Dissimilarities are likely to exist between experimental rotating cylinders and commercial scale rotating drums like cement kilns. Consequently, it is quite difficult to draw conclusions from these experiments and apply them directly to industrial configurations; the scale factor cannot be ignored. In the present paper a numerical study is carried out to investigate the dynamic characteristics and the rheology of a granular viscous flow in rotating cylinders. The equations are mainly based on the continuum representation of solid particles extended with the kinetic theory of granular flow and taking into account the solid frictional forces. The software Fluent is used to solve the latter model. The simulation reproduces the experimentally observed configurations of solid flows for the intended aim, i.e., rolling and cascading regimes. The predicted values are in reasonable agreement with the experimental data of Ding et al. (2001). A new phenomenon is highlighted; velocity vectors of particles at the bed surface are not always parallel to the latter but make an angle. This means that particles, when they move, do not slide or roll but jump. The validation of the model for a commercial scale rotating cylinder is treated in the second part of this study. Similarity Criteria have been inspired from the works of Henein et al. (1983). Thus, the model could be validated for industrial scales and will be used to provide suitable conclusions for a real cement rotary kiln. (C) 2012 Elsevier B.V. All rights reserved.