I have a couple questions regarding the reference pressure under numerics and pressures specified at pressure outlets. First, is the pressure at the outlet gauge or absolute static pressure? If it is gauge pressure, does it use the reference pressure specified under numerics? For example, if I want 1 atm (101325 Pa) at my outlet, do I need to specify it or is it assumed by the software that 0 Pa at the outlet is gauge pressure with respect to 1 atm as a reference pressure? Thanks for your help!
The incompressible single phase code operates on kinematic gauge pressure, whereas the compressible single phase code absolute dynamic pressure. A zero fixedValue at outlet for an incompressible single phase computation is one of the appropriate settings.
Hello!
As @dylan pointed out, for incompressible single phase simulations we are interested only in pressure differences- the reference is arbitrary. On top of that, the incompressible solvers of OpenFOAM use pressure / density as a field for N-S equations. You can note that if the unit of pressure is m2/s = Pa / (kg/m3).
The reference pressure that you can find in the “numerics” section is required for systems that do not have a fixed pressure value defined at any boundary- for example a closed cavity. Since we deal with pressure differences, we need a reference point. It will be selected at chosen control volume with a prescribed reference value. I personally like to change the “reference pressure” for all my simulations, regardless if they have fixed pressure at boundary or not.
Hope this helps a bit 
Ciao!