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-   -   Cannot decide value of yplus (http://www.cfd-online.com/Forums/cfx/93638-cannot-decide-value-yplus.html)

td2e09 October 21, 2011 09:30

Cannot decide value of yplus
 
Good afternoon. I have done a lot of reading on yplus, and am aware of its calculations and why it is important. However I have not been able to see from literature how it is possible to determine what is the yplus value required for any given flow problem, so that the mesh can be constructed accordingly.

Does anyone have any idea how I can examine my flow problem and determine what value of yplus is required, so that I can adjust my mesh accordingly. Or does anyone know any good literature or references on choosing an appropriate value for yplus. Thanks and best wishes.

mauricio October 22, 2011 02:18

I also had this question sometime ago and i believe it was glenn h. who told this depends a bit on your turbulence model. In my case i wanted to use k-w which needs a very small (close to 1) yplus when i could have used the more advanced (and recomendded by the ansys documentation) SST. I'd also love to find any papers on this subject as well. If you can't find anything and you'd like to ensure the best yplus,
why don't you add it as a parameter for a sensibility analysis? make a mesh with some settings, run a simple converging simulation and write down results, change the yplus, see how the results change, and go on until the results are not depending on your mesh, or you start getting limited by your own computational resources.

ghorrocks October 22, 2011 05:58

The answer is all over this forum, and in the FAQs as well - see http://www.cfd-online.com/Wiki/Ansys...publishable.3F

The answer is a sensitivity analysis. Different flows have different sensitivities to the boundary layer, so of course their requirements for BL meshing is different. That is why there is a not a universal answer.

td2e09 October 25, 2011 09:20

Thats great, many thanks to you both for your help :)

Far October 27, 2011 09:03

Quote:

Originally Posted by ghorrocks (Post 328997)
the answer is a sensitivity analysis. Different flows have different sensitivities to the boundary layer, so of course their requirements for bl meshing is different. That is why there is a not a universal answer.

Let say If we are analyzing the flow over airfoil for following flow regimes (Reynolds number is sufficiently high so that the flow can be assumed fully turbulent)

1. subsonic
2. transonic
3. supersonic

can we set yplus for the different airfoils based on above study or again we have to make the sensitivity study for each new configuration?

ghorrocks October 27, 2011 16:48

I would check this with a simple sensitivity test, but I reckon you will find you are pretty insensitive. As long as your y+>11 and is smallish then it should be fine.

Edit: Oh, except for all the exceptions - separated flow, heat transfer, multiphase etc etc :)


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