Melt flows in combined rotating and traveling magnetic fields

The liquid metal flow in a cylinder driven by poloidal and azimuthal magnetic body forces is studied experimentally. For this purpose we apply the ultrasound Doppler velocimetry in the bulk and a particle image velocimetry on the free surface. The body forces are generated by axial traveling and rotating magnetic fields of considerably different frequencies. The transition between poloidal to azimuthal force governed states proceeds in two intermediate stages. At first, a pronounced swirl enhancement and concentration is observed. As the azimuthal forcing is further increased the axial vorticity accumulates on a ring inside of which the poloidal flow changes its direction. This transition occurs at a surprisingly low azimuthal forcing of just about one per cent of its poloidal counterpart. The two characteristic velocity components, in turn, have comparable magnitude. The disproportionality of both force strengths is, thus, a consequence of a considerably higher hydraulic resistance for the poloidal flow. The observed swirl concentration during the regime change is explained by the phenomenon of vortex stretching.