User field function "if" condition
 

Hi,
I'm having one interface between two rooms at different temperature.
Natural convection is accuring between the rooms.
Hot air is entering at the top of the interface and cold air is leaving at the bottom.
My question is how can I define an unser field function (and surface integral) to measure the flow entering the interface.
I tried the following but didn't work :
($$Velocity[0] >= 0) ? $$Velocity[0] : 0
Can you help me, I'm not confortable with field function.
Thanks,
Djack

Hi
I've tried the problem you described - and it works fine exactly with the field function you wrote and Surface Integral over the Interface between rooms.
Describe the problem, please

You're absolutly right !
Thanks