Skipping rope in the cookie jar

HZDR scientist discovers a staggering phenomenon in liquid metals

Anyone who heats a liquid invariably causes turbulence: the hot fluid rises and mixes with the colder residue, but in some cases, several vortices can combine to form a larger structure, a large-scale circulation. Colleagues at the University of California at Los Angeles (UCLA), with Tobias Vogt (Dresdner Latest News: "Stir Doctor Vogt ") from Dresden-Rossendorf Helmholtz-Zentrum (HZDR) Surprising: In their experience, this structure was moving quite like a skipping rope. The result could help to better understand some phenomena on the solar surface.

Researchers in Dresden and California have been able to demonstrate that the so-called large-scale circulation forms a vortex that moves like a skipping rope. – Image © HZDR, T. Vogt

This is particularly evident in a lava lamp: as soon as its liquid warms up, bubbles form and mix. Experts call this phenomenon a convection temperature. Under certain conditions, thermal turbulence can even meet and form an extended and superordinate structure – large-scale circulation.

An example of this can be found in the cloudy sky. If the wind blows in a certain direction, several cumulus clouds can line up for tens of kilometers, creating a cloudy road. The phenomenon can also be observed in the sun: if you look at its surface through a special telescope, it appears as an accumulation of many grains. This "granulation" occurs by convection: from the bottom up, a warm and bright-looking material rises to the surface, where it cools and sinks to the edge of a grain as a dark material. These grains measure up to 1,000 kilometers and only exist for a few minutes.

"Until now, the experts have badumed that these large-scale circulations were more or less two-dimensional structures," he explains. Tobias Vogt of the HZDR Institute for Fluid Dynamics. "However, our experience now challenges this idea." The starting point was a travel grant that Vogt won in 2016, together with the Helmholtz Doctorate Award. It took UCLA three months. With local experts from the Department of Earth Sciences, Planets and Space, he developed a test setup with which he was able to study in detail large-scale traffic.

The heart of the experiment consisted of a cylindrical container the size of a cookie jar containing liquid gallium, a metal that melts at just under 30 degrees Celsius. "It conducts heat very well and is three times thinner than water," says Tobias Vogt. "The phenomena of convection can therefore appear very clearly." While the bottom of the container could be heated to 70 degrees Celsius, the lid could be cooled to around 30 degrees Celsius. Due to this temperature difference, the liquid metal began to stir: the hot fluid rose and turbulence formed all over the box.

Turbulence like streamers

To observe the event, the team had to use a special technique on ultrasound: "Since gallium is not transparent, laser procedures were out of the question," says Vogt. "Instead, we used a procedure that doctors typically use to make blood flow visible in the vessels." Specifically, the researchers sent short pulses of ultrasound into the container. Depending on the flow, the pulses were reflected differently, which could be measured with sensors. The result was three-dimensional flow profiles of the turbulent liquid metal, supplemented by numerical simulations on a supercomputer.

Jump Rope Swirls: (A) Conditionally averaged 3D visualization of current lines in a cylindrical tank. The current lines surround the kernel of the skipping rope, the color of the current line indicating the local speed. In addition, the color contours in the backplane wafer indicate velocity, and the velocity lines are visualized by full line convolution (LIC). The cycle of the skipping rope is indicated in B, C, D and E. The cross sections are halfway up. The vortex nucleus of the large-scale circulation (LSC) has a minimal speed (pink). In B, the LSC is limited to the median plane. In C, she left the medium plane. The highest speeds (green) in C / E also show the clear division of upward and downward currents. Source: pnas.org/content/early/2018/11/21/1812260115

In all the profiles, large-scale circulations were clearly recognizable – they recall a stream snake, which fills the whole box. "To our surprise, we discovered that this structure looked like a skipping rope," says Vogt. "This is constantly turning – the movement and structure of the large-scale circulation are clearly three-dimensional." This raises doubts about the common theoretical descriptions – that the phenomenon had been treated as a quasi-two-dimensional problem and that it now needed to be re-examined.

Finally, to check if jump rope structures are also formed with other fluids, scientists have launched a series of computer simulations. The result: "This effect also occurs with water," says Tobias Vogt. "But as the water is more viscous than liquid gallium and conducts heat less well, the phenomenon is much weaker."

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