Interpretation of LEVI velocity signals in 3D MHD flows

For experimental determination of velocities in magnetohydrodynamic (MHD) liquid metal flows, probes that measure electric potential gradient are often used. They are known as Liquid-metal Electromagnetic Velocity Instruments (LEVI) or as Conduction Anemometers (CA) and they have been used in the past preferentially for investigating more or less fully developed flows in poorly conducting ducts and flows with smooth variations along the channel axis. For such applications, where electric current density is negligible, the probe was assumed to give reliable results since the potential gradient signals can be directly interpreted as a velocity measure. If the flow varies along its path on very short length scales, like in ducts with abrupt change of cross-section or in manifolds, 3D electric currents may occur that are not negligible any more so that the LEVI readings may become inaccurate. Moreover the presence of the probe itself may introduce already strong perturbations to the flow field since internal layers, which touch the shaft and body of the probe at both sides develop along magnetic field lines. This effect perturbs the flow not only locally in the vicinity of the probe tips but also along an entire region located between the internal layers. The present paper aims at quantifying these effects in order to obtain reliable velocity data.