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mukushla

  • 3 years ago

how to choose repeating variables in Buckingham Pi Theorem...in particular

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  1. mukushla
    • 3 years ago
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    nondimensional groups for heat transfer in a pipe...effective parameters : \[h : Mt^{-3}T^{-1}\]\[u : Lt^{-1}\]\[k : MLt^{-3}T^{-1}\]\[c_p : L^2t^{-2}T^{-1}\]\[\mu : ML^{-1}T^{-1}\]\[\rho : ML^{-3}\]\[d : L\]

  2. mukushla
    • 3 years ago
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    we have 7-4=3 nondimensional group and 4 repeating variables..how to choose them considering the dimensions i gave u

  3. mukushla
    • 3 years ago
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    and sorry \[\mu : ML^{-1}t^{-1}\]

  4. mukushla
    • 3 years ago
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    @experimentX

  5. mukushla
    • 3 years ago
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    we have a rule that No repeating parameter should have dimensions that are a power of the dimensions of another repeating parameter. why ...and how apply it here?

  6. eashmore
    • 3 years ago
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    I seem to have forgotten this from fluids (or maybe I never really learned it). You might find this helpful: http://www.eng.wayne.edu/legacy/forms/4/Buckinghamforlect1.pdf

  7. eashmore
    • 3 years ago
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    It says that the repeating variables that you choose cannot be manipulated into dimensionless groups. Define the above parameters for me please.

  8. mukushla
    • 3 years ago
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    sorry i was out h: heat transfer coefficient u: flow mean velocity k: heat conductivity coefficient cp: specific heat capacity \(\mu\): viscousity \(\rho\) : density d: diameter of pipe

  9. eashmore
    • 3 years ago
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    Oh. Those are the units after each parameter. Let's see. We need 3 recurring variables. Choose d, u, and \(\rho\) Those should work.

  10. mukushla
    • 3 years ago
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    emm...we need 4 if im not wrong

  11. eashmore
    • 3 years ago
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    I'm working out of that document I linked and it's ringing some bells. It looks like we have 7 variables\[f(h, u, k, c_p, \mu, \rho, d) \] Between these 7 variables, we have 4 different dimensions. Mass (M), time (t), temperature (T), and length (L). n = 7; m = 4 n-m gives 3 dimensionless groups. We then need to choose 3 variables (these three cannot be formed into dimensionless groups themselves. That is to say that we cannot manipulate these three independently to create a dimensionless group. For example, if we had length and diameter, we couldn't choose these are two of our three. ) Once we choose three variables, we rewrite the four dimensions in terms of the three variables we chose. Does this follow?

  12. akifersoylu
    • 2 years ago
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    ı think 3 repeating variables occur; u, d, ρ,

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