Fractals in the air

Turbulence is often thought of as chaotic. Yet fluid mechanics expert Gerrit Elsinga believes it is very ordered and consists of layers.

Whether on the wingtips of an airplane, in oil pipes or at the rear of your car, turbulence can be a real drag. And unfortunately researchers have a hard time figuring out the dynamics of this annoying phenomenon.

Using a tomographic particle image velocimetry, Dr Gerrit Elsinga, of the fluid mechanics laboratory (3mE faculty), obtained new interesting insights. With this technique, which involves several high-speed cameras and lots of tiny fluorescent particles, high resolution 3D videos can be made of turbulent liquid or gas.

Elsinga found that tiny whirls were organized in rows. Between the whirls he saw what he calls “turbulent layers”. Whirls on either side of this layer rotated in opposite directions.

“This pattern seemed to repeat itself on different scales, kind of like fractals,” says Elsinga, who received the Leen van Wijngaarden Prize this month for his work on tomographic particle image velocimetry.

The findings, which were recently published in the Journal of Fluid Mechanics, could prove useful for improving models on turbulence, the researcher says: “For one, these kind of insights can be helpful if we want to translate results from experimental set ups, where we experiment on small scales, to the real world.”

Elsinga and a couple of his colleagues are about to start such an experiment in the fluid dynamics laboratory’s wind tunnel. The experiment is called DisTUrbe (dispersion by turbulence in urban environment), and its goal is to produce better urban air pollution forecasts by determining how air moves around at a scale of 10 to 1000 metres. The experimental set-up looks like a puppet theater with buildings only a few centimeters high and tiny cars.  

Techniek blijft een afschrikwekkende werking hebben op sommige vrouwen. Althans, dat blijkt uit de vragen die dinsdagavond beantwoord werden op de Ladies Night voor scholieres. Bijvoorbeeld of de techniek echt zo’n mannenwereld is, of je wel iets met mensen kunt doen met een technische studie en of het wel gezellig is op de TU. Sprekers als Hester Bijl, de eerste hoogleraar van de faculteit Luchtvaart- en Ruimtevaarttechniek, probeerden de meisjes gerust te stellen. 

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