Nav: Home

Understanding stars: How tornado-shaped flow in a dynamo strengthens the magnetic field

May 23, 2017

WASHINGTON, D.C., May 23, 2017 -- The massive, churning core of conducting liquids in stars and some planets creates a dynamo that generates the planetary body's magnetic field. Researchers aim to better understand these dynamos through computer simulations and by recreating them in the laboratory using canisters of rapidly spinning, liquid sodium.

A new simulation based on the von-Kármán-Sodium (VKS) dynamo experiment, run jointly by the French Atomic Energy Commission (CEA), the National Center for Scientific Research (CNRS) and the École Normale Supérieure (ENS) from Paris and Lyon, takes a closer look at how the liquid vortex created by the device generates a magnetic field. Researchers investigated the effects of fluid resistivity and turbulence on the collimation of the magnetic field, where the vortex becomes a focused stream. They report their findings this week in the journal Physics of Plasmas, from AIP Publishing.

The study is the first to examine the flow inside the churning blades at high resolution, and can offer ways to improve laboratory dynamos so that they more accurately recreate stellar astronomical observations.

"We hope that, in the future, we can give a better description of the flows," said lead author Jacobo Varela, now a postdoctoral researcher at the Oak Ridge National Laboratory. "Using this approach, we can begin to understand the dynamo that is observed in the stars."

Dynamos turn kinetic energy into magnetic energy by transforming the rotation of an electrically conducting fluid or plasma into a magnetic field. In the VKS dynamo, two impeller blades on either side of a cylinder filled with liquid sodium create turbulence, which can generate the magnetic field.

The mechanisms that create that field, however, are poorly understood. Other researchers have performed global simulations of sodium dynamos, but the models yielded low-resolution results. This research models the vortex-shaped flow within a small region next to an impeller inside the VKS dynamo.

"The helical flows between the impeller blades collimate the flow that strengthens the magnetic field and generates the field observed in the device," Varela said.

The researchers simplified the device's geometry and built focused magnetohydrodynamic simulations to understand how the flow turbulence and the device's material characteristics affect the magnetic field collimation.

"We found that when you use magnetized ferromagnetic materials, there is an effective increase in the magnetic field collimation, resulting in a lower dynamo threshold, and this is what they observed in the experiment," Varela said.

In contrast, using conducting materials in the simulation weakened field collimation. This finding may explain why researchers can trigger dynamo action in VKS experiments more easily when using soft iron impellers.

The researchers also analyzed their results in the context of the mean-field dynamo theory, which attempts to explain how stars and planets sustain their magnetic fields. As the turbulence increased in the simulation, the magnetic field shifted from a steady 1-to-1 with periodic oscillations, such as the ones observed in certain stars. The magnetic field of the sun, for example, switches polarity approximately every 11 years, which is a product of its turbulence and the speed of its rotation.

Varela and his colleagues at CNRS continue to develop the model to reflect the actual device's geometry. They plan to investigate additional parameters, such as the blade shape and the magnetic field background, so that they can more closely simulate device performance and test ways to optimize the machine.

"The simulation we are performing is just the very first step, but with the model we have now, we can catch a lot of the physics they observe in the VKS dynamo experiment," Varela said. "Our observations and data from the machine will give us much more evidence of the dynamo loop in stars and other astronomical objects."
The article, "Effects of turbulence, resistivity and boundary conditions on helicoidal flow collimation: Consequences for the Von-Kármán-Sodium dynamo experiment," is authored by Jacobo Varela, Sacha Brun, Bérengère Dubrulle and Caroline Nore. The article will appear in Physics of Plasmas May 23, 2017 [DOI: 10.1063/1.4983313). After that date, it can be accessed at


Physics of Plasmas is devoted to the publication of original theoretical, computational, and experimental contributions to the dynamics of gases, liquids, and complex or multiphase fluids. See

American Institute of Physics

Related Magnetic Field Articles:

Massive photons in an artificial magnetic field
An international research collaboration from Poland, the UK and Russia has created a two-dimensional system -- a thin optical cavity filled with liquid crystal -- in which they trapped photons.
Adhesive which debonds in magnetic field could reduce landfill waste
Researchers at the University of Sussex have developed a glue which can unstick when placed in a magnetic field, meaning products otherwise destined for landfill, could now be dismantled and recycled at the end of their life.
Earth's last magnetic field reversal took far longer than once thought
Every several hundred thousand years or so, Earth's magnetic field dramatically shifts and reverses its polarity.
A new rare metals alloy can change shape in the magnetic field
Scientists developed multifunctional metal alloys that emit and absorb heat at the same time and change their size and volume under the influence of a magnetic field.
Physicists studied the influence of magnetic field on thin film structures
A team of scientists from Immanuel Kant Baltic Federal University together with their colleagues from Russia, Japan, and Australia studied the influence of inhomogeneity of magnetic field applied during the fabrication process of thin-film structures made from nickel-iron and iridium-manganese alloys, on their properties.
'Magnetic topological insulator' makes its own magnetic field
A team of U.S. and Korean physicists has found the first evidence of a two-dimensional material that can become a magnetic topological insulator even when it is not placed in a magnetic field.
Scientists develop a new way to remotely measure Earth's magnetic field
By zapping a layer of meteor residue in the atmosphere with ground-based lasers, scientists in the US, Canada and Europe get a new view of Earth's magnetic field.
Magnetic field milestone
Physicists from the Institute for Solid State Physics at the University of Tokyo have generated the strongest controllable magnetic field ever produced.
New world record magnetic field
Scientists at the University of Tokyo have recorded the largest magnetic field ever generated indoors -- a whopping 1,200 tesla, as measured in the standard units of magnetic field strength.
Researchers discover link between magnetic field strength and temperature
Researchers recently discovered that the strength of the magnetic field required to elicit a particular quantum mechanical process corresponds to the temperature of the material.
More Magnetic Field News and Magnetic Field Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

Why do we revere risk-takers, even when their actions terrify us? Why are some better at taking risks than others? This hour, TED speakers explore the alluring, dangerous, and calculated sides of risk. Guests include professional rock climber Alex Honnold, economist Mariana Mazzucato, psychology researcher Kashfia Rahman, structural engineer and bridge designer Ian Firth, and risk intelligence expert Dylan Evans.
Now Playing: Science for the People

#541 Wayfinding
These days when we want to know where we are or how to get where we want to go, most of us will pull out a smart phone with a built-in GPS and map app. Some of us old timers might still use an old school paper map from time to time. But we didn't always used to lean so heavily on maps and technology, and in some remote places of the world some people still navigate and wayfind their way without the aid of these tools... and in some cases do better without them. This week, host Rachelle Saunders...
Now Playing: Radiolab

Dolly Parton's America: Neon Moss
Today on Radiolab, we're bringing you the fourth episode of Jad's special series, Dolly Parton's America. In this episode, Jad goes back up the mountain to visit Dolly's actual Tennessee mountain home, where she tells stories about her first trips out of the holler. Back on the mountaintop, standing under the rain by the Little Pigeon River, the trip triggers memories of Jad's first visit to his father's childhood home, and opens the gateway to dizzying stories of music and migration. Support Radiolab today at