title: Role of boundary conditions in helicoidal flow collimation: Consequences for the von Kármán sodium dynamo experiment.
abstract: We present hydrodynamic and magnetohydrodynamic (MHD) simulations of liquid sodium flow with the PLUTO compressible MHD code to investigate influence of magnetic boundary conditions on the collimation of helicoidal motions. We use a simplified cartesian geometry to represent the flow dynamics in the vicinity of one cavity of a multiblades impeller inspired by those used in the Von-Kármán-sodium (VKS) experiment. We show that the impinging of the large-scale flow upon the impeller generates a coherent helicoidal vortex inside the blades, located at a distance from the upstream blade piloted by the incident angle of the flow. This vortex collimates any existing magnetic field lines leading to an enhancement of the radial magnetic field that is stronger for ferromagnetic than for conducting blades. The induced magnetic field modifies locally the velocity fluctuations, resulting in an enhanced helicity. This process possibly explains why dynamo action is more easily triggered in the VKS experiment when using soft iron impellers.
authors.affiliation: AIM, CEA/CNRS/University of Paris 7, CEA-Saclay, 91191 Gif-sur-Yvette, France.|Laboratoire d'Informatique pour la Mécanique et les Sciences de l'Ingénieur, LIMSI, CNRS, Université Paris-Sud, Université Paris-Saclay, Bât 508, Campus Universitaire F-91405 Orsay, France.
authors.forename: J
authors.initials: J
authors.lastname: Varela
authors.affiliation: AIM, CEA/CNRS/University of Paris 7, CEA-Saclay, 91191 Gif-sur-Yvette, France.
authors.forename: S
authors.initials: S
authors.lastname: Brun
authors.affiliation: SPEC/IRAMIS/DSM, CEA, CNRS, University Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France.
authors.forename: B
authors.initials: B
authors.lastname: Dubrulle
authors.affiliation: Laboratoire d'Informatique pour la Mécanique et les Sciences de l'Ingénieur, LIMSI, CNRS, Université Paris-Sud, Université Paris-Saclay, Bât 508, Campus Universitaire F-91405 Orsay, France.
authors.forename: C
authors.initials: C
authors.lastname: Nore