I found here:https://www.cfd-online.com/Forums/openfoam-solving/77256-non-zero-divergence-incompressible-flow.html that for incompressible flow the divergence of velocity is not zero since only conservatism of the flux is enforced. To monitor the divergence one can look at the timeStepContinuityError. That also means that my visualization of vorticity is flawed since it is also calculated with the cell centered velocity. It is recommended to use the vorticity function object in Openfoam.