Modeling liquid aspiration
 

Hello,
I am trying to model aspiration through a small tube within a larger tube (for example: drawing blood through a needle with the needle inside the vein). The boundary condition of the vein is a velocity inlet with a UDF to capture pulsatile flow. I am unsure of the best way to model the aspiration: can I set it as an inlet with a velocity opposite in direction to the boundary? Will fluent correctly model this as a constant velocity of fluid exiting the "needle" boundary?
In the past I have used an "outflow" condition, however with pulsatile (changing) flow, the outflow ratio would also have to change.

Thank you!
Marissa