# One more time: wall functions - SST

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September 29, 2016, 09:56
One more time: wall functions - SST
#1
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Hi everyone,

I struggle choosing the right wall functions. I am using SST in my simulation, since it combines the advantages of k-epsilon and k-omega. As far as I searched for wall functions and SST in this forum I found:

http://www.cfd-online.com/Forums/ope...ion-usage.html

Quote:
 nutUSpaldingWallFunction (standard for SA turbulence model, called nutSpalartAllmarasWallFunction in earlier version, original reference is doi:10.1115/1.3641728): continuous wall-function which should cover the complete y+ range from O(1) to somewhere of O(10). Might be the best choice (together with low Re kEpsilon, kOmegaSST or SA, when y+ varies for different parts of the wall.
https://www.comsol.com/blogs/which-t...d-application/

Quote:
 Finally, the SST model is a combination of the k-epsilon in the free stream and the k-omega models near the walls. It does not use wall functions and tends to be most accurate when solving the flow near the wall.
http://www.cfd-online.com/Forums/ope...estigated.html

Quote:
 Note that the k-omega SST model we provide is in high-Re form and does not include the wall-damping terms often included in the k-omega model for near-wall and low-Re flow. However, you can still use the k-omega SST model for low-Re and near wall flow for a range of resolutions if you use a continuous wall-function (which in OpenFOAM-1.7.x is named nutSpalartAllmarasWallFunction for historical reasons) and this should be used as the wall BC in nut. The BC of k for the continuous wall-function should be kqRWallFunction.

I am wondering now: one source says SST uses wall functions, one says: no . So one is wrong or do I confuse anything?

The range of my y+ values are for a really fine grid on some walls 0.01 and on other walls y+=45.

So when I use nutSpalartAllmarasWallFunction , what range should my y+ be?

Member vkrastev says:
Quote:
 Well, the good news are that the SST model CAN actually be employed to resolve boundary layers with y+<1 (as it is supposed to), both with the omegaWallFunction BC as well as with directly imposing Menter's BC for the viscous sublayer.
So my y+ value does not play a role when using nutSpalartAllmarasWallFunction ?

Could someone bring light into the dark (dark for me).

By the way: currently I am using mutkWallFunction which makes trouble for very fine meshes with SST. So that's why I have doubts about it.

Sorry I am a rookie in CFD :-/

Thank you guys!

October 5, 2016, 10:58
#2
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Quote:
 Originally Posted by CellZone The range of my y+ values are for a really fine grid on some walls 0.01 and on other walls y+=45.
Hi, I am just wondering. Is there something like a too small y+ value?

Best regards,

Kate

Last edited by KateEisenhower; October 5, 2016 at 11:02. Reason: typo

 October 7, 2016, 08:52 #3 Member   Join Date: Apr 2016 Posts: 91 Rep Power: 10 I think yes. If the grid is too fine, the wall laws can lead to wrong results. Does anyone have some suggestions?

 February 3, 2017, 07:07 #4 New Member   Martin Join Date: Oct 2013 Location: Newcastle Posts: 21 Rep Power: 12 Hi There, did you manage to clarify the above? I'm looking for some more information regarding yplus levels in combination with KomegaSST and the nutUSpaldingWallFunction. I'm running simulations at the moment with yplus ranging from <1 to <100. If someone can clarify or suggest limits on yplus in this approach would be very helpful as i'm trying to work with automatically generated meshes in Openfoam which makes mesh manipulation quite hard to find good yplus. Thanks

 February 3, 2017, 11:34 #5 Senior Member   Join Date: Mar 2014 Posts: 112 Rep Power: 12 I have done a test to understand the same issue about 2 years ago with 2.3.1. The set up was a 2D external case with naca0012 profile. The kwSST model in combination with nutUSpladingWallFunction run on 11 different meshes (structured O-grid / Re 1,8e06) which have following average yPlus values: 1, 5, 15, 30, 50, 70, 100, 150, 300, 500 and 1000. The first two have diverged and/or produced unphysical results and all others worked and produced acceptable results. If I remember correctly average yPlus 30 was the best against the wind tunnel data. The same setup worked well with SA model on all mesh range since the SA implementation on OpenFOAM can work on low and high Re modes but kwSST can not. There are also some lowRe implementations of kwSST for OpenFOAM around though and I guess you can find it in this forum... Clément_G and R.Tanaka like this.

 February 3, 2017, 13:34 #6 Senior Member   Mikko Join Date: Jul 2014 Location: The Hague, The Netherlands Posts: 243 Rep Power: 12 I have simulated NACA0012 with recent foundation and foam-extend versions and my results are close to NASA's. I used k-omega SST turbulence model without wall functions (y+<1) and following boundary condition for the wall: nut=0, k=0 and omega=omegaWallFunction. Clément_G, pgh, kuprat and 1 others like this.

February 8, 2017, 07:29
#7
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jey
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Quote:
 Originally Posted by Flowkersma I have simulated NACA0012 with recent foundation and foam-extend versions and my results are close to NASA's. I used k-omega SST turbulence model without wall functions (y+<1) and following boundary condition for the wall: nut=0, k=0 and omega=omegaWallFunction.

Dear Flowkersma

I can understand that since y+~1, you don't have to use wall functions (as you already did for nut and k). But why did you still use wall functions for omega? Is there a specific reason for that?

Kind regards

 February 8, 2017, 08:59 #8 Senior Member   Mikko Join Date: Jul 2014 Location: The Hague, The Netherlands Posts: 243 Rep Power: 12 Note that omega is not zero at the wall and therefore you cannot use fixedValue boundary condition. See more from Menter's original paper or from here . jeytsav, saeed jamshidi and openfoam_aero like this.

 February 9, 2017, 09:00 #9 Senior Member   Join Date: Mar 2014 Posts: 112 Rep Power: 12 I was not aware that the standart kOmegaSST model in OpneFOAM is capable to work on on low-Re grids and had given up after the failed results with the continious wall function (nutUSplading) on this model as decribed my above post. I have just tested the standart kOmegaSST without wall functions on y+ ~1 meshed cases and got good results with that approcach so thanks for the information to Flowkersma. kuprat and AMR96 like this.

 February 9, 2017, 09:50 #10 Member   jey Join Date: Nov 2016 Location: Greece Posts: 30 Rep Power: 9 Dear Flowkersma and mzzmrt I am using the conditions that Flowkersma suggested (post#6) for a y+~1 case, but I can not make it work (it crashes after some time) But.. I tried using a fixedValue for omega at wall (as suggested by Menter - see attached picture) and the simulations works fine until now. omegaAtWall.PNG I am really frustrated that I can not make it work with omegaWallFunction while you people can do it. I am probably missing something? Please find here my /0 and /system folder, in case you can help with any suggestions. p.s.1: The value I am calculating at y=0 according to Menter, is of the same order of magnitude as the one that omegaWallFunction sets at wall. p.s.2: This is the way I implement the omegaWallFunction type. I noticed that the function is completely independent of the "value" entry. So, the "value" entry is just a placeholder. Is it correct? Code: ``` { type omegaWallFunction; value \$internalField; }``` p.s.3: I have already tested my case with SA model and it works. Now I am trying to switch to kwSST. Thank you in advance! Kind regards

 February 9, 2017, 17:42 #11 Senior Member   Mikko Join Date: Jul 2014 Location: The Hague, The Netherlands Posts: 243 Rep Power: 12 The value field is the initial condition. Generally, a good initial value results in faster convergence. Bad initial value may result in divergence. In this case it probably does not play a big/any role. I use first order upwind scheme for divergence for the turbulence quantities. Give it a try. jeytsav likes this.

December 16, 2017, 06:39
no wall function
#13
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Guilherme
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Quote:
Tobi, would you mind teaching us how I do not put wall functions? Many files within the CFDOnline forum mention the komegaSST model and some wall functions for y+ ~1. As you have very clearly stated your point of view, I would like more information on how to '' model '' my problem.

 December 17, 2017, 06:01 #14 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,708 Blog Entries: 6 Rep Power: 51 Hi, actually I am not familiar with turbulence modeling. As far as I understand the problem --- which should be stated in my last post --- in a numerical/mathematical way, if you have y+~1, then you should not apply wall functions (sure, the subsequent cells should resolve the viscous sub layer). Therefore, I do not know which value one has to set. ZeroGradient? I read somewhere that you fix k to very small value such as 0.001. But please don't ask me about that. There are much more advanced users here who should give a fundamental and clear statement. __________________ Keep foaming, Tobias Holzmann

 April 19, 2018, 17:42 #15 Senior Member   Join Date: Mar 2018 Posts: 115 Rep Power: 8 Hello Tobias How can I disable wall functions in OpenFOAM if I have y+ << 1 ?

 June 29, 2018, 00:00 #16 New Member   Juan Felipe Monsalvo Salazar Join Date: Apr 2013 Location: Medellín / Colombia Posts: 4 Rep Power: 13 Dear Tobias Just to be sure, reviewing the following link https://en.wikipedia.org/wiki/Law_of_the_wall I saw that the log law is only valid in the range between [ 30 < y+ < 200 or 300] and not at [ 0 < y+ < 16.8] as you state. Neverthless I'm not sure if you was referring to the wall of law when talking about log law Sincerely, Juan Felipe Monsalvo

June 29, 2018, 16:44
#17
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Juan Felipe Monsalvo Salazar
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Quote:
 Originally Posted by Tobi Hi, actually I am not familiar with turbulence modeling. As far as I understand the problem --- which should be stated in my last post --- in a numerical/mathematical way, if you have y+~1, then you should not apply wall functions (sure, the subsequent cells should resolve the viscous sub layer). Therefore, I do not know which value one has to set. ZeroGradient? I read somewhere that you fix k to very small value such as 0.001. But please don't ask me about that. There are much more advanced users here who should give a fundamental and clear statement.
Dear Tobias

Just to be sure, reviewing the following link https://en.wikipedia.org/wiki/Law_of_the_wall I saw that the log law is only valid in the range between [ 30 < y+ < 200 or 300] and not at [ 0 < y+ < 16.8] as you state.

Neverthless I'm not sure if you was referring to the wall of law when talking about log law

Sincerely,
Juan Felipe Monsalvo

 January 20, 2020, 02:49 #18 Senior Member   Mehdi Baba Mehdi Join Date: Jan 2011 Posts: 158 Rep Power: 15 A good reference for why we need wall function near wall is this link Tobi and Raphael_Santos like this.

 September 21, 2020, 07:25 #19 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,708 Blog Entries: 6 Rep Power: 51 Nice summary but a small review would increase its qulaity significantly in terms of readablity, typo and others. __________________ Keep foaming, Tobias Holzmann

 March 30, 2021, 19:53 #20 Senior Member   Join Date: Jun 2011 Posts: 205 Rep Power: 16 also it would be very helpful (and practical) if one knows and lists the Y+ values of the grid needed (i.e. providing the highest theoretical accuracy) when using different turbulent models (the most popular ones). The information in the links above is too difficult to digest to a simple conclusion especially by not CFD professionals. As Tobias said a short summary on this important subject would be very helpful to a lot of users.