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Turbulent flow

  1. Dec 26, 2008 #1

    wolram

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

    Quote.
    Until now, scientists assumed that a turbulent flow travelling 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?
     
  2. jcsd
  3. Dec 27, 2008 #2
    Ever thought of a laminar flow going thurbulent?
     
  4. Dec 27, 2008 #3
    What it is his Name Richardson was not crazy! Dimensionless numbers and all its consecuences to Englisch is not quite understood. It could be that inches and foots are not the right measures at all. Miles not to speak of, in all the different forms they exist. Euro and meters and seconds as a standard measure ±10 years to go then I keep 10 years of silence!
     
  5. Dec 28, 2008 #4

    rcgldr

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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?
     
  6. Dec 28, 2008 #5

    stewartcs

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    I'm not convinced it would!

    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
     
  7. Dec 29, 2008 #6

    Andy Resnick

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    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.
     
  8. Mar 28, 2009 #7
    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
     
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