Pressure boundary condition issue
 

1. I am modeling a manifold of an engine with an inlet and 4 outlets (cylinders)

2. I used UDF to define transient pressure boundary condition at four outlets.

3. My goal is to analyze airflow from intake to 4 cylinders at different time steps. Some cylinders are closed and some are open during different times.

4. For example, in first 20 steps air should flow only to cylinder 1 since remaining cylinders are closed. I modeled this by assigning same pressure at intake and other 3 cylinders.

5. My issue is I see a airflow from cylinder 2,3,4 to cylinder 1. It appears that this is happening due to the pressure drop between intake and cylinder 1 is same as pressure drop between cylinder 2 or 3 or 4 and cylinder 1.

Please let me know if this is due to boundary condition specification issue. I don't see any problem in UDF since I modeled the same with steady pressure boundary condition and I face the same issue.

Any help is appreciated.