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Wall functions and openfoam

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Old   November 17, 2015, 21:29
Default Wall functions and openfoam
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Matej Muller
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Hi!

I know, there are already meany threads on this topic on the forum, but I'm still a bit confused when it comes to wall functions in openfoam. Can anyone explain what is the difference between calculated and nutkWallFunction for the nut BC . I've noticed that if I use

k --> kqrWallFunction
epsilon --> epsilonWallFunction
nut --> calculated

instead of

k --> kqrWallFunction
epsilon --> epsilonWallFunction
nut --> nutkWallFunction

I get better results according to my measured data. If the wall function is already included in k and epsilon, why do we have to use it also in the nut file... Or is this approach with the calculated BC totally wrong (the y+ values are greater than 30).

Also, if I use the k-omegaSST model, do I have to use the nutUSpaldingWallFunction for the nut BC if my y+ values are somewhere smaller than 5? I know the omegaWallFunction works well for lowRe and HighRe flows, but I'm not sure if that is true for the nutkWallFunction? But even if I use omegaWallFunction and nutUSpaldnigWallFunction, I still have to use the kqrWallFunction, which I know is not good for lowRe flows. So what would be the right approach in using the k-omegaSST model?

Any comments would be appreciated.

Regards, matej
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Old   November 19, 2015, 12:08
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Eric Robertson
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Quote:
Originally Posted by matejmuller View Post
Hi!

I know, there are already meany threads on this topic on the forum, but I'm still a bit confused when it comes to wall functions in openfoam. Can anyone explain what is the difference between calculated and nutkWallFunction for the nut BC . I've noticed that if I use

k --> kqrWallFunction
epsilon --> epsilonWallFunction
nut --> calculated

instead of

k --> kqrWallFunction
epsilon --> epsilonWallFunction
nut --> nutkWallFunction

I get better results according to my measured data. If the wall function is already included in k and epsilon, why do we have to use it also in the nut file... Or is this approach with the calculated BC totally wrong (the y+ values are greater than 30).

Also, if I use the k-omegaSST model, do I have to use the nutUSpaldingWallFunction for the nut BC if my y+ values are somewhere smaller than 5? I know the omegaWallFunction works well for lowRe and HighRe flows, but I'm not sure if that is true for the nutkWallFunction? But even if I use omegaWallFunction and nutUSpaldnigWallFunction, I still have to use the kqrWallFunction, which I know is not good for lowRe flows. So what would be the right approach in using the k-omegaSST model?

Any comments would be appreciated.

Regards, matej
What do you quantify as "better results?"

For SST, you should be able to use a direct condition of nut ~ 0 on the wall if your y+ is that small.

Why is kqrWallFunction not good for low-Re formulations?
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