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-   -   BC's for k-omega SST (low / hi Re setups) (http://www.cfd-online.com/Forums/openfoam-solving/107375-bcs-k-omega-sst-low-hi-re-setups.html)

Jonathan September 25, 2012 19:22

BC's for k-omega SST (low / hi Re setups)
 
1 Attachment(s)
hi everyone,

i have been testing my k-omega SST setup for OF using a simple flat plate geometry, and BC's i have worked out from looking at a number of other posts on the issue. although there is a *lot* out there on the setups you need to use, i dont seem to get the impression that there has been widespread agreement on how the model is supposed to be implemented :D

anyway, so i have a low Re and a hi Re mesh. the BC's for the two meshes are:

For the hi Re mesh:
Code:

'k'
WALL
{
  type  kqRWallFunction;
  value  uniform 1;
}

'omega'
WALL
{
  type  omegaWallFunction;
  value  uniform 1;
}

'nut'
WALL
{
  type  nutkWallFunction;
  value  uniform 0;
}

and for the low Re mesh:

Code:

'k'
WALL
{
  type  fixedValue;
  value  uniform 1e-12;
}

'omega'
WALL
{
  type  omegaWallFunction;
  value  uniform 1e9;
}

'nut'
WALL
{
  type  nutUSpaldingWallFunction;
  value  uniform 0;
}

Also attached is a plot of the non-dimensional parameters for the wall, using the standard definitions (i.e. not Cmu etc!). As you can see, the low Re result looks ok, but the hi Re BL is too think.

Using yPlusRAS to check the meshes, i see that the first node points for each mesh are in the correct region, so i cant quite work out what the problem is. BC's perhaps??

anyway, i would appreciate any feedback on anything obvious i have gotten wrong + am quite keen to 'finalise' the 'proper' approach for the current implementation of k-omega SST (OF v2.1.1)

thanks a lot in advance!
Jonathan

Jonathan September 26, 2012 09:33

never mind ... solved it ...

mrt September 28, 2012 17:37

Hi Jonathan,

How did you resolve this?

openfoam_user October 4, 2012 07:39

Hi Jonathan,

what formula do you use to set 'value uniform 1e9'
for k at the wall for your low Reynolds mesh (y+~1) ?

Best regards,
Stephane.

Jonathan October 4, 2012 08:06

Hi Stephane,

I didnt use a formula - looking through the forums and theory, i realise that omega tends to infinity at the wall - therefore, i just used a large number as the omega BC ...

regards
jonathan

kiddmax February 2, 2014 19:15

Hi Jonathan

I also did this validation with komega SST turbulence model. But with Low Re and high Re model both failed. There is mismatch in the log-law region. Can you please give a hint how you solve it?

Best regards,
Ye

Jonathan February 3, 2014 04:07

hi Kid,

I havent looked at OF for a little while, but in the last version i was using (2.1.1), i finally used the following for a low Re mesh:

'k' -> set to SMALL no at wall
'omega' -> use omegaBC built into OF. This calculates omega at the wall using the correct formula from Menter

'nut' -> I used 'calculated' condition and not nut wall function as in my first post

PS I didnt do anymore with the hi-Re meshes, as my project needed low-Re only.

hope this helps a little.

cheers
jonathan

PPS I dont know what the latest versions of OF are using for turbulence wall BC's these days. Might pay to download the latest UG and look through the release notes and code to see what is actually going on ... :) cheers jon

Quote:

Originally Posted by kiddmax (Post 473038)
Hi Jonathan

I also did this validation with komega SST turbulence model. But with Low Re and high Re model both failed. There is mismatch in the log-law region. Can you please give a hint how you solve it?

Best regards,
Ye


kiddmax February 10, 2014 16:36

Hi Jonathan,

Thanks a lot for your reply. I finally got the correct result after several trying.

Best regards,
Ye


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