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Accessible ranges of turbulent and transitional flow in electromagnetic levitation experiments
A. K. Pauls, G. P. Bracker and R. W. Hyers
Electromagnetic Levitation in reduced gravity has demonstrated great utility in the measurement of thermophysical properties and the study of solidification of metallic melts. Internal flow in the liquid metal samples is a critical parameter in many of these experiments. For example, turbulent flow in the sample prevents measurement of viscosity by the oscillating droplet method, as the measured damping of oscillations is a property of the flow and not of the fluid. In solidification, it has been demonstrated that internal flow changes the lifetime of the metastable phase in stainless steels by almost two orders of magnitude over the experimentally accessible range. For these experiments, the flow was quantified by CFD models; however, these models require as an experimental input whether the flow is laminar or turbulent.
To date, the only study of the transition to turbulence in EML comes from experiments on the Space Shuttle in 1997 on MSL-1 TEMPUS on a palladium-silicon alloy. On each melting cycle, tracer particles reveal the laminar or turbulent nature of the flow. However, this phenomenon was only noticed and examined after the flight, so observations are available only for a very narrow range of conditions. As these results were the only ones available, they have been boldly extrapolated to conditions far from the original experiment. Furthermore, while evidence from measurements on germanium shows that the positioner alone can drive turbulent flow, there are no available measurements of the turbulent transition in positioner-dominated flows.
On a more fundamental level, EML flows are constrained by the free surface of the drop, like the walls of internal flows. However, the free surface of the EML drop does not support shear stresses, unlike the walls in internal flows. Better quantification of the transition to turbulence in EML flows may lead to insights into the nature of turbulence and turbulent transition.
Closing this gap requires a combination of experiments and models that will quantify the turbulent transition over as wide a range of experimental conditions as possible in ISS-EML. Modeling results are presented here; experiments are planned for ISS-EML Batch 4, estimated for 2024.
Keywords: electromagnetic levitation, magnetohydrodynamics, turbulent transition
DOI: 10.32908/hthp.v52.1331