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Navier Stoke Problem

  1. Jul 8, 2012 #1
    hi every body, im doing an analysis of a paper about a tool used in oil well drilling, they use 3D navier stoke equation and by lubrication aproximation the get this

    -(1/r) ∂p/∂θ + k (∂( 1/r ∂(rw)/∂r )/∂r )=0

    its necesary to get w so i use integral but i obtain an irreal i used mathematica wolfram. but in the paper they obtain

    w= 1/2k ∂p/∂θ (ln r - 1/2) + C1 1/2 r + C2/r

    somebody could confirm this
  2. jcsd
  3. Jul 8, 2012 #2
    sorry imaginary
  4. Jul 9, 2012 #3
    -(1/r) ∂p/∂θ + k (∂( 1/r ∂(rw)/∂r )/∂r )=0
    ∂( 1/r ∂(rw)/∂r )/∂r = (1/k) (∂p/∂θ) (1/r)
    1/r ∂(rw)/∂r = (1/k) (∂p/∂θ) ln(r) + C1
    ∂(rw)/∂r = (1/k) (∂p/∂θ) r ln(r) + C1 r
    (rw) = (1/k) (∂p/∂θ) [(r²/2) ln(r)-r²/4] + C1 r²/2 + C2
    w = (1/2k) (∂p/∂θ) [r ln(r)-(r/2)] + C1 r/2 + C2/r
  5. Jul 9, 2012 #4
    I have issues with this. In particular, you have:

    [tex]\int \left(\frac{1}{k} \frac{\partial p}{\partial \theta} \frac{1}{r}\right)dr=\int \partial\left(\frac{1}{r}\frac{\partial}{\partial r}(rw)\right)[/tex]

    I do not see how you can integrate the left side with respect to r and get:

    [tex]\int \left(\frac{1}{k} \frac{\partial p}{\partial \theta} \frac{1}{r}\right)dr=\frac{1}{k}\frac{\partial p}{\partial \theta} \ln(r)+c[/tex]

    You're assuming [itex]\frac{\partial p}{\partial \theta}[/itex] is not a function of r and I do not think you can assume this. Also, wouldn't the constant of integration be an arbitrary function of theta in this case?
    Last edited: Jul 9, 2012
  6. Jul 9, 2012 #5
    Hi every body thanks for the reply Jacqueline u obtain the same as the paper And jack in the doc they assume p doesnt change in r But as You say a c depent in theta What do u think?
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