Setup

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

The important lines, establishing Periodic (or cyclic) boundary conditions are these:

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

Check also:

less 0/p
less 0/U

The relevant lines are, in 0/p:

    inout
    {
    type            zeroGradient;
    patchType cyclic;
    value uniform 0;
    }

And in 0/U:

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

Finally, set up the mesh, and view it:

blockMesh
checkMesh
paraFoam

Run the simulation by typing icoFoam.