# controlling BC with conditions

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 April 2, 2009, 03:15 controlling BC with conditions #1 New Member   Marat Join Date: Mar 2009 Posts: 2 Rep Power: 0 Hi, everyone, I'm doing my simulation which is like drainaging water from a tank with a fixed inlet flow and a outlet flow which increases with time. From experiences of previous experiment, this process could only be steady state after few repetitions. But if I want to investage what was happening before it achieve steady state, one of the issues is the control of BCs esp the outlet BC. It would be quite interesting that the outlet hole can be closed, i.e. the mass flow becomes zero, once the volume fraction of the liquid at outlet bounder is less than unit, and then it's open again after a few timesteps. After reviewing some threads, I found it's impractical as some of experienced user suggest using stead state is a better option. Anyway, if it is possible, would anyone can give some hints about it? If not, I'll try to persuade my supervisor. Thanks a lot.

 April 2, 2009, 20:14 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,709 Rep Power: 98 Hi, If you are running a VOF simulation with gravity then won't the flow automatically stop when there is no liquid over the outlet? Glenn Horrocks

 April 2, 2009, 21:01 #3 New Member   Marat Join Date: Mar 2009 Posts: 2 Rep Power: 0 Hi, Glenn, thanks for your reply. This is another question I feel confused. During the solving process, the volume fraction of the liquid is monitored as well as its mass flow at outlet. The mass flow indeed goes to zero but it happenes after few time steps at which the volume fraction becomes less than one, i.e., they are not consistent with each other. (perhaps I'll try to make my time interval smaller to investigate it). Nevertheless, I think it could be better if I can close the outlet immediately after the volume fraction goes to less than unit. I guess the wall is put at outlet based on other criterion instead of volume fraction in CFX.

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