[mostafu]

Hello everyone,

I have made a pipe with a periodic boundary condition (translational) and when I define a mass flow rate, everything goes fine and I can get the pressure drop. However, I have a problem where I want to define a pressure gradient this time and get the mass flow rate as an output. The mass flow rate box gets de-activated, I cannot initialize the problem from the periodic boundary, the convergence is too slow, and finally I cannot get the mass flow rate unless I go to the reports and get the number, which turned out to be away lower than the expected and totally wrong. Does anyone have an idea why this happens?

[LuckyTran]

You should be retrieving mass flow rate from a report. You shouldn't be initializing from a periodic boundary, use standard initialization and and directly give the fields. The convergence is not any slower, it's just your initialization is off. All working as intended.

[mostafu]

Originally Posted by LuckyTran

You should be retrieving mass flow rate from a report. You shouldn't be initializing from a periodic boundary, use standard initialization and and directly give the fields. The convergence is not any slower, it's just your initialization is off. All working as intended.

Thanks a lot for the answer, that makes sense. I have another question if you don't mind. Can I solve with the same approach for multiphase, it seems to work (by defining only pressure gradient, not mass flow). However, I cannot reach flow separation or any other 2 phase phenomenon and the flow is totally homogeneous at the end of the run. Do you have an idea how I can do it in the right way?

[LuckyTran]

You seem to be under the impression that they are somehow different. The archetype of the periodic BC is the pressure gradient. The mass flow option is the same BC using a pressure gradient, but Fluent iterates the pressure gradient until the set massflowrate is achieved. If it can be done with massflow rate, then it can definitely be done using the pressure gradient approach. If it doesn't work with a pressure gradient then it definitely won't work with the massflow option.

[mostafu]

Originally Posted by LuckyTran

You seem to be under the impression that they are somehow different. The archetype of the periodic BC is the pressure gradient. The mass flow option is the same BC using a pressure gradient, but Fluent iterates the pressure gradient until the set massflowrate is achieved. If it can be done with massflow rate, then it can definitely be done using the pressure gradient approach. If it doesn't work with a pressure gradient then it definitely won't work with the massflow option.

Yes yes i got this point, it might be my initial values are a bit odd. But I am wondering if that can be used with multiphase also (VOF)? To get a fully developed flow there.