Experimental Investigation of Orbitally Excited Interfacial Waves in Two- and Three-Layer Systems
Not least due to their high practical importance interfacial instabilities have a leading role in the wide field of Magnetohydrodynamics. It is known for decades that dangerous interfacial waves can be excited between liquid aluminum and the cryolith floating on above during the aluminum production process within so-called Hall-Héroult reduction cells. This instability is driven by a complex interaction of the strong electrolysis currents with external magnetic fields.
In the last years, very similar interfacial instabilities were observed also in liquid metal batteries, which consist, in contrast to aluminum reduction cells, of three stratified fluid layers that can be hydrodynamically coupled. While both systems are extensively studied numerically and analytically, the interfacial wave dynamics is still lacking of insufficient experimental validation.
For that reason, we have constructed a new experimental set-up to investigate the dynamics of interfacial wave in multi-layer fluid systems. Due to the fact that it is very difficult to excite interfacial waves electromagnetically in lab-size experiments, we used an orbital shaker that can mechanically excite exactly the same rotational wave modes occurring in circular aluminum reduction cells and liquid metal batteries by performing steady circular motions.
A modified version of the Kuhner Shaker LS-X is used for enabling a continuous adjustment of the shaking amplitude up to 70 millimeters. Different cylinders made of acrylic glass can be installed on the shaking table. Various combinations of silicon oils, liquid metals and water solutions are available to realize different two- or three-layer stratifications within the cylinder providing distinct interfaces. The cylinder can be enclosed by differently sized caps allowing to modify the aspect ratio of the observation volume. For measuring interfacial waves amplitudes we used the acoustic measurement technique Ultrasound Doppler Velocimetry (UDV) allowing to track the wave motion in optically inaccessible liquid metals as well. To facilitate dual measurements from both sides, ten holes for UDV-sensors are engraved in the top and bottom enclosing caps allowing to reconstruct the wave's amplitudes as well as the excited waves modes by tracking the reflected ultrasound echos.
Interface deflections can be detected as well by using the background-oriented schlieren technique (BOS or "synthetic schlieren"), which provides information about the deformation of the entire interface. This technique was recently used to detect spiral wave structures in a strongly damped system for the first time. The structures found experimentally confirmed the high accuracy of the theoretical prediction.