Help in VOF Modelling of manholes
 

Hi guys!

I am new to ANSYS Fluent and I have a question regarding the VOF Model. I was trying to model a flow through a manhole wherein water will enter on an upstream pipe and a lateral through the manhole and will exit into another pipe. The top of the manhole should be open such that gauge pressure should be 0.

The problem I encounter is that the the pressure exerted by air in the manhole is not zero which I expected it to be. I tried putting a pressure outlet boundary on the top of the manhole but the result was still the same. It seems like the flow is always pressurized no matter what boundary conditions I used. Any thoughts?

Thank you all in advance!