Fluxes of molten metal can generate magnetic fields. This so-called dynamo effect creates cosmic magnetic fields, like those found on planets, moons, and even asteroids. In the coming years, researchers are conducting a unique experiment in which a steel drum containing several tons of liquid sodium rotates around two axes to demonstrate this effect. It will be carried out in the new DRESDYN facility at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), an independent German research laboratory. A study recently published in Physical examination letters, reports the chances of success of the experiment.
Similar to a bicycle dynamo converting motion into electricity, conductive fluids in motion can generate magnetic fields. The so-called magnetic Reynolds number (the product of fluid flow velocity, expansion, and conductivity) primarily determines whether a magnetic field is actually generated.
During the experiment, scientists in Frank Stefani’s team at the HZDR Institute for Fluid Dynamics aim to achieve the critical value required for the appearance of the dynamo effect. For this, a steel cylinder two meters in diameter containing eight tons of liquid sodium will rotate around an axis up to 10 times per second and once per second around another axis which is inclined relative to the first . The technical term for this movement, which is often compared to a tilting top, is precession.
“Our experience in the new DRESDYN installation aims to demonstrate that precession, as a natural motor of flow, is sufficient to create a magnetic field”, explains AndrÃ© Giesecke, lead author of the study. In its simulations and in the accompanying water experiments using a mockup six times smaller than the experimental setup, the scientists examined the structure of the flow caused by the precession.
âTo our surprise, we observed a double-role symmetrical structure in a specific range of the precession rate, which should provide a dynamo effect at a magnetic Reynolds number of 430,â explains the physicist.
The center of the Earth is made up of a solid core surrounded by a layer of molten iron. âThe molten metal induces an electric current, which in turn generates a magnetic field,â explains Giesecke. The common belief is that the convection driven by buoyancy, as well as the rotation of the Earth, is responsible for this geodynamo. However, the role played by precession in the formation of the Earth’s magnetic field is still unclear.
The Earth’s axis of rotation is tilted 23.5 degrees from its orbital plane. The axis of rotation changes position over a period of approximately 26,000 years. This precessive movement through space is considered to be one of the possible sources of energy for geodynamo. Millions of years ago, the moon also had a strong magnetic field, as indicated by rock samples from the Apollo missions. According to experts, precession could have been the main cause.
The experiments on liquid sodium at the HZDR are expected to start in 2020. Unlike previous experiments in the geodynamic laboratory, there will be no propeller inside the steel drum, as was used in the first successful experiment. dynamo event in Riga, Latvia, in 1999, in which scientists from the HZDR participated. This experience and others in Karlsruhe, Germany and Cadarache, France provided groundbreaking research for a better understanding of geodynamo.
âIn principle, we can define three different parameters for the experiments at DRESDYN: the rotation, the precession and the angle between the two axes,â explains Giesecke. He and his colleagues hope to get answers to the fundamental question of whether precession actually produces a magnetic field in a conductive fluid. In addition, they are interested in finding out which components of the flux are responsible for creating the magnetic field, and the point at which saturation occurs.
Dual role in the Container
âIn the simulations, we found that standing inertia waves occur over a wide range of parameters. Within a certain range, however, we have now noticed a characteristic dual-role structure that is proving to be extremely effective for the dynamo effect In principle, we are already aware of such a speed structure thanks to the French dynamo experiment, in which it was artificially produced by two propellers, while in our precession experiment it should emerge naturally. “
HZDR researchers used special ultrasound technology to measure the structure of the flow. âWe were very surprised to see how closely the data from the experiment matched the results of the simulation. So we have an extremely robust prediction for the big DRESDYN experiment. For example, we know at what rotational speeds the dynamo effect occurs and what magnetic field structures we can expect, âsays Giesecke.
The scientific community involved in the dynamos is eagerly awaiting the results of the planned experiment, which will run to the limits of technical feasibility in many ways. “We are also awaiting detailed information on the general dynamics of liquid metal fluxes under the influence of magnetic fields. This will allow us to draw conclusions about fluxes in the industrial sector,” according to Giesecke.
Finally, the magnetic flux tomography developed at HZDR as part of its research on dynamo is of interest to many fields of steel foundry and crystallization.
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AndrÃ© Giesecke et al, Large-scale nonlinear flow in a precession cylinder and its ability to drive dynamo action, Physical examination letters (2018). DOI: 10.1103 / PhysRevLett.120.024502
Quote: The occurrence of magnetism in the universe (March 13, 2018, retrieved October 30, 2021 from https://phys.org/news/2018-03-occurrence-magnetism-universe.html
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