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-   -   due to wall function, continue refining mesh until when (http://www.cfd-online.com/Forums/cfx/71824-due-wall-function-continue-refining-mesh-until-when.html)

zandi January 16, 2010 15:50

due to wall function, continue refining mesh until when
 
Hi
I'm doing a free surface flow CFD modeling project that the velocity and pressure near the bottom (wall boundary) are important parameters. for evaluating the pressure result I have data and after refining mesh I saw that the pressur was not sensitive to it and I reached to a good answer.

but for velocity I do not have any data and just want to reach an answer that not depedent on mesh size.

I use k-epsilon standard in CFX code

so my limitation for y+ is 20 and 100 (CFX help)
  • now i'm reaching to y+ under 20 and still my velocity profile near the boundary changes due to make the meshes smaller. what can I do
please help

ghorrocks January 16, 2010 17:21

I would switch to the SST turbulence model then you can use the scalable automatic wall functions. Then it would matter when y+ goes under 11 and you can refine as much as you like.

Is there a reason you wish to use k-e?

zandi January 17, 2010 07:59

Dear G. Horrocks Salam
and thanks a lot
actually my masters insist on it maybe they're not so friendly with low Rey models. I'm not too!
they said the k-e is sufficient for my case.

by the way
  • if I change my turbulent model how much change result in my work. does my pressure or mass flow rate change. do I have compute any parameters before
best regards Zandi

ghorrocks January 17, 2010 17:20

SST is not a low Reynolds number turbulence model! It is a general purpose turbulence model and works well for a wider range of flows than k-e. If you are having problems with mesh refinement in k-e then I recommend you go to SST as the wall function issue is not a problem for SST.

As for whether the change in turbulence model will affect things - how can I possibly know that? You have not described at all what you are doing. It is straightforward to work that out for yourself - do a test run using SST and find out for yourself whether it makes a difference. For most flows it takes the same run time as k-e and converges the same so it should be a easy test to do.

zandi January 18, 2010 03:53

thank you very much from your advice

paulo January 18, 2010 10:16

Quote:

Originally Posted by ghorrocks (Post 242859)
(...) For most flows it takes the same run time as k-e and converges the same so it should be a easy test to do.

it can be even faster, if you use the k-epsilon results to start your simulation.

Best Regards,

Paulo Rocha

zandi January 18, 2010 14:29

Quote:

Originally Posted by paulo (Post 242943)
it can be even faster, if you use the k-epsilon results to start your simulation.

Best Regards,

Paulo Rocha

thank you friend but could you tell me how I could use the result
maybe when we want write the solver but I don't know which option should I use
thanks again

ghorrocks January 18, 2010 17:47

Why use k-e at all? Just use SST then you don't have any issues about wall functions with mesh refinement. The SST model has superseded the k-e model for most general applications.

zandi January 23, 2010 05:16

thank you, nice of you


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