Floating Point Error in FGM combustion transient

- October 21, 2022 at 9:23 am
  
  Nick Pester
  Subscriber
  
  Hi,
  I am currently performing a reacting case of a swirling combustor. My model is (partially premixed combustion) FGM diffusion model. I ran an initial steady case, and it worked perfectly fine converging well, and when comparing velocity profiles versus experiment, they were really good. I am now trying to do the same case but transient, to do this, I used my steady state solution and ran the transient case with a time step of 1e-6 to keep the courant number well below 1. The turbulence model used in k-e realizable. When running the transient case at the first time step, the solution diverges, and a floating point error is given. I am using a coupled solver with a very low URF, but I still have the same issue. I tried using SIMPLEC as well but still same error. I also tried to patch the progress variable to 1 and still diverged, and then tried to patch it to 0 and still diverged. Any ideas about what could be the issue and how to sort it out?
  Thank you,
  Nick
- October 21, 2022 at 10:37 am
  
  Rob
  Forum Moderator
  
  I'm not aware of anything that'll cause that in the FGM model, I also asked one of the European experts. Have you got any expressions, profiles or UDFs attached to the case?
- October 21, 2022 at 10:47 am
  
  Nick Pester
  Subscriber
  
  Hi Rob,
  Nothing attached. Just the mechanism file and thermodynamics at the beginning to create the PDF table for the FGM. Are there any good practices you recommend when doing combustion transient (setup, timestep, etc.)?
- October 21, 2022 at 12:05 pm
  
  Rob
  Forum Moderator
  
  If it worked steady we're not sure why transient isn't working. Please can you add screen shots of the species & combustion panels.
- October 21, 2022 at 12:20 pm
  
  Nick Pester
  Subscriber
  
  Hi Rob,
  Please find below the images.
  Thank you,
  Nick
- October 21, 2022 at 2:09 pm
  
  Rob
  Forum Moderator
  
  That looks fine. Which version of Fluent are you using?
- October 21, 2022 at 2:38 pm
  
  Nick Pester
  Subscriber
  
  2021 R1
- October 24, 2022 at 8:18 am
  
  Rob
  Forum Moderator
  
  Some comments from over the weekend. A couple of colleagues have seen an error but it's not repeatable enough to diagnose.
  Try restarting after switching to transient, ie switch over, write case & data. Close Fluent. Open Fluent & load files.
  Double check you have got the time step set correctly. There's a GUI glitch which can void the setting, check the ribbon AND tree menus are seeing the same result.
- October 24, 2022 at 8:30 am
  
  Nick Pester
  Subscriber
  
  Will try that. Thank you very much for your time and help, Rob.