Generation of isolated vortices in a rotating fluid by means of an electromagnetic method

We present a method for generating isolated monopolar vortices in rotating tank experiments. The technique is based on the electromagnetic forcing commonly used in nonrotating systems, which consists of setting a vertical magnetic field—parallel to the rotation axis—and a horizontal density current in an electrolytic fluid layer. The magnetic field is provided by a permanent magnet placed underneath the central point of the fluid container, while a radial density current is established between a central electrode and a number of opposite-sign electrodes at the periphery. The resulting azimuthal Lorentz force creates a monopolar vortex. It is shown that the generated vortices are axisymmetric and isolated, that is, their total circulation is zero. Cyclonic or anticyclonic vortices can be generated by choosing the appropriate polarity of the electrodes or the orientation of the magnet. The strength of the vortices is regulated by the magnitude of the density current and by the forcing time. This method allows the systematic study of the unstable evolution of isolated vortices, which is characterized by the formation of multipolar vortices.