Parametric study and optimization trends for the Von-Kármán-Sodium dynamo experiment

Abstract

We present magneto-hydrodynamic simulations of liquid sodium flow with the PLUTO compressible MHD code. We investigate the influence of the remnant magnetic field orientation and intensity, impinging velocity field due to Ekman pumping as well as the impeller dimensions on the magnetic field collimation by helical flows in between the impeller blades. For a simplified cartesian geometry we model the flow dynamics of a multi-blades impeller inspired by the Von-Karman-Sodium (VKS) experiment. The study shows that a remnant magnetic field oriented in the toroidal direction is the less efficient configuration to collimate the magnetic field, although if the radial or vertical components are not negligible the collimation is significantly improved. If the intensity of the remnant magnetic field increases the system magnetic energy is larger but the magnetic field collimation efficiency is the same, so the gain of magnetic energy is smaller as the remnant magnetic field intensity increases. The magnetic field collimation is modified if the impinging velocity field changes: the collimation is weaker if the impinging velocity increases from Gamma=0.8 to 0.9, slightly larger if the impinging velocity decreases from Gamma=0.8 to 0.7. The analysis of the impeller dimensions points out that the most efficient configuration to collimate the magnetic field requires a ratio between impeller blade height and base longitude between 0.375 - 0.5. The largest enhancement of the hypothetical alpha^2 dynamo loop, compared to the hypothetical Omega-alpha dynamo loop, is observed for the model that mimics TM 73 impeller configuration rotating in the unscooping direction with a remnant magnetic field of 10^-3 T orientated in the radial or vertical direction. The optimization trends obtained in the parametric analysis are also confirmed in simulations with higher resolution and turbulence degree.