Magnetohydrodynamic flow induced by arrays of rotating permanent magnets

• Experimental investigation of liquid metal (GaInSn) stirring in cylindrical configuration.
• Flow induced by arrays of rotating permanent magnets (localized magnetic fields).
• Free surface oscillations for specific experimental conditions.
• Flow characterization by UDV and image processing.

The main objective of the study is to explore the motion induced by arrays of rotating permanent magnets on a shallow liquid metal layer (GaInSn). We explored arrays from one to five magnets, located underneath the layer, uniformly distributed at a constant distance roro from the rotation axis, the magnet’s diameter (12.7 mm) being much smaller than the inner diameter of the container (191.2 mm). Observations were made at ro=29ro=29 mm and 42 mm and magnet rotation frequency varied from 0.4 to 6.1 Hz. The radial velocity component of the flow was recorded using Ultrasonic Doppler Velocimetry (UDV) while visualization of the free surface deformations was also used to characterize the flow. For magnet arrays placed at the largest radius, a rotating flow with a frequency an order of magnitude smaller than the forcing frequency was observed. The behavior of the flow generated by the array of rotating magnets located at the smaller radius is more complex. For small forcing frequencies, the flow features are similar to those observed with magnet arrays at the larger radius, but for large forcing frequencies, an eccentric rotating vortex can be inferred from the UDV readings. Regions in the layer with large content of kinetic energy which actively participate in the stirring are quantitatively described. It was observed that the free surface is deformed and that the perturbations in the otherwise flat surface rotate with the forcing frequency for discrete intervals of this variable which depends on the number of magnets in the array.