A simulation study was carried out on the free surface granular flow occurring in partially filled tumbler mixers. Three different 2D geometries were considered: a drum, a double cone mixer and an asymmetric double cone mixer. The solid-air two phases flow was simulated using an Eulerian continuum approach, including a suitable rheology for the granular flow. We focused on the kinematics of the active layer where mixing occurs. The modification of the surface active layer, as a consequence of mixer geometry and rotation, was studied. The analysis of (a) the active layer depth and area, (b) the velocity field and (c) the level of granular activation (given by granular temperature) suggested that the less conventional geometry, i.e. the asymmetric double cone mixer, has the potential to give better mixing performances than the traditional tumblers. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.