Backflow in multiphase model is wrong phase

- September 8, 2023 at 5:06 pm
  
  bv169
  Subscriber
  
  Hello,
  I am running an open channel multiphase model. When any backflow enters the outlet, the model crashes as there is divergence in the residuals. I finally figured out it is due to the wrong phase entering as backflow. Is there a way to control this? or turn off backflow?
  Thanks,
  Breanna
- September 11, 2023 at 1:40 pm
  
  SRP
  Ansys Employee
  
  Hi,
  In the boundary condition, you can set the volume fraction for the backflow.
  You can check fluent user manual for more detail: 25.2. Steps for Using a Multiphase Model (ansys.com)
  Thank you.
- September 11, 2023 at 1:46 pm
  
  Rob
  Ansys Employee
  
  If you look in the phase options in the outlet you'll find what you're looking for. However, if you're using Open Channel VOF you should find options to set the free surface level.
- September 11, 2023 at 4:04 pm
  
  bv169
  Subscriber
  
  Yes, I am setting the free surface level at the inlet and outlet. Despite that it seems my phase from above that level is entering as backflow below that level. When open channel is turned on, I don't see the option to set the volume fraction for backflow...
- September 11, 2023 at 4:17 pm
  
  Rob
  Ansys Employee
  
  It shouldn't unless you have another boundary that's not been set? Pictures will help us as we're guessing at present.
- September 11, 2023 at 6:05 pm
  
  bv169
  Subscriber
  
  This is the outlet after initialization:
  and this is the outlet after the model vof diverges and crashes:
- September 12, 2023 at 12:11 pm
  
  Rob
  Ansys Employee
  
  If you're using open channel why is the domain full of water? The second image looks to be a diverged result so not overly diagnostic.
- September 12, 2023 at 1:37 pm
  
  bv169
  Subscriber
  
  The domain isn't completely full, but it is mostly full. The air phase is only about 5 meters at the top of the domain. Thanks for the info regarding the contour, I can try to evaluate where the divergence in vof is coming from before it gives the error to get a better diagnosis of the issue.
- September 12, 2023 at 2:06 pm
  
  Rob
  Ansys Employee
  
  What did you set on the top boundary as the backflow phase?
- September 12, 2023 at 2:44 pm
  
  bv169
  Subscriber
  
  The top was set to a wall in this run. I just looked into changing the top boundary to an outlet/outflow and don't see the volume fraction option in any of the boundary types.
- September 12, 2023 at 2:56 pm
  
  Rob
  Ansys Employee
  
  Pressure outlet - there should be a phase option. Do NOT use outflow, it's an old boundary and isn't recommended for use in most cases.

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