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-   -   RNG K-epsilon and wall function (http://www.cfd-online.com/Forums/fluent/124528-rng-k-epsilon-wall-function.html)

PedFr0 October 7, 2013 08:53

RNG K-epsilon and wall function
 
Hi All

I am currently investigating external flow around a heliostat with Re, based on the length of the heliostat, of around 3.5x10^6. I am using the RNG K-epsilon model for now as the Fluent documentation says that it can deal with rapidly strained and swirling flows, which is what I expect to encounter. Along with this model I am using the non-equilibrium wall functions as these are recommended for complex flows involving separation and impingement. I would like to know if this is the correct approach as I vaguely remember reading that the non-equilibrium functions are recommended for turbulence models that are valid near the wall, and I am uncertain as to if the RNG k-epsilon model satisfies this condition. Ideally I understand that I should be using enhanced wall treatment,which I plan to do at a later stage.

Many Thanks for your help
Adhikar

oj.bulmer October 14, 2013 11:58

The non-equilibrium wall function is inherently an improvement over standard wall function (Launder/Spalding) in that it is sensitive to pressure gradient and hence the predicts the separation better. At the same time, it divides the wall adjacent regions into two layers. The inner viscosity dominated layer is treated with one equation model, while the outer turbulence dominated layer is solved using the k-eps or Reynolds stress transport closure. These are the only two closures available, and I don't think the model you are using has any bearing over the closures in these layers. I think RNG should be fine.

That said, when your physics starts having very severe pressure gradients or complex 3D flows very adjacent to the walls etc, these wall functions lose their relevance and no better than standard wall function.

OJ

PedFr0 October 14, 2013 12:17

Hi OJ

Thanks for the reply, its really helped clear things up with regards to the wall function. I just have been having issues with enhanced wall treatment and am looking for the next best thing just for now.

Thanks
Adhikar

Far October 14, 2013 13:05

Quote:

Originally Posted by oj.bulmer (Post 456879)
At the same time, it divides the wall adjacent regions into two layers. The inner viscosity dominated layer is treated with one equation model, while the outer turbulence dominated layer is solved using the k-eps or Reynolds stress transport closure. These are the only two closures available, and I don't think the model you are using has any bearing over the closures in these layers. I think RNG should be fine.

That said, when your physics starts having very severe pressure gradients or complex 3D flows very adjacent to the walls etc, these wall functions lose their relevance and no better than standard wall function.

OJ

I want to add bit more :D

It is from theory guide

1. Launder and Spalding’s log-law for mean velocity is sensitized to pressure-gradient effects.

2. The two-layer-based concept is adopted to compute the budget of turbulence kinetic energy ( *, *) in the wall-neighboring cells.

PedFr0 October 15, 2013 03:49

Hi Far

Thanks for the reply, I've found the non-equilibrium wall treatments in the theory guide however I vaguely remember reading that it should only be used with models valid in the near-wall region but it looks like I was mistaken. Thanks for the help.


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