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Why Menter's SST model low-Re issue has not been seriously investigated?

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Old   July 30, 2013, 10:16
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sivakumar selvaraju
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Sure, I am doing it.

I have a small doubt, for epsilon do I need to use epsilonWallFunction??

top0 (wall)
{
type epsilonWallFunction;
value uniform 0.26295;

}
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Old   July 30, 2013, 10:18
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If you have y+~1, you can resolve the full boundary layer, so that you don't need to use any wall function.

Simply put k=1e-9 and epsilon = 1e-9 at the wall and you should be good!
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Old   July 30, 2013, 10:21
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the key word should be fixedValue right?

top0 (wall)

type fixedValue;
value uniform 1e-9;
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Old   July 30, 2013, 10:22
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yep, that's right.
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Old   July 30, 2013, 13:47
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Out of curiosity, I tried to use the k-omega model on some basic test cases. The version implemented in OpenFOAM is the original Wilcox 1988 model. Impossible to get a solution!

Has someone already faced this problem? I checked in the .C and .H, but could not find any mistake in the implementation so far...
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Old   July 30, 2013, 14:09
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Actually, it seems to work. I just needed to be more careful with the initial and boundary conditions...
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Old   August 1, 2013, 05:45
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Dear All,
As Joachim and others instruction, I have tried both LaunderSharmaKE and kOmegaSST for low re calculation. but none of them working fine.

the Boundary conditions which i used are

For kEpsilon model:

0/k:

internalField uniform 0.39;

boundaryField
{
inlet
{
type fixedValue;
value uniform 0.39;
}
wall
{
type fixedValue;
value uniform 1e-12;

0/e:

internalField uniform 0.26295;

boundaryField
{
inlet
{
type fixedValue;
value uniform 0.26295;
}

wall
{
type fixedValue;
value uniform 1e-12;
}

0/nut:

internalField uniform 0;

boundaryField
{
inlet
{
type calculated;
value uniform 0;
}

wall
{
type nutLowReWallFunction;
value uniform 0;
}

for kOmegaSST:

0/k:

internalField uniform 0.39;

boundaryField
{
inlet
{
type fixedValue;
value uniform 0.39;
}
wall
{
type fixedValue;
value uniform 1e-12;


0/Omega:

internalField uniform 3.7;

boundaryField
{
inlet
{
type fixedValue;
value uniform 3.7;
}

wall
{
type omegaWallFunction;
value uniform 1441;
}

0/nut:

internalField uniform 0;

boundaryField
{
inlet
{
type calculated;
value uniform 0;
}

wall
{
type nutkWallFunction;
value uniform 0;
}

it seems for low re kepsilon the boundary conditions are fine but the simulation crashes due to floating point, the same error for kOmegaSST.

I tried different BC as well but none of them working fine.

can you guys spend few minutes to check my BCs to find what is wrong?

Thanks,
Sivakumar
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Old   August 1, 2013, 09:48
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Hello Sivakumar!

I did some low Re simulations with k-omega-SST model and it worked fine for me. Your bc's look good to me (only refering to k-omega-SST), so there may be another problem with your simulation.
For k you could also try a zero gradient bc. For nut you could try to set the value to zero (like in the nutLowReWallFunction) or just don't correct it (calculated).
Maybe your freestream bc's are not appropriate but thats hard to say without knowing the characteristics of the flow.

Regards, Andre
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Old   August 1, 2013, 11:28
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Hi Andre,
Thanks for your reply, I believe that the problem could be mesh.
however checkMesh is not giving any warning and error message.

Thanks,
Sivakumar
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Old   October 22, 2013, 06:18
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Sivakumar, did you resolve the problem? I have the same issue. I have two meshes which, in essence, are identical but one is fine with around 2000k cells and the other is coarse with around 500k cells. The fine mesh spends most of the extra cells near the walls and so the y+ min/max/avg are significantly lower.

OpenFOAM 2.2.2 blows up on the fine mesh but solves perfectly on the coarse one. I've been changing nut/k/omega BCs to whatever is suggested here on cfd-online without luck. I will continue to investigate on the mesh distribution close to the walls since some say that smoothness is important for OF. My mesh is made from blockMesh so it is already quite smooth, but there is certainly stretching near walls. No checkMesh issues.

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Old   October 22, 2013, 08:09
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Dear Mads,
Yes, I have solved that problem, the cases are running fine.
I have few questions, which turbulence model you want to use? what is the yplus value for the fine mesh? , I think KOSST will be fine with all kind of y+ values, but for LaunderSharmaKE we need be little bit careful.

my problem was yplus value, it was less than 1(people usually says y+ less than or equal to 1 will be fine, but in my experience not), that is why my case was diverging, now I changed the y+ value to in between 3 to 5 its working fine.

I hope you will be fine with BC's, if you need any info regarding that please go through the below link.

Launder Shrma Low Reynolds turbulence model case setup


Thanks,
Sivakumar
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Old   October 22, 2013, 09:17
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Dear Sivakumar,

thank you for your reply. My geometry is quite complex so y+ varies quite a lot from one area to another. I try to get as low on y+ as possible because I know that I need low-Re treatment in certain areas but in other areas I save cells and get a larger y+ (maybe even into the hundreds).
A code like CFX copes nicely with this using their automatic wall functions but I am not sure how this works out in OpenFOAM - it crashes for now It seems that it should be ok to use omegaWallFunction as it "automatically" adopts to local, sensible, y+ whereas I am more uncertain about the wall treatment for k. I am not going to use LaunderSharmaKE.

I think I will try to remesh so I don't have y+<1 anywhere which seems to have been your problem.

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Old   April 4, 2014, 05:07
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Joachim,

I ran the case in Fluent and get the following:
k-omega-sst-tau395.png

And also compared to Spalart-Allmaras:
tau_w.jpg
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Old   May 9, 2014, 10:17
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Jonathan, I don't know if you still use it, but the function you call "gammaStar0" is not correct. At least not compared to Fluent.
In Fluent they don't use the "SST" beta_i but the regular "k-Omega" beta_i for the "beta_i / 3.0" calculation. So you neet to change it to

return 0.072 / 3.0;

instead of

return betai(F1) / 3.0;
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Old   May 13, 2014, 03:31
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Please find a working version of the incompressible sst model with low-Re wall damping functions here:
kOmegaSST with lowRe-damping (as in Fluent)
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Old   May 14, 2014, 06:10
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Hi Phillip,

thanks for the post + correction. I havent had time to check the correction you made, but surely it is correct - I didnt get anyone willing to help debug first time around

when i get a chance i will look and see why i wrote it like i did!

cheers and regards
jonathan
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Old   May 14, 2014, 06:14
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It's not your fault. It's the Fluent manual that is misleading.
There is also a second typo in the Fluent manual in the omega production term, that I corrected in my code.
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Last edited by RodriguezFatz; May 14, 2014 at 08:18.
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Old   June 12, 2014, 21:21
Default Problems with validating kOmegaSST for channel flow
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Hi Vesselin and Joachim

While using OpenFOAM 2.2x to implement two dimensional developed, channel flow with kOmegaSST, I have encountered the same results as Joachim and have found it quite difficult to figure out the issue.

Here's a post where I talk about it.

Using k-omega SST to simulate simple channel flow

Maybe I should try the same on OF 1.7x as suggested by Vesselin.
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