  Studies of granular materials, both theoretical and experimental, are often
restricted to convex grain shapes. We demonstrate that a non-convex grain shape
can lead to a qualitatively novel macroscopic dynamics. Spatial crosses
(hexapods) are continuously sheared in a split-bottom container. Thereby, they
develop a secondary flow profile that is completely opposite to that of
rod-shaped or lentil-shaped convex grains in the same geometry. The crosses at
the surface migrate towards the rotation center and sink there, mimicking a
`reverse Weissenberg effect'. The observed surface flow field suggests the
existence of a radial outward flow in the depth of the granular bed, thus
forming a convection cell. This flow field is connected with a dimple formed in
the rotation center. The effect is strongly dependent on the particle geometry
and the height of the granular bed.
