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BCs for k and epsilon at walls with y+=1

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Old   October 15, 2012, 15:18
Default BCs for k and epsilon at walls with y+=1
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Vangelis Skaperdas
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Hello everybody,

I would like to know the following for OpenFOAM 2.1.1:

when setting up a turbulent flow case with a mesh that leads to y+ values of 1 I do not need to use wall functions.
What is the correct BC for k and epsilon in such a case
Are they fixedValue=0 or
zeroGradient?

Similar question for k-omega models also.

I have found this post
k-omega SST and wall functions in OpenFOAM-1.5-dev
but it refers to previous OF releases

Thanks!

Last edited by vangelis; October 15, 2012 at 16:44.
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Old   October 16, 2012, 03:22
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Roland
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Hi Vangelis,

To my understanding you would (theoretically) set both at fixedValue = 0, as there is no turbulence possible in the viscous sublayer. And as there is no turbulence, you also can't have dissipation of turbulence.

I say theoretical because actually setting the BC's to 0 would result in a division by zero in the OpenFOAM implementation. In practice you would therefore set them to a very small value, for example fixedValue = 1e-12.

Best regards,
Sylvester
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Old   October 16, 2012, 03:51
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Vangelis Skaperdas
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Hi Sylvester!

Thank you very much for your reply.

Best regards

Vangelis
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Old   October 16, 2012, 08:57
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Vangelis Skaperdas
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Hi Sylvester

I forgot to also specify that
nut should also be set to fixedValue=0
like k, right?

Vangelis
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Old   October 16, 2012, 09:14
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Hi Vangelis,

Or use nutLowReWallFunction, which makes the patch accessible from yPlusRAS.
From nutLowReWallFunctionFvPatchScalarField.H:
Quote:
Description
Boundary condition for turbulent (kinematic) viscosity when for use with
low Reynolds number models.

Sets nut to zero, and provides an access function to calculate y+.
regards,
Sylvester

ps. Can we perhaps expect improved OpenFOAM pre-processing in ANSA v13.3?
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Old   October 16, 2012, 10:25
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Vangelis Skaperdas
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We are trying to do this.
And in this sense thank you for your help!

so you mean that it is better to use
nutLowReWallFunction

so that afterwards I can get values for y+?

Otherwise it would be the same as setting fixedValue=0 for nut at walls?
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Old   October 16, 2012, 10:52
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Hi Vangelis,

Yes, from what I can gather it is better to use the nutLowReWallFunction. But I haven't done much work with low Re turbulence models, so I may be wrong here.

Best regards,
Sylvester

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Old   October 16, 2012, 11:45
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Vangelis Skaperdas
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Thanks again Sylvester!
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