  Granular material flowing through bottlenecks like the openings of silos tend
to clog and to inhibit further flow. We study this phenomenon in a
three-dimensional hopper for spherical and shape-anisotropic particles by means
of X-ray tomography. The X-ray tomograms provide information on the bulk of the
granular filling, and allows to determine the particle positions and
orientations inside the silo. In addition, it allows to calculate local packing
densities in different parts of the container. We find that in the flowing zone
of the silo particles show a preferred orientation and thereby a higher order.
Similarly to simple shear flows, the average orientation of the particles is
not parallel to the streamlines but encloses a certain angle with it. In most
parts of the hopper, the angular distribution of the particles did not reach
the one corresponding to stationary shear flow, thus the average orientation
angle in the hopper deviates more from the streamlines than in stationary shear
flows. In the flowing parts of the silo shear induced dilation is observed,
which is more pronounced for elongated grains than for nearly spherical
particles. The clogged state is characterized by a dome, i. e. the geometry of
the layer of grains blocking the outflow. The shape of the dome depends on the
particle shape.
