  Resistance force exerted on an obstacle in a gravity-driven slow granular
silo flow is studied by experiments and numerical simulations. In a
two-dimensional granular silo, an obstacle is placed just above the exit. Then,
steady discharge flow is made and its flow rate can be controlled by the width
of exit and the position of obstacle. During the discharge of particles, flow
rate and resistance force exerting on the obstacle are measured. Using the
obtained data, a dimensionless number characterizing the force balance in
granular flow is defined by the relation between the discharge flow rate and
resistance-force decreasing rate. The dimensionless number is independent of
flow rate. Rather, we find the weak shape dependence of the dimensionless
number. This tendency is a unique feature for the resistance force in granular
silo flow. It characterizes the effective flow width interacting with the
obstacle in granular silo flow.
