Abstract: This work analyses in detail the transition process from the fixed to the fluidized state for homogenous and binary mixtures of particles differing in density, with different initial configurations. The phenomenon is studied in cylindrical columns fed with water, with the aid of CFD-DEM numerical simulations. With regard to the homogenous case, the well-established knowledge is substantially confirmed. The transition to the fluidized state strongly depends on the initial solid configuration in the case of same density binary-solid mixture. For the segregated case, the experimental behaviour reported by Di Felice and Scapinello in 2010 is closely reproduced, whereas for the initially perfectly mixed case, the contemporary presence of a fixed and a fluidized region is observed. A simple model that predicts the extent of these two regions as a function of the fluid velocity is proposed.