The three simulated reactors are characterized by a free surface. For all the simulation the free surface is considered flat, i.e. undeformable. In this way the air liquid interface is implemented in the form of a free shear condition at the top of the liquid level. This introduces some inaccuracies in the flow field and turbulence quantities calculated in the region near the free surface [Ciofalo et al., 1996, Serra et al., 2000]. Nevertheless it is a necessary choice to contain the time of computation since explicit simulation of the free surface deformation is very computationally intensive [Issa and Ubbink, 1999, Jeong and Yang, 1998, Scardovelli and Zalesky, 1999].