  We present hydrodynamic and magneto-hydrodynamic simulations of a liquid
sodium flow using the compressible MHD code PLUTO to investigate the magnetic
field regeneration in the Von-Karman-Sodium dynamo experiment. The aim of the
study is to analyze the influence of the fluid resistivity and turbulence level
on the collimation by helicoidal motions of a remnant magnetic field. We use a
simplified cartesian geometry to represent the flow dynamics in the vicinity of
one cavity of a multi-blades impeller inspired by those used in the
Von-Karman-Sodium (VKS) experiment. We perform numerical simulations with
kinetic Reynolds numbers up to 1000 for magnetic Prandtl numbers between 30 and
0.1. Our study shows that perfect ferromagnetic walls favour enhanced
collimation of flow and magnetic fields even if the turbulence degree of the
model increases. The location of the helicoidal coherent vortex in between the
blades changes with the impinging velocity. It becomes closer to the upstream
blade and impeller base if the flow incident angle is analogous to the TM73
impeller configuration rotating in the unscooping direction. This result is
also obtained at higher kinetic Reynolds numbers when the helicoidal vortex
undergoes a precessing motion, leading to a reinforced effect in the vortex
evolution and in the magnetic field collimation when using again perfect
ferromagnetic boundary conditions. We estimate the efficiency of a hypothetical
dynamo loop occurring in the vicinity of the impeller and discuss the relevance
of our findings in the context of mean field dynamo theory.
