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Seb October 1, 2003 19:24

Degassing BC in Fluent
Hi all,

With Fluent 6, I would like to know if it is possible to impose a degassing BC at an outlet as we do in CFX. In more details, for two-phase flows, an outlet where the dispersed phase (usually a gaz) can escape and the other fluid (the liquid) cannot.


thomas October 2, 2003 04:06

Re: Degassing BC in Fluent
Hey, Ya man it can be done only by UDF using define profile. Then at the outlet you define a velocity inlet boundary condition and apply your UDF parameter. Can I know from what country are you from ?

regards thomas

Seb October 2, 2003 08:10

Re: Degassing BC in Fluent
Thanks Thomas,

I still have a problem with this procedure : the profil of the volume fraction and the velocity of the gas are unknown at the outlet. Hence, their profil cannot be defined.

I think I will have to expand the domain and add an "hold-up" of gaz. Hence, the outlet will be well defined. However, It will be more costly.

P.S. I am from Canada (Quebec)

thomas October 2, 2003 08:52

Re: Degassing BC in Fluent
Hey, Thanks to let me know where you are from, you name sounds like french so I wanted to figure out ... cause i am french. Okay you should explain to me a little bit more what you want to simulate ? are you trying to simulate bubble column or something like that ?


Seb October 2, 2003 10:33

Re: Degassing BC in Fluent
Hi Thomas,

Let say we have two rectangles (a small and a big one). The small rectangle is in the big one. THe domain is filled with water. The bubbles are produced underneath the small rectangle (inlet) and escape along its sides. The top of the big rectangle is the outlet. The remaining sides are walls. How do I manage to only let the bubbles escape from the top and not the liquid?


eric October 2, 2003 11:10

Re: Degassing BC in Fluent
hi, seb.

Fluent has recently launched a User Defined Functions (UDF) archive on Fluent's User Services Center (USC). a UDF for imposing a de-gassing boundary condition for the eulerian multiphase (euler/euler) & mixture models can be found there.

go to the USC (, click on the UDF archive link, then choose "UDF examples". a brief desciption and source listing is provided.

good luck!


Seb October 2, 2003 12:14

Re: Degassing BC in Fluent
Thanks. All I need is to ask my boss to buy a C compiler.

thomas October 3, 2003 04:01

Re: Degassing BC in Fluent
hey, effectively get the udf from the fluent user web site is the best you can do. This udf will help you to realize what you want 'bubble escape when the liquid does not'. This UDF will set the water vertical velocity component at 0 -> to create a water recirculation, like that the water will not leave the domain. Then it is the contrary for the bubble, the gas horizontal velocity will be set at 0. The velocities value correspond to the value of the last cell before the outlet. The UDF you might get could be more complicate than what I explained (essentially because of free-surface geometry or repartition) but it consists in the same way.

hope this will hep you ! thomas

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