Problem with Spalart Allmaras turbulence model

david24 · July 31, 2023, 17:05

Hello everyone, I wonder if I could get suggestions to solve the following issue.

I did a simulation of a centrifugal compressor using the SST turbulence model in CFX without any problem (the mesh was ok and the y+ was lower than 1) however when I changed to Spalart Allmaras, the simulation crashed after, more or less, 15 iterations. To solve it, I tried to simulate with a less refined mesh in order to use it for the correct one but it failed. Besides, I tried using upwind and first-order schemes instead of high resolution it didn't work. I even tried to use the results from the SST simulation to initialize this one but it crashed.

Can anyone give some recommendations about what I could try?

ghorrocks · July 31, 2023, 20:10

The Spalart Allmaras run crashed - but why? What clues was there leading up to it? Was it diverging? The linear solver failing? Or did it suddenly crash? Did it give any error or warning messages?

Please upload your output file if you want us to look at this.

siw · August 1, 2023, 02:51

Slightly off-topic, but isn't the S-A model in perpetual beta status in CFX? What (if any) wall treatment does CFX use with the S-A model?

david24 · August 1, 2023, 04:01

Oh I'm sorry, I will post it on the forum adding the output file

david24 · August 1, 2023, 04:13

Quote:

Originally Posted by siw

Slightly off-topic, but isn't the S-A model in perpetual beta status in CFX? What (if any) wall treatment does CFX use with the S-A model?

Yes it is, I had to activate beta features to use it and the wall function that uses the so-called automatic near-wall treatment as default but it can be changed also to the scalable wall function

siw · August 1, 2023, 04:43

I'd not be surprised there are problems as Ansys clearly aren't going to bring this out of beta status (not that Ansys bother much with CFX anymore anyway).

Searching through the Ansys Customer Portal Knowledge Resources for the S-A model in CFX for any hints does bring up three instances:

#610201 What is the y+ requirement for Spalart Allmaras model in CFX?
#2029289 How to set boundary conditions for the Spalart-Allmaras-Model in CFX?
#2056102 How to set up a simulation, which is using the Spalart-Allmaras turbulence model, in CFX? (includes a PDF).

I'm not going to upload them here but I assume you have access as an Ansys customer.

david24 · August 1, 2023, 04:55

Quote:

Originally Posted by siw

I'd not be surprised there are problems as Ansys clearly aren't going to bring this out of beta status (not that Ansys bother much with CFX anymore anyway).

Searching through the Ansys Customer Portal Knowledge Resources for the S-A model in CFX for any hints does bring up three instances:

#610201 What is the y+ requirement for Spalart Allmaras model in CFX?
#2029289 How to set boundary conditions for the Spalart-Allmaras-Model in CFX?
#2056102 How to set up a simulation, which is using the Spalart-Allmaras turbulence model, in CFX? (includes a PDF).

I'm not going to upload them here but I assume you have access as an Ansys customer.

I'm not sure if as a student I can access there but I will check it. Thank you

July 31, 2023, 17:05	Problem with Spalart Allmaras turbulence model	#1
david24 New Member David Join Date: Jul 2023 Posts: 4 Rep Power: 3	Hello everyone, I wonder if I could get suggestions to solve the following issue. I did a simulation of a centrifugal compressor using the SST turbulence model in CFX without any problem (the mesh was ok and the y+ was lower than 1) however when I changed to Spalart Allmaras, the simulation crashed after, more or less, 15 iterations. To solve it, I tried to simulate with a less refined mesh in order to use it for the correct one but it failed. Besides, I tried using upwind and first-order schemes instead of high resolution it didn't work. I even tried to use the results from the SST simulation to initialize this one but it crashed. Can anyone give some recommendations about what I could try?

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July 31, 2023, 20:10		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The Spalart Allmaras run crashed - but why? What clues was there leading up to it? Was it diverging? The linear solver failing? Or did it suddenly crash? Did it give any error or warning messages? Please upload your output file if you want us to look at this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

August 1, 2023, 02:51		#3
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 742 Rep Power: 26	Slightly off-topic, but isn't the S-A model in perpetual beta status in CFX? What (if any) wall treatment does CFX use with the S-A model?

August 1, 2023, 04:01		#4
david24 New Member David Join Date: Jul 2023 Posts: 4 Rep Power: 3	Oh I'm sorry, I will post it on the forum adding the output file

August 1, 2023, 04:43		#6
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 742 Rep Power: 26	I'd not be surprised there are problems as Ansys clearly aren't going to bring this out of beta status (not that Ansys bother much with CFX anymore anyway). Searching through the Ansys Customer Portal Knowledge Resources for the S-A model in CFX for any hints does bring up three instances: #610201 What is the y+ requirement for Spalart Allmaras model in CFX? #2029289 How to set boundary conditions for the Spalart-Allmaras-Model in CFX? #2056102 How to set up a simulation, which is using the Spalart-Allmaras turbulence model, in CFX? (includes a PDF). I'm not going to upload them here but I assume you have access as an Ansys customer.