I see Raijin is asking the same question about degassing boundaries for ASM. I suspect neither Raijin or Antanas understand what I am saying - degassing boundaries do not make physical sense for ASM models, that is why you cannot apply it.

ASM assumes the disperse phase only has a relative slip to the continuous phase. At a degassing boundary the continuous phase flow must be tangential to the boundary (as it is a wall to the continuous phase). If the continuous phase has no component normal to the boundary then the disperse phase cannot have any component normal to the boundary either. This is because the disperse phase is just following the continuous phase, with a slip. If the disperse phase has no component normal to the boundary then no disperse phase fluid can pass through the boundary.

This is why you cannot apply degassing boundaries to ASM models, and also why all the work arounds you are discussing will not work either. If the disperse phase does pass through the boundary and degas then ASM is not the appropriate model for it as you have significant non-drag forces (for instance buoyancy).

I understand you Glenn. I asked about an alternative to this BC when modelling bubble column using ASM. Is it impossible to model such device using this model?

The only thing I can think of is to put a source term in a volume region at the top of the domain, just below where you want to put a degassing boundary. You can use a source term to push the disperse phase volume fraction to zero. This source region needs to be big enough that the continuous phase can enter and leave it, as that will drag the disperse phase into the source region with it.

Do you mean free slip wall with mass source term? How fast should this source eliminate dispersed phase? What about relevant momentum source? Do we need it? What do you think about opening BC in such case?

Yes, the top boundary can be a free slip boundary, but the source term to gobble up the disperse phase volume fraction will need to be a volume just below the boundary to ensure the disperse phase actually enters the source term region.

As it is a source term you can define it to gobble up volume fraction at any rate you like. Just make sure you keep bounded ie don't push the volume fraction below 0 or above 1.

No need for a momentum source term unless something is affecting the momentum. Based on what yo uhave described you do not appear to have this, so no momentum source term is required.

You certainly can use an opening boundary on the top face and avoid all these hassles, but of course that means the continuous phase will flow in and out of it. But you can fix the pressure at the location which may mean you have better location of the free surface anyway. Whether this is a good idea will depend on exactly what you are modelling.

I found the tutorial (link) where a bubble column is modelled using mixture model in Fluent. In this tutorial pressure outlet BC is used, I believe that this is equivalent to opening BC in CFX. I successfully reproduced this tutorial in Fluent, but have troubles in CFX. For some reason gas tends to rise along the wall that is close to inlet.

I noticed that operational density in this tutorial is set to the density of the lighter phase. It seems that this has similar purpose as ref. density in CFX, but in CFX help it is recommended to set this to the density of the continuous phase if there is not much dispersed phase.

I can't get good convergence even with small time step. I uploaded my cfx file here. Maybe you could take a look at this and see what I've missed.

What BC is more correct from the physical point of view? Degassing or opening? I mean in common, not concerning ASM. What do you think?

The degassing boundary acts as a wall to the continuous phase, but an outlet to the disperse phase. This means the continuous phase is constrained like a wall, which means it can generate pressure. It is usually used to model a free surface which has a known location and does not move. If there are flows strong enough that the free surface deviates from the modelled location significantly then this is not a good choice. The big advantage of degassing boundaries is that it is very numerically stable and easy to converge.

An opening will allow flow in and out and set the pressure at the surface. Note that you can set the static pressure in both directions or the total pressure in the forward direction - you have a few options. This has the advantage of making the static head of the free surface in the domain closer to the ideal flat free surface (again assuming it does not move). But this approach is less numerically stable as you may well have flow in different directions at different locations on the same boundary (ie forward flow and backward flow) and this is always numerically tricky and will be hard to converge.

Which is more correct? As always, that depends on what you are modelling and what is important. There is no general answer to that. And don't forget numerical stability is an important issue as well - if it is more physically correct but never converges it is not much good to you.

But if the rate will not be enough then volume fraction will grow which we don't want. I think that we must kill all this gas during one time step.

What about something like recoil?

OK Glenn, I see it. Thank you.

Then just use a source term to fix the volume fraction to 0. Then whatever VF enters the region will just get zapped to 0.

What is recoil? Recoiling against what? How is that relevant?

What is recoil force mentioned in degassing UDF in fluent?
 

Fluent UDF manual has given following description. Then in UDF code is defined for these forces. Can anybody guide me what is the significance of recoil force for degassing BC?

"The following UDFs are used to define the bottom surface as a standard velocity inlet for the gas
(primary) phase. The inlet VOF of the droplet phase is 0 and a negative source term for secondary phase
mass conservation is set for the layer of cells next to the outlet. The source removes all secondary phase
mass in the cell during one time step. The recoil force due to the mass source is also calculated".

Try the fluent forum.