Aug 4, 2017 | By Tess

A team of physicists from the University of Chicago recently achieved a breakthrough in the field of experimental vortex dynamics. Using 3D printed hydrofoils to create thin-core vortices, and with the lucky help of a red Sharpie marker, the team was able to capture measurements of total helicity, a fundamental property of fluid flow, for the first time.

Total helicity, described as “the measure of when two vortex rings intertwine,” had prior to this point been impossible to measure in a laboratory. Now, however, thanks to a happy accident caused by a red marker, UChicago has accomplished the impossible.

Graphic of a helical vortex leapfrogging through a vortex ring in water

(Image: William Irvine / University of Chicago)

As they explain it, they were able to create thin-core vortices, similar to ones found in aircraft wakes or insect flight, by 3D printing hydrofoils. A red Sharpie was unwittingly used to label the hydrofoils, the ink of which became fluorescent when exposed to laser light. This reaction was caused by the rhodamine dye in the marker, which also began to diffuse when the hydrofoil was placed in water.

Within the water tank, and once the hydrofoil was accelerated, the dye from the marker got “sucked into the core of the newly created vortex.” This accidental but innovative method was recorded using a high-speed laser scanning tomography system, which enabled the researchers to measure the helicity of the hydrofoil.

Through their experiments, which were recently published in the journal Science, the physicists found that helicity maintains a constant value during the flow of viscous fluids. "The fact that we have some measurements for the first time that show helicity can be preserved, especially in the presence of stretching, can translate directly to those efforts," explained William Irvine, an associate professor in physics who worked on the project.

The research, helped in part through 3D printing technology, marks an important step forward in vortex dynamics. A relatively new field of study, vortex dynamics has a number of applications that affect our daily lives: meteorologists consider helicity as contributing to the formation of supercell tornadoes, for instance.

As part of the research, the scientists analyzed three types of helicity: writhing, linking, and twisting. "If you take a piece of rope or a telephone cord and you coil it up, then we would say that the center of this rope or telephone cord is writhing," said Irvine. "And if we then took this thing that we coiled up and we pulled it straight, you would see twisting along its length."

Up until now, simulating helicity has remained difficult, and most research has relied on theoretical and hypothetical situations. Thanks to the seemingly innocuous use of a red Sharpie, however, the University of Chicago team was able to discover something new about helicity and vortex dynamics.

Though it is not the first time that dye has been used in similar experiments, the red Sharpie demonstrated a more precise way of measuring the helicity of a hydrofoil. “We hadn't really realized that that was a possibility until we saw dye bleeding off the hydrofoil," said Martin Scheeler, the study’s lead author.

"There really is no playbook, and that's really exciting. You get to try out all sorts of different things, and sometimes it's the wackier stuff that works,” he concluded.



Posted in 3D Printing Application



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