Publications

We present results from numerical reconstructions of magnetic obstacle experiments performed in liquid metal flows. The experimental setup consists of an open rectangular container filled with a thin layer of liquid metal (GaInSn). A permanent magnet is installed on a rail beneath the container and is moved with a constant velocity U0, which in turn induces a flow inside the liquid metal due to Lorentz forces. The setup allows experiments in a parameter range that is accessible by direct numerical simulations (DNS). We present results from realizations with four different parameter sets, covering flows with stable stationary vortex structures in the reference system of the moving magnet as well as time-dependent flow regimes. Although the liquid metal layer is very thin, the flow shows a highly three-dimensional character in the near and in the far wake of the magnetic obstacle. We conclude that the streamline visualization in the experiment (using gas bubbles at the surface of the liquid metal layer) is insufficient to picture the flow structure occurring in the liquid metal. To underpin our conclusions, we introduce a modified numerical model which aims to mimic the movement of these gas bubbles. Although this model is a strong simplification of the highly complicated behavior of bubbles at a fluid-fluid interface, it captures the main effects and provides a good reproduction of the experimental results. Furthermore, transient effects are investigated when the flow is initiated, i.e., when the magnet approaches the container and crosses its front wall.We conclude that the process of vortex formation is accompanied by a decrease of the streamwise component of the Lorentz force compared to the time when the fluid is still quiescent. This decrease occurs only for flows with stable vortex structures, which might be of interest for practical applications like Lorentz force velocimetry. The Lorentz forces obtained from our DNS are in good agreement with the values measured in experiment.

Transition in a pressure driven magnetohydrodynamic duct flow, subjected to a uniform transverse magnetic field is studied with direct numerical simulations. The electric boundary conditions correspond to a Hunt's flow regime, with perfectly insulating side walls and perfectly conducting Hartmann walls. The flow forms strong side wall jets at already moderate fields, represented here by Hartmann number Ha = 100. While increasing Reynolds number Re, various unsteady regimes are identified: Ting-Walker (TW) vortices at the side walls, elongated vortices, jet detachment and finally, fully turbulent side-wall jets. We have also found hysteresis behaviour in a broad range of Re.

We present results from numerical reconstructions of magnetic obstacle experiments in which a moving permanent magnet drives a free-surface flow in a container filled with liquid metal (GaInSn). The results cover different flow regimes, such as stationary (in the reference system of the moving magnet) so-called six vortex structures, as well as time-dependent flows. The numerical results show that - although the liquid metal layer is thin - the flow structure is highly three dimensional. We conclude that the experimental technique for streamline visualization (gas bubbles at the surface of the liquid metal) is insufficient to picture the occurring flow structure. To underpin our conclusion, we introduce a modified numerical model that aims to mimic the movement of the gas bubbles. The results of the modified model and the experiments are in excellent agreement. Furthermore, Lorentz forces obtained from simulations match well with those from the experiments.

A streamwise magnetic field leads to turbulent drag reduction in channel flow of a conducting liquid due to the selective Joule damping of certain flow structures. Near the walls, the turbulent mean velocity profile retains the logarithmic layer but the von Kármán constant decreases with increasing magnetic field strength. In the outer region, the flow is characterized by persistent streaky structures of large streamwise extent, which lead to a rather flat mean velocity profile. In addition, the streamwise velocity fluctuation profiles develop a pronounced second peak upon increasing the magnetic induction as well as a second logarithmic layer that increases in steepness.