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-   -   buoyancy production term in turbulence model (http://www.cfd-online.com/Forums/openfoam-solving/87600-buoyancy-production-term-turbulence-model.html)

braennstroem April 24, 2011 14:29

buoyancy production term in turbulence model
 
Hi,

I know, that for natural convection problems additional buoyancy production terms are used/needed in the turbulence equations

http://my.fit.edu/itresources/manual...ug/node482.htm

Related to this I have a few questions:
1) What is your experience with this additional production term?
2) How important is this for other turbulence models like the v2-f or k-omega-SST model?
3) Is there a difference for compressible/ideal gas and incompressible boussinesq solvers for using this additional production term (I expect not)?
4) What is your experience of using boussinesq approximation for higher temperature differences? For a difference of up to 100K I saw quite identical results evaluated by Darioush Gohari Barhaghi / Chalmers.

It would be nice, if you have some ideas and could recommend any papers related to these questions!
Best Regards!
Fabian

Btw. it seems, that the only available turbulence model for openfoam is the implementation of http://openfoamwiki.net/index.php/Co...EpsilonViollet

eugene April 26, 2011 07:27

Hi Fabian,

Here is a presentation from Launder et al that goes into some detail about the issues related to buoyancy and turbulence:
http://www.powershow.com/view/5c86-Z...t_presentation

By all accounts it is not just the turbulence model which needs modification, but also the wall functions if you are running a high Re mesh. I suggest you read the related papers as well.

Certainly, our experience with the Fluent source term approach has not shown any noticeable benefits for natural convection dominated problems typical of HVAC applications. I imagine the influence will be more pronounced at larger dT though.

It is quite difficult to give a clear-cut range of applicability for the Boussinesq approximation as its behaviour is entirely case dependent. If you look at the derivation of the Boussinesq approximation, it is not necessarily the gravity term that causes the largest deviations due to the assumption of constant pressure. The secondary effects of non-constant density can be quite pronounced, especially in its effect on the heat capacity via Cp. In general, the smaller the pressure range is, the larger the accurate temperature range will be. Larger amounts of forced convection will also reduce the relative impact of the Boussinesq approximation.

eugene April 26, 2011 07:29

I forgot to add this: a great article detailing the derivation of the Boussinesq approximation with some discussion of its range of applicability:

http://articles.adsabs.harvard.edu/c...IF&classic=YES

braennstroem April 27, 2011 14:39

Hello Eugene,

thanks for your reply and for the interesting links! Yes, you are correct with the wall treatment for higher yplus values. Related to the production term, I remember, that there were some study of Kriegel with Fluent for displacement ventilation playing around with the productions term for the dissipition equ. using a k-epsilon variant... unfortunately I do not know, if it changed a lot.

Thanks again!
Fabian

mabinty May 25, 2011 11:22

Hi,

a while ago I implemented a buoyancy augmentation for the standard k-eps model in OF based on the Generalized Gradient Diffusion Hypothesis. Comparison with experimental results of a vertical round turbulent plume showed improved results, similar to the study of Maele and Merci: Application of two buoyancy-modified k-eps turbulence models to different types of buoyant plumes, Fire Safety Journal, 2006, 41, 122-138.

Aram

ToulBraz January 13, 2012 06:43

Hi,

I've downloaded Viollet's k-e model from :
http://openfoamwiki.net/index.php/Co...EpsilonViollet

I made some mods in order to make it useable with OpenFOAM 2.0. I think I succeeded as it seems to work (but I'm quite new to OF so,....).
However, I lately understood that the implementation is only for incompressible flows, so I can't use it with buoyantSimpleFoam (only with boussinesqBuoyantSimpleFoam).

Does anyone have some hints on how I should proceed to make it work for compressible flows ?

Thanks in advance


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