  We present hydrodynamic and magneto-hydrodynamic 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 multi-blades impeller inspired by those used in the
Von-K\'{a}rm\'{a}n-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.
