Kinematics of confined granular flows: effect of particle shape anisotropy

In this work, granular media made of shape anisotropic particles are investigated in heterogeneous, confined flow conditions. Experiments are performed in an annular shear cell with a rotating bottom wall and a top wall permitting confinement of the flow. The granular samples are composed of 3D printed spherocylinders with three different aspect ratios. Kinematic profiles as well as particle orientation statistics are computed by means of a particle tracking algorithm. We analyze at first the vertical profiles of the kinematic variables. In analogy with spherical particles the translational velocity shows an exponential decay, and remarkably seems not influenced by the particle shape. Conversely, angular velocity is significantly affected by the particle shape. Rotations are frustrated by the particle elongation and, on an average basis, the angular velocity systematically decreases with the particles’ elongation. The confined nature of the flow induces a broad variation of the shear rate, and differences are observed between the zone with low shear rate and the zone of shear localization. Particularly, angular velocity, once rescaled by the local shear rate, shows a decreasing trend moving farther from the shear zone, thus pointing out a stronger inhibition of particles rotation in the creep-like region of the flow. We turn then our attention to the orientational ordering of the particles. In highly confined flow conditions, a strong collective re-orientation of the particles is observed and these show a misalignment with the streamlines that tends to decrease when increasing the particles’ elongation. Samples composed of more elongated particles display a higher orientational order, and, at the particle scale, the correlation between angular velocity fluctuations and local orientation becomes stronger with the particles elongation. Furthermore, orientational order seems slightly shear rate dependent with a more ordered state in zones characterized by a lower shear rate. Finally, numerical 3D discrete element analyses are compared with the experimental data, permitting extension of the results on a wider range of aspect ratios. Numerical data allow a detailed characterization of the rotational dynamics and velocity fluctuations at the particles scale.