Setup

cd $FOAM_RUN/tutorials/canal/Poiseuille
./clean.sh
less constant/polyMesh/blockMeshDict

The important lines, establishing Periodic (or cyclic) boundary conditions are again, as in Couette:

    cyclic inout
    (
        (0 4 7 3)
        (2 6 5 1)
    )

Check also:

less 0/p
less 0/U

The file 0/U is as in Couette:

    inout
    {
      type cyclic;
      value uniform (0 0 0);
    }

But now 0/p differs:

    inout
    {
    type fan;
    patchType cyclic;
    f List<scalar> 1(-0.005); // p_OF = p_real / rho
    value uniform 0;
    }

In a nutshell, we now specify a pressure drop across the system. Type fan is introduced for this special boundary conditions (this is what a fan would do). The f list, with only one element equal to $-0.005$, specifies the pressure drop in units of pressure divided by density (this is always the convention for pressure in incompressible OpenFOAM). The $0$ value is just for $t=0$ (same as for the velocity, and all previous cases.)

As usual by now, set up the mesh, and view it:

blockMesh
checkMesh
paraFoam

Then, run the simulation by typing icoFoam.