Significance
Under magnetic fields, the formation and evolution of jets is usually a common phenomenon that presents an important problem in modern astrophysics. For example, it is speculated that cosmic gamma-ray bursts are a result of explosions characterized by the formation of thin jets. From the magnetohydrodynamics (MHD) perspective, jet flows are characterized by a high magnetic Reynolds number representing the ratio between the advection and diffusion flux of the magnetic field. As a result, the effects of MHD on fluid jets are also a key consideration in various fields like nuclear fusion, though on smaller scales.
Magnetic fields have considerable effects on jet flows. Jets spread over a large distance under the axial magnetic field than without magnetic fields. In addition, magnetic fields delay the transition to turbulence due to their strong stabilizing effect. The jet profiles become more unsteady in the presence of magnetic fields due to the generation of traveling waves by the interaction of magnetic fields and secondary radial flows. Besides axial fields, transverse fields have also drawn more research attention, especially in continuous casting (CC). Nevertheless, only a limited amount of theoretical and experimental studies of submerged jets have been reported despite their practical significance.
Although the impact of magnetic fields has been investigated, the role of the transverse magnetic field in reducing velocity fluctuation is yet to be confirmed. Industrial investigations have also shown that not all magnetic fields produce quality metals. Further investigations have also been carried out to clarify the underlying physical mechanisms of jet flows. Most importantly, the theoretical analysis of low magnetic Reynolds number has revealed the development of long thin sheetlike structures along the direction of the magnetic as well as the occurrence of reverse flows. The existence of reverse flows has also been confirmed through continuous casting and large eddy simulations. However, the flow characteristics of submerged jets, including the occurrence of the thin sheetlike structures, are yet to be fully clarified.
To this note, Austrian researchers from University of Leoben (Montanuniversität Leoben): Associate Professor Abdellah Kharicha, Dr. Alexander Vakhrushev, Dr. Ebrahim Karimi-Sibaki, Professor Menghuai Wu and Professor Andreas Ludwig studied the occurrence of reverse flow and flattening of submerged jet under transverse uniform magnetic field. The flow characteristics for Reynolds (Re < 4500) and moderate interactions (N < 0.1) were numerically simulated. The original research work is currently published in the journal, Physical Review Fluids.
The research team findings revealed more complex phenomena than the widely expected MHD damping effects, which was in agreement with the previous predictions. The inlet round jet underwent anisotropic flattening along the direction of the magnetic field, while reverse jet flow zones were formed adjacent to the main jet. Even at large distances of ~800d, the decay rates and jet dynamics remained very sensitive to external electrical boundary conditions. Whereas the small-scale turbulent fluctuations were suppressed, the magnetic field formed within the shear region aligned with the large coherent vortices at the boundary between the main and adjacent reverse jets.
These fundamental findings have direct industrial applications, for instance in the continuous casting of steel, where the applied electromagnetic braking (EMBr) is used as a common flow control technique. It was revealed by the authors, that a typical double-roll flow in the CC mold can be transformed to the opposite single roll pattern with the growth of the EMBr power and consequent Hartmann numbers. These reverse zones develop from the jets towards a free-surface and occupy the top region of the CC mold in a form of an undesired reverse meniscus flow.
In summary, the Austrian study numerically investigated the spatial revolution of submerged jet flow in transverse magnetic fields. It was clear from their results that a viscous stress increase or porous does not provide a full explanation of the electromagnetic field-induced momentum redistribution. The full explanation was only achieved by taking into account the loops created by the induced electric current. In a statement to Advances in Engineering, Dr. Abdellah Kharicha explained their findings advance our understanding of submerged jet flow frequently observed in metallurgical industries where magnetic fields are used to control the liquid jets. A complex MHD phenomenon, happening in the CC mold, is currently investigated, aiming to assist the EMBr systems development for the metallurgical applications.
References
Kharicha, A., Vakhrushev, A., Karimi-Sibaki, E., Wu, M., & Ludwig, A. (2021). Reverse flows and flattening of a submerged jet under the action of a transverse magnetic field. Physical Review Fluids, 6(12), 123701-123715.
Vakhrushev, A., Kharicha, A., Karimi-Sibaki, E., Wu, M., Ludwig, A., Nitzl, G., Tang, Y., Hackl, G., Watzinger, J., & Eckert, S. (2021). Generation of Reverse Meniscus Flow by Applying an Electromagnetic Brake. Metall Mater Trans B, 52, 3193–3207.