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Skin friction coefficient with simpleFoam

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Old   December 2, 2022, 09:29
Exclamation Skin friction coefficient with simpleFoam
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Kermit
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Dear all,

I'm working on a 2D backward-facing step at Re_h = 5100 with the simpleFoam solver and I'd like to compute the skin friction coefficient C_f on the wall.

To do so, I use the non-dimensionalized wall shear stress \tilde{\tau}_w (with m^2/s^2 units) obtained with the postProcessing utilities and then I compute C_f as follows:

(1) : C_f = \frac{\tilde{\tau}_w}{0.5 U_{0}^{2}}

instead of using the conventional formulation

(2) : C_f = \frac{\tau_w}{0.5  \rho U_{0}^{2}}
where \tau_w is the dimensional form of the wall shear stress.

However, as it can be seen in the attached figures, my results are better when I use the conventional formulation and I dont understand why... Does anyone have an explanation for this?


Thanks for your help!


With Eq. (1): cf.png
With Eq. (2): cf_conventional.png
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Old   May 19, 2023, 02:34
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Hi Kermit

Did you find your answer? I almost have the same issue!
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Old   May 22, 2023, 11:46
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Hello saturn_K,


I assumed that the first formulation was the correct one, since it is dimensionally good. What do you think of it ?


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Old   May 22, 2023, 12:34
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I'd also like to point out that the following tutorial supports the first relationship:


https://develop.openfoam.com/Develop...FlatPlate/plot


Best regards.
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