  The kinematic flow pattern in slow deformation of a model dense granular
medium is studied at high resolution using \emph{in situ} imaging, coupled with
particle tracking. The deformation configuration is indentation by a flat punch
under macroscopic plane-strain conditions. Using a general analysis method,
velocity gradients and deformation fields are obtained from the disordered
grain arrangement, enabling flow characteristics to be quantified. The key
observations are the formation of a stagnation zone, as in dilute granular flow
past obstacles; occurrence of vortices in the flow immediately underneath the
punch; and formation of distinct shear bands adjoining the stagnation zone. The
transient and steady state stagnation zone geometry, as well as the strength of
the vortices and strain rates in the shear bands, are obtained from the
experimental data. All of these results are well-reproduced in exact-scale
Non-Smooth Contact Dynamics (NSCD) simulations. Full 3D numerical particle
positions from the simulations allow extraction of flow features that are
extremely difficult to obtain from experiments. Three examples of these, namely
material free surface evolution, deformation of a grain column below the punch
and resolution of velocities inside the primary shear band, are highlighted.
The variety of flow features observed in this model problem also illustrates
the difficulty involved in formulating a complete micromechanical analytical
description of the deformation.
