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Homework Help: Reynolds Number

  1. Feb 12, 2015 #1
    1. The problem statement, all variables and given/known data

    I don't fully understand the Reynolds number and it has arisen in a problem. It says:

    Fluid with viscosity µ and density ρ fills the gap between two parallel plates at z = 0 and z = h. The upper plate at z = h moves with speed V in the x direction, while the lower plate at z = 0 is stationary. The fluid is also subject to a pressure gradient −G in the x direction.

    2. Relevant equations

    I have solved for the velocity u (no idea if this is needed for the Reynolds number bit at all) to find

    u(z) = Gz(h-z)/2µ + Vz/h.

    3. The attempt at a solution

    The question then says:

    What is the Reynolds number for this flow? What are the conditions on V and G for it to be small?

    Can anybody help at all? It would be much appreciated.

    All my notes really say is that Re = ρUL/µ or Re = ρL^2/µT but I don't understand the U, L or the T.
  2. jcsd
  3. Feb 12, 2015 #2


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  4. Feb 16, 2015 #3
    That Wiki helps a little but I'm still confused by this. So the Reynolds number is given by Re = ρUL/µ? As U is the velocity scale it makes to take U as V here. But after this I don't know how G comes into the Reynolds number.
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