  In this paper we report the results of simulations of a 2D gravity driven,
dissipative granular flow through a hopper system. Measurements of impulse
distributions P(I) on the simulated system show flow-velocity-invariant
behavior of the distribution for impulses larger than the average impulse <I>.
For small impulses, however, P(I) decreases significantly with flow velocity, a
phenomenon which can be attributed exclusively to collisions between grains
undergoing frequent collisions. Visualizations of the system also show that
these frequently colliding particles tend to form increasingly large linear
clusters as the flow velocity decreases. A model is proposed for the form of
P(I), given distributions of cluster size and velocity, which accurately
predicts the observed form of the distribution. Thus the impulse distribution
provides some insight into the formation and properties of these ``dynamic''
force chains.
