Steady and Unsteady Vortex Flow Generated by Electromagnetic Forcing - Selected Topics of Computational and Experimental Fluid Mechanics - page 410

form current within the range of 3-200mA circulates in the fluid. The magnetic field
is produced by Neodymium permanent dipolar magnets of cylindrical shape with
a diameter of 0.92 cm and 0.63 cm of height, axially magnetized with a maximun
magnetic strength on their surface of 0.33T. Magnets are located below the bottom
wall of the container in arrays of three to ten magnets placed equidistantly on the
perimeter of a circle with diameter of 7.5 cm, so that for each array, the magnets rest
on the vertices of a regular polygon.
Figure 1 a, b shows schematically the top and lateral views of the experimental
setup, respectively. For illustration, only two magnets with opposite orientations are
shown, the normal component of themagnetic field being perpendicular to the bottom
wall. The interaction of the current and the non-uniform magnetic field originates
Lorentz forces in the fluid that are perpendicular to both the current direction and
the normal component of the magnetic field. Evidently, the direction of the force
depends on the orientation of the magnets and the polarity of the electrodes. We
now show different arrays of magnets that give rise to flow patterns with varying
degrees of complexity. Figure 2 a shows schematically an array of four magnets with
alternated orientation, north orientation being represented by blue solid circles and
south orientation by red solid circles. Dipolar vortices generated by electromagnetic
forcing are represented by curved lines with the arrows indicating the direction of
rotation. Figure 2 b shows the experimental visualization with dye of the steady flow
obtained for this configuration with a current of 10 mA. Note that exterior vortices
remain mainly unaffected while interior vortices interact strongly. Figure 3 ashows
the experimental visualization of the steady flow pattern obtained by applying an
electric current of 3mA to an array of three magnets with north orientation placed at
the vertices of a equilateral triangle inscribed in the circle whose center is denoted by
a cross. Since the separation between magnets is large and the electric current rather
small, dipolar vortices created by each magnet persist mainly independently without
Fig. 1 Sketch of the experimental device used to investigate the interaction of vortices driven by a
D.C. uniform current and localized magnetic fields produced by different distributions of permanent
magnets. a Top vi ew. b Lateral view