The Sensitivity to Axial Alignment of a Flow Driven by a Travelling Magnetic Field

The present study, both experimentally and by numerical simulation, concerns the sensitivity of a flow that is driven by a travelling magnetic field (TMF) with respect to a stepwise applied mismatch between the axes of the TMF and of the cylindrical liquid metal column. The TMF is generated by six equidistantly arranged cylindrical coils powered with an AC current having a phase shift of 60° between them. Because the flow induced by a TMF is basically of a torus type, vertical velocity components are representative for the motion in the meridional plane. Ultrasound Doppler Velocimetry (UDV) was applied to acquire the velocity component along the beam axis in the centre of the liquid metal column and at various azimuthal positions for a fixed radial coordinate. The measurements are compared to the flow data gained by large eddy simulations. Computations were based on the usual approximations to simplify the magnetohydrodynamic equations, which are low frequency and low induction, and on an analytical expression for the Lorentz force considering the shift between the fluid volume and the field. The results show that even a small shift between the axes may result in a distinct three-dimensional constituent of the flow structure which changes completely the usually assumed axisymmetric torus-type flow.