Flow organization at small scales

Monodisperse and bidisperse granular flows in rotating drums (cylinder, sphere, double cone) are studied numerically to better understand the influence of the geometry of these mixers and optimize powder mixers. 
This research is carried out in collaboration with

Richard Lueptow de Northwestern University et 
Nathalie Thomas de l'IUSTI, Marseille. 

In addition to highlighting the influence of wall roughness, these numerical studies have shown the existence of convection cells in rotating drums. For a monodisperse granular material, a convection cell develops on each side of the equator (plane of symmetry) in spherical and double-cone geometries. In a short cylinder, 2 cells are also found, but for longer cylinders, 2 new additional convection cells, contra-rotating with respect to the first ones, appear in the center of the drum. The outer convection cells, close to the side walls, are highly efficient, inducing recirculation in around a hundred rotations. The central cells are far less efficient, requiring more than 1,000 rotations to sweep the entire cell. In the case of a bidisperse granular medium, we have shown that in a sphere, convection cells are responsible for reversing the segregation pattern. In cylindrical geometry, convection cells explain the appearance of axial segregation in the vicinity of side walls, and explain why this segregation is always composed of a band of large particles at wall level. Finally, we have shown that for cylinders with large aspect ratios, the central convection cells lengthen, but no new cells appear.

Contact : Umberto d’Ortona