- **Fluent UDF and Scheme Programming**
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- - **UDF to constrain one of the phase to stay in the domain**
(*https://www.cfd-online.com/Forums/fluent-udf/106186-udf-constrain-one-phase-stay-domain.html*)

UDF to constrain one of the phase to stay in the domainHas anyone ever written a UDF to constrain one of the phase (in a multiphase simulation) not to leave the domain?
I have got this two-phase simulation where my secondary phase (solid particles) are prevented from coming out the the flow cell with a wire mesh and I would like to simulate that with a UDF. Any help and/or ideas are very welcome. Cheers |

Just to mention that my outlet is a pressure outlet boundary condition.
Cheers |

UDF to constrain one of the phase to stay in the domainHi Koromajor
What do you mean by flow cell? If you mean to say that your secondary phase does not come out of the domain then at the outlet you have to define volume fraction as zero which resist the outward flow of secondary phase. best wishes Mvee |

Hi Mvee,
Thanks for your reply. By flow cell I meant my computational domain, so you understood it right :) How do you define a zero volume fraction at a pressure outlet? You can define the backflow volume fraction but this is different from the actual volume fraction of the phase. The other option would be to change the outlet boundary to mass flow inlet, then you can specify zero mass flow rate of secondary phase through the outlet, but I don't want to do that because the next step of my project is to introduce another secondary phase that will have no constraint unlike my first secondary phase. So if one changes the outlet boundary to mass flow inlet, you will still be stuck on what mass flow rate you will need to specify for the primary phase and the second secondary phase at the outlet. My feel is that, the solution of the problem is through a UDF, but like I said any help /suggestions are welcome, either via UDF or other ways of doing it. Cheers, Koromajor |

I suggest to define the back flow volume fraction as 1 which will resist the secondary phase to go out. Why do you feel this as different from realistic picture?
Through UDF (DEFINE_ADJUST) you can adjust the volume fraction as zero. Best wishes MVee |

The backflow volume fraction is only used by the solver in case reverse flow occurs during calculation, so backflow volume fraction set to 1 cannot prevent the secondary phase to exit the domain.
I don't think DEFINE_ADJUST (setting volume fraction to 0) can solve the problem because what you will only do is telling the solver to use 0 volume fraction at the outlet at the beginning of every iteration, but at the end of the iteration the solver could calculate non-zero volume fraction of secondary at the outlet (which means flow out). |

Hi Koromaor,
Did you solve your problem? i face the same problem that i am trying to model steam condensation inside shell and tube condenser. I face reverse flow problem that when i set the back flow volume fraction for secondary phase equal to 1 , the domain is filled with secondary phase. So i want to prevent any phases to enter the domain when reverse flow. Also, the pressure outlet can permit any phases to go out and nothing enter the domain. Thanks |

Go to Cell Zone conditions>select the phase>edit>mark fixed values>select constant in x and y velocity> put values to zero.
Now, after putting velocity and volume fraction to zero at inlet boundary condition, initialize the system with volume fraction of stationary phase as zero. you can then patch the domain with required volume fraction of stationary phase Hope this helps. |

Hi dear all
I have the same proble I want that secondary phase displce the primary phase but the volume fraction of primary phase doesn't go below the predefined value in each cell or voulme fraction of secondary phase never goes above the certain value I would be appreciated if you could help me |

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