Interaction of electromagnetic fields with electrically conducting fluids constitutes the basis for several technological applications and natural phenomena. For instance, electromagnetic forcing is helpful in the exploration of the fundamental flow phenomena including instabilities, turbulence, and mixing. In addition, conducting fluids driven by electromagnetic forces are of great interest in modeling flows of astrophysical and geophysical relevance. This is due to the potential of generating and tuning motion without interfering with mechanical devices.
Electromagnetic forces generally appear due to the interaction of a magnetic field with electric currents, either induced or applied. As a result, electrically conducting fluids (electrolytes or liquid metals) exposed to an external magnetic field can be stirred by applying an electric current. In liquid metals, owing to their high conductivity, electromagnetic forces can be generated by time varying magnetic fields. In most industrially relevant situations these flows are characterized by small values of the magnetic Reynolds number indicating that the induced magnetic field is much smaller than the field applied. Previously, researchers used Lorentz forces to produce three-dimensional swirling tornado-like flows in liquid metals and electrolytes.
Moreover, in geophysical applications, rotating flows have been extensively studied, and magnetohydrodynamic flows were found to be appropriate for modeling large-scale oceanic and atmospheric flow structures. Experimental observations of an azimuthal electrolyte flow driven by Lorentz force in a thin annular fluid layer positioned on top of a magnet indicate that it generates a robust vortical pattern close to the outer cylindrical wall. This is a result of previously undocumented instabilities developing on a background of steady axisymmetric flow.
Associate Professor Sergey Suslov from Swinburne University of Technology in Australia in collaboration with Dr. James Pérez-Barrera and Dr. Sergio Cuevas at National Autonomous University of Mexico set up a scene for a future comprehensive stability study of these flows. They discussed in the study the depth averaged and quasi-two-dimensional approximate solutions taking advantage of the thin-layer assumption. However, the researchers found that such approximations were easily invalidated when the layer thickness increased and could not adequately define even qualitative features of the flow. While the flow remained axisymmetric it became three-dimensional, and had to be treated as such by employing the necessary numerical algorithms. Only fully three-dimensional solutions could shed light on experimentally observed flow instabilities.
The authors confirmed that when working with electrolytes, the assumptions of a fixed logarithmic electric potential distribution between the electrodes and those of free surface remaining flat could be used satisfactorily without the inherent danger of misinterpreting the observed flows. For this reason, when modelling experimentally observed formation of vortices that appear close to the outer cylinder, the stability of the three-dimensional axisymmetric flat-surface flows described in this study can be used satisfactorily. These research findings were published in the Journal of Fluid Mechanics and the experimental image of the explored flow instability was the Editor’s choice for the cover of the October 2017 issue.
Sergey A. Suslov, James Pérez-Barrera and Sergio Cuevas. Electromagnetically driven flow of electrolyte in a thin annular layer: axisymmetric solutions. Journal of Fluid Mechanics, volume 828 (2017), pages 573–600.
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