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Hydrodynamic entrance length: independent of Re?

  1. Aug 26, 2010 #1
    Hi all,
    I have learned that the hydrodynamic entrance length of a channel (to form fully developed laminar flow) is correlated to the Reynolds number, because the shear effects have to propagate inwards from the walls of the channel. However recently I found out that there is a 'minimum' to the dependence on the reynolds number, for example in Deen (Analysis of transport phenomena) I find for a cylindrical tube:
    [tex]\frac{L_v}{R}=1.18+0.112 Re[/tex] with [tex]Re=\frac{2 U R}{\nu}[/tex]
    With R the radius and [tex]L_v[/tex] the entrance length.

    The Reynolds dependent term in this equation I can understand (and derive), but I do not know what effect is responsible for the 'offset'. Can anyone explain to me why this occurs?

    Thanks in advance!
     
  2. jcsd
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