Faced With A Crossroads, Water Forms A Spiral Vortex

When two flows of water meet within a cross-shaped device, they behave in an intriguing way.

AsianScientist (Apr. 29, 2016) – In many plumbing and pipework systems, there are junctions and connections to move liquids such as water in different directions. But have you ever thought about what happens to the water in those fluid intersections?

A team of researchers from Okinawa Institute of Science and Technology Graduate University (OIST) and collaborators have found an unexpected spiral vortex phenomenon that occurs in the intersections of cross-shaped devices when liquid flows through the channels in a particular way. The team has published their results in Physical Review E.

The team moved water through a cross-shaped device by simultaneously pumping water into two channels facing each other, so that they met at an intersection. When the two streams of water meet at the intersection they are compressed by the force of the flow and then extended by being pulled out through the remaining two pathways, creating a spiral vortex in the water.

This is an example of the cross-shaped device that the team pumped water through. The arrows show which way the water was flowing, which side included dye, and where the spiral vortex forms in the middle. Credit: OIST
This is an example of the cross-shaped device that the team pumped water through. The arrows show which way the water was flowing, which side included dye, and where the spiral vortex forms in the middle. Credit: OIST

The researchers could visualize this by inserting fluorescent dye into the water that entered from one of the two pathways. Also, the team was able to perform numerical simulations that could predict the spiral vortex phenomenon computationally.

When the flow starts up, the spiral forms, spinning either clockwise or counter clockwise with equal probability. Through the combinations of the experiments and numerical simulations, the scientists were able to classify this behavior as an unusual kind of flow instability.

“On the surface it is a really simple geometry,” said author Professor Amy Shen of the Micro/Bio/Nanofluidics Unit at OIST. “But no one has ever captured or visualized such striking flow structures like this before.”

“We are starting to think that this kind of instability would exist in any kind of intersecting geometry,” said first author and group leader, Dr. Simon Haward.

The team discovered that the spirals appear as the flow rate is increased and disappear again if the flow rate is decreased, as shown in the OIST video below. However, the flow rates corresponding to appearance and disappearance are not necessarily the same.

By varying the aspect ratio of the channel, or the depth of the channel divided by its width, they saw differences in how the spiral formed and collapsed. Specifically, with a smaller aspect ratio, the spiral would form and collapse again at the same flow rate. But, in devices with large aspect ratios, the spiral collapsed at a lower flow rate than that at which it had formed, exhibiting what is called hysteresis.

Interestingly, the team identified a very specific value of the aspect ratio, called a tri-critical point, and the growth and collapse of the spiral vortex happens very suddenly over a narrow range of flow rate. All of this information could be useful in understanding and optimizing basic fluid transport processes, predicting fluid instability conditions, and enhancing the mixing of fluids.

“In any channel, we can predict when the spiral will form, how big it will grow, and the mixing quality that will result,” Haward said.

This could also have implications in other research: for example, in microfluidic devices, it can be difficult to begin the mixing process, according to Shen.

“Our results suggest that if we make the channel dimensions deeper, then it is easier to induce mixing,” she said.

The article can be found at: Haward et al. (2016) Tricritical Spiral Vortex Instability in Cross-Slot Flow.


Source: Okinawa Institute of Science and Technology Graduate University; Photo: Tony Hisgett/Flickr/CC.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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