Electromagnetic stirring

It is well known that an alternating magnetic field (either single phase or multiphase) applied to a conductor, whether solid or fluid, will induce electric currents in the conductor, and hence a Lorentz force distribution. This Lorentz force is in general rotational, and if the conductor is fluid, it is set in motion. Thus the magnetic field acts as a nonintrusive stirring device and it can, in principle, be engineered to provide any desired pattern of stirring. Stirring may also be effected through the interaction of a steady current distribution driven through a fluid and the associated magnetic field. When the field frequency is high, the Lorentz force is confined to a thin electromagnetic boundary layer, and the net effect of the magnetic field is to induce either a tangential velocity or a tangential stress just inside the boundary layer. The distribution of velocity or stress is related to the structure of the applied field. Symmetric configurations may lead to patterns of stirring in which the streamlines lie on toroidal surfaces; more generally, however, the streamline pattern is chaotic. Examples of both types are presented, and applications are discussed. A somewhat related problem arises if electrically conducting particles are suspended in a nonconducting fluid, the whole being subjected to the same type of alternating magnetic field. Currents are now induced in the particles, and each particle experiences a force and a couple. When several particles are in suspension, the movement of each is influenced by the presence of the others, and a problem somewhat analogous to the problem of interacting point vortices arises. For a suspension of conducting particles, inhomogeneity of the field leads to inhomogeneity of the concentration of particles, and a bulk flow is, in general, induced. The general principles underlying this behavior are briefly summarized.