Hi Sulman,

First of all thank you for your nice question and happy new year. I am sorry the late response since I was in long holiday.

Lets go step by step:

1. This is incompressible solver and in incompressible solvers of OpenFOAM, we are using kinematic pressure instead of absolute pressure. Therefore, we are not interested in the exact pressure value for an incompressible flow in a channel, but only pressure difference.

2. zeroGradient means that the gradient of respective quantity is zero, meaning that the actual value is constant. Ususally, pressure is taken to be zeroGradient at an inlet, because the pressure is assumed to be constant there, whereas we will set the pressure to be 0 at the outlet.

3. In OpenFOAM you can use generally Dirichlet vs Neumann BCs. Moreover, at the inlet usually a velocity value is defined as fixed uniform value in OpenFOAM where pressure is assumed to be constant and assingned Neumann BCs which is zeroGradient.

4. Last thing is that why we assing 0 for pressure is related to NS equations that only the alteration of the pressure is presented, e.g. dp/dx, which means that only for pressure differences are being solved. In postprocessing is it just easier to handle the pressure distribution about zero

I hope that it is clear for you now.

Best.

Baris