New evidence suggests turbulent flow may become laminar over time

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wolram
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http://www.sciencedaily.com/releases/2008/11/081121101003.htm

Quote.
Until now, scientists assumed that a turbulent flow traveling with a constant speed will always remain turbulent. However, scientists from Göttingen and Delft have now found evidence that points to the contrary. "Our measurements show that every turbulent flow in a pipe will inevitably become laminar", says Dr. Björn Hof from the Max Planck Institute for Dynamics and Self-Organization. Depending on the exact geometry of the pipe this transition may take many years. But just like a ball inside a hollow, that always rolls back into the equilibrium position, only the laminar flow is stable.

This seems extraordinary, a flow can become laminar after years of being turbulent,
why would this transition happen?
 
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It would seem that the random nature of the real world would continue to keep triggering turbulent flow in a pipe. Would the pipe have to remain free of any form of externally induced movement such as vibration over a period of years to test this theory?

Does this only relate to flow in pipes, or only certain types of pipes? I assume that solids moving through the air, such as a sphere, would continue to generate turbulent flow indefinately?
 
wolram said:
This seems extraordinary, a flow can become laminar after years of being turbulent,
why would this transition happen?

I'm not convinced it would!

For their measurements the scientists let water flow through glass pipes of up to 14 meters length and only a few millimetres in diameter. With the help of a short water pulse from the side they created a turbulent eddy in the otherwise perfectly laminar flow. They then monitored closely, how this eddy changed as it traveled down the pipe. From the probability with which it reached the end of the pipe they could derive the basic principles that govern turbulence.

"In order to discern whether turbulence is stable or only has an extremely long lifetime, our measurements had to be very exact", says Hof. For example, it was crucial to keep the temperature of the water absolutely constant during the experiment. The measuring accuracy which the scientists achieved in this way exceeded all previous experiments. Even computer simulations cannot provide such precise data.

It is still unclear whether the new results also hold true for flows outside of pipes. But even now the results could help in ending turbulence in pipes in a controlled way. "Turbulent flow consumes more energy than do laminar ones. In many applications such as oil pipelines they are therefore bothersome", explains Hof. Since the flows aspire to turn laminar on their own, it could be possible to shorten the long lifetime of the turbulence with the help of a small perturbation. This could help save energy.

Without reading the whole journal article, and based solely on this excerpt that I just perused through, it would seem that they started with a laminar flow, perturbed it, and then waited for it to become laminar again. It seems to me if they started with a turbulent flow, this would not have happened.

Also, they state that their test and measurement conditions were better controlled than a mathematical model could be, specifically that the temperature was held absolutely constant. I find that extremely difficult to believe.

Lastly, things in nature tend to become less stable, not more (e.g. entropy always increases in the universe).

CS
 
Interesting... I need to read the Phys Rev article, but off the top of my head, it's not too far-fetched; viscosity should act to damp out neighboring regions of large velocity gradients over time.
 
stewartcs said:
Without reading the whole journal article, and based solely on this excerpt that I just perused through, it would seem that they started with a laminar flow, perturbed it, and then waited for it to become laminar again. It seems to me if they started with a turbulent flow, this would not have happened.
CS
Hello stewart
Do you mean that they know that laminar flow under the same conditions is possible? If one starts with a turbulent flow conditions have to be created to get it laminar: broadening of the pipe? Cleaning the inner surface of the pipe? Changing the form of the cross-section of the pipe? Why would a turbulent flow not get laminar under one of these conditions?
greetings Janm