Cannot decide value of yplus

td2e09 · October 21, 2011, 10:30

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, 03:18

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, 06:58

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, 10:20

Thats great, many thanks to you both for your help

Far · October 27, 2011, 10:03

Quote:

Originally Posted by ghorrocks

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, 17:48

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

October 21, 2011, 10:30	Cannot decide value of yplus	#1
td2e09 New Member Join Date: Aug 2010 Posts: 6 Rep Power: 16	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.

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October 22, 2011, 03:18		#2
mauricio Member Mauricio Labarca Join Date: Aug 2009 Posts: 35 Rep Power: 17	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.

October 22, 2011, 06:58		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	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.

October 25, 2011, 10:20		#4
td2e09 New Member Join Date: Aug 2010 Posts: 6 Rep Power: 16	Thats great, many thanks to you both for your help

October 27, 2011, 17:48		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	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