Heaping and secondary flows in sheared granular materials

Abstract

Cylindrical containers with a rotating bottom disk (so-called split-bottom geometry) are well established devices to shear granular materials in a continuous way, and to generate well-defined localized shear bands in the granular bed. When material composed of shape-anisotropic grains is sheared in such a container, a secondary flow is generated that leads to the formation of a considerable heap of material near the rotation center. We demonstrate that this effect can be found not only with prolate grains, as shown in a previous study, but also for oblate particle shapes. In addition, the quantitative influence of geometric and dynamic parameters is studied systematically. It is shown that the fill height of the container has considerable influence on the time scale for heap formation, but much less effect on the heap height. Results of numerical simulations agree with the experimental findings and provide insight in the particle dynamics.

Heaping and secondary flows in sheared granular materials

“…The central part of the bottom can be rotated with respect to the outer container, the granulate develops a broad shear zone [21]. Observation and analysis of the particle arrangements and dynamics can be performed optically at the granular surface [19][20][21][22][23], or in an index-matching fluid with a Laser sheet [24], destructively by excavation [25][26][27], or non-invasively by X-ray CT [26,27,30,31] and Magnetic Resonance Imaging [28,29].…”

“…A camera mounted vertically above the container allows us to record the horizontal displacement of the Laser reflec-tion. From this shift, the radial profile of the emerging heap is calculated [20]. With an array of Laser lines (Figure 2), accomplished by a combination of a line Laser and a diffraction lattice, multiple scans of the granular surface can be recorded simultaneously during the shear experiment.…”

“…The optimal fill height for heap formation is when the granulate at the surface above the revolving disk rotates with approximately half the disk velocity. A convective flow was detected by means of X-ray CT. Fischer et al [20] showed that heaping is also found in oblate material. They demonstrated the influence of the fill height on the time scales of heap formation and presented the results with numerical simulations.…”

“…Recently, the phenomenon of secondary flow has attracted the interest of researchers in different fields of granular matter physics (e.g. [7][8][9][10][11][12][13][14][15][16][17][18][19][20]). One prominent example where secondary flow effects manifest themselves is the heaping of elongated grains under shear in a cylindrical geometry [19].…”

“…One prominent example where secondary flow effects manifest themselves is the heaping of elongated grains under shear in a cylindrical geometry [19]. The formation of these structures as well as the dynamics of the secondary flow and its dependence on particle shapes and geometrical parameters of the setup have been studied in detail by Fischer et al [20]. Wortel et al [19] gave the following description of the phenomenon: Elongated grains form a heap in the center, which reaches a saturation height after a certain number of rotations.…”