CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   Simple questions (http://www.cfd-online.com/Forums/cfx/19358-simple-questions.html)

Eva December 4, 2002 04:20

Simple questions
 
Hi!

I have a few question about Build in CFX5.5.

I want to use the k-eps model with a low Re model instead of a wall function near the wall. How do I do that? Iīve read the manual and donīt get it.

How do I check if my mesh is good? Can I only use a tetrahedral mesh in Build?

Can you do a 2D mesh in Build?

Thanks in advance!

Astrid December 4, 2002 07:20

Re: Simple questions
 
- I think you require the SST turbulence model. So go to the domain definition and then switch turbulence models. Use automatic wall functions.

- You should check Yplus which is a postprocessing variable. With automatic wall functions your y+ should at least not exceed 300, although this depends on your flow problem. Stricktly speaking, you should refine your mesh several times until your solution does not change anymore. The solution is then 'mesh independent'. It is easier to use mesh adaption. Based on your input, the mesh will then be refined automatically where needed. Unfortunately it is not possible to adapt the mesh on the variable Yplus. Why not? Good question, I don't know.

Astrid

Eva December 5, 2002 04:32

Re: Simple questions
 
SST is a k-omega model near the wall, so that doesnīt work. I can choose the k-eps model, but then I can only choose between scalable and standard wall functions. Why canīt you do a simple low Re model in CFX-5?


Robin December 5, 2002 07:52

Re: SST
 
Hi Eva,

Low Re number models are very sensitive to near wall resolution; if your Yplus values are not around 0.1, the skin friction prediction will be way off. The k-omega model has the desirable property of allowing you to integrate through the boundary layer, with a reasonable Yplus requirement of about 2, but is not so good in the free stream.

The SST model only uses k-omega near the wall. Away from the wall it uses k-e. Furthermore, the automatic wall treatment will allow you to integrate to the wall where your Yplus is small enough, and blends smoothly to a wall function treatment where your mesh is coarser.

So the only difference between the low Re model you are looking for and SST is in the details of how it integrates to the wall and less stringent meshing requirements. The SST option works better than a typical low Re model and is easier to mesh. I suggest going with it.

Regards, Robin



All times are GMT -4. The time now is 10:31.