[Isaac]

Hello,

I am trying to get a hang of OF and have been trying to set up some simple simulations. I have a simple case for a tank with a hole at the bottom draining water purely due to gravity.
It is a simple set up. Tank dia of 3 m, height 0.75 m, 0.1 m dia drain hole at the bottom center. The tank is initially filled with water upto height of 0.5 m and the rest above it is air. At time 0, the tank starts to drain purely under gravity.
What boundary conditions do I use for the atmosphere (top) and the drain, for U and p_rgh?
I would like to add that I have done a good bit of searching, including on this forum and while there are several "similar" questions and attempted responses, I feel that somehow the question hasn't been answered for someone at my level. I feel rather stupid not being able to figure this out but the truth is I could not. Each answer has confused me even more. The top boundary (atmosphere) is very similar to the dam break tutorial I think and is OK. But the bottom drain isn't that clear for me.
Can someone please help with a little more clear direction?

Thank you very much.

Isaac