We presented simulations of a 2D granular model material, falling inside a vertical container with parallel walls.
Many cracks occur and interfere.
We followed in detail the events which lead to this crack. This happens when the surface of a particle with small angular velocity sticks on the wall. Sticking means here that the velocites of particle surface and wall surface are adapted. The momentum wave, starting from such a particle, leads to a region of large pressure, which spans the width of the system, i.e. a dynamic arch. Due to the strong pressure, the dynamic arch is slowed down by friction with the walls.