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-   -   Doubt about calculating the frictional velocity over an airfoil (https://www.cfd-online.com/Forums/fluent/109267-doubt-about-calculating-frictional-velocity-over-airfoil.html)

 Jorg November 13, 2012 21:14

Doubt about calculating the frictional velocity over an airfoil

Hi there,

Recently I read a post in a blog about estimating the initial cell height in a simulation of an aerofoil, this estimation includes the calculation of the frictional velocity over the aerofoil wall but the equation is not clearly written and I don't know if the right form is:

Vt = Vinf*2*(Cf)^0.5

or:

Vt = Vinf*(Cf*2)^0.5

Vinf : is the flow velocity
Cf : is the skin friction coefficient

Any help will be appreciated!

 LuckyTran November 14, 2012 01:32

Quote:
 Originally Posted by Jorg (Post 391956) Hi there, Recently I read a post in a blog about estimating the initial cell height in a simulation of an aerofoil, this estimation includes the calculation of the frictional velocity over the aerofoil wall but the equation is not clearly written and I don't know if the right form is: Vt = Vinf*2*(Cf)^0.5 or: Vt = Vinf*(Cf*2)^0.5 Vinf : is the flow velocity Cf : is the skin friction coefficient Any help will be appreciated!
It looks mostly correct, however, I think it should be:

Vt = Vinf(Cf/2)^0.5

Starting with definition of Cf
Cf(0.5*density*Vinf^2) = Tau

Vt = sqrt(Tau/density) = sqrt ( 0.5*Cf*Vinf^2) = Vinf*sqrt(Cf/2)

That particular work may have used a different definition of friction coefficient so that a factor of 4 may have been introduced. Which would yield the formula you already provided.

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