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April 9, 2013, 08:35 
motorBike Residuals for SST komega... and mine

#1 
Senior Member
Jose Rey
Join Date: Oct 2012
Posts: 131
Rep Power: 11 
Hi,
I have been fighting for a while trying to bring the residuals for a model I am running. I am using simpleFoam and SST komega turbulence model. I decided to plot the residuals for the simpleFoam motorBike (OF2.2.0) and their residuals don't look too good either. I've followed some of the strategies in the forum to bring them down, and the strategies have worked to some degree (the best I've gotten is about one order of magnitude better than the image for my residuals). If I make a very rough model of a sphere suspended in the air with the same settings as my model and a fairly coarse mesh, I get amazingly low residuals. Any comments? Is there a general criteria for residuals for SST komega? Thanks in advance Here are the residuals for the OF2.2simpleFoam motorBike tutorial. You could run this sucker forever, and the residuals will stay moreless the same: Here are my residuals: Thanks very much in advance. Last edited by JR22; April 9, 2013 at 11:33. Reason: fixed CFD language 

April 17, 2013, 14:43 

#2 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 237
Rep Power: 10 
Check the yPlus values of your models. To do so you can use the yplusRAS command to calculate it. To use the k omega you need to have a maximum yPlus of about 4.


July 30, 2013, 09:39 

#3 
New Member
Stefan Gaerling
Join Date: Dec 2012
Posts: 22
Rep Power: 6 
Hey JR,
I am currently facing the same problems regarding komegasst with simpleFoam. Other than fredo490 I don't think that you need an y+ = 4. It is nowhere said definitely that the implemented komegasst model works for LowRe only. Most threads say that it is capable as well for HighRe and LowRe simulations. But I have to say that your residuals dont seem "amazingly low" to me like you said. This is a residual plot from my benchmarking regarding wall treatment, different y+ values and boundary layer mesh which i am currently struggling with: https://www.dropbox.com/s/9q8ubx9fg3...st%20order.png Please let me know if you get any further with this because i'm really interested in getting komegasst to work reliable 

July 31, 2013, 08:08 

#4 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 237
Rep Power: 10 
KOmega SST works with y+ > 4 because it switchs to a high Reynolds model. So don't expect to take advantages of Komega SST if you have a y+ > 4 ! Just take a standard kepsilon and it's done. Moreover, try to avoid to have 4 < y+ < 20 because the model might give you some strange behavior (it is also true on Fluent). You have to choose: y+ < 4 or y+ > 20.
The only way to predict the transition laminar/turbulent is to use a y+ < 1 with Komega SST Plus, there are some effects that are impossible to model if your y+ > 4. 

August 1, 2013, 01:41 

#5 
New Member
Stefan Gaerling
Join Date: Dec 2012
Posts: 22
Rep Power: 6 
So you're saying that komegasst with wall functions on a mesh with Y+> lets say 30 gives no advantage to standard kepsilon?
Why then did henry state out that the highreversion of komegasst is currently implemented in openfoam? What would be the use of a highre komegasst which is  as you say  just some instable version of standard kepsilon with no additional use? I'm rather confused right now. 

August 1, 2013, 08:57 

#6 
Senior Member
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 237
Rep Power: 10 
I've never said that it is an "instable version of standard kepsilon"...
1) I said that no matter what, it is dangerous to work with a 4 < y+ < 20 (which is the area near y+ = 11 that is the limit of the log law) ! If you work with a y+ between 4 and 20, the program will locally switch from a wall function to a "classic" boundary condition and this might give you a messy results. Moreover, many cases report some mesh dependency when the y+ is between 1 and 4 for kOmega SST . 2) KOmega SST theoretically works for all y+ < 300... So yes, it works for y+ > 4 but in this case the results are fairly (very ?) similar to a kepsilon model since it will use a similar wall functions and that the free stream will use a kepsilon behavior. 3) The main purpose of using KOmega SST is to predict the adverse pressure gradients and separating flow areas (but for this, you need a y+ < 4 and ideally y+ < 1). 

August 1, 2013, 09:08 

#7 
New Member
Stefan Gaerling
Join Date: Dec 2012
Posts: 22
Rep Power: 6 
Thanks a lot for this clarification and sorry that i misunderstood your points


Tags 
motorbike, openfoam 2.2.0, residuals, sst komega 
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