  The drag of two cylindrical intruders in a two-dimensional granular
environment is numerically studied by the discrete element method. We find the
yield force, below which the intruders cannot move because of interactions with
the surrounding particles. Above the yield force, on the other hand, the
intruders can move at a constant speed. We investigate the relationship between
the drag force and the steady speed of the intruders, where the speed becomes
higher as the distance between the intruders decreases. We confirm that the
origin of the yield is the Coulombic friction between the particles and the
bottom plate by changing the value of the friction coefficient. We also find
that the yield force is almost proportional to the friction coefficient, which
means that the number of particles determining the yield force is almost
constant. On the other hand, the two-dimensional elasticity is applicable to
determine the stress fields around the intruders. We confirm that fields
asymmetric with respect to the drag direction are reproduced by using the
information of the stresses on the surfaces of the intruders by introducing
bipolar coordinates.
