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Why is turbulence so little understood?

  1. May 26, 2010 #1
    As ole' Feynman said: "Turbulence is the most important unsolved problem of classical physics."

    So why is it so, that this everyday phenomena is so little understood by scientists?
     
    Last edited: May 26, 2010
  2. jcsd
  3. May 26, 2010 #2
    Because it presents seemingly no pattern. Almost everything about it seems random.
     
  4. May 26, 2010 #3
    Randomness is also found in other fields of physics like QM and radioactivity, yet it is well understood and we have some very accurate theories to describe those phenomena.
     
  5. May 26, 2010 #4
    That's no answer; the kinetic theory if gasses is based on the random actions of many molecules which add up to the classical laws we recognise.

    I'm not sure that you can't make a case for similar properties attributable to turbulence.
     
  6. May 26, 2010 #5
    We can model tubulence quite well now. We can not efficienctly but practically run large eddy simulations that accurately model larger scale turbulence. The problem with it that it is pseudorandom (I suppose chaotic is a more correct term), there is no classical equation that can be solved to decribe it. The turbulene and Navier stokes equations have to be solved using numerical methods.

    Modern computing power allows that.


    What exactly are you getting at with the question OP?
     
    Last edited: May 26, 2010
  7. May 27, 2010 #6
    As to my knowledge, it is possible to simulate turbulence - but there exist no good explanation for why turbulence appear as it does. Since it may be to much to ask for an explanation of turbulence (I guess that answer would give you a Nobel), I'm asking what's the reason for this inability to explain the phenomena. I figured maybe someone has a sound answer, so that's where Im trying to get at :)
     
  8. May 27, 2010 #7
    Are you saying that no explanation for turbulence exists? And I can get a nobel just for answering this question? Man :/ tempting.
     
  9. May 27, 2010 #8
    As you get to smaller scales, microscopic changes in flow conditions will 'trip' a flow into turbulence. With low energy flow, small changes in flow conditions will not cause a destabilising effect. As flow energies increase, smaller and smaller discrepancies cause molecular diffusion to become a significant factor. This is why turbulence is not connected directly to Reynolds number. The flow is dependent on the exact situation and conditions.

    The reason why it was so difficult to understand for years is that the changes simply could not be seen or measured. So you had one flow that looked like it had exactly the same conditions as a second flow, one was turbulent and the other was laminar. This was becuase on a scale smaller and quicker than was measurable the flow conditions were altered. With computer simulation direct solution you can slow 'time' down to see the point where turbulence is tripped.

    We certainly don't understand turbulence fully, but it wasn't understood nearly as well in Feynmans time because they simply didn't have the technology to see or try to predict what was going on. To an extent we still don't, it's only just getting tothe point where we can directly solve the equations that govern a flow and they replicate reality to the highest degree of accuracy possible at this time.

    As an analogy it's similar to gravity and weak force. On large scales gravity dominates. in fluids viscous and inertia effects dominate. When you get to very very small scales, weak force dominates. In fluid flows molecular attraction and diffustion effects donimate over viscous and inertia effects.

    (although they are acutally understood) :D

    EDIT: Although we suspect that diffusion effects casue turbulence, we don't know the exact mechanism or how or why. (Which I suppose is more to the point of your original question).
     
    Last edited: May 27, 2010
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