For model A simulations, the viscous force is applied in addition to the particular drag model (Schiller Naumann, Koch Hill, DiFelice or Gidaspow). I have read over the paper by Zhou (2010) 'Discrete particle simulation of particle-fluid flow: model formulations and their applicability' and I have seen that this viscous force term is included in their eqn 2.13, the sum of forces on a particle.
However, I am still puzzled as it seems that the viscous force is included in the drag force and thus 'captured' by the use of the particular drag force model selected. Can you please explain what this term is and why it is not part of the drag force?