Open boundary condition for VOF in CFX
Dear all,
I am using CFX for modeling the transient free surface flow, but I have a problem if I specify the open boundary condition for the domain. In CFX, you must specify the value of volume of fraction for each phase, nevertheless, you don't actually know the exact value of the volume of fraction in a transient case. How can I do that if I have to specify an open boundary condtion or can CFX specify a zerogradient for fraction? It will be grateful if you can give me any further suggestions and thank you very much for your attention! Cheers, Zhihua 
Re: Open boundary condition for VOF in CFX
Place the BCs where the VF is known a priori.
If this is not possible you may have to calculate the BC hydrostatic pressure profile on the fly. 
Re: Open boundary condition for VOF in CFX
Thanks for your response. It is easy if the VF is known a priori. I t is possible to calculate the hydrostatic pressure if the BC is on the right of a 2D domain, but if the BC is on the top, how can I specify it? As a transient case, it will be complicated if calculating the hydrostatic pressure every time step. Anyway, thanks for your advice and I also wonder how to specify the zerogradient for VF at the boundary. Does anyone have some further suggestion? Thanks.

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