Setting boundary conditions for simple pipe flow with flow direction changing in time
 

Hi friends
I'm pretty new to CFD simulation so please be patient with me. Thanks:p

I'm trying to model airflow in the lung airways during quiet respiration process. Essentially what I'm looking at is a continually bifurcating pipe system that has one big inlet (the trachea) and multiple small outlets (the bronchioles). The flow model is incompressible, isothermal laminar flow. The data available are the flow histories from all the outlets through time, and naturally they change flow directions periodically between the two phase of respiration. Also available are the driving pressure history at the inlet as well as the total inlet flow (summing all outlet flows together due to the incompressible assumption). I did some google search and I got some idea regarding how to simulate the steady state inspiration flow (specify velocity at the inlet and flow rate weightings at all the outlets by using outflow BC), but if I want to move on to transient analysis I have no idea how to proceed to the expiration phase. Also during the expiration phase the velocity profile at the inlet (or should I call it outlet now:confused:) should no longer be a constant value any more. Actually I wonder if it is possible to get a unique solution of expiration phase with all the information I currently have? Or do I need to make additional assumptions about the flow field?

As I said I'm still quite ignorant in this field so if some of the questions are stupid please don't laugh at me. Additionally If you see any error in my understanding of CFD in my problem description please let me know. Thanks!

A good place to start is to check the literature for similar cases and have a close read of their methods section. Have you found any papers which use CFD of the lung airways in a transient analysis?