# boundary condition

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 October 1, 2009, 20:18 boundary condition #1 Member   Join Date: Mar 2009 Posts: 48 Rep Power: 9 Hi All I have bifurcation model including two branches with different sizes. So I can not estimate the pressure or velocity at these two and assuming pressure zero is not correct. I was wondering can we add some subroutine to define boundary condition at outlet. In that case how can I do that? Many Thanks

 October 2, 2009, 01:09 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,718 Rep Power: 99 Well, you have to know something about it! It is pointless even thinking about CFD until you have a set of boundary conditions which properly specify the flow. You need to have a think about how this flow is defined and where your boundaries should go. If you don't know either velocity or pressure then it is not a good place to put a boundary. You should move the boundary to somewhere where some conditions are known.

 October 2, 2009, 01:14 #3 Member   Join Date: Mar 2009 Posts: 48 Rep Power: 9 Hi Glenn, I mean physiologically we couldn't measure the velocity and pressure at outlet. Therefore we have to calculate the flow pressure or velocity according to the diameter of the branches. My question is how I can assign this boundary in cfx -pre as an additional variable or I don't know....

 October 5, 2009, 18:41 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,718 Rep Power: 99 Do you know the velocity or pressure? Is it a function of something? You can set the velocity of a velocity boundary or the pressure of a pressure boundary to be an expression so you can link it to other flow features if you like.

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