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-   -   Wallfunctions onoff (https://www.cfd-online.com/Forums/openfoam-solving/60603-wallfunctions-onoff.html)

gjesing April 28, 2005 04:44

Hi, A simple question. H
 
Hi,

A simple question.

How is wall-functions for RANS disabled and enabled?

To me, it looks like in turbulenceProperties that wall-functions can be chosen on the same level as choosing the k-epsilon model. But I can also see in the implementation of the turbulence models that the source files for the wall-functions are included.

Thanks in advance,
Rasmus

henry April 28, 2005 04:47

All high-Re RANS turbulence mo
 
All high-Re RANS turbulence models include wall-functions because it is inappropriate to use them without. Only the low-Re models operate without wall-functions as they include model-specific wall treatments.

imano May 4, 2005 03:14

Sorry, I forgot to mention abo
 
Sorry, I forgot to mention about my calculation setup's detail.

1. I'm running high-Re RANS.
2. I need to apply the rough wall function only to a ground and other walls
should be calculated with normal slip wall function. So I would like
to change wall functions whether a wall is a ground or not.

Since I'm beginner about both OpenFOAM and C++ language;;,
I would like to copy and modify the wall function sources directly,
but I still need to switch wall functions with using information of
the wall.

[Q] How can I switch wall functions according to wall's
information(name?) in RANS wall function sources?

Thanks for your advice,
Masashi IMANO

muellea February 10, 2010 11:01

Production term
 
Dear Henry and RANS-experts,

first of all, thank you for the improved near wall treatement (NWT) of kOmegaSST. I really appreciate it. :)
However, to have a reliable NWT, 2 questions may arise:
1) Production term:
  • From my point of view, the production term of k (called G) looks correct for small y+ and within the log-law region, but the if-condition may cause problems, if y+ is in the buffer layer.
  • Some paper propose a smooth blending of this term, which is perhaps less problematic than the implemented if-condition.
  • As far as I know, Menter never said (mentioned) a single word about this term. "Just one of this annoying secrets of CFX ;)"
  • Fluent proposes that one can use the following formula everywhere, as long as tau_w is consistent with the velocity gradient based on the "automatic wall function". G=[(tau_w)^2]/(kappa*rho*(C_mu)^0.25*sqrt(k)*y)
  • What do you think about that? Is the implemented G unproblematic with respect to the buffer-layer?
2) Turbulent thermal diffusivity:
  • If we use wall functions for alphat, the following formula is used: alphat=mu_t/Pr_t
  • A lot of papers use the temperature profile proposed by Kader. Do you think such an adaptation leads to significant better results in cases with adiabatic walls?
Any comments are welcome. Thanks

Andi

Ps.: As english is not my mother tongue, some statements may sound a bit rude. If this is so, don't take it amiss. It was never my intention.

muellea February 10, 2010 11:17

Sorry
 
Sorry, wrong thread:)

panda60 February 11, 2010 00:55

I think it is very difficult to understand how wall function works in OpenFOAM.

When you select high Renolds number kEpsolon model, it will be automatically be used ?

Or when you select nutWallFunction, it will be automatically used ?

If velocity wall function will be automatically used, then temperature wall function how to use ?
for example, buoyantBoussinesqSimpleFoam, It doesn't have a variable for temperature to use wall function. As far as I know, alphat wall function can only be used for compressible flow. Am I right?


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