J. -L. Guermond,F. Plunian
Physics , 2008, DOI: 10.1103/PhysRevLett.101.104501
Abstract: In the VKS2 (von K\'arm\'an Sodium 2) successful dynamo experiment of September 2006, the magnetic field that was observed showed a strong axisymmetric component, implying that non axisymmetric components of the flow field were acting. By modeling the induction effect of the spiraling flow between the blades of the impellers in a kinematic dynamo code, we find that the axisymmetric magnetic mode is excited and becomes dominant in the vicinity of the dynamo threshold. % The control parameters are the magnetic Reynolds number of the mean flow, the coefficient measuring the induction effect, $\alpha$, and the type of boundary conditions (vacuum for steel impellers and normal field for soft iron impellers). We show that using realistic values of $\alpha$, the observed critical magnetic Reynolds number, $Rm^c \approx 32$, can be reached easily with ferromagnetic boundary conditions. We conjecture that the dynamo action achieved in this experiment may not be related to the turbulence in the bulk of the flow, but rather to the alpha effect induced by the impellers.
Electromagnetic induction in non-uniform domains
A. Giesecke,C. Nore,F. Luddens,F. Stefani,G. Gerbeth,J. Léorat,J. -L. Guermond
Physics , 2010, DOI: 10.1080/03091929.2010.507202
Abstract: Kinematic simulations of the induction equation are carried out for different setups suitable for the von-K\'arm\'an-Sodium (VKS) dynamo experiment. Material properties of the flow driving impellers are considered by means of high conducting and high permeability disks that are present in a cylindrical volume filled with a conducting fluid. Two entirely different numerical codes are mutually validated by showing quantitative agreement on Ohmic decay and kinematic dynamo problems using various configurations and physical parameters. Field geometry and growth rates are strongly modified by the material properties of the disks even if the high permeability/high conductivity material is localized within a quite thin region. In contrast the influence of external boundary conditions remains small. Utilizing a VKS like mean fluid flow and high permeability disks yields a reduction of the critical magnetic Reynolds number for the onset of dynamo action of the simplest non-axisymmetric field mode. However this decrease is not sufficient to become relevant in the VKS experiment. Furthermore, the reduction of Rm_c is essentially influenced by tiny changes in the flow configuration so that the result is not very robust against small modifications of setup and properties of turbulence.
Ambivalent effects of added layers on steady kinematic dynamos in cylindrical geometry: application to the VKS experiment
F. Stefani,M. Xu,G. Gerbeth,F. Ravelet,A. Chiffaudel,F. Daviaud,J. Léorat
Physics , 2005, DOI: 10.1016/j.euromechflu.2006.02.002
Abstract: The intention of the ''von Karman sodium'' (VKS) experiment is to study the hydromagnetic dynamo effect in a highly turbulent and unconstrained flow. Much effort has been devoted to the optimization of the mean flow and the lateral boundary conditions in order to minimize the critical magnetic Reynolds number and hence the necessary motor power. The main focus of this paper lies on the role of ''lid layers'', i.e. layers of liquid sodium between the impellers and the end walls of the cylinder. First, we study an analytical test flow to show that lid layers can have an ambivalent effect on the efficiency of the dynamo. The critical magnetic Reynolds number shows a flat minimum for a small lid layer thickness, but increases for thicker layers. For the actual VKS geometry it is shown that static lid layers yield a moderate increase of the critical magnetic Reynolds number by approximately 12 per cent. A more dramatic increase by 100 until 150 per cent can occur when some rotational flow is taken into account in those layers. Possible solutions of this problem are discussed for the real dynamo facility.
Influence of high permeability disks in an axisymmetric model of the Cadarache dynamo experiment
A. Giesecke,C. Nore,F. Stefani,G. Gerbeth,J. Léorat,W. Herreman,F. Luddens,J. -L. Guermond
Physics , 2011, DOI: 10.1088/1367-2630/14/5/053005
Abstract: Numerical simulations of the kinematic induction equation are performed on a model configuration of the Cadarache von-K\'arm\'an-Sodium dynamo experiment. The effect of a localized axisymmetric distribution of relative permeability {\mu} that represents soft iron material within the conducting fluid flow is investigated. The critical magnetic Reynolds number Rm^c for dynamo action of the first non-axisymmetric mode roughly scales like Rm^c({\mu})-Rm^c({\mu}->infinity) ~ {\mu}^(-1/2) i.e. the threshold decreases as {\mu} increases. This scaling law suggests a skin effect mechanism in the soft iron disks. More important with regard to the Cadarache dynamo experiment, we observe a purely toroidal axisymmetric mode localized in the high permeability disks which becomes dominant for large {\mu}. In this limit, the toroidal mode is close to the onset of dynamo action with a (negative) growth-rate that is rather independent of the magnetic Reynolds number. We qualitatively explain this effect by paramagnetic pumping at the fluid/disk interface and propose a simplified model that quantitatively reproduces numerical results. The crucial role of the high permeability disks for the mode selection in the Cadarache dynamo experiment cannot be inferred from computations using idealized pseudo-vacuum boundary conditions (H x n = 0).
Towards an experimental von Karman dynamo: numerical studies for an optimized design
Florent Ravelet,Arnaud Chiffaudel,Fran?ois Daviaud,Jacques Léorat
Physics , 2004, DOI: 10.1063/1.2130745
Abstract: Numerical studies of a kinematic dynamo based on von Karman type flows between two counterrotating disks in a finite cylinder are reported. The flow has been optimized using a water model experiment, varying the driving impellers configuration. A solution leading to dynamo action for the mean flow has been found. This solution may be achieved in VKS2, the new sodium experiment to be performed in Cadarache, France. The optimization process is described and discussed, then the effects of adding a stationary conducting layer around the flow on the threshold, on the shape of the neutral mode and on the magnetic energy balance are studied. Finally, the possible processes involved into kinematic dynamo action in a von Karman flow are reviewed and discussed. Among the possible processes we highlight the joint effect of the boundary-layer radial velocity shear and of the Ohmic dissipation localized at the flow/outer-shell boundary.
Attention Patterns in Children with Attention Deficit Disorder with or without Hyperactivity
Gil Zalsman,Orat Pumeranz,Gabriel Peretz,David H. Ben-Dor
The Scientific World Journal , 2003, DOI: 10.1100/tsw.2003.94
Probabilistic Verification over GF(2m) Using Mod2-OBDDs  [PDF]
J.L. Imana
Intelligent Information Management (IIM) , 2010, DOI: 10.4236/iim.2010.22012
Abstract: Formal verification is fundamental in many phases of digital systems design. The most successful verification procedures employ Ordered Binary Decision Diagrams (OBDDs) as canonical representation for both Boolean circuit specifications and logic designs, but these methods require a large amount of memory and time. Due to these limitations, several models of Decision Diagrams have been studied and other verification techniques have been proposed. In this paper, we have used probabilistic verification with Galois (or finite) field GF(2m) modifying the CUDD package for the computation of signatures in classical OBDDs, and for the construction of Mod2-OBDDs (also known as ?-OBDDs). Mod2-OBDDs have been constructed with a two-level layer of ?-nodes using a positive Davio expansion (pDE) for a given variable. The sizes of the Mod2-OBDDs obtained with our method are lower than the Mod2-OBDDs sizes obtained with other similar methods.
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