# The role of boundary conditions on helicoidal flow collimation: consequences for the Von-K\'arm\'an-Sodium dynamo experiment

## TL;DR

This study uses simulations to explore how magnetic boundary conditions influence helicoidal flow collimation in liquid sodium, revealing effects on magnetic field enhancement and implications for dynamo experiments.

## Contribution

It demonstrates how magnetic boundary conditions affect flow collimation and magnetic field amplification in a model of the VKS experiment, highlighting the role of impeller material.

## Key findings

- Helicoidal vortices form at impeller blades, influenced by flow incident angles.
- Magnetic field lines are collimated by vortices, enhancing radial magnetic fields.
- Ferromagnetic blades produce stronger magnetic field effects than conducting blades.

## Abstract

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.