Skin friction coefficient with simpleFoam

tyskies · December 2, 2022, 09:29

Dear all,

I'm working on a 2D backward-facing step at

with the simpleFoam solver and I'd like to compute the skin friction coefficient

on the wall.

To do so, I use the non-dimensionalized wall shear stress $\tilde{\tau}_w$ (with

units) obtained with the postProcessing utilities and then I compute

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

saturn_K · May 19, 2023, 02:34

Hi Kermit

Did you find your answer? I almost have the same issue!

tyskies · May 22, 2023, 11:46

Hello saturn_K,

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

Kermit

tyskies · May 22, 2023, 12:34

I'd also like to point out that the following tutorial supports the first relationship:

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

Best regards.

December 2, 2022, 09:29	Skin friction coefficient with simpleFoam	#1
tyskies New Member Kermit Join Date: Jan 2021 Location: France Posts: 19 Rep Power: 6	Dear all, I'm working on a 2D backward-facing step at with the simpleFoam solver and I'd like to compute the skin friction coefficient on the wall. To do so, I use the non-dimensionalized wall shear stress $\tilde{\tau}_w$ (with units) obtained with the postProcessing utilities and then I compute 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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May 19, 2023, 02:34		#2
saturn_K New Member Join Date: Sep 2019 Posts: 14 Rep Power: 7	Hi Kermit Did you find your answer? I almost have the same issue!

May 22, 2023, 11:46		#3
tyskies New Member Kermit Join Date: Jan 2021 Location: France Posts: 19 Rep Power: 6	Hello saturn_K, I assumed that the first formulation was the correct one, since it is dimensionally good. What do you think of it ? Kermit

May 22, 2023, 12:34		#4
tyskies New Member Kermit Join Date: Jan 2021 Location: France Posts: 19 Rep Power: 6	I'd also like to point out that the following tutorial supports the first relationship: https://develop.openfoam.com/Develop...FlatPlate/plot Best regards.