Boundary Conditions in a gas stirred tank.
 

Hi everyone,
I am a beginner using Fluent. I am trying to simulate a gas stirred tank using Eulerian multiphase model. The case is about a tank full of water and the tank had a gas inlet in the center of the bottom of the tank. The Modelo is 2D. I am using the pressure-based solver. The tank is open to atmosphere, allowing the escape of injected gas. How can I set the atmospheric pressure (101325 Pa) in the top of the tank? What operating pressure condition should I establish?
The gas inlet is a velocity-inlet type of boundary and the value is 0.61 m/s. However, the condition needs a initial pressure gauge, What is the value that I have to set?

I hope you can help me and thank you for all your comments.

if it is open to the atmosphere just leave them as 0. both for the inlet and the outlet. (i think)