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m0h November 16, 2013 16:45

Multiphase: Opening: prevent 1 Fluid of leaving the domain
Hello all,

I am runing a multiphase simulation in ansys CFX with a Rotor and a Stator domain. In these two domains I want to have an opening to have an airiation to the system.

My two fluids are oil and air. I am looking for a possiblity to prevent the oil going out of the domains. air is allowed to leave the domains.

I miss understood the "Volume Fraction" values in the 3rd tab (Fluid Values). I thought, if I set oil to 0 and air to 1 that this solves my issue but it does not, because this defines which Fluids can come back from the "outside".

Then i modeled a subdomain and tried to define two different permeabilities for the fluids. Air got 0.1 mē and oil got 1E30. I assumed a low value for air and a very high for the oil. But i am still loosing oil through this opening

Does anyone have an idea how to apply an opening where just one fluid is allowed to go out?

Thank you for your time,

ghorrocks November 17, 2013 08:01

Is a degassing boundary suitable?

m0h November 17, 2013 11:21


Thank you for the fast reply,

I checked the degassing boundary for outlet in the ansys help. it is available if one of the two fluids is a dispersed one. But in my case both fluids are "continous fluid" so this option is not available for me.

Thank you

ghorrocks November 17, 2013 17:07

Can you use air as a dispersed phase? Why do they both need to be continuous?

m0h November 21, 2013 14:19


Thank you for the tip. my system is now working. the only problem i have now is the following:

On the Outlet Degassing i am building up a negaitve pressure which influences my result dramatically. with the degassing boundary it is not possible to set the pressure at this surface. With the opening i had the possiblity with opening / entrainment to set the relative pressure to 0.

How can i solve this issue?

regards robert

ghorrocks November 21, 2013 17:46

The degassing boundary assumes you know the degassing surface (maybe the free surface). This sounds like you do not know the degassing surface and in that case the degassing boundary is not appropriate.

If motion of the free surface is important then you might need to use a free surface model and put a pressure boundary above it. This will be a much more complex simulation so you would only do this if you absolutely had to.

m0h November 23, 2013 11:17

Hello all,

i solved my issue by splitting the opening surface into small surfaces and just selected one where no oil can leave the domain. this is not the perfect sollution, but it works for my model so thank you for taking your time to this issue.

Regrads robert.

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