I am trying to simulate a vacuum pumpdown on a complicated geometry but having little luck. I want the pressure in the cavity to start at 100,000 Pa and pump down (transient) to 20,000 Pa (absolute), but Star is changing the pressure across the entire field during the first iteration. I'm currently using a time step of 1e-6, but have even tried 1e-9 and it doesn't matter. Conceptually it's very simple, a cavity with a single pressure boundary, but the results are disappointing.

Any ideas on what I could be doing wrong?

Sounds simple enough.

Are you using a time dependant pressure boundary? How is it prescribed - bcdefp.f or a table?

Where is your reference pressure specified?

Are you looking at absolute pressure or relative pressure?

I thought so too... until I tried it.

Physically the geometry is very small and we have a large vacuum reservior so I am just changing setting the pressure boundary on the GUI to the new value (-80kPa relative).

I believe my reference pressure (which is 100kPa absolute) is specified as one of the cells in the domain. (I say believe because now I am unsure of how the reference pressure works.)

I don't particularly care if it is absolute or relative. Perhaps I don't understand the question though.