CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   CFX v12 adding reattachment modification to sst model (http://www.cfd-online.com/Forums/cfx/88302-cfx-v12-adding-reattachment-modification-sst-model.html)

ovechkin May 13, 2011 05:18

CFX v12 adding reattachment modification to sst model
 
Hello,

I need to add the reattachment modification to the Shear Stress Transport model, but Im not sure how to do it. I assume i need to add some lines in the definition file..
can someone give me a clue how to do this?

thanks a lot,
ovechkin

ovechkin May 13, 2011 06:56

..got it. just add "SST Reattachment Modification = On" to Turbulence Model in Command Editor..

cfd_newbie May 13, 2011 06:59

Quote:

Originally Posted by ovechkin (Post 307510)
..got it. just add "SST Reattachment Modification = On" to Turbulence Model in Command Editor..

Hi,
Do you know what is the SST Reattachment Modification how is it different from the standard SST ? Why and where is it used. Can you share some references ?

ghorrocks May 13, 2011 07:08

I am not familiar with the reattachment model. In fact I did not even know it was in there. Do you know any details about it, or have any documentation on it?

ovechkin May 13, 2011 07:45

Unfortunately I don't have any references/documentation on it. All I know is that the Reattachment Model accounts for an additional production of kinetic energy in seperated shear layers.
We use it for simulation of a turbulent premixed combustion process

Far June 27, 2011 15:50

It is basically related to fact that the baseline SST (original SST) predicts the larger separation bubble than the experimental values. This is due to the wrong prediction of reattachment point (boundary layer attachment at further downstream location). Therefore the RM modification was introduced in production term to account for the realistic separation bubble length and hence earlier attachment. It is to be noted that this limiter is not activated in the attached flow and hence base line sst is fully recovered.

Keeping in view above discussion, base line SST model has difficulties in predicting the stall point correctly and simulation often stops due to premature numerical stall.


Best Regards
Far

swm March 4, 2014 04:59

Quote:

Originally Posted by ovechkin (Post 307510)
..got it. just add "SST Reattachment Modification = On" to Turbulence Model in Command Editor..

In version 14.5,it seems that it is NOT allowed to add this to TURBULENCE MODEL?

alinik June 3, 2016 19:51

folks,

I am using CFX v15.1
I cannot add this line to the command editor.
I constantly receive this error:

ERROR
CCL validation failed with message:
Error: Sub-object 'FLUID MODELS' is not allowed in /

any ideas? any kind of help is much appreciated.

ghorrocks June 4, 2016 00:52

Why not just set it up in CFX-Pre and see what syntax it uses?

alinik June 4, 2016 15:35

CFX 12 and 15 which I have on my machine do not have this option in the advanced turbulence control. According to this post there should be a way to get away around this problem.
I use the same syntax that people have recommended here but I am getting the error I said.
I suppose some people have already solved this problem.
Really appreciate it if someone can help me out with this issue.

Thanks,
Ali

ghorrocks June 5, 2016 06:20

My understanding is the options are available in CFD-Pre if your licensing allow those options. If you can't see the options then your licence does not support the advanced turbulence models and therefore putting it in via CCL will not help as the solver won't accept it.

alinik June 5, 2016 15:40

I do not think it is a license issue while I have the full license.
There should be some other way. The way this thread had initially started suggests that there is some alternate method for enabling this option using command editor.
Any kind of help is much appreciated folks.

Thanks,
alinik

ghorrocks June 5, 2016 20:13

If you look in the <CFX_ROOT>/etc/RULES file and search for the term it gives you some hints to the syntax. This might help you.

If that does not work post the CCL you are trying to use on the forum.

alinik June 5, 2016 23:12

Thanks man for the info.
I checked the file and found the syntax in the file.
It is actually the 8th line in the following text.
in the command editor I write this line according to ovechkin and get the error I said.
"SST Reattachment Modification = On"

CONTEXT: SST
Optional Parameter List = \
Eddy Viscosity, \
BC TKI Factor, \
BetaStar Coefficient, \
Compressible Production, \
A1 Coefficient, \
SST Reattachment Modification
Optional Child List = \
BUOYANCY TURBULENCE, \
K OMEGA REGIME, \
K EPSILON REGIME, \
TRANSITIONAL TURBULENCE, \
PRODUCTION LIMITER, \
CURVATURE CORRECTION
END

ghorrocks June 5, 2016 23:17

Further down in the RULES file is where it shows the syntax of the specific element. It says the default value is "No". So I assume the values it is looking for is "Yes" or "No". That's just a guess.

alinik June 5, 2016 23:30

This is what I found further down.
What shall I do? Shall I change the "No" to "Yes"?


PARAMETER: SST Reattachment Modification
Description = Determines whether the reattachement modification is \
used in the SST turbulence model
Parameter Type = Logical
Solver Name = LSSTRM
Default = No

ghorrocks June 5, 2016 23:43

This is a beta option and so is not documented anywhere. You have to figure it out.

In this case the easiest way to activate it is in CFX-Pre. Go to edit/options/general and select "enable beta features". Then go to the turbulence model options, select SST and the reattachment model will be visible under advanced turbulence control.

Far June 7, 2016 10:12

As per my discussion with F. Menter, he said that "Reattachment model is not physically correct".

Thats why now I dont use this option anymore.

ghorrocks June 7, 2016 21:33

Did he explain what is wrong with it?

alinik June 8, 2016 13:41

Glenn

Thank you for your reply. I have enabled beta features but there is still no sign of SST Reattachment modifier. Any clue what to do?

Sijal

Thank you also for your comment. What do you suggest we should do if we want to control separation?

Thanks,
Ali


All times are GMT -4. The time now is 05:48.