Please help- unexpected fluid entering boundary
 

This has been stumping me for months. Every so often it will happen again and I can't figure out why except that it might be a problem with the software.

I have a simple tank with several outlets and several vents. It is gravity feeding with no pressure. All the boundaries are set to "symmetry" except for the outlets and vents which are set to "specified pressure" with no pressure entered and the "stagnation pressure" box checked.

Normally things run just fine. But sometimes fluid enters the vents at the top and flows into the tank. Where the *&&^^ is the fluid coming from?

It must be a software glitch. But perhaps I'm missing something?

To ensure fluid does not enter you need to define fraction of fluid to 0. Fraction of fluid 1 indicates to use fluid 1 properties and fraction of fluid equal to 0 indicates to use fluid 2 properties if a 2 fluid problem or void region.