Local velocity measurements in electromagnetically forcedconfined flows

For a physical modelling of hot and chemically aggressive metallic melts in industrial processes, the according experiments are done mostly on water. Besides the fact that the important influence of magnetic fields on the flow cannot be studied at all, the majority of these results are meaningless owing to the non-matching similarity criteria, such as for instance the Prandtl number in the case when thermal gradients are involved. Realistic models rely on low melting point metals the flow of which is not easily measurable by „of the shelf“ equipment.
Rather than giving a systematic review of the measuring techniques available for liquid metal flows, the presentation will concentrate on electric potential probes (EPP) and the up and coming ultrasonic Doppler velocimetry (UDV). These methods will be compared with respect to their applicability and characteristics such as sensitivity, dynamic range, and spatial as well as temporal resolution. Several practically orientated examples have been selected to demonstrate the state of the art performance of both these techniques.
In crystal growth processes rotating magnetic fields (RMFs) are often employed for homogenisation purposes while it is mandatory to keep the flow almost stationary. Well below the threshold of linear stability, the transition from the Stokes-regime to a laminar boundary layer flow was quantified by EPP.
The flow mapping capabilites of UDV have turned out to be a powerful tool for the determination of topological details of the convective pattern.
Scaling laws known from theory may be proved by precision measurements. This was done for the ~Ta2/3-dependence in the RMF by EPP and the ~B-characteristic in a single-phase AC (pulsating) magnetic field by UDV.
Whereas „Kolmogorov-like“ spectra were obtained from the swirling flow, large scale structures with the according low frequency oscillations were found within the recirculation. A video prepared from a sequence of the correlated velocity information contained in each UDV-profile impressively shows the spatio-temporal behaviour of such coherent structures.
To complete with the modern developments in velocity measuring techniques in electrically conduction fluids, the principles of the inverse reconstruction from measurements of the flow-induced magnetic field outside the fluid volume will be outlined. A first benchmark of this method clearly shows its robustness and the ranges of applicability.