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Jump rope in a jar for cookies

HZDR scientist discovers a striking phenomenon in liquid metals

Anyone who heats the fluid invariably causes turbulence: the hot fluid rises and mixes with the cooler residue. In some cases, several eddies can merge, forming a larger structure – large-scale circulation. Colleagues at the University of California at Los Angeles (UCLA) with Tobias Vogt (Latest Dresdner News: "from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) Surprisingly: in its experiment, this structure is very similar to the rope. The result can help to better understand some of the phenomena on the surface of the sun.

In experiments with liquid gallium, researchers from Dresden and California were able to show that the so-called large-scale circulation forms a vortex that moves like a jump rope. – Image © HZDR, T. Vogt

This is especially noticeable in the lava lamp: as soon as its liquid is heated, the bubbles rise and mix. Experts refer to this phenomenon associated with temperature as convection. Under certain conditions, thermal turbulence can even merge and form an ultra-high, extended structure — large-scale circulation.

An example of this can be found in a cloudy sky. If the wind blows from a certain direction, several cumulus clouds can line up to tens of kilometers – creating a cloudy road. This 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 through convection: from the bottom a hot, bright material rises to the surface, where it cools and descends along the edge of the grain as a dark material. These granules are up to 1000 kilometers in size and exist only a few minutes.

"Until now, experts have assumed that these large-scale circulations are more or less two-dimensional structures," explains Tobias Vogt from the Institute of Fluid Dynamics HZDR. "However, our experiment now calls into question this idea." The starting point was a travel scholarship, which Vogt won in 2016 along with the Dr. Helmholtz Prize. He took it to LA for three months. With local specialists from the Department of the Earth, Planetary and Space Science, he developed a test setup, with the help of which he was able to investigate the large-scale circulation in detail.

The core of the experiment was a cylindrical container about the size of a cookie jar filled with liquid gallium, a metal that melts just below 30 degrees Celsius. “It tolerates heat well and is three times thinner than water,” explains Tobias Vogt. "Thus, convection phenomena can manifest themselves very clearly." Although the bottom of the container can heat up to 70 degrees Celsius, the lid can cool to about 30 degrees Celsius. As a result of this temperature difference, the liquid metal began to mix: hot liquid was rising, and turbulence was forming everywhere in the can.

Turbulence such as streamers

To observe the event, the team had to use a special ultrasound technique: “Since gallium is not transparent, laser procedures are out of the question,” says Vogt. "Instead, we used a procedure that doctors basically use to make blood flow in the vessels visible." In particular, the researchers sent short short pulses of ultrasound into the container. Depending on the flow rate, the pulses were reflected in different ways, which could be measured by sensors. As a result, there were three-dimensional flow profiles of a turbulent liquid metal, supplemented by numerical simulation on a supercomputer.

Passing rope twists: (A) conditionally averaged three-dimensional visualization of streamlines in a cylindrical tank. The current lines surround the core of the skipping rope, with the color of the current line indicating local speed. In addition, the color contours in the middle section indicate the speed, and the speed lines of the line are visualized using linear-integral convolution (LIC). The hopping cycle is shown in B, C, D, and E. Cross sections at half height are shown. The vortex core of large-scale circulation (LSC) has a minimum speed (pink). In B, the LSC is bounded by the median plane. In C she left the middle plane. The highest speeds (green) in C / E also show a clear separation of currents up and down. Source:

In all profiles, large-scale circulations were clearly recognizable – they resemble a hand snake that fills the entire jar. “To our surprise, we found that this structure looks like a jump rope,” explains Vogt. “He was constantly circling — the movement and structure of the large-scale circulation are obviously three-dimensional.” This raises doubts in the general theoretical descriptions, namely, that considered the phenomenon as a quasi-two-dimensional problem and now should be revised.

Finally, to test whether other fluids form the structure of the missing cables, the scientists initiated a series of computer simulations. The result: “This effect also occurs with water,” explains Tobias Vogt. "But since water is more viscous than liquid gallium, and heat is less good, the phenomenon is much weaker."

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