  We investigate numerically how the motion of an intruder within a
two-dimensional granular system affects its structure and produces drag on the
intruder. We made use of discrete numerical simulations in which a larger disk
(intruder) is driven at constant speed amid smaller disks confined in a
rectangular cell. By varying the intruder's velocity and the basal friction, we
obtained the resultant force on the intruder and the instantaneous network of
contact forces, which we analyze at both the cell and grain scales. We found
that there is a bearing network that percolates forces from the intruder toward
the walls, being responsible for jammed regions and high values of the drag
force, and a dissipative network that percolates small forces within the
grains, in agreement with previous experiments on compressed granular systems.
In addition, we found the anisotropy levels of the contact network for
different force magnitudes and regions, that the force network can reach
regions far downstream of the intruder by the end of the intruder's motion,
that the extent of the force network decreases with decreasing the basal
friction, and that the void region (cavity) that appears downstream the
intruder tends to disappear for lower values of the basal friction.
Interestingly, our results show that grains within the bearing chains creep
while the chains break, revealing the mechanism by which bearing chains
collapse.
