Multiphase Flow Boundary + Initial Conditions

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shmulik90
Posts: 2
Joined: Fri Jan 24, 2020 10:42 pm

Multiphase Flow Boundary + Initial Conditions

Post by shmulik90 »

Hi guys,

Im new to this forum and to CFD as well. I tried some tutorials and read a lot but somehow there isn't much information about setting up a simple transient multiphase (air + water in my case) simulation.

I am confused about 3 things:

1) Which boundary conditions should i set for e.g. a quadratic channel which has an inlet (water coming in) and an outlet (air / water going out)?

2) How to make use of these "initialization zones" - i often see that they are defined much larger than the geometry, is that a must? Do i have to use these zones for a multiphase simulation?

3) Why can i choose under boundary conditions "inflow volume fractions": What do they mean and why can i only choose 1?

I have attached the files.
Attachments
multiphase_test.FCStd
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thschrader
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Posts: 3129
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

Re: Multiphase Flow Boundary + Initial Conditions

Post by thschrader »

Hi and welcome to the forum!

Your second question:
All cells you want to initialize must be in the initialisation zone. See dambreak tutorial from openfoam manual
and tutorials from Josef Nagy.
https://www.youtube.com/channel/UCjdgpu ... Vvw/videos

The violet cubes initialize the cells with the waterfraction, the rest is air. So the violet cubes must be bigger
than the part of the mesh to be initialized.
Keep on foaming... :)
cube_inflow.JPG
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dambreak.JPG
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multiphase.JPG
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shmulik90
Posts: 2
Joined: Fri Jan 24, 2020 10:42 pm

Re: Multiphase Flow Boundary + Initial Conditions

Post by shmulik90 »

thank you @thschrader!

I checked some of the videos and I am succesfully setting up a model with inflow and initial conditions (alpha is water half of the channel height, air the rest).

Now i am having trouble setting the right outlet boundary. Ideally i would like to have an atmosphere boundary on top ( i guess i take an "open boundary" == ambient pressure = 0) and on one face i would like to have a boundary where water can leave the domain. I am having toubles implementing this as i do not fully understand what e.g. the volume fraction value "water" or "air" should be for this boundary... see screenshot below.

If i put water = 1 -> does this mean only water can leave / enter??
Capture.PNG
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thschrader
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Posts: 3129
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

Re: Multiphase Flow Boundary + Initial Conditions

Post by thschrader »

shmulik90 wrote: Fri Jan 31, 2020 9:35 pm If i put water = 1 -> does this mean only water can leave / enter??
To be honest: I dont know.
I never use the interfoam-solver, I did the tutorials month ago just for fun.
Have a look at the openfoam tutorials in the bluecfd-core installation, dam-break tutorial.
The upper side of the square is the atmosphere and the parameters below are used as boundary-condition.
You can construct your model, write the case and before running have a look at the generated alpha-water
file. You can overwrite the boundary parameters, then run the calculation (dont close solver window in FC!)
atmosphere_damBreak.JPG
atmosphere_damBreak.JPG (22.15 KiB) Viewed 1465 times
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