Flow control by tailored magnetic fields

Electromagnetic fields provide a tool to influence the flow and, in turn, the heat & mass transfer in electrically conducting fluids. An attractive feature for metallurgical or crystal growth applications is the contact-less form of this control. There have been many studies in the past analysing the changes in the transport phenomena which take place by application of some type of magnetic field. In general, steady magnetic fields suppress flows and alternating magnetic fields drive some motion. The variety of magnetic field actions is very big which allows for an inverse approach: a pre-defined flow control is possible by tailored magnetic field systems. There is a basic fluiddynamic interest in such type of flow control, but applications in crystal growth, solidification, metal casting, welding, seawater flow control and others are very close, too.
On the other hand, there is a growing community dealing from a theoretical and computational point of view with optimization, optimal or sub-optimal control and flow control in general. We expect that Magnetohydrodynamics (MHD) establishes an interesting example for it as it provides an active, well-controllable influence on the flow which can directly be tested in experiments. The flow itself is often not of direct interest but acts as a kind of intermediate agent for more general goals like the resulting heat & mass transfer, the resulting microstructure in solidification, or integral results as drag of lift. Hence, it is in many cases a highly non-trivial question which flow field might be a desirable one for the more general objectives of the various processes.
Such type of inverse MHD approach has not yet been realized on its full scale, i.e., starting from a theoretical optimization problem and its numerical implementation up to the experimental demonstration. However, several examples exist of partly addressing this approach, and some of them will be shortly presented: Cz-Si crystal growth with AC and DC magnetic fields, aluminum investment casting with magnetic field control, tailored DC field stabilization of the melt extraction process for metallic fibres, electromagnetic levitation with DC field sample stabilization, seawater flow control for drag reduction and lift enhancement, and float-zone crystal growth with a tailored magnetic field control in order to shape the solid-liquid phase boundary. The latter case will be presented in more detail demonstrating its capabilities for the float-zone crystal growth and solidification studies with NdFeB alloys.