  Placing a round obstacle above the orifice of a flat hopper discharging
uniform frictional discs has been experimentally and numerically shown in the
literature to create a local peak in the gravity-driven hopper flow rate. Using
frictionless molecular dynamics (MD) simulations, we show that the local peak
is unrelated to the interparticle friction, the particle dispersity, and the
obstacle geometry. We then construct a probabilistic Tetris-like model, where
particles update their positions according to prescribed rules rather than in
response to forces, and show that Newtonian dynamics are also not responsible
for the local peak. Finally, we propose that the local peak is caused by an
interplay between the flow rate around the obstacle, greater than the maximum
when the hopper contains no obstacle, and a slow response time, allowing the
overflowing particles to converge well upon reaching the hopper orifice.
