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Pioneering insights into viscous fluid flow

Volumetric image of a helical vortex leapfrogging through a vortex ring in water, with dye-blob tracks overlaid in warm colors, credit: William Irvine, University of Chicago
Volumetric image of a helical vortex leapfrogging through a vortex ring in water, with dye-blob tracks overlaid in warm colors, credit: William Irvine, University of Chicago

An ‘elusive yet fundamental’ property of fluid flow has been documented for the first time, by researchers from the University of Chicago.

The findings could eventually go on to be hugely important in a range of fluid handling applications, as they offer valuable insight into viscous fluid flow. By showing that the total helicity (the measure of when two vortex rings intertwine) maintains a constant value during the flow of viscous liquids, the University of Chicago physicists have shown that it is possible to measure that helicity.

Luck played a role in the team’s discovery. The scientists created thin-core vortices - the kind found in aircraft wakes and insect flight – by producing hydrofoils using a 3D printer. By chance, the red Sharpie marker used to label the hydrofoils contained rhodamine dye, which fluoresced when illuminated by laser light. When the hydrofoils were placed in a water tank, the dye began to diffuse, and when the hydrofoil was accelerated, the dye got sucked into the core of the newly created vortex. This process was recorded via high-speed laser scanning tomography.

"We hadn't really realised that that was a possibility until we saw dye bleeding off the hydrofoil," said Martin Scheeler, the study's lead author.

Until now, scientists have found it extremely difficult to measure helicity in viscous fluid flow, with previous research being largely theoretical and relying on hypothetical, simpler fluids lacking in viscosity. Although calculations showed that helicity was conserved in the hypothetical tests, viscosity emerged as a significant factor in actual flow of fluids.

"One of the core problems is that you need to sample or measure features of the flow that exist on very different length scales," said Scheeler.

Those scales range from the diameter of a vortex (approximately 30cm) to the diameter of its thin core (around 1mm).

“You need to measure the flow inside the core as well as the overall shape evolution of that vortex,” said William Irvine, a co-author of the study.

The findings could prove hugely influential in a range of applications. Meteorologists, for example, view helicity as a factor that contributes to the formation of supercell tornadoes. It could also influence the pumping of viscous fluids, and their flow measurement, alongside other fluid handling applications.

The teams research has been published in the journal Science.

 

Volumetric image of a helical vortex leapfrogging through a vortex ring in water, with dye-blob tracks overlaid in warm colors, credit: William Irvine, University of Chicago