Preventing backflow from occuring
 

Im am working on a horizontal pipe resembling part of the lungs bronchioles. Instead of working with a pressure difference (between the inlet and outlet) I am focusing on volumetric flow rates therefore I am using a velocity inlet and a flow split outlet.
I am entering velocities using a time table as it is time dependent, air is entering the tube at a certain velocity for a certain time period. Then when the time is reached the velocity drops to 0 m/s and air no longer enters the tube.

The problem is that when the velocity drops to zero m/s the air starts to flow backwards, this does not occur in the body.
Is there anyway of entering a function or table to ensure the air flows out the tube once the velocity drops to 0m/s
I want to work like an on/off switch

Any help would be greatly appreciated

you don't give the pressure difference at the pressure outlet, the pressure is depending on the wall shear stress (in a pipe). So use pressure outlet and you don't have problems with backflow. Because of continuity i think you can not deactivate the backflow.

Greetings Thomas