# Expression as boundary condition

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 June 15, 2010, 14:33 Expression as boundary condition #1 New Member   kits youhara Join Date: May 2010 Posts: 14 Rep Power: 8 Hello Everybody! I would like to pose a question. I am trying to implement different boundary along the same wall. e.g. velocity of 0[m/s] for the first 10mm and 5[m/s] for the next 10mm. First of all, is this done by an expression? If yes, how exactly? is there the possibility to insert a branch equation in the expressions? Couldn't really find something in the help section...... Any ideas welcome. Ty!

 June 15, 2010, 15:07 #2 Senior Member     Attesz Join Date: Mar 2009 Posts: 355 Rep Power: 9 Slice your wall surfaces, then mesh it again, and you will get in CFX-pre this surfaces separately (in mesh regions as primitive's or "named" faces). Now you can set them different velocities. Or if you want to create a continous velocity function along a direction?

 June 15, 2010, 18:26 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,638 Rep Power: 98 You can split the wall as Attila suggests, but you can also use a CEL expression to do it. You can use the step() function or if/then statements to generate the function, or you can use a 1D interpolation function with distance along the wall as the input variable. All these options are very simple to do.

 June 16, 2010, 08:45 #4 Senior Member   Join Date: Jan 2010 Posts: 110 Rep Power: 8 Just to complement the previous answers you can also divide the wall using the Operation Imprint Faces in the extrude feature from the 3D features toolbar. I think itīs always a good practice to use the least number of "blocks" as possible... Regards and good luck

 June 16, 2010, 09:04 #5 New Member   kits youhara Join Date: May 2010 Posts: 14 Rep Power: 8 Hello and thank you for your answers! I already tried a user function and it seems to work! Maybe I'll also try antonio's suggestion but I never done something like that again! Thanks anyway!

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