Hydrodynamics: Turbulent at times

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Hydrodynamics: Turbulent at times
Hydrodynamics: Turbulent at times

Sometimes turbulent

Technicians don't like them at all: turbulence. After all, these turbulences increase the frictional resistance and make flowing systems unpredictable. But the wild river can also be steered back into calmer channels.


Wall! walle

some routes, that, for the purpose, water flow

and with a rich, full flow

to which bathe pour …

These lines by Johann Wolfgang von Goethe (1749-1832) from the poem about a sorcerer's apprentice who escapes from violence over his conjured little helpers must have been memorized by many pupils in the Penne. These phrases may come to mind for some engineers and scientists who study the dynamics of liquids when the solution, caustic or broth they are studying begins to run wild, forming unpredictable vortices and eddies. The fact is: Above a certain flow speed, a flow begins to become turbulent when it has to squeeze through a narrow passage.

You can observe this particularly well on water in nature. A gentle stream in a level bed often flows calmly. The speed of the individual water molecules increases relatively evenly from the edge to the middle, which can be easily demonstrated by small sticks or leaves thrown into the rivulet. But if we follow the course of the river for a bit, large masses of water, which flow towards the brook from all sides, soon push their way through a relatively small bed. It's getting cramped for the individual droplets, and like shopaholics at the opening of a shop, they push forward, push each other to the side and squeeze through the smallest gap. The water becomes restless, a spray spurts out, whirlpools and whirlpools appear.

Scientists call this flow behavior turbulent. Nothing can be precisely predicted or calculated. Sticks or leaves perform a wild dance on the waves, they move forwards, then backwards, dive under and then up again - pure chaos. The river only calms down again when it has a wider bed in front of it.

Researchers have so far assumed that a flow in a pipe with a given geometry or in a hose does not calm down again once the liquid has started to get violent: Once turbulent, always turbulent, that was the doctrine.

Attempts by an experimental team led by Björn Hof from the British University of Manchester now speak a different language. The scientists have discovered that turbulence can subside again - if sometimes only after a relatively long time. The American Daniel Perry Lathrop from the University of Maryland, for example, who examined the work of Hof's research group, calculated that it would take 103000 years for a turbulent flow to develop in a perfectly normal sewer a diameter of around 60 centimeters would have calmed down again. The lifetime of the universe, which is estimated at a good 1010 years, would not have been nearly enough for this.


The working group around Hof was content with its experiments with a thirty meter long tube with a diameter of only four millimeters. They found that the flow in the tube remained laminar up to a Reynolds number of over 2750. The dimensionless Reynolds number depends on the flow velocity, the length of the pipeline and the viscosity - the toughness - of the liquid and characterizes the transition to turbulent movement. The experimenters deliberately disrupted the even flow of water by briefly injecting a jet of water perpendicular to the direction of flow through one of twenty holes with a diameter of around 0.6 millimeters.

From the way the water left the long tube, the scientists could now tell whether the turbulent disturbance forced its way through the entire tube or whether it had calmed down beforehand. They determined that the turbulence apparently only has a certain lifetime depending on the Reynolds number and disappears again according to an exponential law, similar to radioactive decay.

These findings are very interesting because the phenomenon of turbulence runs through all natural phenomena: starting with the water cycle, cloud formation, the aerodynamics of jet plane wings and the formation of solar systems or galaxies.

Lathrop believes that the results must first be confirmed by other research groups. However, if it is true that turbulence can calm down again, then facilities or equipment could possibly be devised that accelerate this process - which promises great technical benefits. Because turbulence always has something to do with friction and resistance. If it is possible to reduce these, vehicles that get by with less fuel, for example, are attractive. A lot would be gained if one could simply learn to assess these chaotic movements better in the future.

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